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 mid-summer, 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.