When do meadow vipers (Vipera ursinii) become sexually dimorphic? – ontogenetic patterns of sexual size dimorphisms
Abstract
Contrary to an increasing number of papers that document sexual dimorphism in size (and/or shape) in adults, studies dealing with sex differences in newborn and juvenile snakes are surprisingly scarce. Data about ontogenetic shifts in sexual dimorphism are generally lacking and hence, it is unclear whether sex differences are set at birth or arise post-natally. In this study, we analyzed patterns of sexual dimorphism in body size, head dimensions and tail length (TL) among newborn, subadult and adult meadow vipers (Vipera ursinii) from the Bjelasica Mt. in Montenegro. Patterns of sexual size dimorphisms differed among traits. There was no significant difference in head dimension of males and females, but adult snakes were sexually dimorphic in body size. Sexual differences in TL were evident since birth but changed in degree throughout ontogeny. Neonate meadow vipers presented highly significant inter-litter variation in the sexual dimorphism of all traits we have measured. Such family effects may have an important influence on extent of inter-sexual differences in snakes and should be included in analyses of sexual dimorphism.
Introduction
More than two centuries ago, Charles Darwin (1871) has already suggested that a difference between the sexes was one of the important sources of natural intraspecific variability in body sizes. Reptiles express a broad spectrum of patterns of sexual size dimorphism. For example, tortoises tend to be moderately dimorphic (Willemsen and Hailey 2003), whereas considerable variation in direction and magnitude of sexual dimorphisms is observed in lizards (Cox et al. 2003).
The direction and extent of sexual size differences in adults can originate via sexual selection, natural selection, selection on fecundity (Andersson 1994) and phylogenetic constraints (Krause and Burghardt 2007), and these processes are already mentioned as probably shaping sexual dimorphisms in body, head and tail sizes in adult snakes (e.g. Shine 1978; Forsman 1991; Vincent et al. 2004). Sexual size dimorphism in snakes is widely reported, with more than 370 species being described as dimorphic in at least some morphometric features (Shine 1993, 1994). Among the species of European vipers, different patterns of sexual size dimorphisms have been reported – males exceed females in body size in Vipera ammodytes (Tomović et al. 2002); females are in average larger than males in Vipera berus, Vipera seoanei and Vipera ursinii and both sexes of Vipera aspis and Vipera latastei attain similar body size (Saint Girons 1978; Shine 1978; Madsen and Shine 1994; Bonnet et al. 1998; Brito and Rebelo 2003).
Although sexual size dimorphism is well-documented, for many species of snakes, in most studies, sub-adults and newborns are not included, hence the timing of expression of sexual size dimorphisms is unknown. Although sexual size dimorphism at birth or hatching seems to be rare (Shine 1993), recent studies have documented its occurrence in some species (e.g. Naulleau 1970; King et al. 1999; Schuett et al. 2005; Krause and Burghardt 2007), thus three ontogenetic patterns of sexual dimorphism are possible (King et al. 1999): (1) inter-sex differences are present only in adults; (2) sex differences are set at birth and their direction and degree are similar in neonates and adults; and (3) sexual dimorphism is present in both neonates and adults, but differs in degree and/or direction between age classes.
Phenotypic differences among individuals, including cases of sexual dimorphisms in neonates, may result of the genetic background and/or environmental influences. The maternal identity (also referred as clutch/litter effects or family effects) may play an important role on those differences and may be based either on genetic or environmental differences (e.g. basking regimes of the mother in each pregnancy event).
Available data show that family effect was included only in a minority of studies dealing with variation in head and body dimensions and sexual dimorphisms in snakes (Seigel 1992; Clark and Pendleton 1995; Weatherhead et al. 1995; King et al. 1999; Krause and Burghardt 2007). Excluding such family effect when testing sexual dimorphism in neonates could lead to erroneous assumptions: inter-sex differences can be blurred by the differences among families, or could be assumed when actually they do not exist and are due to family effect (King et al. 1999).
In this study, we investigate the occurrence of ontogenetic patterns in sexual size dimorphisms in a population of V. ursinii, specifically testing family effects.
Material and Methods
Species, study site and data collection
The meadow viper (V. ursinii) is a small-sized (up to 60 cm) viviparous snake with a highly fragmented distribution in Europe, occurring in lowlands of dry, meadow-steppe regions or in mountains of alpine-subalpine meadow habitats (Nilson and Andrén 1997, 2001). They feed mainly on grasshoppers and crickets (Baron 1997), and small mammals and lizards occasionally (Arnold and Ovenden 2002). Females attain larger body size and have a greater number of ventral scales than males (Kramer 1961), and males tend to have greater number of subcaudal scales (Saint Girons 1978; Nilson and Andrén 2001).
