Sounds Wild and Broken, page 13
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. The female oars her way through the water, gluing peppercorn-sized eggs to submerged vegetation, fertilizing each with sperm from the male clinging to her back. 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 mother’s nourishment in the egg yolk and the parents’ DNA is all the inheritance the tadpoles receive. 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. She endows each one with a supply of yolk, draining her hard-won stores of fat and nutrients. Early spring is a lean time, and so these stores date to the warm, insect-filled days of autumn. 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. 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. The spring peeper’s body—from toes with adhesive disks for climbing to sticky insect-catching tongues—is built without wasted energy and material. 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. In plants, large petals saturated with pigments and fruits loaded with colored nutrients faithfully signal the plants’ condition to pollinating and fruit-dispersing animals. Even the costly red-colored leaves of autumn may signal the quality of trees. 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. Singing uses between 10 and 25 percent of a Carolina wren’s daily energy budget, and of all the wren’s daily activities, only flying requires more energy than singing. A singing wren also pays an opportunity cost because time spent in song is time not spent feeding or preening. Predators such as sharp-shinned hawks may cue in on the wren’s song, finding the singer among concealing tangles of vegetation, just as tachinid flies do with katydids. Bird nestlings clamoring at their parents from the nest draw the attention of predators. When a leafhopper blasts its vibratory signals through its legs into plant stems, the insect’s energy usage goes up twelve times. A skylark that sings while it flies away from an attacking merlin uses precious breath and time. In each case, listeners receive information about sound makers. Female spring peepers assess the fat stores and muscular condition of potential mates. Wrens infer one another’s health. Parent birds understand the vigor and hunger of their nestlings. Leafhoppers communicate body condition. The merlin understands the fleetness of the lark and gives up when it hears song streaming from its quarry.
As we walk our neighborhoods and hear the varied sounds of animals around us, we’re participating in a network of flowing information. With a little attention, we can hear some of the meanings in these sounds. In a chorus of insects or frogs, the healthiest animals have the loudest or most persistent voices. Among breeding birds, the individuals with the most diverse repertoire may be those that leave the most surviving offspring. In the song sparrow, a bird species common across North America, for example, males that sing with wide range whistles and trills leave more grand-offspring than those with simpler songs.
Naturalists are taught to recognize animal species by ear, a practice that opens us to the diversity of creatures around us. When I first learned the frog and bird sounds around my home, I felt my sensory boundaries expand. Suddenly I was in contact with the conversations of dozens of species. But, at first, I neglected to go beyond the names of species and attend to the sonic nuances within the sounds of each. Arriving at a name, I stopped. Yet every voice carries meaning. Some individual differences, like the melodic and rhythmic variations of the song sparrow, are easy to pick out after only a few minutes’ listening. Others are harder, such as the seemingly infinite complexity of crow and raven sounds or the subtle differences among frog calls. By giving individual animals around our homes the gift of our attention, we can learn much about the meanings of their sounds.
A largely uncharted area for future study is the relation between sound making and the diversity of animal sexuality. Nearly all field studies of song assign sex to individuals with the untested dualistic and heteronormative assumptions that all animals exist in either male or female bodies, and that all pairs are between males and females. Neither assumption is true. Many species have nonbinary individuals, either as third or fourth “sexes” within the species or as unions in one animal body of male and female sex cells, body forms, and behaviors. The frequency of these intersex individuals varies from 1 to 50 percent in most vertebrates. Many “male” frogs, for example, have egg-making cells within their testes. I assumed that the spring peeper whose throat sac I watched was a male, but the animal’s hormonal and cellular bodily reality may have been a mix of male and female. A number of frog species also have two types of males: singers and silent “satellites.” The silent males are often smaller and sit close to the singer. As I listen to the spring peepers from the boardwalk, about 10 percent of the vocal males likely have a satellite nearby. These males contribute no effort to calling. From a human perspective, such lurking seems perhaps creepy and parasitic. But spring peeper females have their own sexual aesthetic and sometimes choose to mate with these edgy, silent types. In spring peepers, the roles of singer and satellite are flexible, with individuals switching as conditions change. In other species of frog, and in some insects and birds, animals stick with one of multiple within-sex identities for an entire breeding season or lifetime.
