Froggy went a-hikin'

Natural History, March, 2007 by Norm Condit, Tracie A., Anton Seimon

In "Living the High Life" [9/06], Kevin Krajick describes the discovery of tadpoles in ponds as high as 17,700 feet in Peru's Cordillera Vilcanota. Assuming they came from lower-elevation ponds, what could have led at least one reproducing pair to ascend to such suicidal heights? And how could they have survived the ascent?

Norm Condit

Staten Island, New York

BIOLOGISTS TRACIE A. AND ANTON SEIMON REPLY: Three species of frog occur in the high alpine watershed of Sibinacocha, a large lake at 16,000 feet: the aquatic Telmatobius marmoratus, and two terrestrial species, Pleurodema marmorata and Bufo spinulosus. We have found the same three species at 17,200 feet in ponds that feed a small stream flowing into the lake. Thus the stream provides a conduit between the lake and the high ponds for all three species. No surface watercourses link those ponds to even higher ponds, and we have not observed T. marmoratus (the aquatic species) above that elevation.

In contrast, the two terrestrial species can migrate to higher ponds, and we have observed P. marmorata tadpoles at a cluster of small ponds at 17,700 feet. Because reaching those isolated ponds would require arduous traverses, it is plausible that they landed there by other means, perhaps as accidental passengers aboard birds or mammals.

Yet in two visits to the highest ponds we found no adults, which might indicate that adults only migrate to the high ponds seasonally to breed. The high ponds' isolation may reduce the threat of predation for the adults, and the intense midday solar heating may accelerate the metamorphosis of tadpoles into adults. A more speedy metamorphosis would also reduce the tadpoles' chances of being eaten, and enable them to leave the seasonal ponds before they dry out. In any case, no one should assume that these well-adapted species find their environments inhospitable.