Among most male frogs and toads, croaking is a way to say, “Don’t mess with my gal.” But when rushing streams and waterfalls drown out these warnings, more than 40 species have come up with a different strategy: They stick their rear legs up and out, a bit like dancers doing the “can-can,” researchers reported here earlier this month at the annual meeting of the Society for Integrative and Comparative Biology.

This “foot flagging” likely mimics the movements of a possible predator, giving potential intruders pause. “It would be like getting in a bar fight and being able to roar like a lion to scare off your rival,” says the study’s lead author, Nigel Anderson, a graduate student at Brown University. But some rival frogs have wised up to this scare tactic, he has now discovered.

The work “helps [us] understand how behavioral displays evolve, and how predictable evolution is,” says Yusan Yang, an evolutionary ecologist at the University of South Florida who was not involved in the research.

An animal’s environment can have a strong impact on the type of communication signal it uses. Deep-water fish respond more to red hues, for example, because reds are easier to see where sunlight is weaker. Thus these fish have a so-called perceptual bias toward seeing reds and that bias shaped their evolution: African cichlid species living in deep water have developed red coloration to attract mates instead of the blues that their shoal-dwelling relatives use, Yang explains.

Anderson and his adviser, Matthew Fuxjager, long suspected that perceptual bias might underlie the evolution of foot flagging in frogs. Their fellow anurans, toads, tend to advance on horizontal objects, which tend to be prey in their environment, such as worms. But toads freeze or back away from objects that are vertical in shape—such as a human that’s reached down to grab them, for example. Did foot flagging—with a leg shooting up vertically—evolve to mimic this frightening motion as a way to deter rivals?

Foot flagging has evolved at least six times among anurans. It arises in species living in noisy environments, such as under waterfalls, and thus “may have evolved to solve the same environmental and sensory problem,” Anderson explains. Such multiple origins of the same behavior represent convergent evolution, which has long mystified biologists. Anderson suspects that perceptual biases may underlie many convergent traits: “Why evolve something completely new when you can modify existing systems to evolve something new,” he points out.

Experiments conducted by Anderson and his colleagues in 2021 suggested this is what might have happened in foot-flagging frogs. In many vertebrates, higher levels of testosterone correlate with more aggressive mating behavior. For example, toads with extra testosterone tend to croak louder. When Anderson and Doris Preininger, an integrative zoologist at the Vienna Zoo who keeps a colony of these frogs, injected testosterone into the bellies of Bornean rock frogs (Staurois parvus), a foot-flagging species, the animals lifted their legs higher and in a wider arc—that is, more vertically. All of this suggested foot flagging was strongly tied to territorial behavior.

The Asian common toad (Duttaphrynus melanostictus) chases after a horizontal bar, thinking it’s a worm, but ignores and turns away from a vertical one. Evan Donnachie

Not all rivals were scared away by vertical shapes, however. When Anderson and colleagues tested the responses of foot flagging and species to black rectangles moving across a screen in different orientations, all of the animals attacked a flat rectangle moving horizontally—as expected. The non–foot-flagging toads and frogs moved away from more vertically oriented rectangles, as if threatened by them.

But foot-flagging frogs showed a range of responses to the vertical shape: Some froze, whereas others attacked the vertical bars, Anderson reported at the meeting. “We think the brain overcomes the effects of ‘seeing a predator’ and initiates a follow-up attack,” Anderson says. In response to a rival’s foot-flagging dance of deterrence, he notes, some males apparently have figured out that these flaggers are really just rival males.

Patrick Green, an integrative biologist at the University of California, Santa Barbara, is excited about the work, particularly the variation Anderson observed. That variation “is really important for evolution,” he points out. What seems to happening is that as more males ignore this signal, more exaggerated scare tactics—such as higher leg lifts or lifting both legs simultaneously—are evolving, he says. “I think there is more to be explored, especially looking across more of the foot-flagging species.”