Behavioural adaptations in egg laying ancestors facilitate evolutionary transitions to live birth

Live birth is a key innovation that has evolved from egg laying over 100 times in reptiles. One significant feature in this transition is the thermal conditions experienced by developing embryos. Adult lizards and snakes often have preferred body temperatures that can be lethal to developing embryos and should prevent egg retention: how has viviparity repeatedly evolved in the face of this pervasive mismatch? Here we resolve this paradox by conducting phylogenetic analyses using data on thermal preference from 224 species. Thermal mismatches between mothers and offspring are widespread but resolved by gravid females behaviourally down-regulating their body temperature towards the thermal optimum of embryos. Importantly, this thermoregulatory behaviour evolved in ancestral egg-laying species before the evolutionary emergence of live birth. Maternal thermoregulatory behaviour therefore bypasses constraints imposed by a slowly evolving thermal physiology and is likely to have been a key requirement for repeated transitions to live birth.