Higher Energy Demands and Lower Body Condition in a Hot Spring: Comparing Relict Leopard Frogs at Two Thermally...

Following range-wide decline, all remnant populations of the relict leopard frog (Rana onca) in the eastern Mojave Desert of the southwestern USA occupied geothermally influenced hot springs. In recent years, however, several large populations have been established at cold-water sites. There has, however, been little research into the thermal ecology of the species. We monitored body temperature (T_b) of adult frogs at a pair of hot and cold springs using implanted temperature data loggers, and measured body condition across seasons. We determined temperature preference using a thermal gradient, and measured mass-specific oxygen consumption rates to estimate energy demands in the wild. We found that frogs at the two sites had similar temperature preferences, but the thermal profiles in the wild were seasonally 2.5°C to 5.7°C warmer at the hot site. In summer, frogs at the hot site mostly sought the lowest temperatures available, which were still warmer than measured thermal preferences. Frogs at the cold site maintained cooler temperatures at night, but warmed up during the early part of the day, likely moving in and out of cold water. At both sites, frogs in winter used two overwintering strategies, manifested mostly as differences in T_b variability. Frogs with low T_b variability thermally reflected cold water temperatures throughout most of winter and were interpreted as mostly inactive. Frogs with higher T_b and variability were interpreted as mostly remaining active throughout winter. We estimated that the energy demands at the hot site were 1.27-fold higher in summer and 1.14-fold higher in winter than at the cold site. Frogs at the hot site had significantly lower body condition than those at the cold site. We concluded that thermal conditions at the hot site were not favorable to R. onca and required higher energy investment in physiological maintenance, presumably at the expense of growth and reproduction.