Warning coloration paved the way for louder, more complex calls in certain species of poisonous frogs
Frogs are well-known for being among the loudest amphibians, but new research indicates that the development of this trait followed another: bright coloration. Scientists have found that the telltale colours of some poisonous frog species established them as an unappetizing option for would-be predators before the frogs evolved their elaborate songs. As a result, these initial warning signals allowed different species to diversify their calls over time.
Zoologists at the National Evolutionary Synthesis Center (NESCent), the University of British Columbia, and other research universities assembled an acoustic database to analyse more than 16,000 calls from 172 species within the poison frog family, Dendrobatidae. The paper, which will appear in the December issue of the Proceedings of the Royal Society B, is now available online.
The study included both frogs that display bright colours and others that rely on camouflage for protection. Each call was examined in terms of pitch and duration, and researchers also factored in the size of the frogs and their visibility to predators. They found that because warning coloration protected them from predators, they were better able to attract a mate with low-pitch, pulsing vocalizations in plain sight than their quieter, darker-hued relatives.
Scientists already understood that predators shied away from brightly coloured frogs because of visual cues, but Santos (who worked formerly at NESCent) and his colleagues hypothesized that some species evolved to include audio signals, as well. Such a warning system is not unprecedented: Tiger moths emit ultrasonic chirps to communicate their unsavoury taste to bats. Without a similar ability, frogs navigate a precarious dilemma in which they must either risk detection by predators or forgo possible courtship.
Initially the researchers expected that audio warnings preceded coloration, but the results indicate the opposite. Using molecular data and statistical analyses, they were able to infer a phylogenetic tree and pinpoint which trait came first. Their findings indicate that visual traits established the frogs as poisonous and cleared the way for louder, more elaborate calls.
Species relying on camouflage for defense will not invite attention with boisterous calls, while their protected relatives — including non-poisonous frogs that mimic the appearance of their toxic counterparts — can be loud and more nuanced.
“The type of colour they have is in the range of the noisy ones,” Santos said. “When you’re mimicking somebody that’s already protected, you have some freedom to be found by potential mates.”
These calls require high energy expenditures, but the boon of attracting females without predatory threats makes it a rewarding behaviour for males. Less is known about the reasons females are attracted to the noisier males and how they appraise the various calls. Santos explained that if the females are being especially picky, it will drive male diversity by pushing them to create even more complex songs.
(Source: NESCent press release, 24.10.2014)