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.
Monthly profiles of mean (±SD) precipitation (A) and air temperature (B) from 1998–2008 of the regions where CB (Bellingham, WA) and SR (Dallesport, WA) are located. These data were obtained from nearby weather stations via the Western Regional Climate Center (WRCC).
Linear regressions of mass and SVL for both populations. Mass and SVL were correlated, and analyses of log-transformed data revealed CB lizards were significantly heavier per unit SVL (F1,110 = 11.80, P < 0.01).
Estimation of the number of predominantly sunny days with ≥4 hours of available activity per month for CB and SR. On average, CB (17.0±9.7 d m−1) lizards have fewer days of activity relative to SR lizards (21.0±11.1 d m−1). We estimate that CB lizards had 102 days of activity in 2008, while SR lizards had 189 d of activity.
Hourly temperature profiles (x̄±SD) of lizard microhabitats during the 2008 study period. The top two figures represent exposed microhabitats found at CB (A) and SR (B). The dotted lines represent the mean field-active body temperatures of lizards at each population. The bottom two figures represent unexposed microhabitats found at CB (C) and SR (D).
Estimations of available activity time for CB (A) and SR (B) during the 2008 activity season. Estimations were based on weather station data, microhabitat temperature data, and field observations. The vertical dotted lines enclose the estimated activity for lizards during the study period. These data show available activity time was similar between study sites (CB = 5.70±2.32 h d−1, SR = 4.92±2.4 h d--1).
Allometric relationship of log(DEE) as a function of log(Mb). This relationship was significant (F1,12 = 40.03, P < 0.001) with log(Mb) accounting for 77% of the variation in log(DEE). Open circles represent lizards from SR and filled circles represent lizards from CB.
Comparisons of DEE rates and DEI rates between populations (x̄±SD). Daily energy expenditure rates for CB lizards (n = 4, 193.31±18.65 J g−0.85d−1) were similar to DEE rates measured for SR lizards (n = 10, 174.46±19.48 J g−0.85d−1; W = 32.0, P = 0.106). Mean DEI rates for CB lizards (n = 20, 211.08±64.85 J g−0.85d−1) were significantly higher than SR lizards (n = 22, 141.23±43.36 J g−0.85d−1; t1,40 = 4.136, P < 0.001). Within populations, DEE rates and DEI rates were not different at CB (W = 36.0, P = 0.607). At SR, the difference in DEI rates and DEE rates were not significantly different (W = 142.0, P = 0.067), but DEI rates trended lower than DEE rates. Capital letters represent statistical comparisons of DEE rates between sites, and lowercase letters represent statistical comparisons of DEI rates between sites. Significant differences are indicated by different letters.
Contributor Notes
Associate Editor: C. Bevier.