Are Tropical Highland Frog Calls Cold–adapted? The Case of the Andean Frog Hyla labialis1

Across a wide range of temperatures established in the laboratory, we tape–recorded the advertisement calls of 76 freshly caught Hyla labialis males from three elevationally separated populations in the Eastern Andes of Colombia. Each male was tested once at a single temperature and returned to his capture site after measurement of his snout–vent length. We measured and averaged three characteristics of five to ten successive calls for each individual: number of pulses per call, pulse repetition rate, and call duration. We found that calling activity occurred within temperature ranges that overlapped among frogs from different elevations, but widened and shifted downward with increasing altitude of origin. Males from all sites called at temperatures higher, but not lower, than those naturally occurring during their nightly activity period. No decline in vocal performance was apparent when frogs extended their calling activity into the range of high temperatures selected for basking. Both snout–vent length and temperature affected pulse repetition rate and call duration, while the number of pulses per call was temperature–independent. Compared to the smaller males from lower elevations, the larger, high–mountain males had calls with significandy more pulses, a lower pulse repetition rate, and longer duration. Within each population, rising temperatures caused pulse repetition rate to increase and call duration to decrease significantly, whereas the number of pulses per call remained unchanged. Pulse repetition rate of highland males was the factor least affected by temperature, and it was less sensitive to night temperatures than to day temperatures. This, together with their capacity to call at low temperatures, suggests that highland frog calls are cold adapted.