Further, in many species, females also sing. Yet the vast majority of scientific studies of song in the breeding season focus on males. The bias against noticing and studying female sounds has both cultural and geographic roots. We project onto “nature” our preconceptions. Victorian naturalists saw quiet domesticity in females and loud, conquering vitality in males. In the Reaganite and Thatcherite years of the 1980s, biologists described song as the result of an economic battle of the sexes. In a free market of competing individuals, silent females assessed which garrulous males might best serve their interests. Now the idea that females are, by nature, mostly silent has been overturned.
The peculiarities of animal behavior in the temperate climate of northern Europe and northeastern North America, where, until recently, most scientists studying animal behavior lived, add to this bias against studies of female vocal displays. There, male birds and frogs dominate the soundscape of gardens and forests. But female birds in the tropics and in warm-temperature regions in the Southern Hemisphere are often just as vociferous as males. The birds of temperate Europe and North America, then, are unusual. A survey of birds around the world shows that females sing in more than 70 percent of songbird families, and a reconstruction of the songbird family tree shows that female song was likely present in common ancestors of all modern species. In the developing embryo, the song centers of the bird brain develop in both sexes. The evolutionary and embryological roots of song are thus present in all adult birds. Among frogs, song is heavily male biased, but females make sounds during social interactions, some of which seem to individually identify each female. In the vibratory world of insects communicating on plants, males and females often duet, passing tremors back and forth along stems or through leaves. Both male and female mice give ultrasonic sounds during breeding interactions, part of a much wider array of sonic communication within their social networks.
In On the Origin of Species, Charles Darwin wrote that female birds, “standing by as spectators,” might select “the most melodious or beautiful males,” causing evolution to elaborate the songs and plumage of males. He was right that evolution sculpts sexual displays, but his cultural context constrained his view of sexual diversity and possibility.
Blinders of our own time no doubt also narrow our views today, underscoring the need to question assumptions about sexual roles. We can expand Darwin’s vision and recognize that all sexes—male, female, nonbinary—of vocal species use sound to mediate social interactions. This more expansive view is an invitation and challenge. As we listen to the voices of animals around our homes, can we leave behind our preconceptions and hear the richly varied forms of sexuality in nature? Around me in the glorious pealing thunder of the spring peeper’s chorus are not only males and females, enacting a simple story of male advertisement to female spectators. Each individual has its own sexual nature—a blend of “male” and “female” for many—and each its own agency. The sounds that so lift my winter-worn spirits are the information-rich mediators of behavior in this complex sexual web.
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There are two great puzzles and wonders about the songs of breeding animals. The first is why any animal would expend energy and advertise itself to predators by making loud and persistent sounds. This seemingly wasteful and dangerous activity allows singers both to reach potential mates over a vast area and, in some cases, to reliably communicate information about health. The second puzzle is the great diversity of sonic forms in breeding displays. A loud and repeated grunt would suffice to advertise the location and vitality of any animal. Yet even among closely related species, sound takes on timbres, tempi, and melodic patterns whose marvelous diversity surpasses what is needed to reveal the location and strength of singers.
The spring peeper’s relative, for example, the upland chorus frog, makes a raspy sound, like running a fingernail along the teeth of a plastic comb. Another relative, the northern Pacific tree frog, calls with a rising, two-part krek-ek! Sounds of more distant tree frog kin include the rapid tap of flinty stones in the northern cricket frog, stuttering beeps like a Morse code machine gone mad in the European tree frog, and a groaning waar from the Mediterranean tree frog. If the sound of these species’ calls had been shaped only by a need to show off vigor and fat reserves, the tree frogs would all sound similar, perhaps a peep varying only in its pitch in different-sized species. The frogs all call from similar habitats, and so it is unlikely that differing demands of sound transmission would have created such a diverse array of calls.
