Lucas Henrique Carvalho Siqueira 

 
 
 
 
 
 
 
 

MORPHOLOGICAL VARIATION IN BOTHROPS JARARACA AND B. 

INSULARIS: SEXUAL DIMORPHISM AND ONTOGENY  

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 

 
São José do Rio Preto 

2021

 

Câmpus de São José do Rio Preto 



 

  

 

 Lucas Henrique Carvalho Siqueira  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

MORPHOLOGICAL VARIATION IN BOTHROPS JARARACA AND B. 

INSULARIS: SEXUAL DIMORPHISM AND ONTOGENY 

 

 

 

Tese apresentada como parte dos requisitos para 
obtenção do título de Doutor em Biologia Animal, 
junto ao Programa de Pós-Graduação Biologia 
Animal, do Instituto de Biociências, Letras e 
Ciências Exatas da Universidade Estadual 
Paulista “Júlio de Mesquita Filho”, Câmpus de São 
José do Rio Preto. 
 
Financiadora: CAPES 

 

 

 

 

 
Orientador: Profº. Drº. Otavio Augusto Vuolo 
Marques 
Coorientador: Carla Piantoni 

                                                         

 
 

São José do Rio Preto 
2021



 

  

 

 
 
 
    
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

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Instituto de Biociências Letras e Ciências Exatas, São José do Rio Preto. Dados 

fornecidos pelo autor(a). 

 
Essa ficha não pode ser modificada. 

S618m 

Siqueira, Lucas Henrique Carvalho 

MORPHOLOGICAL VARIATION IN BOTHROPS 

JARARACA AND B. INSULARIS: SEXUAL DIMORPHISM AND 

ONTOGENY / Lucas Henrique Carvalho Siqueira. -- São José 

do Rio Preto, 2021 

115 f. : il., tabs., fotos, mapas 

 

Tese (doutorado) - Universidade Estadual Paulista (Unesp), 

Instituto de Biociências Letras e Ciências Exatas, São José do 

Rio Preto 

Orientador: Otavio Augusto Vuolo Marques 

Coorientadora: Carla Pintoni 

 
1. Ecologia Animal. 2. Herpetologia. 3. Morfologia. I. Título. 



 

  

 

Lucas Henrique Carvalho Siqueira 

 
 MORPHOLOGICAL VARIATION IN BOTHROPS JARARACA AND B. 

INSULARIS: SEXUAL DIMORPHISM AND ONTOGENY 

 

 

Tese apresentada como parte dos requisitos para 
obtenção do título de Doutor em Biologia Animal, 
junto ao Programa de Pós-Graduação Biologia 
Animal, do Instituto de Biociências, Letras e 
Ciências Exatas da Universidade Estadual 
Paulista “Júlio de Mesquita Filho”, Câmpus de São 
José do Rio Preto. 
 
Financiadora: CAPES 

 

 

 

 

Comissão Examinadora 
 
Prof. Dr. Otavio Augusto Vuolo Marques 
UNESP – Câmpus de São José do Rio Preto - SP 
Orientador 
 
Prof. Dr. Márcio Roberto Costa Martins 
USP - Instituto de Biociências 
 
Prof. Dr. Fausto Errito Brabo 
Instituto Butantan 
 
Prof. Dr. Daniel Fernandes da Silva 
UFR J - Instituto de Biologia 
 
Profª. Drª. Verônica Alberto Barros 
Ministério do Meio Ambiente 
 
 
 
 

São José do Rio Preto 
17 de setembro de 2021 

 
 



 

  

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 

 
 
 
 
 
 
 
 
 
 

                                

  Dedico a minha família e meus amigos mais próximos que foram pilares sem os 
quais este trabalho não estaria de pé  

. 

 



 

  

 

AGRADECIMENTOS 
 
Ao meu orientador Otavio Marques, que esteve comigo em mais esta jornada e por 
acreditar mais uma vez na minha capacidade. 
 
À minha co-orientadora Carla Piantoni, pela colaboração do projeto até a tese, e por 
todas as dicas e sugestões que foram muito valiosas e enriquecedoras, mas 
principalmente por toda a motivação que me deu durante o processo. 
 
Agradeço à banca, por disponibizar parte do seu valioso tempo e por aceitar avaliar o 
meu trabalho. 
 
Á Giuseppe Puorto e Felipe Grazziotin por conceder acesso ao material biológico sob 
seus cuidados. 
 
Ao grande amigo Varldi Germano e Livia Correa, por sempre ser tão disposto a ajudar, 
sempre compartilhando seus conhecimentos e técnicas que hoje fazem parte do meu 
trabalho. 
 
Agradeço à Carlos Jared por ceder o uso do seu laboratório e a Luciana Sato pela 
ajuda e paciência com o uso dos equipamentos. 
 
À Selma Almeida-Santos por ceder o uso das dependências dos laboratórios do 
GERES e a Poliana Correa, por toda a colaboração no processo de histologia. 
 
A Kalena Barros pelo esmero e cuidado com os animais utilizados durante o trabalho 
 
A equipe da seção de Biologica Celular e Molecular do Leev, principalmente à Maria 
José e Mariana Guilard e Leonardo Kobashi pelos ensinamentos, uso dos 
equipamentos e reagentes. 
 
Á toda equipe do LEEV que sempre foi tão solícita e prestativa. 
 
Aos meus queridos amigos, por me ajudar não somente na parte acadêmica e coleta 
de dados, mas por todas as risadas, e presença nas mesas de bar. 
 
Agradeço muito as minhas queridas amigas, que são também irmãs, Silara Batista, 
Natália Torello e Karina Banci. Estas que foram imprescindíveis desde minha chega 
ao Butantan, conselhos, piadas ruins, às “sacudidas e banhos de água fria”,  
 
Agradeço à minha família por topo apoio emocional, principalmente à minha mãe que 
é meu porto seguro e me manteve firme por todo este tempo. 
 
Ao Ibilce pela oportunidade e toda prestimosa atenção. 
 
O presente trabalho foi realizado com apoio da Coordenação de Aperfeiçoamento de 
Pessoal de Nível Superior - Brasil (CAPES) - Código de Financiamento 001 
 
 

 



 

  

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
 
 

 

 

 
 
 

 

 

 

 

 

“[...] Venha, o amor tem sempre a porta aberta 

E vem chegando a primavera 

Nosso futuro recomeça 

Venha, que o que vem é perfeição.”  
(Legião Urbana, 1993) 

 



 

  

 

 
RESUMO 

 

A morfologia é um dos traços mais variáveis nas serpentes. Ela é altamente 

correlacionada a vários traços biológicos e também a pressões ambientais. Usei duas 

técnicas complementares, morfometria linear e geométrica, para avaliar a variação 

morfológica em Bothrops jararaca e B. insularis dentro, e entre populações e espécies, 

para testar o efeito do dimorfismo sexual, distribuição geográfica e tendências 

microevolutivas. Medi entre 11 a 17 variáveis lineares de cada indivíduo. Além disso, 

19 landmarks anatômicas foram posicionadas na cabeça, usando uma imagem 

fotográfica da vista dorsal. Em B. jararaca, fêmeas foram geralmente maiores para as 

medidas do corpo e da cabeça, ao passo que machos foram maiores para as variáveis 

da cauda. Encontrei efeito significativo da população, sendo que a população do 

planalto alcançou maiores tamanhos do que no litoral, e fêmeas apresentaram a 

cabeça com uma região pós-occipital em formato de flecha. Ambas as populações 

mostraram marcada alometria ontogenética, e a trajetória variou para cada traço 

medido. Um padrão de dimorfismo sexual similar ocorreu em B. insularis, mas não 

houve diferença no formato da cabeça, porém machos apresentaram olhos maiores. 

Os sexos tiveram trajetória ontogenética sobreposta para o formato do corpo, mas 

com inclinação diferente para o formato da cabeça. Comparações interespecíficas 

indicaram uma cabeça mais comprida e com focinho mais proeminente em B. 

insularis, mais similar à da população do planalto. A trajetória ontogenética também 

foi paralela com a do planalto e convergente com a população do litoral. A partição de 

nicho é uma explicação a para algumas das diferenças dos padrões aqui detectados. 

Da mesma forma, a disponibilidade de presas e ecologia comportamental podem 

produzir diferentes fenótipos em cada população ou espécie. Atribuo as diferenças na 

trajetória ontogenética principalmente à eventos de maturação heterocrônica e 

variação temporal nas mudanças ontogenéticas. 

 

Palavras-chave: Jararaca. Forma. Morfometria geométrica. Alometria. Jararaca-

Ilhoa. 

 
 
 

 
 

 



 

  

 

 
ABSTRACT 

 

Morphology is one of the most variable traits in snakes. It is highly related to several 

biological traits and also environmental pressures. I used two complementary 

techniques, linear and geometric morphometrics, to evaluate morphological variation 

in Bothrops jararaca and B. insularis within and among populations and species, to 

test the effects of sexual dimorphism, geographic distribution and microevolutionary 

trends. I measured from 11 to 17 linear variables from each individual. Moreover, 19 

anatomical landmarks were placed in the head using a photographed image of the 

dorsal view. In females were generally larger than males for body and head measures, 

while males were larger for tail variables. I found a significative effect of population, 

being that the highland population reached larger sizes than coastal population, and 

females presented a larger post-ocular region and a more arrow shaped head. Both 

populations showed a marked ontogenetic allometry and ontogenetic trajectory varied 

depending on each variable. A similar sexual dimorphism pattern occurred in B. 

insularis body, but with no difference in head shape, however males had larger eyes 

than females. Sexes had overlapped ontogenetic trajectory in body shape, but with 

different slopes in head shape. Interspecific comparisons indicated a longer head and 

prominent snout in B. insularis, closer to the highland population. Ontogenetic 

trajectory also was parallel with highland and convergent with coastal population. 

