Sound Reveals Species Identity At A Distance

Sound reveals species identity at a distance, making the search for mates far less dangerous. Exploiters of mating signals underscore the perils of mistaken identity. Predatory katydids in Australia mimic the mating sounds of female cicadas, luring amorous males to their deaths. The function of sound changes when the spring peeper’s trek across the forest floor brings her to the wetland. She now listens for information embedded in individual voices. Males are spaced every ten to one hundred centimeters and so she lops and swims through an array of pealing sound and bulging throat sacs. Most of the calls are peeps, but if males get too close to one another, they joust with rough reeeps, grappling for territory with sound. One cluster is tuned to the frequency of the male’s sound. A second has a wider range, presumably for detecting the diverse sounds of the forest. Males, curiously, have ears tuned higher than their calls, perhaps to better tolerate many nights sitting in the cacophony or to listen for the higher rustling sounds of approaching danger. It is also possible that the males’ ears are seeking subtle differences in acoustic structure that reveal the identity of neighbors. Bullfrogs recognize familiar calls and respond more vigorously to strangers.

Playing With Fire

Male peepers remember how aggressive their neighbors are, reeeping to those that suddenly ramp up the pushiness. These synchronous duets occasionally expand into groups of up to five males with closely aligned tempi. We do not yet know whether spring peepers recognize individual voices. Female spring peepers prefer calls that are loud and rapidly repeated. The sonic vigor and pacing of the peep have their evolutionary origins in this preference. Loud males are easy to detect and locate. Evolution has thus cranked the blast of sound about as loud as it can from lungs the size of a pea. At temperatures just above freezing, males give about twenty peeps per minute. On balmy nights, their peeping rate increases to eighty peeps every minute. But regardless of whether the night is cool or warm, some males vocalize at up to twice the rate of others. Females sense these differences and swim or hop over to the faster callers. In doing so, they select the healthiest males in the swamp.

Learn To Be Still

Some males deliver more than thirteen thousand peeps in a night, each one powered by strong muscular contractions. Fats stored in these muscles supply 90 percent of the energy needed for calling. Males who cannot supply ample fat to their muscles have little stamina. They also tend to show up night after night, rather than sporadically through the spring. Her choice made, the female approaches a male, taps him, then, in a flail of limbs, he clambers onto her back, forelegs clasping her neck. Unlike many other frogs that lay eggs in clusters, spring peepers place most of their eggs singly, perhaps to prevent predators from finding and eating them all. Once the eggs are deposited, the parents leave them to their fates. The female’s acoustic preferences for extreme calling rates have a practical result for her offspring, uniting her genetic material with that of vigorous males. She may also reap a benefit for herself in the shorter term, avoiding sickly males that might transmit their ailments while they are clamped onto her back. Over the breeding season, the female spring peeper lays up to one thousand eggs. The egg yolk supplies energy for the developing embryos and a boost when the tadpoles first hatch. The male’s singing is exhausting too, depleting his reserves and exposing him to predators.

Something In The Air

His investment brings no food or other physiological benefit to the young. Instead, it enforces a kind of honesty in the communication system between males and females. Only healthy males can afford to sing loud, fast, and long. An inexpensive call could be given by any male and the sound would therefore carry no reliable message about body size and condition. The high cost of calling, then, ensures that the spring peeper’s call carries worthwhile information. By using sound to make their mating choices, females select males whose genetic qualities are likely to be helpful for their offspring. The costs of singing have lodged both the females’ preferences and the males’ songs at the center of the species’ breeding behaviors. This is not how costs usually affect evolution. But for the spring peepers’ calls, costs are an essential part of the function of the signal. Without them, the communication system would fall apart. Costs of singing, then, have two opposing effects. For slow, defenseless animal species, making a loud sound likely brings death. This is too high a cost for any sonic signaling system, no matter how much information the sound reveals about the health of the singer. But for species that can leap or wing away from danger, costs of sound making ensure that the sound is meaningful and thus favored by evolution. Evolution will not endow spring peepers with signals so extreme that they all but guarantee death. But it will tax the frogs enough to reveal the vitality of each singer. Costs play a foundational role in communicative signals across the animal kingdom. The bright colors of bird feathers and lizard throats, and the heavy antlers of deer, reveal the health and vigor of their bearers. The costs of these structures are too severe for feeble animals to bear. Many of these signals are closely tied to the body sizes of animals. The volumes of the lungs and throats of frogs and deer, for example, are revealed by the depth and vigor of their calls. For small individuals, the price of mimicking the call of a large animal is prohibitively large. When gazelles flee predators, they sometimes interrupt their runs with upward leaps. These prances advertise fleetness and tell pursuers that a chase is unlikely to succeed. Aphids fare poorly on trees with fiery displays and avoid them when they can. For vocal signaling, costs take several forms. A calling spring peeper depletes energy reserves, pushes muscles and lungs to the limit, and reveals its location to predators.