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Risky Ripples Allow Bats and Frogs to Eavesdrop on a Multisensory Sexual Display

Science  24 Jan 2014:
Vol. 343, Issue 6169, pp. 413-416
DOI: 10.1126/science.1244812

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Abstract

Animal displays are often perceived by intended and unintended receivers in more than one sensory system. In addition, cues that are an incidental consequence of signal production can also be perceived by different receivers, even when the receivers use different sensory systems to perceive them. Here we show that the vocal responses of male túngara frogs (Physalaemus pustulosus) increase twofold when call-induced water ripples are added to the acoustic component of a rival’s call. Hunting bats (Trachops cirrhosus) can echolocate this signal by-product and prefer to attack model frogs when ripples are added to the acoustic component of the call. This study illustrates how the perception of a signal by-product by intended and unintended receivers through different sensory systems generates both costs and benefits for the signaler.

It's Complicated

Animals have evolved impressive displays used in mate selection. Although intended for the opposite sex of the same species, the potential for eavesdropping is significant. In cases where the sensory signature is the sexual signal itself (such as a bird call), selection from harmful eavesdroppers could result in a reduction in signal intensity that represents a balance between the cost and benefit of the signal. Halfwerk et al. (p. 413), however, show that the physical by-product of a signal can also act as a cue to both intended and eavesdropping recipients. Ripples in the water made by throat sac expansion in calling túngara frogs signal their presence both to rivals and to predatory bats. This physical signature of the call itself cannot be modified; thus, it represents a cost-benefit ratio to calling that cannot be shifted through selection pressure from either side. Thus, physical by-products of sensory signaling create significant complexity in the evolution of sexual signaling.

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