EFFECTS OF LOW-TEMPERATURE ON EMBRYONIC-DEVELOPMENT OF SCELOPORUS LIZARDS

The most widely accepted explanation for the evolution of viviparity a t 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 embr yos warmer than they would be in nests and, thus, enhance their develo pment. 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 test ed the hypothesis that short-term exposure to cold temperatures has le ss 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: Sc eloporus scalaris (Arizona, 1460 m) and Sceloporus aeneus (Mexico, 280 0 m) from the scalaris species group; and Sceloporus undulatus (Virgin ia, 600 m) and Sceloporus virgatus (Arizona, low and high elevation po pulations at 1800 and 2400 m) from the undulatus species group. We inc ubated eggs under simulated natural temperature regimes, but experimen tal eggs were exposed to cold (8, 11, 14, or 17 C) for five days to de termine the mortality and the delay in hatching relative to control eg gs that were incubated under the same simulated natural temperature re gimes. The mortality of eggs that were exposed to cold temperatures du ring incubation did not differ from that of control eggs, and mortalit y did not vary with elevation. These experimental eggs hatched later t han control eggs, but the delay in hatching was again not related to e levation. The hypothesis of physiological adaptation to cold by embryo s was thus rejected.