The effect of signal complexity on localization performance in bats that localize frog calls

Abstract

The fringe-lipped bat, Trachops cirrhosus, uses frog mating calls to detect and locate its prey. The tungara frog, Physalaemus pustulosus, a preferred prey species of this bat, produces two types of sexual advertise- ment calls, simple and complex, and both female frogs and predatory bats prefer complex calls to simple ones. Complex calls differ from simple ones in that they contain chucks: short, broadband suffixes with distinct onsets and offsets, acoustic properties that should maximize binaural comparisons and facilitate localization. We investigated the hypothesis that frog-eating bats prefer the complex calls of tungara frogs to simple ones because they find complex calls easier to localize. We tested bats under experimental con- ditions that mirror the conditions they encounter in nature: we broadcast tungara frog calls with and with- out background noise and with and without intervening obstacles. We broadcast calls either continuously during the hunting approach or only prior to the bat's flight to mimic the conditions under which frogs have detected an approaching bat and ceased calling. Bats showed a trend for better localization perfor- mance of complex calls than of simple ones under all treatment conditions. We found significant differ- ences in localization performance in some but not all levels of localization task complexity. This study is the first to offer evidence that an eavesdropping predator shows better localization performance for a pre- ferred signal variant of its prey.