During the years of 2002–2006, we sampled 158 individuals of V. ursinii (including pregnant females) in Biogradska Gora National Park, Bjelasica Mt., eastern Montenegro (see complete description of the study site in Tomović et al. 2004).
For each snake, we recorded: sex (detected by tail shape and hemipenis inversion), snout-vent length (SVL), tail length (TL), head length (HL: from the tip of the snout to the articulation point of the lower jaw and quadrate), head width across the widest part of the head (HW), mouth length (ML: from the posterior edge of the posterior-most upper labial scale to the tip of the rostrum), mouth width across the widest part of the mouth, when closed (MW), body mass (M) and age class (newborns, sub-adults and adults). Adults were identified by body length, more than 24.5 cm and 31.5 cm of the SVL, for males and females of V. ursinii, respectively (Baron 1997). All measurements were taken by the same person (Lj. T.) to avoid biases. SVL and TL were measured to the nearest millimetre using a flexible measuring tape, head dimensions were obtained with a digital caliper to the nearest 0.01 mm, adult and sub-adults were weighted with a 0.5 g accurate Pesola® scale (AAS / Pesola, Ontario, Canada) and neonates with a 0.01 g accurate electronic balance. Individuals containing evident prey items in the gastrointestinal tract (detected by palpation) were placed in cloth bags and kept until the contents were defecated (up to two days) before recording the body mass. Pregnant females (detected by palpation) were kept in terrariums containing water ad libitum, feed on small rats once a week, until parturition. After birth, we recorded the morphometrics described before, for the mother and offspring. All snakes were scale clipped for individual recognition and released at the point of collection after recording the morphological traits.
Data analyses
Data from recaptured individuals were not used in any of the analyses.
Indices of sexual dimorphism (SDI) were calculated for each morphological character for newborns, sub-adults and adults. For SVL and M, SDI was calculated as female mean/male mean (non-transformed values); for head dimensions and TL, SDI was computed as female ‘adjusted’ mean/male mean (non-transformed values). Female ‘adjusted’ means were calculated as follows: we obtained regression equations relating female TL (and head dimensions) to female SVL and then used them to calculate expected TL (and head dimensions) for a female with an SVL equal to the mean SVL of males (King et al. 1999).
Prior to all analyses, equality of slopes was tested and all variable were transformed to their natural logarithms.
We used multivariate analyses of variance (manova) with sex as factor to analyze the pattern of sexual dimorphism in body dimensions (SVL and M combined). Multivariate analyses of covariance (mancova) with sex as factor and SVL as covariate were run to investigate variations in head dimensions (HL, HW, ML and MW pooled together). Analyses of covariance (ancova) with sex as factor and SVL as covariable were used to analyze sexual dimorphism in TL. These statistical tests were run independently for each age class and family was added as a random factor for the neonates’ analyses. Also, because we did not detect any significant difference in the morphometry of live and still-born (fully developed and non-malformed), animals they were polled together in our analyses, and only litters with at least three offspring were included in these analyses. Because number of litters per year was small, our analyses do not consider inter-year variation.
Multivariate significance was tested using Wilks’s λ for body and head dimensions and by univariate F-tests for TL; p-values <0.05 were considered as statistically significant. All statistical analyses were done using Statistica 5.1 (Statsoft, Tulsa, OK, USA).
Results
We sampled a total of 96 adults (32 males, 64 females), 62 sub-adult individuals (30 males, 32 females) and 124 neonates (60 males and 64 females) from 22 litters: five in 2003, seven in 2004, six in 2005 and four in 2006.
Indices of sexual dimorphism were similar for most morphological traits in newborns, sub-adults and adults (Table S1), with the exception of SVL and M. The degree of sexual dimorphism was low for head dimensions (values close to 1) and high for TL (values around 0.7) in all three age classes. A progressive increase in the degree of sexual dimorphism was found for body dimensions of newborns to adults (SVL: 1.01-1.06-1.20; mass: −0.97-1.20-1.78).
Sub-adult and adult males exceeded females in TL, whereas females exceeded males in body dimensions (SVL and M) (Tables S1 and S2). Significant inter-sex differences in head dimensions were found in sub-adults, but not in adults. Head dimensions and TL covaried significantly with SVL in both sub-adults and adults (Table S2).
When we analyzed sexual dimorphism of newborns including family effect, both body dimensions and TL showed highly significant differences between males and females. Head dimensions were not sexually dimorphic. For all morphometric traits, the family effect was highly significant and the sex-by-family interaction was significant only for head dimension (Table S2).
