A global analysis of viviparity in squamates highlights its prevalence in cold climates

Aim: Viviparity has evolved more times in squamates than in any other
vertebrate group. Therefore, squamates offer an excellent model system to
study the patterns, drivers, and implications of reproductive mode
evolution. Based on current species distributions we examined three
selective forces hypothesized to drive squamate viviparity evolution: (1)
cold climate, (2) variable climate, and (3) hypoxic conditions, and tested
whether viviparity is associated with larger body size. Location: Global.
Time period: present day. Taxon: Squamata. Methods: We compiled a dataset
of 9,061 squamate species including their distributions, elevation,
climate, body mass, and reproductive modes. We applied species-level and
assemblage-level approaches for predicting reproductive mode, globally and
within biogeographical realms. We tested the relationships of temperature,
interannual and intra-annual climatic variation, elevation (as a proxy for
hypoxic conditions), and body mass with reproductive mode, employing path
analyses to account for correlations among the environmental predictors.
Results: Viviparity was strongly associated with cold climates at both
species and assemblage levels, despite the prevalence of viviparity in
some warm climates. Viviparity was not clearly correlated with climatic
variability or elevation. The probability of being viviparous was weakly
positively correlated with body size. Conclusions: Although phylogenetic
history is important, potentially explaining the occurrence of viviparous
species in presently warm regions, current global squamate distribution is
characterized by a higher relative abundance of viviparity in cold
environments – supporting the prediction of the ‘cold-climate’ hypothesis.
The roles of climatic variation and of hypoxia are less important and not
straightforward. Elevation probably exerts various selective pressures and
influences the prevalence of viviparity primarily through its effect on
temperature rather than on oxygen concentration.