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.

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