Are viviparous lizards more vulnerable to climate warming because they have evolved reduced body temperature and heat tolerance?

Lizards may experience population declines and extinctions on a similar scale to that experienced by amphibians, and climate warming is one hypothesis proposed to explain these declines and extinctions. Within lizards, viviparous species are hypothesized to be more vulnerable to climate warming, because they have evolved reduced body temperature and heat tolerance, but this idea remains untested. To test this hypothesis, we conducted three temperatures (20, 24, and 28 °C) × two species [Phrynocephalus przewalskii (oviparous) and P. putjatia (viviparous)] factorial design experiment that simulated warming on oviparous versus viviparous lizards. Our manipulation of ambient temperature affected activity and thermal preference in both species, birth date in P. putjatia, and egg mass in P. przewalskii; other examined traits (fecundity, reproductive output, and size, morphology, and sprint speed of offspring) were not affected. Neither in P. putjatia nor in P. przewalskii behavioral responses to rising temperatures differ between the sexes. The viviparous species thermoregulated more actively than did the oviparous species, but the two species did not differ in thermal preference. Warming reduced the activity time allotted for thermoregulation in both species, but the effect was more dramatic in the viviparous species. Our data support one of the central predictions that lead to the hypothesis that viviparous lizards are more vulnerable to climate warming; however, this is not because viviparous lizards have evolved reduced body temperature and heat tolerance, but, because warming constrains activity more dramatically in viviparous species.