Evolution of thermal physiology and growth rate between populations of the western fence lizard (Sceloporus occidentalis).

Hatchling Sceloporus occidentalis from northern populations (central Oregon) grow more slowly than hatchlings from southern populations (southern California) in nature. In this study, I determine whether this difference in growth rate results from differences in thermal environment and/or in thermoregulatory behavior. To determine the degree to which the thermal environment affects growth rate among populations, I reared hatchings from the northern and southern populations in a cycling thermal regime in one of three experimental treatments differing in access to radiant heat (6, 9, or 12 h radiant heat; remainder of 24 h at 15°C). I also measured the body temperature that each individual voluntarily selected over the course of the daily activity cycle. Growth rate varied positively with duration of access to radiant heat. Within the three treatments, individual growth rate was positively correlated with body temperature. Moreover, the difference in growth rate between the northern and southern populations was due in part to differences in behavior - individuals from northern populations selected lower body temperatures. I found that significant variation in body temperature was associated with family membership, suggesting that thermal physiology has a genetic basis. Moreover, growth rate was correlated with body temperature among families in each population suggesting a genetic correlation underlies the phenotypic correlations. Thus, genetically based variation in thermal physiology contributes to differences in growth rate among individuals within a population as well as to differences among populations.