Niche partitioning is an explanation for the differences in the observed patterns. 

Accordingly, prey availability and behavioral ecology may produce different 

phenotypes on each population or species. I attribute differences in ontogenetic 

trajectories mainly due heterochronic maturation events and different onset on 

ontogenetic changes. 

 

Keywords: Pitviper. Shape. Geometrical morphometrics. Allometry. Golden 

Lancehead. 

 
 
 
 
 
 
 
 



 

  

 

 

ILUSTRATION LIST 

 

CHAPTER 1   

Figure 1 - Map for specimens’ distribution 27 19 

Figure 2 - Schematics for head measures 27  

Figure 3 - Boxplots for sexual dimorphism 37 19 

Figure 4 - Linear discriminant analysis for sexual dimorphism 39 24 

Figure 5 - Ontogenetic trajectories for linear variables 42  

Figure 6 - Principal component analysis for morphology 43 15 

CHAPTER 2  20  

Figure 1 - Schematic of landmark configuration 55  

Figure 2 - Adult centroid size variation 57  

Figure 3 - Principal component analysis for adult head shape 58  

Figure 4 - Adult static allometry for head shape 59  

Figure 5 - Adult wireframes for size scaling 60  

Figure 6 - Principal component analysis for ontogeny in head shape 63  

Figure 7 - Ontogenetic trajectory for head shape 64  

Figure 8 - Ontogenetic wireframes for size scaling 65  

CHAPTER 3   

Figure 1 - Boxplots for juvenile and adult sexual dimorphism 81  

Figure 2 -Principal component analysis for body shape variation 84  

Figure 3 - Principal component analysis for head shape variation 85  

Figure 4 - Ontogenetic trajectory for body shape 86  

Figure 5 - Ontogenetic trajectory for head shape 87  

Figure 6 - Principal component analysis for interspecific head shape  88  



 

  

 

Figure 7 -Interspecific ontogenetic trajectory for body and head shape 89  

APPENDIX A   

Figure 1 -Prey type and tail luring frequency 104  

Figure 2 - Proportion of individuals with tail luring 106  

Figure 3 - Logistic model for tail luring probability 108  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



 

  

 

 

LIST OF ABBREVIATIONS AND ACRONYMS 

 

A.S.L. – Above sea level 

IBSP – Instituto Butantan São Paulo 

SVL – Snout-vent length 

VS – Ventral scales 

SS – Subcaudal scales 

TL – Tail length 

TW – Tail width 

MW – Middle width 

HW – Head width 

DBE – Distance between eyes 

DBL – Distance between loreals 

DBN – Distance between nasals 

DEN – Distance eye to nasal 

DEL – Distance eye to loreal 

DLN – Distance loreal to nasal 

HL – Head length 

DRL – Distance rostral to labial 

HH – Head height 

ED – Eye diameter 

F – Female 

M – Male 

SDI – Sexual dimorphism index 

LDA – Linear discriminant analysis 

ANOVA – Analysis of variance 

ANCOVA – Analysis of covariance 

PCA – Principal component analysis 

CS – Centroid size 

MANOVA – Multivariate analysis of variance 

SSD – Sexual shape dimorphism 

BC – Body circumference 

RLD - Rostrum-labial Distance 



 

  

 

QGI – Queimada Grande island 

Regscore – Regression score 

RTL – Relative tail length 

MAT – Mean annual temperature 

MAP – Mean annual precipitation 

 

 

 

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 



 

  

 

SUMMARY 
 

 
 

1 - General Introduction 13 

2 - CHAPTER 1: Morphological variation on the common lancehead 

Bothrops jararaca populations: sexual dimorphism and ontogenetic 

patterns. 

24 

2.1 - Introduction 25 

2.2 - Methods 26 

2.3 - Results 30 

2.4 - Discussion 43 

2.5 - References 47 

3 - CHAPTER 2: Sexual dimorphism and ontogenetic variation on 

the head shape of two neighboring populations of the common 

lancehead Bothrops jararaca: a geometric morphometric approach 

51 

3.1 - Introduction 52 

3.2 - Methods 54 

3.3 - Results 56 

3.4 - Discussion 65 

3.5 - References 69 

4 - CHAPTER 3: Morphological variation in the golden lancehead 

Bothrops insularis: sexual dimorphism, ontogeny and 

microevolutionary trends. 

76 

4.1 - Introduction 78 

4.2 - Methods 79 

4.3 - Results 81 

4.4 - Discussion 91 

4.5 - References 95 

 APPENDIX A: intrinsic and geographic influences on the probability 

of born and retain conspicuous tail tip color (tail luring) in the 

common lancehead Bothrops jararaca 

101 



13 

 

 

 

 

GENERAL 

INTRODUCTION 

 

Extant snakes are characterized by a unique morphology related to others vertebrates. 

It has an elongated and cylindrical body associated to a multiplication in body vertebrae 

and an almost total reduction on limbs and girdle elements, which are most attributed 

to an evolution to a more terrestrial than aquatic habits (Apesteguías and Zaher 2006; 

Müller et al. 2010). Thereby, one may mistakenly think that snakes have a simplified 

morphology with little variation, however several studies have shown otherwise, with a 

large variation in size and shape mainly attributed to their functional biology, 

phylogenetic relationships and ecological pressures (Gans 1961; França et al. 2008; 

Hampton 2011; Esquerré and Keogh 2016).  

 In reptiles, morphological variation is expected not only on large scale, but occur 

in closely related species (Zamudio 1998; Wüster et al. 2005), and even 

intraspecifically, among sexes (Camilleri 1990; Shine and Shetty 2001; Brown et al. 

2017) or populations (Hoge et al. 1976; Zamudio 1998; Shine et al. 2012). Body size, 

the most prominent morphological trait, is often biased toward the sex that receive 

advantage for being larger (Shine 1994). For example, females are larger due an 

increase in fecundity, and males are the large sex when male-male combat is present 

(Shine 1978; Shine 1993; Shine 1994). On the other hand, the ecological hypothesis 

also provides good explanation for sexual dimorphism. Sexes often diverge due a 

niche partitioning, such as diet or habitat use, accordingly, the sex that consume larger 

prey often attain larger sizes (Shine 1986; Shine and Fitzgerald 1991; Shine 1998; Cox 

2007). Although body size be the most studied trait, dimorphism also occur in a variety 

of other traits, such as tail and head size and shape, scalation and coloration (Shine 

1993; Shine 1991; Shine and Shetty 2001). 

 The hypothesis stated above also account for morphological variation between 

populations of the same sex and also for sexual dimorphism degree. In Australia the 

python Morelia spilota is widespread all over mainland and islands and, unusually 

among reptiles, populations diverge in mating systems, being that populations of 

northeast, males present combat and are larger than females, whereas in southeast, 

no evidence of combat are known and females grew twice than males and reach 



14 

 

 

 

 

almost 10 times their mass (Shine and Fitzgerald 1995; Pearson et al. 2002a). 

Furthermore, comparing populations with a single mating system (no combat), females 

were always larger than males, however the degree of sexual dimorphism greatly 

varies, associated mainly with prey resources in each population (Pearson et al. 

2002b). 

 Sexual dimorphism is rather studied in samples composed only by adult 

specimens, however a comprehensive analysis on postnatal ontogenetic growth is 

important to understand the onset of diversification as sexes may have different growth 

rates and size/age at maturation (Beaupre et al. 1998; Taylor and Denardo 2005; 

Pearson et al. 2002a). Additionally, several traits exhibit a significative allometric 

association with size, and differences in sex, population or species may rise as 

differences in ontogenetic trajectories (Scanferla 2016; Strong et al. 2019). 

Accordingly, patterns of allometry are strongly related to species phylogeny and 

ecology, such as diet, foraging behavior and habitat use (Taylor and Denardo 2005; 

Urosevic et al. 2013; Sherrat et al. 2019). 

 Notwithstanding, phenotypes may diverge even in overlapped allometric 

trajectory, through heterochronic events. In its seminal review, Klingenberg (1998) 

argued about the concept of heterochrony, and although it is still under discussion, 

from a developmental point of view, heterochrony may be summarized as changes in 

rate and/or timing of ontogenetic allometries between groups. Heterochrony is the 

proximate cause responsible for several cases of morphological variation, being that 

groups may be paedomorphic or peramorphic in relation to each other (Klingenberg 

1998; Piras et al. 2011). In snakes, heterochronic processes are known to drive 

phenotypical convergence or divergence even in megadiverse clades, as the skull 

shape of microcephalic sea snakes (Sherrat et al. 2019) and the body and head shape 

of pythons (Esquerré et al. 2017). 

 In Brazil, the pit vipers of the genus Bothrops are one of the most diverse. About 

30 species are recognized and are widespread in all country (Costa and Bérnils 2018; 

Nogueira et al. 2019). The rapid diversification rates and radiation of the pit vipers to 

the New Word in late Oligocene and early Miocene (Alencar et al. 2016), enabled the 

occupation of several niches, and consequently, morphological adaptations (Alencar 

et al. 2016; Alencar et al. 2017). Species with enhanced arboreal habits are generally 

in intermediate sizes, are slander bodied and present larger tails than terrestrial 



15 

 

 

 

 

species (Martins et al. 2001; Alencar et al. 2017). Also, there is a broader variation in 

diet of pit vipers, with a wider range of prey types, generalist or specialist species, and 

presence or absence of a conspicuous ontogenetic change (Martins et al. 2002). 

Females are usually larger, and no male-male combat are rare (Barros et al. 2020). In 

this group, two species have been the subject of several researches, the common 

lancehead B. jararaca and its sister clade B. insularis, yet, little is known about 

morphological variation and developmental processes in these two species. 

