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Eavesdropping Bats Make Frogs Pay A Price For Recycling Air When Calling

Many species of animals have elaborate sexual performances to attract mates. These complicated mating displays are often observable in multiple sensory modalities.

Many species of male birds of paradise, for example, sing and dance in the most lavishly dressed feathers. The females then use their visual and auditory systems to judge and select the best song and dance. Although singing birds are a familiar day-time example of mating displays, many other animals sing too – sometimes at night.

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During the evening, many species of frogs form large aggregations from which to call for mates. The túngara frog (Physalaemus pustulosus), for example, is a neotropical frog that gathers in pools of water. Many male calls overlap as they vie for nearby females. With such abundant competition, males must continually call if they hope to find a mate. This repeated calling is energetically very taxing. These males, however, don’t get out of breath. Instead, they have evolved a large vocal sac that allows them to recycle air. The air passes from their lungs, over their vocal cords and into the vocal sac, which expands like a balloon from the throat. Then, the elasticity of the vocal sac rebounds, and the same air is forced back into the lungs of the frogs. This means that frogs don’t have to waste time and energy filling their lungs with new air before each call. This also allows males to call more, which makes them more attractive to females.

Females, however, are not the only ones listening. Frog-eating bats (Trachops cirrhosus), are sit-and-wait predators that eavesdrop on túngara frog mating choruses. The bats often roost in trees until they hear a calling túngara frog. Once they have located their prey, they take flight and use echolocation to navigate through the forest toward the frog. While bat predators continue to use the male frog call as a cue throughout the hunt, it turns out that the bats can also spot the moving vocal sac of the frog via echolocation. When bats can perceive both the mating call (via what is known as passive listening) and the vocal sac cue (via echolocation), they can more effectively find their prey.

A research team from the Smithsonian Tropical Research Institute used robotic frog models to understand how these frog-eating bats use their sensory systems to find their túngara frog prey. What was found is that these bats actually prefer to attack frogs when the vocal sac is moving, likely because it is easier to find and pinpoint prey with this additional cue. This makes the vocal sac more costly for the male frogs than previously thought. Although female frogs sometimes use the vocal sac as a visual cue to find mates, they are not as attracted to it as the bats are. So, while calling males are able to recycle air with their vocal sacs, enabling them to call more often to females, bats often make them pay with their lives.

These findings are described in the article entitled Multimodal weighting differences by bats and their prey: probing natural selection pressures on sexually selected traits, recently published in the journal Animal BehaviourThis work was conducted by Dylan G. E. Gomes, currently at Boise State University, W. Halfwerk from VU University, Amsterdam, R. C. Taylor from Salisbury University, M. J. Ryan from the University of Texas Austin, and R. A. Page from The Smithsonian Tropical Research Institute.

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