Evaporative cooling via panting and its metabolic and water balance costs for lizards in the American Southwest.

In lizards there is considerable variation in the ability to dissipate environmental/endogenous heat loads through evaporative cooling via panting, which effects how long lizards can spend exposed to high solar heat loads. We recently described the differing capacities of lizards to depress body temperature (Tb) through evaporative cooling via panting. Here, we link panting and Tb depression with rates of evaporative water loss and its metabolic costs under high heat loads. We used flow-through respirometry to measure evaporative water loss rates and metabolism of 17 lizard species from the American Southwest while simultaneously measuring Tb. We exposed lizards to air temperatures (Ta) ranging from 35°C to their critical thermal maximum (CTmax) while marking the onset of panting. We then estimated pre-panting Q10 values for metabolism to partition increases in metabolism associated with the van't Hoff effect from the mechanical cost of panting with increasing heat loads. We found that evaporative cooling costs substantially varied among species, with panting effort significantly affecting lizards' evaporative capacity. Lizard evaporation rates ranged from 0.32 to 1.5 g H2O h-1, with individuals losing as much as 6% h-1 of body mass while panting. Lizards also experienced an increase of up to 7.9-fold in metabolic rate while panting, although the overall energetic costs of panting remained relatively low compared with evaporative water costs. Across species, there was a significant positive relationship between the overall rate of evaporative heat loss and the maximum Ta-Tb gradient a species could maintain. While evaporative cooling may be an effective mechanism for reducing Tb and extending activity in hot environments for many species, it has significant metabolic and water balance costs that should be considered, as habitats with high environmental heat loads can be especially costly to an animal's water budgets.

The functional significance of panting as a mechanism of thermoregulation and its relationship to the critical thermal maxima in lizards.

Because most desert-dwelling lizards rely primarily on behavioral thermoregulation for the maintenance of active body temperature, the effectiveness of panting as a thermoregulatory mechanism for evaporative cooling has not been widely explored. We measured changes in body temperature (Tb) with increasing air temperature (Ta) for 17 species of lizards that range across New Mexico and Arizona and quantified the temperatures associated with the onset of panting, and the capacity of individuals to depress Tb below Ta while panting, and estimated the critical thermal maxima (CTmax) for each individual. We examined these variables as a function of phylogeny, body mass and local acclimatization temperature. We found that many species can depress Tb 2-3°C below Ta while panting, and the capacity to do so appears to be a function of each species' ecology and thermal environment, rather than phylogeny. Panting thresholds and CTmax values are phylogenetically conserved within groups. Understanding the functional significance of panting and its potential importance as a thermoregulatory mechanism will improve our understanding of the potential for species' persistence in an increasingly warmer world.

The evolution of different maternal investment strategies in two closely related desert vertebrates.

We compared egg size phenotypes and tested several predictions from the optimal egg size (OES) and bet-hedging theories in two North American desert-dwelling sister tortoise taxa, Gopherus agassizii and G. morafkai, that inhabit different climate spaces: relatively unpredictable and more predictable climate spaces, respectively. Observed patterns in both species differed from the predictions of OES in several ways. Mean egg size increased with maternal body size in both species. Mean egg size was inversely related to clutch order in G. agassizii, a strategy more consistent with the within-generation hypothesis arising out of bet-hedging theory or a constraint in egg investment due to resource availability, and contrary to theories of density dependence, which posit that increasing hatchling competition from later season clutches should drive selection for larger eggs. We provide empirical evidence that one species, G. agassizii, employs a bet-hedging strategy that is a combination of two different bet-hedging hypotheses. Additionally, we found some evidence for G. morafkai employing a conservative bet-hedging strategy. (e.g., lack of intra- and interclutch variation in egg size relative to body size). Our novel adaptive hypothesis suggests the possibility that natural selection favors smaller offspring in late-season clutches because they experience a more benign environment or less energetically challenging environmental conditions (i.e., winter) than early clutch progeny, that emerge under harsher and more energetically challenging environmental conditions (i.e., summer). We also discuss alternative hypotheses of sexually antagonistic selection, which arise from the trade-offs of son versus daughter production that might have different optima depending on clutch order and variation in temperature-dependent sex determination (TSD) among clutches. Resolution of these hypotheses will require long-term data on fitness of sons versus daughters as a function of incubation environment, data as yet unavailable for any species with TSD.