 

BOTHROPS JARARACA 

The common jararaca Bothrops jararaca is a medium sized pit viper that reaches until 

1,600 mm in total size (Campbell and Lamar 2004). It is widely distributed in South 

America, occurring in Paraguay, Argentina, and in Brazil, from Rio Grande do Sul to 

southern Bahia (Campbell and Lamar 2004; Nogueira et al. 2019). This species 

horizontally, inhabits mainly the Atlantic Forest, however may occur in open areas and 

even in modified and highly urbanized regions, and vertically from sea level up to 

1,200m A.S.L. (Puorto et al. 1991; Marques et al. 2019; Nogueira et al. 2019). B. 

jararaca is largely a nocturnal species and may be found almost the entire year, 

although show a seasonal peak of activity during the rainy season (Sazima 1992; 

Campebell and Lamar 2004; Siqueira et al. 2021). 

 The species pass through a marked ontogenetic change in diet, with juveniles 

feeding mostly on frogs such as Hylids and Leptodactylids, however small rodents, 

lizards and centipedes are also eaten less frequently, and as adults rely mainly on 

small rodents (Sazima 1992; Marques et al. 2019). Ontogenetic changes are also 

evident in some morphological traits, such as tail tip color, that are often white or 

yellowish contrasting with the body color in juveniles and are used as a bait to attract 

ectothermic prey such as frogs and lizards, and the tail become suffused as snake 

grows (Sazima 1991; Sazima 1992; Martins et al. 2002). B. jararaca is an ambush 

predator that probably actively forage occasionally (Sazima 1992). Juveniles usually 

bite to envenom and hold their harmless prey to avoid them to scape (frogs jumps and 

chemical clues became difficult to follow), however, adults face usually more 

dangerous species, therefore release the prey after the bite, and follow chemical clues 

to find them after subjugation (Sazima 1989; Sazima 1991; Hartman et al. 2003). 



16 

 

 

 

 

 Females are larger and heavier than males (Sazima 1992; Matias et al. 2011), 

have a larger head and smaller tail (Wüster et al. 2005). A previous study also indicates 

that morphology may vary between population, with a trend of larger females in a small 

and urbanized fragment than in larger and connected one (Siqueira et al. 2018). 

Females usually mature at larger sizes than males (750 mm for females and 650 mm 

for males), and growth rates vary from 5 to 18 mm monthly, but no sex differences are 

known, and life span are estimated from 10 to 12 years (Sazima 1992). 

  

BOTHROPS INSULARIS 

The golden lancehead Bothrops insularis is an insular species endemic on Queimada 

Grande island, located about 33 kilometers far from the Southwestern Brazilian coast 

(24° 30' S, 43° 42' W; Duarte et al. 1995). The island is considered an inhospitable 

place due its inaccessibility and hostile environment with no fresh water spring and the 

presence of the venomous and snake (Amaral 1921b; Duarte et al. 1995). Although B. 

insularis appears to be abundant relatively with any other continental snake, the first 

populational estimative was below those once speculate in literature (less than 2500 

individuals in the island; Martins et al 2008). Unfortunately, few years later a detailed 

demographic survey found evidence for a populational decline trend (Guimarães et al. 

2014), which brought new concerns about the species knowledge and conservation. 

The endemism, small island area (430,000 m²) with suitable habitat, populational 

decline, and biopiracy make this snake one of the most threatened species in the entire 

word (Marques et al. 2004). 

 The most accepted hypothesis for the origin of the species is that B. insularis 

share a common ancestor with its sister clade B. jararaca. About 11,000 years ago, in 

the quaternary period, a glaciation has enhanced the sea level, isolating one 

population of the ancestor on what would be today, the Queimada Grande island, and 

due different selective pressures of the mainland, the island population suffered an 

allopatric speciation (Marques et al. 2002; Wüster et al. 2005). The golden lancehead 

has a more diurnal and arboreal habits than the general in the genus Bothrops (Amaral 

1921; Amaral 1921b; Marques et al. 2019). It feeds on small ectotherms as juvenile, 

such as centipedes and small frogs, and whereas adults, rely most on migratory 



17 

 

 

 

 

passerine birds that visit the island twice a year, moreover, its ontogenetic shift in diet 

is less conspicuous than B. jararaca (Martins et al. 2002; Marques et al. 2012). 

 Several morphological adaptations have accompanied the increased arboreal 

habits, such as smaller size, larger tail and slender body than the mainland sister 

species (Martins et al. 2001; Alencar et al. 2017). Due to morphological constraint, this 

species matures at small sizes and produces small clutches (Marques et al. 2013). 

Also, the feeding apparatus is adapted to its feeding habits, such as the larger head 

and smaller fangs than B. jararaca (Wüster et a. 2005). Sexual dimorphism includes a 

female biased body, head and fang length, and also an anteriorly positioned hearts, 

and males had a larger tail (Wüster et a. 2005; Marques et al. 2013).  

 In this work we use linear and geometric morphometric techniques to analyze 

morphological variation in B. jararaca and B. insularis. Specifically, we test hypothesis 

that there are morphological disparities in adult body and head shape between sexes, 

populations and species. Also, we seek for ontogenetic scaling in several 

morphological traits and test for differences in allometric trajectories between groups. 

 

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24 

 

 

 

 

CHAPTER 1 

MORPHOLOGICAL VARIATION ON THE COMMON LANCEHEAD BOTHROPS 

JARARACA POPULATIONS: SEXUAL DIMORPHISM AND ONTOGENETIC 

PATTERNS. 

 

ABSTRACT 

The common lancehead Bothrops jararaca is widespread in the Atlantic Forest 

in Brazil. The species is known to show a marked sexual dimorphism pattern, with the 

female being larger than males. However, most efforts in clarifying morphological 

variation between sexes are often focused on a single population. In this paper we 

investigate the effect of populational variation on the sexual dimorphism and 

ontogenetic trajectories of B. jararaca. We measured 17 morphological traits, including 

linear and meristic characters, and the analysis revealed a clear but variable effect of 

sex and population. Females were larger than males in all evaluated populations. 

Furthermore, females in the coastal population were generally smaller than in the 

highland population, but had significantly more scales. Widespread species often 

suffer from differential environmental pressure even in biotic and abiotic factors. We 

attribute the results found herein to specificities in prey availability and climatic 

conditions, which affect the ontogenetic pattern between sexes and populations 

resulting in specific sexual dimorphism pattern. 

 

KEYWORDS 

Geographic variation; Allometry; Growth; Morphometry. 

 

 

 

 

 

 

 

 

 

 



25 

 

 

 

 

2.1 . INTRODUCTION 

 In snakes, sexual dimorphism is a character with ecological impact, being widely 

shared in the Viperidae family (Hendry et al. 2014). Two hypothesis that possibly 

explain the difference between sexes acquire notable prominence in the last decades. 

The sexual selection hypothesis predicts that being larger for one sex carries certain 

advantages. In this case, males are larger in species that have combat behavior and 

females are larger in species where fecundity rate or offspring size is strongly 

correlated with maternal size (Shine, 1993; Shine, 1994). On the other hand, the niche 

partition hypothesis predicts that morphological differences are due to ecological 

differences between the sexes such as habitat use or diet (Camilleri and Shine, 1990; 

Shine, 1986). 

 The relationship between ecology and morphology in snakes is so 

complex that significant adaptations may emerge in a short time period after a drastic 

change in local dynamics (e.g. introduction of new species, environmental change). A 

striking example is the change in body measures of Australian snakes associated with 

the occurrence of the invasive toxic frog Rhinella marina. Over time, batracophagous 

snakes vulnerable to the toxin showed a reduction in the size of the mouth opening 

and an increase in body size (increasing in toxicity tolerance), changes that limit the 

intake of larger and potentially more toxic frogs and enhance survival probabilities 

(Phillips and Shine, 2004).  

The same species interaction can be applied to the allometric relationship 

between morphological traits of the prey and the predator. The parotoid gland of the 

frog grows disproportionately (larger frogs are relatively more toxic), while the snakes’ 

head relatively decreases with size, and this two allometric patterns compensate each 

other (Phillips and Shine, 2006). This means that snakes with relative smaller heads, 

younger individuals, or conspecific of the smaller sex, may become more vulnerable to 

the R. marina poison (Phillips and Shine, 2006). 

 Widely distributed species generally exhibit morphological variations among 

different populations. This pattern is mainly associated with differences in the 

environmental pressures to which each population is subject. These pressures can be 

of biotic origins such as eating habits in different types of prey (Fabien 2004), or of 

abiotic origins linked to climate, geography (e.g. altitude, latitude), or 

phytophysiognomy (Cruz -Elizalde et al. 2017; Zhong et al. 2017; Nóbrega et al. 2016). 



26 

 

 

 

 

 Morphometric studies are often focused only on adult individuals, however the 

morphological pattern studied may have resulted from ontogenetic development. 

Allometric hypotheses contrast growth rates in a given variable with body growth, and 

size dimorphism may arise i) early, if groups are already born in different sizes and 

maintain a parallel trajectory, ii) late, if a group grows for a longer time or iii) late, if the 

groups have different rates of intrinsic growth (see Klingenberg, 1996; Sanger et al. 

2013). 

