The Bogert Effect and environmental heterogeneity.

A classic question in evolutionary biology is whether behavioral flexibility hastens or hinders evolutionary change. The latter idea, that behavior reduces the number of environmental states experienced by an organism and buffers that organism against selection, has been dubbed the "Bogert Effect" after Charles Bogert, the biologist who first popularized the phenomenon using data from lizards. The Bogert Effect is pervasive when traits like body temperature, which tend to be invariant across space in species that behaviorally thermoregulate, are considered. Nevertheless, behavioral thermoregulation decreases or stops when spatial variation in operative temperature is low. We compared environmental temperatures, thermoregulatory behavior, and a suite of physiological and morphological traits between two populations of the southern rock agama (Agama atra) in South Africa that experience different climatic regimes. Individuals from both populations thermoregulated efficiently, maintaining body temperatures within their preferred temperature range throughout most of their activity cycle. Nevertheless, they differed in the thermal sensitivity of resting metabolic rate at cooler body temperatures and in morphology. Our results support the common assertion that thermoregulatory behavior may prevent divergence in traits like field-active body temperature, which are measured during periods of high environmental heterogeneity. Nevertheless, we show that other traits may be free to diverge if they are under selection during times when environments are homogenous. We argue that the importance of the Bogert Effect is critically dependent on the nature of environmental heterogeneity and will therefore be relevant to some traits and irrelevant to others in many populations.

Behavioral thermoregulation is highly repeatable and unaffected by digestive status in Agama atra.

The precision and the extent of behavioral thermoregulation are likely to provide fitness benefits to ectotherms. Yet the factors driving variation in selected or preferred body temperature (Tset ) and its usefulness as a proxy for optimal physiological temperature (Topt ) are still debated. Although Tset is often conserved among closely related species, substantial variation at the individual, population and species level has also been reported. However, the repeatability (calculated as the intra-class correlation coefficient) of Tset is generally low. One factor that influences Tset is feeding status, with fed reptiles typically showing higher Tset , a process thought to aid meal digestion. Here, using experiments simulating realistic feeding and fasting regimes in Agama atra, a heliothermic lizard from southern Africa, we test if Tset and its repeatability under these 2 states significantly differ. Daily Tset ranged from 33.7 to 38.4 °C, with a mean (± SE) of 36.7 ± 0.1 °C for fed and 36.6 ± 0.1 °C for unfed individuals. Comparisons of repeatability showed that females tend to be more consistent in the selection of body temperature than males, but not significantly so regardless of feeding status. We report some of the highest repeatability estimates of Tset to date (full range: 0.229-0.642), and that the weak positive effects of feeding status on Tset did not increase its repeatability. In conclusion, one of the major prerequisites for natural selection, consistent among-individual variation, is present, making the adaptive significance of Tset considerably more plausible.