THERMAL AND REPRODUCTIVE-BIOLOGY OF HIGH AND LOW ELEVATION POPULATIONS OF THE LIZARD SCELOPORUS-SCALARIS - IMPLICATIONS FOR THE EVOLUTION OF VIVIPARITY

Viviparity in squamate reptiles is presumed to evolve in cold climates by selection for increasingly longer periods of egg retention. Longer periods of egg retention may require modifications to other reproduct ive features associated with the evolution of viviparity, including a reduction in eggshell thickness and clutch size. Field studies on the thermal and reproductive biology of high (HE) and low (LE) elevation p opulations of the oviparous lizard, Sceloporus scalaris, support these expectations. Both day and night-time temperatures at the HE site wer e considerably cooler than at the LE site, and the activity period was 2 h shorter at the HE than at the LE site. The median body temperatur e of active HE females was 2 degrees C lower than that of LE females. HE females initiated reproduction earlier in the spring than LE female s, apparently in order to compensate for relatively low temperatures d uring gestation. HE females retained eggs for about 20 days longer tha n LE females, which was reflected by differences in the degree of embr yonic development at the time of oviposition (stages 35.5-37.0 versus stages 31.0-33.5, respectively). These results support the hypotheses that evolution of viviparity is a gradual process, and is favored in c old climates. Females in the HE population exhibited other traits cons istent with presumed intermediate stages in the evolution of viviparit y; mean eggshell thickness of HE eggs (19.3 mu m) was significantly th inner than that of LE eggs (26.6 mu m) and the size-adjusted clutch si zes of HE females (9.4) were smaller than those of LE females (11.2).