ABSTRACT
Sexual dimorphism in snakes has been mainly evaluated for size and number of some morphological traits, but few studies address on sexual shape dimorphism. Here we evaluated the existence of sexual size and shape dimorphism in the semi-fossorial snake Atractus reticulatus. We use linear and geometric morphometrics to evaluate differences between sexes and among different ontogenetic stages (neonates, juveniles and adults). We have shown that A. reticulatus is sexually dimorphic for some traits such as body length and head shape, with females being larger and having more robust heads than males, but the sexes do not differ in head size. Males and females are sexually dimorphic in head shape even in neonates, suggesting that this differentiation is prenatal. Differences in head shape may be associated with trophic segregation, allowing females to feed on larger prey than males. Body size dimorphism progressively increases throughout the ontogenetic stages, which is possibly related to the late sexual maturation of females and/or different growth rates between the sexes. We also found that males and females shows some sex-specific patterns towards static and ontogenetic allometry, with males showing stronger predictive response on static allometry than females, whereas females have ontogenetic allometry, but males do not. Additionally, the allometric slopes in A. reticulatus between sexes converge by presenting similar shapes as head size increases, an expected result for sexes with similar lifestyles. Further investigation on some physiological and natural history aspects in Atractus will be particularly useful for a better understanding of the significance of the morphological differences found in this study.
ABSTRACT
Didelphis albiventris and D. aurita are Neotropical marsupials that share a unique evolutionary history and both are largely distributed throughout South America, being primarily allopatric throughout their ranges. In the Araucaria moist forest of Southern Brazil these species are sympatric and they might potentially compete having similar ecology. For this reason, they are ideal biological models to address questions about ecological character displacement and how closely related species might share their geographic space. Little is known about how two morphologically similar species of marsupials may affect each other through competition, if by competitive exclusion and competitive release. We combined ecological niche modeling and geometric morphometrics to explore the possible effects of competition on their distributional ranges and skull morphology. Ecological niche modeling was used to predict their potential distribution and this method enabled us to identify a case of biotic exclusion where the habit generalist D. albiventris is excluded by the presence of the specialist D. aurita. The morphometric analyses show that a degree of shape discrimination occurs between the species, strengthened by allometric differences, which possibly allowed them to occupy marginally different feeding niches supplemented by behavioral shift in contact areas. Overlap in skull morphology is shown between sympatric and allopatric specimens and a significant, but weak, shift in shape occurs only in D. aurita in sympatric areas. This could be a residual evidence of a higher past competition between both species, when contact zones were possibly larger than today. Therefore, the specialist D. aurita acts a biotic barrier to D. albiventris when niche diversity is not available for coexistence. On the other hand, when there is niche diversification (e.g. habitat mosaic), both species are capable to coexist with a minimal competitive effect on the morphology of D. aurita.