Discussion
In several snake species, family effect in neonates was found for head dimensions (Thamnophis sirtalis–Krause and Burghardt 2007; four natricine species –King et al. 1999) as well as for body size and TL (natricine species –King et al. 1999); however, interactions between family and sex were not significant. In this study, we found highly significant inter-litter differences in all analyzed characteristics of neonatal meadow vipers (body size, head dimensions and TL). Interactions of sex and family were non-significant for body dimensions and TL.
Family effect on setting of sexual size dimorphism at birth was statistically significant for head dimensions. This finding further supports previous studies showing that family effect provide more accurate explanation on the degree and direction of sexual dimorphism in neonates (King et al. 1999; Gregory 2004).
Difference in body size between sexes could be an artefact of unbalanced sex ratio in different families; thus information on litter size and sex ratio should be taken into account while interpreting results of analyses of sexual size dimorphism in newborns.
Morphological differences among newborns belonging to different families could also be attributed to differences of genotype (King 1997; King et al. 1999) or to pre-natal environment (e.g. maternal size, number of offsprings –King 1993; Krause and Burghardt 2007 or incubation temperature –Gutzke and Packard 1987; Burger 1980; Shine et al. 1997). One obvious explanation of inter-litter differences in sexual size dimorphism is the different genetic identity of mothers (genetic background). But, the presence of sexual dimorphism in neonates may be controlled by level of sex-steroid hormones during the pregnancy (Shine and Crews 1988; Osypka and Arnold 2000) or can be influenced by variation in environmental conditions during incubation (e.g. temperature) (Lourdais et al. 2004).
Concerning the ontogenetic pattern of sexual dimorphism in meadow viper, we found different degree of sexual size dimorphism among three age classes (Table S1). Sexual size dimorphism in SVL is not evident in neonates, slightly increases in sub-adults and reaches the highest level in adults. Sex differences in adult body size may be the outcomes of actual sex differences in plasticity in growth trajectories (Bonnet et al. 2001; Krause et al. 2003; Lourdais et al. 2006) as well as in differences in age at maturity and longevity (Shine 1993; Stamps 1993; King et al. 1999; Brito and Rebelo 2003). Due to lack of data about the exact age and sex-specific growth rate of sub-adult and adult individuals, we cannot currently estimate the effect of different growth trajectories on sexual size dimorphism in this population of snakes.
Sexual dimorphism in TL is set at birth and the degree increases slightly but constantly from newborns to adults, whereas SDI remains relatively constant. TL is positively correlated to the presence and size of the hemipenes and may affect the mating success in males (Shine et al. 1999). Two alternative causes are suggested to be responsible for expression of sexual size dimorphism in TL in adult snakes (King 1988; Shine 1993): the morphological constraint hypothesis – greater TL in males is due to presence of copulatory organs, and the reproductive output hypothesis – females have shorter tails because they allocate resources to increase trunk size, instead of TL. The conservatism in the pattern and degree of sexual size dimorphism in TL during ontogeny of meadow viper could result from isometric growth, and the same trends were recorded for Natrix natrix (Gregory 2004).
Sexual size dimorphism in head dimensions was not found in newborns and in adults. A relatively small degree of sexual dimorphism in head dimensions was detected in sub-adults of meadow vipers. This could be related to the difference of sub-adult size limit between males and females, or may be the result of the heterogeneity of sample itself, as regards to age of individuals (all individuals from age +1 to supposed age at maturity i.e. +3 for males and +4 for females –Baron 1997; were treated as sub-adults). Although it is repeatedly suggested that head size dimorphism in snakes has evolved to allow males and females to use different foraging niches (e.g. Shine 1986; Shine and Crews 1988), without precise data about food habits of different sexes and different age classes (i.e. subadults and adults), we cannot give unambiguous explanations for sexual size dimorphism in head dimensions in this population.
In conclusion, we showed that family effect may have an important influence on the extent of sexual dimorphism in snakes and that it should be included in analyses of sexual size dimorphism. In addition, different ontogenetic patterns are present in the development of sexual dimorphism of different morphological traits. Further studies of these phenomena in other populations of this species (as well as of other vipers) are needed for broader conclusions.
Acknowledgements
We are grateful to the authorities and staff of the National Park “Biogradska Gora” (Montenegro) for field work permissions and hospitality. The following people helped us in the field: B. Sterijovski, N. Ristić, M. Djurakić, D. Krsteski, U. Isailović, O. Isailović, G. Tomović and V. Pešić. Many thanks to the local families Damjanović, Marković, Marjanović and Bogavac for their generosity (provided shelter and food) during the rainy and cold days. Our colleague B. Stojković helped with statistical procedures and provided valuable advices and comments. We would like to thank to the anonymous reviewers whose comments greatly improved the manuscript. This work was initially supported by SEH grant “Awards in Herpetology” in year 2003, and was funded by the Ministry of Science of Republic of Serbia, grant No. 143040.
