Thermal Selection in Tadpoles of the Tailed-Frog, Ascaphus truei

Abstract

Tadpoles of the tailed-frog, Ascaphus truei, exhibit distinct responses to temperature when placed in laboratory temperature gradients. Thermo-regulatory behavior is evidenced by an avoidance of temperatures above 22 C. Both first and second year tadpoles select over a wide temperature range (0-22 C), but first year animals tend to congregate below 10 C, whereas second year tadpoles aggregate above 10 C. Thermal acclimation had little influence on the preferendum of either year class, but photoperiod may alter the response of second year tadpoles to a temperature gradient. INTRODUCTION Extensive comparative studies with fish and reptiles have illustrated that many of these poikilotherms have a characteristic range of temperatures (the thermal preferendum) in which they tend to regulate body temperatures by behavioral selection. Despite the importance of such behavior in the ecology of these poikilotherms, there is only scant information concerning thermoregulation in amphibians, particularly larval forms. With little experimental background, Brattstrom (1963) suggested that amphibians behaviorally seek microhabitats of optimal temperatures and avoid deleterious thermal extremes. Some indication of temperature preference in anuran tadpoles can be derived from field observations and tadpoles of several species appear to aggregate at specific temperatures in ponds (Brattstrom, 1962). The host of variables encountered in natural habitats, obfuscate the response of an organism to temperature. Temperature selection in amphibian larvae, under controlled laboratory conditions, has been reported in only four species of anurans (Workman and Fisher, 1941; Herreid and Kinney, 1967; Lucas and Reynolds, 1967) and two species of urodeles (Licht and Brown, 1967; Lucas and Reynolds, 1967). The anuran, Ascaphus truei, was selected for this study because of its proposed primitive position in amphibian evolution; all previous work with anuran tadpoles has been conducted with ranid species. Much of the literature concerned with Ascaphus suggests that the distribution of this species may be primarily dependent on low temperatures; these ideas stimulated questions of how the tadpoles of this species would behave in response to temperature. Various aspects of the life history of A. truei have been previously documented (Gaige, 1920; Noble and Putnam, 1931; Slater, 1931; Ricker and Logier, 1935; Metter, 1964; Bury, 1968). This species occurs chiefly west of the Cascade Mountains from southwestern British Columbia to Mendocino Co., California; it also occurs in the Rocky Mountains of Idaho and Montana and the extreme southeastern portion of Washington and in northeastern Oregon. Records of collection sites indicate that the distribution of Ascaphus is governed by requirements for perennial, swift streams of low temperature to which many aspects of its morphology and life history can be correlated (Bury, 1968). The main objective of the present study was to ascertain whether the tadpoles of Ascaphus exhibited behavioral thermoregulation; this was tested by studying the animals in laboratory thermal gradient chambers. Preliminary field observations were undertaken to ascertain the role of this behavior in the ecology of this species.