GROWTH PLASTICITY AND THERMAL OPPORTUNITY IN SCELOPORUS LIZARDS

We studied the relationship between daily activity time and growth rat e in hatchling sagebrush lizards (Sceloporus graciosus), a mainly mont ane species, and western fence lizards (Sceloporus occidentalis), whic h inhabit a broader range of thermal environments. The study involved two populations of each species along an altitudinal gradient in south ern California and one population of S. occidentalis from Oregon. In t he field daily activity time varied seasonally: activity periods were short during spring and fall and longer during the summer. Activity pa tterns also varied geographically: in montane and high-latitude enviro nments, hatchlings experienced reduced opportunity for growth both dai ly and seasonally, because fewer, shorter days are available before th e end of the activity season. We conducted laboratory experiments to m easure the effect of the thermal environment on hatchling growth rates . Laboratory-incubated hatchlings had access to radiant energy for par t of each day and were kept at 15-degrees-C (too low for activity) the rest of the day. The four experimental treatments (6, 9, 12 and 15 h of radiant energy per day) approximated the range of thermal environme nts encountered by hatchlings in the wild. Food and water were provide d ad libitum. Mass-specific growth rates were plastic and generally in creased with potential activity time in hatchlings from all population s. However, the form of this relationship varied among populations: gr owth rates of California S. occidentalis (''growth opportunists'') inc reased roughly linearly with potential activity time, whereas growth r ates of Oregon S. occidentalis and California S. graciosus (''short-da y specialists'') showed curvilinear responses to potential activity ti me. Interspecific and large-scale intraspecific differences in the for m of the norm of reaction paralleled the differences in thermal enviro nments encountered by hatchlings in nature. Populations of S. occident alis at low elevation in California routinely experience short and lon g days of activity (e.g., 6-13 h/d) and the norm of reaction for growt h rate increases linearly over this range. In contrast, populations of S. graciosus rarely experience days with > 9 h of activity and the no rm of reaction for growth rate plateaus above 9 h/d. Similarly, nor-th em populations of S. occidentalis from Oregon rarely experience days w ith > 9 h of activity and the norm of reaction for growth rate also pl ateaus above 9 h/d. On a large scale, therefore, growth responses appe ar to be appropriate to the thermal environments to which hatchlings a re most commonly exposed. However, conspecific populations from differ ent altitudes in California did not differ in growth response, althoug h hatchlings experience different thermal environments in nature. We o bserved substantial among-family variation in growth rates of S. occid entalis, which may indicate genetic variation, a prerequisite for evol ution by natural selection. Moreover, significant family x treatment i nteraction effects on growth rate that reflect underlying genotype x e nvironment interactions indicate a genetic basis to the observed intra specific and interspecific differentiation in the norm of reaction. Ad ditional experiments demonstrated that (1) reducing food availability by 50% reduced growth rates by 50% in both species, and (2) growth rat es did not differ between hatchlings whose eggs were incubated at 24-d egrees vs. 27-degree-C.