Growth Plasticity and Thermal Opportunity in Sceloporus Lizards

Abstract

We studied the relationship between daily active time and growth rate in hatchling sagebrush lizards (Sceloporus graciosus), a mainly montane species, and western fence lizards (Sceloporus occidentalis), which inhabit a broader range of thermal environments. The study involved two populations of each species along in altitudinal gradient in southern California and one population of S. occidentalis from Oregon. In the field daily activity time varied seasonally: activity periods were short during spring and fall and longer during the summer. Activity patterns also varied geographically: in montane and high latitude environments, hatchlings experienced reduced opportunity for growth both daily and seasonally, because fewer, shorter days are available before the end of the activity season. We conducted laboratory experiments to measure the effect of the thermal environment on hatching growth rates. Laboratory—incubated hatchlings had access to radiant energy for part of each day and were kept at 15°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 environments encountered by hatchlings in the wild. Food and water were provided ad libitum. Mass—specific growth rates were plastic and generally increased with potential activity time in hatchlings from all populations. However, the form of this relationship varied among populations: growth rates of California S. occidentalis (growth opportunities) increased roughly linearly with potential activity time, whereas growth rates of Oregon S. occidentalis and California S. gracious (short—day specialists) showed curvilinear responses to potential activity time. Interspecific and large—scale intraspecific differences in the form of the norm of reaction paralleled the differences in thermal environments encountered by hatchlings in nature. Populations of S. occidentalis at low elevation in California routinely experience short and long days of activity (e.g., 6—13 h/d) and the norm of reaction for growth rate increases linearly over this range. In contrast, populations of S. graciosus rarely experience days with >9 h of activity and the norm of reaction for growth rate plateaus above 9 h/d. Similarly, northern populations of S. occidentalis from Oregon rarely experience days with >9 h of activity and the norm of reaction for growth rate also plateaus above 9 h/d. On a large scale, therefore, growth responses appear to be appropriate to the thermal environments to which hatchlings are most commonly exposed. However, conspecific populations from different altitudes in California did not differ in growth response, although hatchlings experience different thermal environments in nature. We observed substantial among—family variation in growth rates of S. occidentalis, which may indicate genetic variation, a prerequisite for evolution by natural selection. Moreover, significant family x treatment interaction effects on growth rate that reflect underlying genotype x environment interactions indicate a genetic basis to the observed intraspecific and interspecific differentiation in the norm of reaction. Additional experiments demonstrated that (1) reducing food availability by 50% reduced growth rates by 50% in both species, and (2) growth rates did not differ between hatchlings whose eggs were incubated at 24° vs. 27°C.