Communication requires a sender, a receiver, and a message. But communication doesn’t take place in a vacuum. Often, there are unintended receivers listening in and unintentional messages getting across.
Illustrating just how complicated sending a message can be is the example of the túngara frog (Physalaemus pustulosus). Male túngara frogs, native to Central and South America, gather at night in shallow ponds and call to attract females. They space themselves out carefully, each male defending a small calling site. Competition for females is serious business, and males will fight if one horns in on another’s chosen calling site.
A new study shows how the male túngara frog’s call inadvertently creates a multisensory message that can be exploited by both rivals and predators. Researchers from the University of Texas at Austin, the Smithsonian Tropical Research Institute, Leiden University, and Salisbury University first tested whether male frogs use these ripples associated with calling to assess their rivals. The researchers found that male frogs were more likely to respond to a call accompanied by ripples than a call by itself. In response to a call accompanied by ripples outside a male’s zone of defense, a circle about 15 cm across, frogs tended to call twice as fast as they would if they heard just the acoustic portion of the signal. If the call with ripples was inside their territory, frogs called less, often stopping altogether and deflating their vocal sacs, possibly preparing to either fight or flee.
But these courtship signals meant for females aren’t just overheard by rival male frogs. They also provide frog-eating bats (Trachops cirrhosus) with a way to pinpoint the location of their next meal. The frog-eating bat specializes in túngara frogs, using a combination of echolocation and eavesdropping to detect them. A frog will stop calling if it sees a bat flying overhead, but ripples continue to move through the water for several seconds after the call ceases, leaving a “footprint” of the frog’s presence. Could bats detect these ripples?