The common lancehead Bothrops jararaca is one of the most emblematic 

snakes in Brazil, associated with the Atlantic Forest. This forest is located on the coast 

of Brazil at altitudes between 0 and 1,200 A.S.L. Juveniles feed mainly on anuran 

amphibians, while adults eat mostly small rodents (Campbell and Lamar, 2004; 

Marques et al. 2019; Sazima, 1992). Some study has shown populations with larger 

body size or marked sexual dimorphism (e.g., Matias et al. 2011, Siqueira and 

Marques, 2018). Although studies addressing morphological divergences in snakes 

have been published extensively, many are focused only on one population, sympatric 

species or address only one age group (e.g.adults). Therefore, the aim of this work 

was to test the hypothesis that populations of B. jararaca subject to different 

environmental conditions may present divergent morphologies. Specifically, we tested 

the influence of population variation on: i) direction and degree of intra and 

interpopulation sexual dimorphism and ii) ontogenetic allometry as a mechanism of 

morphological divergence. 

 

2.2 . MATERIAL AND METHODS 

 We choose to use an arbitrary but clear criterion for populational categorization. 

Populations found between 0 to 80 m A.S.L., were classified as coastal population, 

and above 800 m A.S.L., as highland population (Fig. 1). We analyzed 211 specimens 

of Bothrops jararaca: 1) 59 females and 51 males from coastal populations; and 2) 50 

females and 51 males the highland populations. All individuals were housed in the 

Herpetological Collection Richard Alphonse Hoge (IBSP), at the Instituto Butantan, 

São Paulo, Brazil. We measured 17 morphological traits, being 15 linear measures 

and 2 meristic traits in all individuals (see Fig. 2 for head schematics and Tab. 1 for 

variables descriptions). All measures were made using a ruler (precision on 1 mm) or 

digital caliper (precision of 0.5 mm). Raw data were used to present mean value and 



27 

 

 

 

 

standard deviation. Then, all measures were log transformed in order to achieve the 

normality requirement before the statistical analysis.  

 

Fig. 1. Localities of Bothrops jararaca included in the study. 

 

 

 

 

Fig. 2. Schematics illustration showing the variables measured in snakes’ head to 

analyze morphological variation among and within populations of Bothrops jararaca in 

A) dorsal and B) lateral view. Head width (HW), Distance between eyes (DBE), 

Distance between Loreals (DBL), Distance between nasals (DBN), Distance eye to 



28 

 

 

 

 

nasal (DEN), Distance eye to loreal (DEL), Distance loreal to nasal (DLN), Head length 

(HL), Distance rostral to labial (DRL), Head height, and Eye diameter. 

 

Table1: Names and description of the morphological variables used to access 

Bothrops jararaca morphological variation. Schematics in Fig. 1. 

VARIABLE DESCRIPTION 

SVL Snout-Vent Length; Measured from the tip of the nose to the anal scale 

VS Ventral Scales; Counted from the first scale post-quadrate bone 

SS 
Subcaudal Scales; Counted from the first post-anal scale to the tip of 

the tail 

TL Tail Length; Measured from the cloaca to the tip of the tail 

TW Tail Width; Measured post-cloaca 

MW Middle Width; Total circumference in mid-body 

HW Head Width; Measured in the larger portion of the head (quadrate bone) 

DBE Distance Between Eyes; Measured from right to left subocular scales 

DBL Distance Between Loreals; Measured from right to left loreal pit 

DBN Distance Between Nasals; Measured from right to left nasal scales 

DEN 
Distance Eye to Nasal; Measured from eye to nasal scales in the right 

side 

DEL Distance Eye to Loreal; Measured from eye to loreal pit in the right side 

DLN 
Distance Loreal to Nasal; Measured from loreal pit to nasal scale in the 

right side 

HL Head Length; Measured from the neck to the tip of the nose 

DRL 
Distance Rostral to Labial; Measured from the tip of the nose to the last 

labia scale in the right side 

HH Head height; Measured in parietal region; 

ED Eyes Diameter; Measured horizontally in the middle of the eye 

 



29 

 

 

 

 

 

 

 

2.2.1. Sexual dimorphism 

We included only adults in the analyses: females larger than 750 mm SVL and 

males larger than 650 mm SVL (Sazima, 1992). We analyzed 109 specimens in total, 

43 from the coastal population (25F and 18M), and 66 from the highland population 

(35Fand 31M). Variation on SVL, VS and SS between sexes and populations were 

tested using ANOVA with sex, population and interactions as factors. The size 

dependent variables were tested using ANCOVA (Table 3 details the variables and 

covariables). Significative triple interactions were further clarified using linear models 

with each dependent variable and its covariate to eliminate the effect of size. Then, the 

residuals of the regression were extracted and an ANOVA were performed with sex 

and population as factors with paired Tukey post-hoc test. 

 The Sexual Dimorphism Index (SDI) for each variable was computed as the 

(mean of female/mean of male) -1 (Shine, 1994). This arbitrary index varies from -1 to 

1, and expresses the relative size difference between sexes, being positive when 

female-biased, negative when male-biased, and zero when sexes are equal sized. 

Additionally, a Linear Discriminant Analysis (LDA) was used in order to observe the 

degree of separation or overlap of the sexes in each population, as well as which 

variables have higher discriminating scores between classes (male or female). 

 

2.2.2. Ontogenetic allometry 

Linear models were built for each sex and population separately, using 14 

variables that co-varied with size. The aim of these models was to test the hypothesis 

of presence of allometry or isometry in each one, and observe the size variation along 

the individual growth. Then an ANCOVA was performed to test the homogeneity of the 

slope. The presence of a significant result in the interaction is indicative of a difference 

in the growth trajectory. Similar slopes with intercept statistically significant, indicate 

parallel trajectories, with premature differentiation between groups. Significantly 

different slopes indicate divergent trajectory, with late differentiation between groups. 

Equal slopes and intercepts indicate no difference in allometric trajectory and any 

difference between groups simply appearing as a device of size magnitude (Sanger et 

al. 2013). 



30 

 

 

 

 

Finally, a Principal Component Analysis (PCA) was used to visualize the 

relationship between groups in the tangent space. Only the variables that showed 

significant results were kept in the analysis. and to avoid bias due to scaling in 

allometric variables, the residuals of the linear models were used. 

 

2.3. RESULTS 

2.3.1. Sexual dimorphism 

In general, females were morphologically larger than males, except for the tail 

variables. Between populations, the coastal population exhibited smaller values than 

the highland, except for meristic traits (mean and standard deviations in Table 2). In 

the same way, statistical analysis revealed a great variation in morphological patterns, 

both between sexes and populations (Table 3, Fig. 3). Six variables were significantly 

different only between sexes, while eight were different only between populations. We 

found significant effect of the triple interaction for HH and ED. However, those results 

did not hold after ANOVA performed in the residuals of linear models and Tukey post-

hoc tests. There was no effect of sex, populations nor interactions for the variables 

HW, DBL, DBN and DLN. 

Females were larger than males for SVL, VS and HL, whereas males were 

larger for SS, TL and TW. Considering populations, coastal females had more ventral 

scales (VS) than males and females from the highland, while coastal males had more 

subcaudal scales (SS) than males and females from the highland. Both sexes from the 

coastal population were smaller for TW, MW and HL, and were larger for DBE, DEN 

and DRL. 

In the coastal population SDI varied from 0 to 0.39, and in the highland, from 0 

to 0.3 (Table 4). The size disparity was larger in the coastal population only for MW, 

suggesting higher equitability between sexes in this population. In the LDA all females 

and males were correctly classified in both populations, and no overlap occurred (Fig, 

4). The best discriminant variables for the coastal population were HL (with negative 

values on the “x” axis) and TL (with positive values in the “x” axis). In the highland 

population the best discriminant variables were VS (with positive values in the “x” axis) 

and SS (with positive values in the “x” axis).  

 

 



31 

 

 

 

 

Table 2: Raw data of morphological variables of Bothrops jararaca on coastal and 

highland populations. F = females; M = males; sd = standard deviation. see 

abbreviations in Fig. 1) 

VARIABLE 

MEAN ± sd  

F COASTAL M COASTAL F HIGHLAND M HIGHLAND 

SVL 987.1±128.9 815.4±110.3 1038.2±124.6 800.4±82.1 

VS 204.9±5.7 199.2±5.9 196.7±5.4 191.7±4.9 

SS 59.1±2.1 63.5±4.6 56.6±3.3 62.1±2.9 

TL 146.7±22.4 129.6±13.2 147.7±18.2 129.8±16 

TW 10.3±2.1 9.4±1.6 11.8±2.4 10.6±1.4 

MW 68.9±37.8 41.7±26.1 104.6±24 73.5±9.9 

HW 27±5.9 21.3±3.3 30.8±3.6 22.2±3.1 

DBE 16.1±2.1 14.1±1.8 17.7±1.7 14.1±1.6 

DBL 12.6±1.9 10.7±1.6 14.4±1.6 11.2±1.3 

DBN 7.6±1.1 6.2±0.8 8.9±1.5 7.2±1 

DEN 11±1.4 9.1±1.2 11.8±1.6 9.3±1.2 

DEL 6±1 5.2±0.8" 7.3±1.1 5.6±0.8 

DLN 4.8±0.7 3.9±0.4" 5.3±1.1 4.1±0.6 

HL 43.1±5.3 32.8±3.7 49.4±5.8 35.8±4.2 

DRL 34.4±4.7 27.3±2.9 38.9±5.4 28.3±3.4 

HH 15.6±2.9 12.5±2 18.2±2.4 13.2±2 

ED 4.9 ±0.7 4.4±0.5 5.2±0.5 4.4±0.5 

 

 

Table 3: ANOVA and ANCOVA results of the morphological variation between sex and 

populations (Coastal and Highland) of Bothrops jararaca (dependent variables and 

predictors shown). F = F-test; P = P-value. (See abbreviations in Fig. 1) 



32 

 

 

 

 

