The Behavioral Ecology of Anuran Communication

Abstract

As the last rays of sunlight disappear from the evening sky, a shallow marsh in Panama begins to come alive with the calls of frogs and toads. Among these is a small yellow and brown hourglass treefrog, Hyla ebraccata (Fig. 3.1C). First, a single male begins giving a tentative series of single-note, buzzlike advertisement calls. Soon other males join the first one, and a chorus begins to develop. The first male responds to the calls of his neighbors by placing his own calls immediately after their calls, and he soon increases his calling rate and begins to add clicklike secondary notes to his calls in an attempt to outsignal his rivals. Suddenly another male calls only a few centimeters away, and the first male responds by modifying the introductory notes of his calls, producing aggressive notes with a pulse repetition rate about three times that of his advertisement calls. As the two males approach each other, they gradually increase the duration of their aggressive calls and eventually stop giving secondary click notes as a short wrestling bout ensues. After a few seconds, the intruding male withdraws, and the first male returns to advertisement calling. Having sorted out spacing within the chorus, most of the males soon settle into a regular rhythm of advertisment calling, punctuated by occasional aggressive calls. Periodically they stop calling as their calls are overpowered by bursts of calling from groups of males of another frog, the small-headed treefrog (Hyla microcephala) (Fig. 3.1D). The males of H. ebraccata have difficulty making their calls audible when surrounded by the other species, and they attempt to place their calls in the silent periods between bursts of H. microcephala calling activity. After two hours of calling, the first male detects the movement of a noncalling frog nearby. Sensing that a female may be approaching, he immediately switches to a rapid series of repeated introductory advertisement call notes. The female turns toward the male, and with a few zigzag hops, approaches his calling site and allows him to clasp her in amplexus. The pair then moves off to find a suitable leaf on which to lay their eggs, positioned a half meter or so above the shallow water where the tadpoles will complete their development.