Thermal Relations of Some Tropical Frogs Along an Altitudinal Gradient

Abstract

Temperate-zone amphibians generally show an acclimation response of tolerance to high temperatures (CTMax), and several wideranging species have been found to have a negative correlation between elevation and CTMax. Fewer tropical species have been examined, but Heatwole et al. (1965) reported that the Puerto Rican frog Eleutherodactylusportoricensis failed to show an acclimation response, but did show a negative correlation between elevation and CTMax. Subsequently, E. portoricensis, as originally understood, was found to consist of a widespread species (Eleutherodactylus coqui) and a species restricted to high elevations (E. portoricensis) (Thomas 1965). CTMax was determined for these two species, and we found that differences previously believed to be attributable to elevation are, in fact, interspecific differences. There were no differences in CTMax of E. coqui from different elevations, but E. portoricensis had significantly lower CTMax. E. coqui failed to show an acclimation response. The few species of tropical amphibians that have been investigated suggest that, unlike most temperate-zone species, some tropical species fail to show an acclimation response, and some fail to show a negative correlation between CTMax and elevation. ANIMALS CAN RESPOND to environmental change by behavioral adjustments (fast response), physiological adjustments such as acclimatization (slower response), or over a longer time, by evolutionary adjustments such as structural and enzymatic changes (Hertz 1981). The relative degree to which animals respond to environmental change at any of these three levels can be qualitatively described by the term plasticity. In this paper we will discuss responses to thermal environments in terms of both physiological plasticity, the ability to acclimate, and evolutionary plasticity, the ability of populations to adjust to local conditions via selection. These two levels of response are not independent because the ability to make physiological adjustments is undoubtedly genetically controlled and is therefore subject to evolutionary change. Given the differences in the thermal regimes of temperate and tropical regions, a better understanding of the physiological plasticity and evolutionary plasticity of organisms is important to our understanding of the evolution of tropical and temperatezone species. Correlation between critical thermal maxima (CTMax) and environmental temperatures suggest that tolerances to high temperatures reflect adaptations of populations to their local thermal environments (Snyder & Weathers 1975, Miller & Packard 1977, Howard et al. 1983). Thus, tropical species tend to have higher CTMax than temperate species (Brattstrom 1968, 1970; Snyder & Weathers 1975). Brattstrom (1968, 1970) found, in general, no difference between temperate and tropical species in their ability to adjust their tolerances to high temperatures (CTMax) by acclimation. However, acclimation temperature does not alter the standard metabolic rates of most tropical amphibians, whereas all temperate-zone amphibians that have been tested show significant acclimation of metabolism (Feder 1978, 1982). For amphibians in temperate regions, individuals from cool areas at high elevation have lower CTMax than conspecifics from lower, warmer sites (Brattstrom 1968 provides an example and an apparent exception; Cupp & Brodie 1972, Miller & Packard 1977, Hoppe 1978, Howard et al. 1983, Delson & Whitford 1973 demonstrated this trend, but only in neotenic individuals). Fewer tropical species have been examined along an elevational transect, but Heatwole et al. (1965) reported that Eleutherodactylus portoricensis from Puerto Rico showed a trend in temperature tolerance similar to that of the temperate species mentioned above, with individuals from high elevations having lower CTMax than individuals from low elevation. However, the tadpoles of another Puerto Rican species (Leptodactylus albilabris) did not show a relationship between elevation and CTMax (Heatwole et al. 1968). Subsequent to the study of Heatwole et al. (1965), E. portoricensis, as then understood, was found to consist of two species that differ in subtle morphological and vocal characters: Eleutherodactylus coqui, widespread throughout Puerto Rico, and E. portoricensis, restricted to high elevations (Thomas 1965). This raised the possibility that the sample of frogs collected by Heatwole et al. (1965) from the high elevation site may have consisted of either E. portoricensis, E. coqui, or a mixture of the two. We reexamined the CTMax of these Puerto Rican frogs to look for interspecific differences and to compare E. coqui from high and low elevations. In addition, we measured the critical thermal minima (CTMin) of the three groups I Received 19 November 1985, revision received 29 November 1986, accepted 15 December 1986. 2 Present address: University College of the Northern Territory, GPO Box 1341, Darwin, NT 0801, Australia. 236 BIOTROPICA 20(3): 236-239 1988 This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 06:09:18 UTC All use subject to http://about.jstor.org/terms of frogs, and E. coqui were acclimated at 20 and 27?C to determine if the CTMax of these frogs were affected by acclimation. To confirm the results of the experiment (performed in April 1984), the CTMax experiment was repeated in late October 1984 on frogs that were tested within 4 hr of capture.