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.

Aim. Clutch size is a key life-history trait. In lizards, it ranges over two orders of magnitude. The global drivers of spatial and phylogenetic variation in clutch have been extensively studied in birds, but such tests in other organisms are lacking. To test the generality of latitudinal gradients in clutch size, and their putative drivers, we present the first global-scale analysis of clutch sizes across of lizard taxa. Location, Global Time period. Recent Major taxa studied. Lizards (Reptilia, Squamata, Sauria) Methods. We analysed clutch-size data for over 3900 lizard species, using phylogenetic generalized least-square regression to study the relationships between clutch sizes and environmental (temperature, precipitation, seasonality, primary productivity, insularity) and ecological factors (body mass, insularity, activity times, and microhabitat use). Results. Larger clutches are laid at higher latitudes and in more productive and seasonal environments. Insular taxa lay smaller clutches on average. Temperature and precipitation per se are unrelated to clutch sizes. In Africa, patterns differ from those on other continents. Lineages laying small fixed clutches are restricted to low latitudes. Main conclusions. We suggest that the constraint imposed by a short activity season coupled with abundant resources is the main driver of large-clutch evolution at high latitudes and highly seasonal regions. We hypothesize that such conditions―which are unsuitable for species constrained to laying multiple small clutches―may limit the distribution of fixed-clutch taxa.