Temperature Modulation in a High-Elevation Amphibian, Rana muscosa

Abstract

Behavior to modulate body temperature was examined in frogs and tadpoles of R. muscosa (Anura: Ranidae) during summer at elevations above 3060 m in the Sierra Nevada of California [USA]. Frogs tended to maximize body temperature at nearly all times during the day, as evidenced by comparisons between frog locations, body temperatures (Tb) and the equilibrium temperatures of model frogs. Frogs modulated Tb by basking in the sun, moving between water and land, depending on which was warmer and selecting macrohabitat, concentrating in the warmer regions along shorelines. As a consequence of behavioral Tb modulation, the daytime Tb of telemetered frogs ranged up to 13.0.degree., 17.5.degree. and 18.6.degree. C above ambient air temperature, wet bulb temperature and water temperature near shore, respectively. Despite behavioral modulation of Tb, frogs in the field rarely achieved the mean selected temperature (MST), determined in a laboratory thermal gradient. Telemetered Tb in the field averaged 17.2.degree. C during the daytime, whereas daytime MST averaged 24.3.degree. C. At night Tb averaged 11.3.degree. C in the field and 21.7.degree. C in the thermal gradient. The maximum Tbs recorded were 28.4.degree. C in the field and 28.6.degree. C in the laboratory thermal gradient. During mid day, almost half the adult R. muscosa in one population formed aggregations of as many as 58 individuals. This behavior should minimize both evaporative water loss and heat loss from the sides of each animal. Tadpoles modulated Tb by habitat selection. Before the spring overturn they remained in the warmer water below the thermocline. Thereafter, they moved each day to warm shallows near shore, where they sometimes aggregated by the hundreds. The maximum average Tb of a population of tadpoles was 25.0.degree. C and the maximum individual Tb was 27.0.degree. C.