Physalaemus pustulosus – The Tungara Frog

1. Physalaemus pustulosus – The Tungara Frog • Ranges from tropical Mexico to northern South America • Averages about 35 mm in snout-vent length • Breeds in temporary bodies of water – even hoof prints

2. Males produce an advertisement call to which females respond. The call consists of two components. One is a frequency-modulated downward sweep (the “whine” component). Whine

3. When several males are calling simultaneously, they may increase the complexity of their call by adding one or more “chuck” components. Playback experiments have shown that females respond preferentially to the calls that have the most chuck components, that is, to the most complex calls. Chuck Why then don’t all males always produce the most complex calls possible, if females “prefer” the more complex calls?

4. The Tungara Frog, among others, is preyed upon by a species of bat, Trachops cirrhosus. In near total darkness, the bat locates the frog on the basis of the frog’s advertisement call. The whine component is difficult to localize, but the source of the chuck component is much more easily located.

5. The complexity of male advertisement calls is therefore a compromise between sexual selection, operating as female choice to favor more complex calls, and natural selection in the form of predation by the bat, favoring simple calls that are difficult for predators to locate. Thus, when few males are calling and there is little competition for females, frogs emit relatively simple calls. Only when male-male competition is intense does it “pay” males to emit the risky complex calls that females favor.