My column about chorus frogs a couple of weeks ago drew a couple of reader inquiries: Why are chorus frogs more resistant to the fungus that is devastating other amphibian species? And how are the chorusers able to stop calling at the same time?
Good questions, both. Unless I’ve missed some recent research, the first question is still open. The fungus in question is Batrachochytrium dendrobatis (BD for short). In a 2006 article, Andrew Blaustein and his colleagues at Oregon State University explained that biologists are not sure how BD kills its victims. It may produce lethal toxins; it may disrupt respiration; it may do both. In some studies, infected amphibians died within two days, suggesting the work of a toxin. In others, death occurred two weeks after exposure. Some frog and toad species are vulnerable at different developmental stages. Blaustein’s group found that Cascade frog tadpoles resisted the fungus but recent metamorphs (ex-tadpoles that had just shed their tails) succumbed.
Conversely, western toad metamorphs showed more resistance to BD than toad tadpoles. Both larval and young adult northern Pacific chorus frogs (Pseudacris regilla) just shrugged it off, even when the fungus was coupled with a hit of ultraviolet-B radiation. They’re tough little guys.
As for the timing, my best answer is that members of a frog chorus pay close attention to their neighbors and are capable of extremely rapid responses. But why are they chorusing in the first place? Some contend that male frogs and toads obtain a collective benefit from calling together: making themselves more detectable by females, or confusing predators. Others say the chorus is an artifact, the result of a bunch of selfish individuals trying to jam each other’s signals.
The best evidence for the collective-benefit model comes from the tropical treefrog Smilisca sila, in which individual calls tend to overlap. This has been interpreted as a defense against their major predator, a species of frog-eating bat. Otherwise, the structure of a frog chorus seems to be most affected by the preferences of the listening females.
Frogs can have fairly large vocabularies. Four call types have been described in the Baja California chorus frog (P. hypochondriaca.) The biphasic call, the well-known “ribbet,” is an advertising call: “I’m here!” The monophasic call is given when a female approaches a calling male. The trill is used in hostile interactions between males. Finally, there’s the release call, the response of a male which has been amplexed, as the herpetologists delicately put it, by another male: “Hey! Get off me!” Lustful amphibians are not very discriminating; male cane toads will amplex roadkill.
Chorus frogs are antiphonal callers. The first biologist to try and parse the structure of the chorus was Woodbridge Foster of UC Berkeley. In the spring of 1964, when other Americans were registering voters in Mississippi or rallying for Goldwater, Foster spent his evenings along Putah Creek in Yolo County listening to Sierran chorus frogs (P. sierra.)
He found that the chorus was composed of small groups of two or three interacting males. A male would respond to the call of his loudest neighbor, and calling would spread from one duo or trio to the next subgroup. Followers would shut up when their leader did. This might occur when a car passed, or when two frogs happened to call at the same time.
A few years later, Frank Awbrey of San Diego State, working with Baja California chorus frogs in San Diego County, reported that individual callers adjusted the timing of their calls to avoid overlapping recorded calls that were played to them. Awbrey concluded that the lead frog acted as a pacemaker, and that the frogs spaced themselves out so two or three neighbors could call without interfering with each other’s vocalizations. Another biologist, Douglas Allan of UC Irvine, reported that chorus frogs in Anaheim used trills at the beginning of a calling session to establish the spacing.
Why do the calling males alternate? Although I couldn’t find any relevant studies, female preferences must have something to do with it. Research on other frog species strongly suggests that audience response determines whether males call simultaneously, antiphonally, or with some degree of overlap.
In Tungara frogs (Physalaemus pustulosus), another New World tropical species, playback experiments show that females prefer the males that call first. That puts pressure on males to match their neighbors’ calls. But female West African running frogs (Kassina fusca) prefer following calls that just slightly overlap the leader’s call. If the follower calls too soon, the female opts for the leader.
Why the variation? Females of different species may have different ways of assessing male fitness, or it may just be a matter of how their sense of hearing is wired. Another good question.