Consider also the red crossbills of the Rocky Mountains and the animals of the Amazon rain forest. The red crossbill sings with complex, inflected melodies, interspersed with buzzes and flourishes, a song far more elaborate than is needed to merely cut through the masking noise of wind in spruce trees. The nighttime chorus of insects and the dawn salvos of bird and monkey sound in the Amazon are astonishingly varied. These species are adapted to the sound transmission qualities of their forests. Their sounds also reflect the ongoing struggle with predators and with ecological competitors. Yet there is more to the diversity of their sounds than can be explained by adaptation to local vegetative and biological conditions, or the need to communicate the vitality of the lungs, blood, and muscles.
Sexual dynamics among animals are creative forces, diversifying sound. This generative power works in three principal ways, none of them exclusive of the other. The first is the sensory biases of each species. The second is the need to avoid breeding with closely related species. The third, and most creative of all, are aesthetic preferences.
Every listening organ is tuned to particular frequencies of sound. These frequencies are usually those that most reliably signal the presence of danger or food. Sexual displays that match these sweet spots are most likely to be noticed and acted on. The listening organs on the legs of water mites, for example, are tuned to the frequency of the swimming motions of small crustaceans. When the water mites sense this distinctive hum, they grab their prey. Males use the same sound frequency to signal to females, using a preexisting bias in the sensory system for courtship.
Small mammals and insects live in proximity to one another, often in dense vegetation. Their hearing range extends into what humans call the ultrasonic because these high sounds reveal useful information about the close-at-hand environment. Social and breeding signals of these animals are therefore also ultrasonic. To human ears, for example, mice and rats seem almost entirely silent, but these animals have rich vocal repertoires including play sounds, calls from pups to mothers, alarms, and breeding songs. Such high-frequency sounds travel very poorly in air, and so these sounds offer rodents good close-at-hand communication without revealing their locations. For animals that interact on larger scales, like humans and birds, lower frequencies work better for long-distance communication. Their ears—and thus breeding songs and calls—are tuned to lower frequencies. The diversity of sonic expression therefore reflects the varied ecologies of each species.
The evolutionary imperative to avoid interbreeding with other species can also be a potent diversifying force. If two closely related species or populations overlap, then interbreeding will produce hybrid offspring that are sometimes deformed and often poorly adapted to either of their parents’ habitats. In this case, evolution will favor breeding displays that clearly differentiate each species, reducing the possibility of ill-advised pairings.
For example, the spring peepers that I listen to in the swamps of upstate New York belong to the eastern population of the species. To the west, in Ohio and Indiana, the peepers are larger and their calls are lower and delivered at a faster clip. Four other populations, one in the Midwest and three along the Gulf Coast, also differ in body size and call style. These six different spring peeper varieties have pedigrees that diverged at least three million years ago, with some hybridization and genetic mixing since. What seemed to human taxonomists one species, labeled with one name, the “spring peeper,” is instead a family of six different genetic lineages with subtly different breeding calls. Where the spring peeper clans meet, in areas of overlap, evolution has made the frogs’ sounds and preferences especially distinctive, slowing genetic mixing among populations.
The sounds of breeding animals, then, can enforce the boundaries between populations. In doing so, they nudge diverging populations toward more complete breaches. These breakups are one of the foundations of biological diversity: the splitting of one species into two.
These examples should in no way be read as supporting the racist laws and cultural biases against so-called miscegenation in humans. Tree frog lineages have been on different evolutionary paths for at least three million years. Humans show no such deep and wide genetic divides within our species. All contemporary human populations share a common ancestry that dates back only a couple hundred thousand years, at most. Compared with other animals, the genetic geographic differences that exist among our populations are slight. Further, children born of parents from different regions show no propensity to increased genetic illness. Instead, the reverse is often true when inbreeding among closely knit human populations unmasks hidden genetic problems. Last, our commitment to equality and the dignity of all human beings makes any discrimination, even if it were founded on some underlying biological patterns, wrong. The behavior of other species is no guide to human morality.
Avoidance of interbreeding can, in some species, cause breeding songs to diverge. But this process is far from universal. In many species, there is no evidence either that hybrid offspring have poor health or that breeding songs are especially divergent when close species live in the same location. Evolution has another trick up its sleeve, the wondrous elaborations wrought by sexual aesthetics.