Dependent Predictors F P 

SVL 

Sex 99.55 <0.001 

Population 0.88 0.34 

Sex:Population 1.96 0.16 

VS 

Sex 27.80 <0.001 

Population 54.27 <0.001 

Sex:Population 0.05 0.823 

SS 

Sex 55.34 <0.001 

Population 9.01 0.003 

Sex:Population 1.06 0.30 

TL 

SVL 160.62 <0.001 

Sex 6.41 0.01 

Population 0.05 0.82 

SVL:Sex 0.22 0.63 

SVL: Population 0.53 0.46 

Sex: Population 1.70 0.19 

SVL:Sex:Population 1.03 0.31 

TW 

TL 47.20 <0.001 

Sex 0.72 0.39 

Population 12.09 <0.001 

TL:Sex 0.30 0.58 

TL: Population 0.34 0.55 

Sex: Population 1.08 0.3 

TL:Sex:Population 1.67 0.19 

 



33 

 

 

 

 

Table 3: Continuation 

MW 

SVL 21.62 <0.001 

Sex 0.36 0.54 

Population 43.00 <0.001 

SVL:Sex 1.26 0.26 

SVL: Population 1.59 0.21 

Sex: Population 0.16 0.68 

SVL:Sex:Population 1.33 0.25 

HW 

HL 364.95 <0.001 

Sex 0.01 0.90 

Population 0.81 0.36 

HL:Sex 0.34 0.56 

HL: Population 0.30 0.58 

Sex: Population 2.58 0.11 

HL:Sex:Population 0.40 0.52 

DBE 

HL 387.69 <0.001 

Sex 0.34 0.55 

Population 6.09 0.01 

HL:Sex 0.20 0.64 

HL: Population 0.35 0.55 

Sex: Population 0.90 0.34 

HL:Sex:Population 0.06 0.79 

 

 

 



34 

 

 

 

 

Table 3: Continuation 

DBL 

HL 393.43 <0.001 

Sex 1.19 0.27 

Population 0.62 0.43 

HL:Sex 0.00 0.97 

HL: Population 0.02 0.88 

Sex: Population 0.82 0.36 

HL:Sex:Population 0.22 0.63 

DBN 

HL 215.46 <0.001 

Sex 3.18 0.07 

Population 3.90 0.05 

HL:Sex 0.03 0.85 

HL: Population 1.04 0.31 

Sex: Population 0.29 0.58 

HL:Sex:Population 0.04 0.83 

DEN 

HL 301.17 <0.001 

Sex 0.27 0.60 

Population 6.91 0.009 

HL:Sex 2.40 0.12 

HL: Population 0.01 0.92 

Sex: Population 0.02 0.87 

HL:Sex:Population 0.03 0.85 

DEL 

HL 269.92 <0.001 

Sex 4.52 0.03 

Population 1.42 0.23 



35 

 

 

 

 

HL:Sex 0.00 0.98 

HL: Population 1.33 0.25 

Sex: Population 0.19 0.65 

HL:Sex:Population 0.00 0.97 

DLN 

HL 167.84 <0.001 

Sex 0.10 0.74 

Population 1.46 0.230 

HL:Sex 0.34 0.5 

HL: Population 0.17 0.68 

Sex: Population 2.53 0.11 

HL:Sex:Population 0.35 0.55 

HL 

SVL 634.97 <0.001 

Sex 25.50 <0.001 

Population 45.24 <0.001 

SVL:Sex 0.12 0.72 

SVL: Population 1.08 0.29 

Sex: Population 0.38 0.53 

SVL:Sex:Population 0.78 0.37 

DRL 

HL 1341.79 <0.001 

Sex 0.96 0.32 

Population 8.09 0.005 

HL:Sex 2.36 0.12 

HL: Population 1.57 0.21 

Sex: Population 2.70 0.10 

HL:Sex:Population 0.00 0.94 



36 

 

 

 

 

HH 

HL 429.74 <0.001 

Sex 0.44 0.50 

Population 0.25 0.61 

HL:Sex 0.33 0.56 

HL: Population 0.03 0.85 

Sex: Population 4.39 0.03 

HL:Sex:Population 2.69 0.10 

ED 

HL 155.19 <0.001 

Sex 0.53 0.46 

Population 5.09 0.02 

HL:Sex 2.64 0.10 

HL: Population 1.44 0.23 

Sex: Population 2.75 0.10 

HL:Sex:Population 4.47 0.03 

 



37 

 

 

 

 

 

 



38 

 

 

 

 

Fig. 3. Boxplots showing sexual dimorphism in two populations of Bothrops jararaca. 

Top panels are raw data, and other plots are residuals extracted from linear models 

between target variable and covariable (see table 3) to exclude the effect from size. All 

variables were previously log-transformed. 

 

Table 4: Sexual Dimorphism Index (SDI) for morphological disparity in two Bothrops 

jararaca populations (calculated as (mean of female / mean of male) - 1). Positive 

values indicate female bias, negative values  male bias, and 0 absence of dimorphism. 

VARIABLE 

SDI 

COASTAL HIGHLAND 

SVL 0.17 0.23 

VS 0.12 0.12 

SS 0.03 0.03 

TL -0.07 -0.1 

TW 0.08 0.1 

MW 0.39 0.3 

HW 0.21 0.28 

DBE 0.13 0.2 

DBL 0.16 0.23 

DBN 0.18 0.2 

DEN 0.18 0.21 

DEL 0.14 0.23 

DLN 0.2 0.23 

HL 0.24 0.27 

DRL 0.21 0.27 

HH 0.2 0.28 

ED 0.1 0.25 

 



39 

 

 

 

 

 

 

Fig. 4. Linear Discriminant Analysis between females and males Bothrops jararaca 

based on morphological data. A) coastal population and B) highland population. Dark 

grey bar = females; Light grey bar = males. 



40 

 

 

 

 

 

 

 

2.3.2 Ontogenetic allometry 

Allometry hypothesis were rejected once for MW in coastal males (r² = 0.07, p 

= 0.08). In general, the percentage of variation explained by the size scaling is quite 

close in both sexes and populations. Only in two of the 56 models built, the percentage 

of variation explained by size was below 70% (64% for DLN and 42% for ED in males 

from the coast). 

Significant effects in the triple interaction between the covariate and the fixed 

factors sex and population were not found, indicating parallel trajectories between the 

groups (Fig. 5). Nevertheless, a significant effect on double interactions occurred in six 

variables, indicating a difference in the inter- or intra-population allometric trajectory. 

For the variables HW, DBL, DEL, and DRL, there was no significant effect of sex, 

population or interactions, with equivalent intercepts, and females reach higher values 

just because they have longer duration of systemic growth. The variable TL had 

significant intercept for the sex factor, with males being the larger one. The variables 

TW, DBE, DBN and ED had significant intercept for the population factor, where the 

coastal snakes being larger and indicating parallel trajectories with early morphological 

divergence. 

The variable HH showed a significant interaction between the factors sex and 

population, which points to parallelism between the trajectories, however with 

alternation of the larger sex, that is, on the coastal population, males are larger 

whereas on the highland population the opposite occurs. The variables MW, DEN and 

DLN showed significant interactions between the covariate and population, indicating 

late divergence between populations. Finally, HL showed significant interactions for 

the covariate and sex and for the covariate and population, indicating late divergence 

between these two factors. 

For the PCA, we used the residuals of the linear models for the ten variables 

above that presented significant results. The first two axes were responsible for 

capturing 48.6% of the data variation (Fig. 6). We found great overlap in the distribution 

of specimens in the tangent space, still, it is possible to observe a clear separation 

between males from the coast and females from the highland. The variables HL and 

MW had the highest negative values on the PC1 axis, while ED and DEN were higher 



41 

 

 

 

 

in the positive direction. This axis is responsible for the segregation on the distribution 

among adults, with males from the coast having mainly greater eye diameter and 

greater distance between eye and loreal pit and females from the highlands were more 

robust and had larger heads. On the PC2 axis, ED and DEN are higher in the negative 

direction and MW and TL in the positive direction. This axis is responsible for the 

greater separation between young and adult specimens, with the former having a 

larger diameter of the eye and distance between the eye and nostrils. Overall adults 

were more robust and had a relatively larger tail. 

 



42 

 

 

 

 

 

Fig. 5. Ontogenetic allometry of morphological traits of females and males Bothrops 

jararaca from the coastal and highland populations. A-D) Equal intercepts and parallel 

trajectory; E-J) different intercepts and parallel trajectory; and K-N) different intercepts 

and non-parallel trajectory. 

 



43 

 

 

 

 

 

Fig. 6. Principal Component Analysis of the ontogenetic morphological variation 

between females and males of Bothrops jararaca from the coastal and highland 

populations. 

 

 

2.4. DISCUSSION 

2.4.1. Sexual dimorphism 

The results above clearly point to a great morphological difference both intra 

and intersexual, but the direction of variation often alternated between groups 

depending on each variable. These results are consistent with other species of viperids 

(Hoyos et al. 2003; Matias et al. 2011; Sasa, 2002; Zhong et al. 2017), showing that 

the sexes have different mechanisms of divergence and are influenced by different 

factors along its distribution. 



44 

 

 

 

 

A previous study carried with Bothrops jararaca in southern Brazil showed that 

several morphological traits differ between the sexes (Matias et al. 2011), with females 

being generally larger than males. However, the average values presented by the 

females were lower than the females of the highland population and similar to the coast 

population. Therefore, different populations of this species are likely to have 

morphological characteristics strongly associated with local environmental pressures. 

The morphological archetype of females with larger body size and smaller tails 

than males is the most common among snakes that lack combat behavior between 

males (King, 1989; Shine ,1993; Shine, 1994) and is largely consistent with the 

hypothesis of sexual selection. Larger females are able to produce more offspring, 

which provides great adaptive advantages. Tail characteristics (size, width and number 

of scales) greater in males, is probably a consequence of the accommodation of 

copulatory organs, an evidence that it is extensively found in snakes (King, 1989). 

