Energetic Consequences for a Northern, Range-Edge Lizard Population

Lizards at the northern, cool edge of their geographic range in the northern hemisphere should encounter environmental conditions that differ from those living near the core of their range. To better understand how modest climate differences affect lizard energetics, we compared daily feeding and metabolism rates of individual Sceloporus occidentalis in two populations during mid-summer. Chuckanut Beach (CB) was a cool, maritime climate in northern Washington State, and Sondino Ranch (SR) was a warmer, drier climate in southern, inland Washington. We found no difference between populations in daily energy expenditure (DEE), as calculated from doubly labeled water estimates. The CB population, however, had significantly higher prey availability and rate of daily energy intake (DEI) as estimated from fecal pellet masses. Consequently, CB lizards had higher size-adjusted body masses than lizards from SR. Within CB, during midsummer, DEE was similar to DEI. Within the SR population, DEE trended higher than DEI during midsummer, but was not significantly different. We found no population differences in lizard activity, active body temperature, or preferred body temperature. Hence, we infer the longer activity season for the SR population may compensate for the low food availability and high daily energy cost of midsummer. Moreover, for the CB population, we infer that cooler temperatures and higher food availability allow the lizards to compensate for the shorter activity. We also suggest the CB population may benefit from the predicted warmer temperatures associated with climate change given the similar activity-period body temperatures and DEE between these lizard populations assuming food availability is sufficient

How do lizards use behavior and physiology to inhabit different climate zones?

Rapid climatic change is expected to pose extreme ecological and physiological challenges on many ectothermic vertebrates. Some ectothermic species are notable, however, for inhabiting wide geographic ranges and variety of climate zones. Studying how exemplars among ectotherms can behaviorally and physiologically accommodate differing temperature ranges should provide useful mechanistic perspectives on climate change challenges for less accomplished ectotherms. The western fence lizard (Sceloporus occidentalis) is one such exemplar, ranging from southern California to northern Washington. In Washington State, a single subspecies of this lizard occupies strongly contrasting climate zones. Thus, the focus of this thesis was to determine how this subspecies uses behavior and physiology to successfully inhabit these very different habitats within these climate zones. I chose to study Sceloporus occidentalis populations from the Sondino Ponds Unit in the Columbia River Gorge (“CRG”; mean max air = 38.9°C), Goat Wall in the North Cascades (“GW”; mean max air = 33°C), and along the coastal shores of the Salish Sea just north of Marysville (“CS”; mean max air = 27.7°C). In summer 2015 and 2016, to compare thermoregulatory capacity in the field among lizards at each of these contrasting climate zones, I measured field-active body temperatures (field-active Tb) of lizards immediately upon capture. To determine whether lizards may have needed to accept field-active Tb that were suboptimal — presumably due to suboptimal thermal conditions — I compared the distribution of a) field-active Tb among the three locales, and b) field-active Tb with preferred v body temperatures of alert-and-active lizards in the lab (lab Tb) where they were free to select precise body temperatures in a thermal gradient. To test for presence of temperature-dependent physiological differences among the three populations of lizards, I used a flow-through respirometry system in lab to measure whole-animal resting metabolic rates (RMR) — lizards with digesting and assimilating food in their guts — at three ecologically and physiologically relevant body temperatures (20°C, 28°C, and 36°C), as well as standard metabolic rates (SMR) — lizards that were fasted and empty of foodstuff — at 28°C Tb. Lizards at the warmest locale, CRG, had significantly higher field-active Tb than those at the cool coastal locale, CS (ANOVA, p=0.05; post hoc, p=0.045), but field-active Tb of lizards at the high-elevation, northern population, GW, were not significantly different from those of lizards at the other two locales. The distribution of field-active Tb of lizards from CRG skewed warmer than lab Tb (t-tests comparing upper quartile,