Effects of low temperature on embryonic development of Sceloporus lizards

The most widely accepted explanation for the evolution of viviparity at high elevations and latitudes (cold climates) is that, by retaining eggs either for short periods (in the transition between oviparity and viviparity) or for the entire gestation period, females can keep embryos warmer than they would be in nests and, thus, enhance their development. However, an increase in the length of egg retention is not the only mechanism that would allow squamate embryos to cope with the low ambient temperatures in nests at high elevations or latitudes. We tested the hypothesis that short-term exposure to cold temperatures has less effect on embryonic development of species or populations from cold than warm climates, indicating physiological adaptation of embryos to cold temperatures. Our experimental subjects were four species (five populations) of Sceloporus lizards from a wide range of elevations: Sceloporus scalaris (Arizona, 1460 m) and Sceloporus aeneus (Mexico, 2800 m) from the scalaris species group; and Sceloporus undulatus (Virginia, 600 m) and Sceloporus virgatus (Arizona, low and high elevation populations at 1800 and 2400 m) from the undulatus species group. We incubated eggs under simulated natural temperature regimes, but experimental eggs were exposed to cold (8, 11, 14, or 17 C) for five days to determine the mortality and the delay in hatching relative to control eggs that were incubated under the same simulated natural temperature regimes. The mortality of eggs that were exposed to cold temperatures during incubation did not differ from that of control eggs, and mortality did not vary with elevation. These experimental eggs hatched later than control eggs, but the delay in hatching was again not related to elevation. The hypothesis of physiological adaptation to cold by embryos was thus rejected.