Nevertheless, not all variations may be explained by the hypothesis of sexual 

selection, especially considering the trophic morphology (e.g. traits of the head). 

Females of the marine species Laticauda colubrinausually feed on few large eels, 

whereas males feed on multiple smaller eels (Shetty and Shine, 2002). The females of 

Acrochordus arafunae forage in deeper waters than the males feeding on more bulky 

fishes (Shine, 1986). Such ecological divergences are accompanied by adaptive 

variations in morphology. Thus, differences in body size and stoutness of B. jararaca 

could be associated with differences in trophic ecology between the sexes. 

Morphological traits may be a result of genetic variation. The number of 

temporal and ventral scales, and color pattern associated with anti-predatory behavior 

in species of Thamnophis sp.is known to have genetic correlation (Broadie III, 1983; 

Dohm and Garland-Jr, 1993). Although the populations of the study belong to the same 

phylogroup (e.g., North clade; Grazziotin et al. 2006) and are geographically close, the 

abrupt altitudinal difference imposed by the Serra do Mar, and consequent 

physiographic variations, possibly represents a barrier, isolating and preventing part of 

gene flow. 

Of the findings reached in this study, the divergence in the number of ventral 

scales is one of the most exceptional. Hoge et al. (1976) reported to B. jararaca a large 

range in the number of ventral scales, however, the latitudinal, and consequently 

climatic, influence is well marked, with specimens from the southern part of the 

distribution having a considerably smaller number ventral scales in relation to those of 



45 

 

 

 

 

the north. Still, individuals in the State of São Paulo have an intermediate number of 

scales, being responsible for most of the overlap in data distribution. Accordingly, the 

most interesting about the populations in this study is the fact that they are very close, 

with little latitudinal, but most altitudinal variation, which may change the climatic 

conditions and consequently the number of scales. 

A macroecological study found a positive correlation between the scale count 

and geographical elevation in the Bothrops genus (Jadin et al. 2019). However, we 

find an opposite intraspecific variation, which means that the scale count may possibly 

vary on smaller geographic scales. The number of ventral and subcaudal scales is 

strongly related to the number of vertebrae, and consequently to the macrohabitat, with 

the density of vertebrae increasing with the arboreal habit (Hamptom, 2011). Thus, the 

largest number of scales for the coastal population suggest a most accentuated use of 

arboreal habitat, however, observational and/or experimental studies are needed to 

better elucidate this issue. 

Several traits of the head varied between populations. The highland snakes 

have a longer head but the distance between eyes, distance from eye to nostril and 

distance from rostral to the last labial scale is greater in the coast population. In snakes 

with generalist diet habits, the type of prey consumed may lead to variations in the 

shape of the head. In the Notechis scutatus, the population that preyed on species with 

greater mass, size and circumference also had a larger jaw and mouth (Fabien et al. 

2004). In this sense, differences found here may be an artifact of prey choice. 

We found no effect of sex or population on the eye size of adults. Although some 

individuals of B. jararaca can be found actively foraging, this species is known to be 

an ambush predator (Sazima, 1992). Thus, relying on other senses, such as 

thermoreception for hunting. Experiments with naturally blind snakes or partially 

deprived of vison, had no impaired biological traits, such as body condition, prey 

capturing rate, and sexual partner meeting (Bonnet et al. 1999; Young and Morain, 

2002), indicating that the size of the eye per se may not undergo strong natural 

selection. 

Coastal females and males are less differentiated from each other than the 

highland population (e.g., degree of sexual dimorphism). The morphological disparity 

between the sexes can be more or less accentuated due to the spectrum of the 

ecological niche occupied by each one in different populations. In sea snakes, for 

example, in regions where large prey are less abundant, the degree of sexual 



46 

 

 

 

 

dimorphism is often reduced (Shine et al. 2002). This suggests a niche partition among 

the populations of this study. 

The linear discriminant analysis shows the marked dimorphism between the 

sexes in both populations. The lack of overlap, however, may have occurred for the 

reduced sample, since the sample used contained only individuals with all the variables 

present. Even so, it is possible to notice that different variables were responsible for 

the separation, which indicates morphological adaptation in each environment. It is still 

necessary to keep in mind that many other factors can contribute to sexual 

segregation, especially demographic ones, which may cause bias in the male-female 

ratio and expression of the SDI, such as parasitism, nutritional stress or physical 

exhaustion (Giery and Layman, 2019). 

 

2.4.2. Ontogenetic allometry 

Females reached larger sizes (with the exception of the tail attributes) by 

different mechanisms. The ontogenetic growth patterns found in this work are very 

similar to the population of southern Brazil (Matias et al. 2011) and other species of 

the genus (e.g., Bothrops atrox; Silva et al. 2017). Thus, it seems to be a very 

conserved trait in the genus Bothrops. In some traits, females are larger from birth. 

This fact also appears to be common in snakes, as in some Natricines where females 

are larger and most sexual differences appear soon in newborns instead of being fixed 

in adults (Gregory, 2004). 

For some traits, even though males were initially equivalent or even larger, 

female growth rate was faster, culminating in relatively larger sizes. In snakes, the 

growth rate is rapid initially and decreases after sexual maturity, and in many cases 

where sexual dimorphism tends towards larger females, they tend to mature later, 

which can result in the observed allometric pattern (Brown and Weatherhead, 1999; 

Shine, 1978; Webb et al. 2003). 

The skewed survival rate for one sex may culminate in size disparities. Although 

this factor has not been explored in the present study, the discrepant allometric 

trajectory between the sexes suggests that the smaller growth rate in males is the most 

likely factor causing dimorphism, rather than a higher mortality rate. Similar results 

were found for Morelia spilota, where females showed extreme values of size, 

however, the recapture rate was equivalent between sexes (Pearson et al. 2002), 

which supports the growth rate hypothesis instead survival rate. 



47 

 

 

 

 

Since snakes have undetermined growth, life expectancy may cause 

morphological differences between populations. Specimens of Elaphe quadrivirgata 

from the island of Tadanae-Jima, Japan, are considered gigantic in relation to those of 

other populations, and take twice as long to reach their maximum size, with a constant 

growth rate (Hasegawa and Mori 2008). The analysis of the ontogenetic trajectories 

together with the PCA helps to illustrate the difference between sexes and populations 

throughout development and supports the hypothesis that specific ecological 

pressures act in each population, considering that the sexes in both are different earlier 

in life. 

Prey availability often fluctuates according climatic variation. Growth rate of 

Liasis fuscus born in years with more food available was higher and constant 

throughout development (Madsen and Shine, 2008). Likewise, prey availability (e.g. 

anurans) and snake fecundity also co-vary annually, and the positive correlation 

between maternal size and litter size makes larger females more sensitive to variation 

in prey availability (Brown and Shine, 2007). Considering that B. jararaca feeds on 

anurans at least in juvenile stages (Sazima, 1992), differences in prey availability 

between populations may induce important ecological variations. 

 

ACKNOWLEDGEMENTS 

We sincerely extend our sincere thanks to everyone who contributed to the 

completion of this work. To all friends who provided valuable help in specimen 

biometrics and reading the manuscript and, and to Vardir Germano and Felipe 

Grazziotin (Instituto Butantan) who generously helped withdata acquisition and material 

supply. 

 

 

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Brodie III, E.D., 1983. Genetic correlations between morphology and atipredator 

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Dohm, M.R., Garland-Jr, T., 1993. Quantitative genetics of scale counts in the garter 

snake Thamnophis sirtalis. Copeia. 4,987–1002. 

Fabien, A., Bonnet, X., Maumelat, S., Bradshaw, D., Schwaner, T., 2004. Diet 

divergence, jaw size and scale counts in two neighboring populations of tiger snakes 

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Gregory, P.T., 2004. Sexual dimorphism and allometric size variation in a population 

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Hampton, P.M., 2011. Ventral and sub-caudal scale counts are associated with 

macrohabitat use and tail specialization in viperid snakes. Evol. Ecol. 25,531-546. 

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Hendry, C.R., Guiher, T.J., Pyron, R.A., 2014. Ecological divergency and sexual 

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between Bothrops atrox and Bothrops asper (Serpentes: Viperidae). Actual Biol. 

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at high elevations? Macroecoogical patterns of scale characters and body size. Ecol. 

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King, R.B., 1989. Sexual dimorphism in snake tail length: sexual selection, natural 

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Loebens, L., Hendges, C.D., Almeida-Santos, S.M., Cechin, S.Z., 2019. Morphological 

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Brazil. Zool. Anz. 280, 42–51. 

Madsen, T., Shine, R. ,2008 Silver spoons and snake body size: prey availability early 

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Marques, O.A.V., Eterovic A., Sazima I., 2019. Serpentes da Mata Atlântica: Guia 

ilustrado para as florestas costeiras do Brasil. Ponto A, Cotia. 

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Nóbrega, R.P., Montingelli G.G., Trevine, V., Franco, F.L., Vieira G.H.C., Costa G.C., 

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CHAPTER 2 

 
SEXUAL DIMORPHISM AND ONTOGENETIC VARIATION ON THE HEAD SHAPE 

OF TWO NEIGHBOURING POPULATIONS OF THE COMMON LANCEHEAD 

BOTHROPS JARARACA: A GEOMETRIC MORPHOMETRIC APPROACH 

 

 

ABSTRACT 

The head are the most important element in trophic ecology among snakes. Head 

shape is affected by several factors both intrinsic as size and sex, as extrinsic such as 

geographic variation. Often different populations are subject do different environmental 

conditions that modulate ecology and are reflected in morphology. Here we investigate 

sexual dimorphism and ontogenetic allometry on Bothrops jararaca head shape in two 

populations. We found a significant effect of sex and population, being that females 

from the highland had a large post-ocular region e more arrow shaped head. Size 

accounted for most variation in shape but diverged between populations. Sexual 

dimorphism in head shape were not found in juveniles, however, ontogenetic trajectory 

varied greatly between populations. In the Bothrops genus, except in early stages, 

females often grow faster and attain larger sizes than males, which may be affecting 

head shape. Also, possible differences in diet and ontogenetic variations between 

populations may be a reasonable cause to specific allometric trajectories. 

 

 

 

 

 

 

 

 

 

 



52 

 

 

 

 

3.1. INTRODUCTION 

 The head figure as one of the most important elements in snakes bauplan. Since 

these animals have an elongated body with almost no limbs, the head becomes 

fundamental for performing many essential functions of the snake biology. In some 

species the triangular shape of the head may act like an antipredator signal, such as 

in vipers and their mimics (Valkonem et al 2011). But most important is the role as a 

feeding apparatus, responsible for capture, holding, manipulating and swallowing of 

the prey (Gans 1961; Cundall 1983). 

 As gape-limited predators, the snake's diet is constrained by the mouth opening, 

and therefore, it is reasonable to assume that the head represents a source of great 

variation in snake fitness. In these group of reptiles, body and head size are widely 

correlated, and in the same way, head size is also correlated with swallowing 

performance. This indicates that longer snakes may take larger prey, but snakes with 

larger relative head size are capable to eat even heavier prey than those with smaller 

heads but with same body size, and which are translated into different energy intake 

(Forsman and Lindell 1993). Following the same idea, head shape may be also 

affected by the prey-predator ratio trade-off. Head shape varies among snakes of 

different size, which allows that different head shaped snakes explore different diets 

(Vincent 2003). Therefore, due to this intricate relationship, the morphology of the head 

becomes a target of strong selective pressures.  

 Differential niche exploitation may be quite advantageous mostly because is a 

major source of competition reduction. Juveniles often shows different patterns then 

adults, both spatially and temporally, having specific diet habits, habitat selection or 

daily activity (Lind and Welsh Jr 1994; Shine et al. 2003; Webb et al. 2005; Székely et 

al. 2020). Furthermore, these partitioning may occur even in the same age class. Male 

and females of different body sizes are able to explore specific prey types or sizes, 

associated or not to different foraging sites (Shine 1986; Shetty and Shine 2002; 

Vincent et al. 2004). Accordingly, in the cottonmouth Agkistrodon piscivorus, males are 

mostly piscivorous, whereas females rely mostly on reptiles, such as snakes, and prey 

size increase as the snake grows (Vincent et al. 2003; Vincent et al. 2004). 

 Nonetheless, both sexes are also subjected to strong sexual selection. Larger 

females generally present a higher reproductive output (e. g. Higher frequency, larger 

litters, larger neonates or higher relative clutch mass) while larger males gain more 



53 

 

 

 

 

access to females in species where male-male combat is common (Luiselli et al. 1996; 

Shine 2003). Nevertheless, some morphological traits such as body and head size, 

may be more or less affected by trophic or reproductive ecology than others (Ford and 

Seigel, 1989; Bonnet et al. 2000), hence, drivers of this sex-biased size and their 

cause-consequence relation are often difficult to determine. 

The developmental processes that result into given final adult sexual 

dimorphism can be often complex. One sex will be larger than the other if it already 

born larger, or grows during the same time interval, but with a higher growth rate, or if 

it grows at a similar growth rate, but during a longer time period. Understanding the 

mechanistic strategies involved in generating those patterns is important, since the 

degree of sexual dimorphism may be expressed differently across species, populations 

or even among different traits in an organism (Badyaev, 2002). For this purpose, 

studying ontogenetic growth that may result in sexual divergences is crucial.  

In studies regarding morphology, the usual linear morphometrics are extensively 

used. However, even though its indubitable value, this method may eventually become 

less appropriate when comparing shape variation, providing poor descriptors mainly in 

studies where the focus are functional traits (Sidlauskas et al 2011; Fabre et al 2014). 

In this regard, geometric morphometrics rises as a complementary approach that 

improves significantly the morphological analyses. In reptiles this method has been 

applied as a powerful analytic tool to shed light on several important biological 

questions concerning species delimitation (Ruane 2015), evolutionary trends (Davis et 

al. 2016), ecological drivers (Manier 2004) and developmental and allometric 

trajectories (Kaliontzoupoulou et al. 2008). 

The common lancehead Bothrops jararaca is a widespread Atlantic Forest 

dweller that occurs from sea level to 1200 m altitude (Sazima 1992; Campbell and 

Lamar 2004), and has a well-known natural history. In this species several biological 

traits pass through a marked ontogenetic change, such as venom action and 

composition (Zelanis et al. 2010), habitat use, and behavior (Sazima 1992; Marques 

et al 2019). Ontogenetic variation is also conspicuous in the diet, as juveniles feed 

mostly on ectothermic vertebrates, such as anurans (e.g. Hylids) and lizards, whereas 

adults rely almost completely on small endotherms like rodents (Sazima 1992; 

Hartman et al. 2003). In the Atlantic Forrest, richness, composition, abundance and 

biomass of anurans are often driven by local environmental factors and generally vary 

in relation to the altitudinal gradient (Giaretta et al 1999; Vasconcelos et al 2014). In 



54 

 

 

 

 

the Serra do Mar Region (Southeastern Coast-Brazil) assemblage composition of the 

Hylid family was found to be greatly variable among the sampling sites at different 

altitudes (Silva et al 2017), which may affect prey availability for B. jararaca. 

 Therefore, based on the premises that B. jararaca i) presents a considerable 

geographic distribution, ii) shows an ontogenetic variation in the diet, iii) depends on 

the prey availability which may vary among sites, and iii)  presents singular both static, 

and ontogenetic allometric trajectories presumably driven by environmental pressure, 

we aim to explore the hypothesis that B. jararaca shows sexual and populational 

differences in the individual head shape, and that allometric pattern vary both intra and 

interpopulationally. 

 

 

3.2. METHODS 

 

3.2.1Data sampling 

Photographs of the dorsal view of the head were taken from 163 B. jararaca 

specimens. Snout-vent Length (SVL) were also measured to the nearest mm using a 

tape. From those, 74 individuals belonged to the coastal population (42 females and 

32 males), and 89 to the highland population (42 females and 47 males). All specimens 

were housed at the Herpetological Collection “Alphonse Richard Hoge” in the Butantan 

Institute, São Paulo, Brazil. 

Nineteenth anatomical landmarks in the right side of the head (to avoid pseudo 

replication) were digitized using the software TPSdig 2 (positioning and landmarks 

types are indicated in Figure 1). The criteria used to select the landmarks focused on 

optimally defining the shape variation among sexes and populations, including most 

relevant morphological characters mainly focused on trophic anatomy. All 

configurations where then subjected to Generalized Procrustes Superimposition 

analysis.  This procedure was used to standardize all specimens subtracting the effect 

of size, positioning and orientation of the coordinates, remaining only shape-derived 

variation. A size variable was extracted from each landmark configuration as the 

Centroid Size (CS). This measure was estimated as the square root of the sum of 

square distances of the landmarks from their barycenter, and were largely used in 

geometric morphometric analysis (Tamagnini et al, 2018; Loebens et al, 2019). 



55 

 

 

 

 

 

Figure 1: Schematic illustration indicating the position of the 19 landmarks used to 

analyze head shape variation in Bothrops jararaca. 

  

 

3.2.2. Sexual dimorphism 

 A nested subset containing only adults was used to assess sexual dimorphism. 

Females were considered adult when larger than 750 mm, and males when larger than 

650 mm length (Sazima 1992). Linear models were used to test the relation between 

CS and SVL in each group, and to test for head size variation a one-way ANOVA was 

carried on CS.  Principal Component Analyses (PCA) were performed on the 

Procrustes Coordinates to visualize groups relationship in the morphospace. The 

presence of intersexual and interpopulation variance on head shape was tested with a 

three-way Procrustes MANOVA using the Procrustes coordinates as independent 

shape variables, with CS as covariate and sex and population as fixed factors. This 

procedure was used to account for the impact of size in the head shape due to static 

allometry.  

 In order to analyze de degree of sexual dimorphism in the two populations, an 

Index of sexual shape dimorphism (SSD) was computed as the Procrustes distances 

between mean female and mean male measurements divided by the maximum 

Procrustes distances between males and females (Tamagnini et al 2018). Static 



56 

 

 

 

 

allometry was tested regressing shape coordinates onto CS using multivariate linear 

models in each sex separately, and wireframes were built to visualize specific shape 

change along CS gradient. 

  

3.2.3. Ontogenetic allometry 

 First, a three-way Procrustes MANOVA was performed to test if head shape 

was different between sex and populations among the juveniles, as well as accounting 

for CS variation. A PCA was then performed with the complete dataset to observe the 

relation of age classes, sex and populations in the morphospace.  

Ontogeny was investigated using a very similar approach as the described in 

the previous section but using the complete dataset (including juveniles). Another 

three-way Procrustes MANOVA was performed to test the significance of the 

interaction between CS, sex and population across all individuals. Significant results 

in the interactions mean different slopes, and consequently different ontogenetic 

trajectories. To verify the assumption of ontogenetic allometry, the impact of CS 

variation in head shape was tested using multivariate linear models separately in each 

sex. Ontogenetic trajectory where visualized using wireframes.  

 

3.3. RESULTS 

3.3.1. Sexual dimorphism 

 In the coastal population, larger snakes also had higher CS values (females r² 

= 0.43, p = 0.002; males r² = 0.83, p>>0.05), however no relation was found in the 

Highland population (females r² = 0.005, p = 0.29; males r² = -0.02, p = 0.5). The 

ANOVA results showed a significant interaction between SVL and population (F = 

10.38, df = 1, p = 0.001), being that in smaller highland snakes the CS was greater, 

whereas in larger highland snakes the CS was smaller (Figure 2).  

The first two PCs captured 47.6% of the shape variation (Figure 3). The PC1 

(25.5% of the variation) explains mostly the post-ocular length, specifically in 

landmarks 9 and 10.  The positive values on this axis indicates shorter post-ocular 

region, with landmarks 9 and 10 close to each other and a longer snout (pre-ocular 

region), and the opposite occurring along the negative values. We found great overlap 

in specimen distributions on this axis, however, the coastal males concentrated more 

in the positive region having a relatively larger pre-ocular region. The PC2 (22.1% of 

the variation) explains head width, mainly in landmarks 5 and 17. The positive values 



57 

 

 

 

 

on this axis indicates broader and most rounded (arrow-shaped) head, with landmarks 

5 and 17 far from each other, whereas negative values, a thinner head. No 

discrimination pattern was clear in this axis. Based on Procrustes distances, SSD on 

coastal population was 0.20, while in highland population SSD was 0.16, which means 

that the morphological displacement on the former was more considerable. 

 

Figure 2: Regression models for Centroid size derived from Procrustes coordinates of 

the dorsal view of the head by snake snout-vent length in Bothrops jacacara 

populations. 



58 

 

 

 

 

 

 

Figure 3: Principal Component Analysis based on Procrustes coordinates showing 

head shape variation in adults Bothrops jararca. 

 

Size variation explained a higher percentage of shape variation in the coastal 

population but had no effect on shape variation in the highland population (table 1). 

The three-way Procrustes MANOVA for adults was found to be significant for all factors 

and the double interactions CSxPopulation even after Bonferroni correction (table 2), 

which means that head shape changes with increasing CS, and were different, and 

parallel between sexes but with different slopes between populations. The general 

static allometry pattern suggests that snakes with smaller CS had thinner heads, while 

greater CS corresponds to more rounded arrow-shaped head (figure 4). Specific 

wireframes highlighted distinct shape variation between populations. The overall shape 

variation pattern indicated a more elongated and rounded head when CS was greater. 

However, the most apparent difference between populaitons was found in the 

backward and lateral displacement of the facial landmarks (e.g. landmarks 5 and 9) in 

individuals of the coastal population, and the forward displacement of the same 



59 

 

 

 

 

landmarks producing greater post-occipital region in individuals of the highland 

population (Figure 5).  

 

 

Figure 4: Static allometry (adults) of head shape in Bohtrops jararaca sexes and 

populations based on the regression of Procrustes coordinates on Centroid Size. 

 

 

Table 1: Static (Adult) and ontogenetic (All sample) allometric models for Bothrops 

jararaca populations with the head shape variation predicted by size. Bold indicates 

significant size effect.  

Data Population Sex Predicted F P 

Adult 

 

coastal 

female 19% 3.53 0.002 

male 19% 3.1 0.008 

highland 

female 3% 1.14 0.31 

male 5% 1.51 0.16 



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All sample 

coastal 

female 13% 6.01 0.001 

male 19% 7.16 0.001 

highland 

female 3% 1.62 0.14 

male 5% 2.47 0.02 

 

 

Figure 5: Wireframes providing visualization of static allometry trajectory using 

Centroid size (CS) as predictor in Bothrops jararaca a) coastal females, b) coastal 

males, c) highland females and d) highland males. 

 

 

 

 

 

Table 2: Three-way Procrustes MANOVA of three datasets, accounting for size, sex 

and population effects on head shape of Bothrops jararaca. Bold indicates significant 

effects after Bonferroni correction. 



61 

 

 

 

 

Data Variable df SS MS RSQ F Z 
P-

adjusted 

Adults 

CS 1 0.006 0.006 0.035 4.025 2.960 0.007 

sex 1 
0.005 0.005 0.030 3.536 2.734 0.014 

population 1 
0.017 0.017 0.102 11.72 4.996 0.007 

CS:sex 1 
0.001 0.001 0.007 0.849 -0.14 1.000 

CS:population 1 
0.011 0.011 0.067 7.739 4.497 0.007 

sex:population 1 
0.003 0.003 0.021 2.487 2.141 0.105 

CS:sex:population 1 
0.001 0.001 0.011 1.331 0.789 1.000 

residuals 83 
0.124 0.001 0.723 

   

total 90 
0.171 

     

Juveniles 

CS 1 0.019 0.019 0.123 11.9 4.82 0.007 

sex 1 0.001 0.001 0.012 1.18 0.59 1.00 

population 1 0.019 0.019 0.119 11.6 4.93 0.007 

CS:sex 1 0.002 0.002 0.018 1.75 1.42 0.588 

CS:population 1 0.006 0.006 0.043 4.17 3.15 0.007 

sex:population 1 0.001 0.001 0.012 1.19 0.65 1.00 

CS:sex:population 1 0.003 0.003 0.022 2.16 1.89 0.196 

residuals 63 0.104 0.001 0.649    

total 70 0.160      

All 

Sample 

CS 1 0.070 0.070 0.187 40.6 7.10 0.007 

sex 1 0.002 0.002 0.005 1.27 0.77 1.00 

population 1 0.026 0.026 0.069 15.1 5.80 0.007 

CS:sex 1 0.002 0.002 0.007 1.68 1.32 0.71 

CS:population 1 0.004 0.004 0.011 2.44 2.16 0.091 



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sex:population 1 0.001 0.001 0.003 0.77 -0.2 1.00 

CS:sex:population 1 0.001 0.001 0.003 0.86 -0.1 1.00 

residuals 161 0.267 0.001 0.710    

total 164 0.376      

 

 

3.3.2. Ontogenetic allometry 

 We found a significant population effect when analyzing the juveniles, although 

there was no effect for sex (table 2). Thus, females and males were pooled together 

on the PCA analysis. PC1 and PC2 explained 51.3% of the total ontogenetic variation 

(Figure 6). Overall, ontogenetic PCA followed a similar pattern of the static PCA, that 

is, PC1 controlled mainly the positioning of the landmarks 9 and 10, stressing a large 

pre-ocular and short post-ocular region in the positive way and the opposite in the 

negative way. Despite a small overlap on this axis, both populations appeared 

separated, being that the coastal individuals occupy mostly the positive portions, with 

juveniles in the extreme of the gradient whereas the Highland individuals were 

distributed mostly in the negative portion despite the ontogenetic trend of variation 

being less evident. PC2 controlled most for head width, and landmarks 5 and 17 

(quadrate bone region) had a major importance on this axis. Individuals had relatively 

broader head (lance-shaped) in the positive way and a thinner head in the negative 

way. We found no clear pattern in this axis. 

   

 



63 

 

 

 

 

 

Figure 6: Principal Component Analysis based on Procrustes coordinates showing 

head shape variation during ontogeny in Bothrops jararaca individuals of two 

populations. 

 

Size variation also explained a higher percentage of shape variation in the 

Coastal population (table 1), but it resulted statistically significant only among the 

males from the highland population. The three-way Procrustes MANOVA performed in 

the complete sample confirmed PCA pattern. We found a significant effect of CS and 

population on head shape variation, but no effect for sex or any interactions (table 2). 

Ontogenetic allometry shown that sex variation where equal, but different between 

populations. However, the lack of significance in interactions indicates parallelism 

between ontogenetic trajectories. In general juveniles had a thinner head and larger 

pre-ocular region, and adults had broader heads and larger post-ocular region (figure 

7). Specific wireframes show a similar head in the juveniles however the most 

prominent change was that in the coastal population landmarks in the ocular region 

moved backward with increasing CS while in the Highland population the opposite 

occurred (Figure 8).  



64 

 

 

 

 

 

Figure 7: Ontogenetic allometry of head shape in Bothrops jararaca sexes and 

populations based on the regression of Procrustes coordinates on Centroid Size. 

 



65 

 

 

 

 

 

Figure 8: Wireframes providing a visualization of the ontogenetic allometry trajectory 

using Centroid size (CS) as a predictor in Bothrops jararaca: a) coastal females, b) 

coastal males, c) highland females and d) highland males. 

 

3.4. DISCUSSION 

3.4.1. Sexual dimorphism 

 Our results showed a distinct pattern of size and shape differences between 

sexes and populations. Since Bothrops jararaca had a clear sexual size dimorphism in 

morphological traits of the head (Matias 2011), shape disparities were not surprising. 

Highland females shown relatively longer pre-ocular region than sympatric males and 

coastal males and females. However, distinct from the coastal, the highland population 

showed no effect of shape explained by size variation (represented by the centroid 

size), thus, this difference in shape is probably an outcome of other sources, as 

ecological pressures. We cannot discard though, a displacement between head and 

body measures.  Highland females were the longest in body size (chapter 1), and head 

shape may be reflecting this pattern, which suggest a different co-variation between 

morphological traits and head shape among populations.  

Similar patterns of sexual shape dimorphism were previously reported for the 

genus. Females Bothrops atrox presented relatively more robust heads than males 



66 

 

 

 

 

(Silva et al 2017). However, populational differences remain unknown. These results 

are congruent with several other linear traits both of the head and the body, as females 

are generally larger in almost all variables measured (Sasa et al 2003; Hartmann et al 

2004; Matias 2011; Leão et al 2014; Bisneto and Kaefer 2019). 

The PCA suggested that neither population was completely sexually dimorphic. 

This appears to be a trend in snakes. For example, a study in two sympatric and closely 

related species Tomodon dors