At The Edge Of The Hawk's Hearing

I can walk for hours in the spruce and fir forests of the Rocky Mountains in summer and hear combinations of the same half dozen bird, two squirrel, and two cicada species. No one knows exactly how many insect species live in the forests around Tiputini, but the count may be near 100 thousand, many of which are sound makers. Frogs and birds are better known. Nearly 600 bird species and 140 frog species live here. The same number of species as inhabit the varied terrain of North America are crammed here into the space of a few square kilometers. The sonic community is thus crowded and richly variegated. The power and diversity of the rain forest’s animal voices reveal sound’s communicative power. Every species here is advertising presence, revealing identity, and conveying meaning to distant others without the danger of being seen. At night, darkness conceals. In the day, the dense profusion of rain forest foliage is almost as effective as a cloak. This is one of the most visually occluded habitats on Earth, perhaps rivaled only by the impenetrably dense thickets of young boreal forest or turbid seawater near a river mouth. No wonder sound blossoms here.

Hope For The Future

Individuals can communicate through the crowds of leaves, all while remaining hidden from predators that hunt by sight. Hundreds of plants in every hectare, smothered in mosses and algae, create habitats of great visual complexity. This, combined with the cryptic color patterns of many insects and other species, makes seeing animals in the rain forest very challenging, even for dedicated and experienced naturalists. What started on the arid plains of the late Paleozoic, 270 million years ago or more, with the thin rasp of Permostridulus and its kin, has now diversified into a thick weave of thousands of sounds in a single place. The sonic grandeur of these forests, though, presents challenges. The costs of vigorous sound making are borne by individual singers and also threaten the viability of sonic communication for the whole community. These dangers drive the diversification of sound in the rain forest, spurring evolution’s creativity. The solution to this problem in Permostridulus’s time was a swift escape. The same is true now. Immobile or slow animals rarely vocalize. But predators and parasites have honed their skills since the Paleozoic. Bounding escape is sometimes insufficient.

Come In From The Cold

Singing insects in the tropics, for example, are plagued by tachinid flies. These hunters have paired eardrums on their undersides, just behind the head, that allow mother tachinid flies to home in on victims. Guided by ears tuned to the particular frequency and tempo of her preferred singing insect host, she alights and spills tiny larvae from her abdomen. These wrigglers swarm the victim and burrow through its exoskeleton. Lodged inside, the larvae grow for a couple of weeks, then burst out, killing their host. Each tachinid fly species has its own sonic preferences, some preferring short trills, others rapidly delivered chirps, and each is sensitive to a particular range of frequencies. For prey, this specificity means that there is an advantage to sounding different from other species. Natural selection therefore favors sonic diversity. By sounding different from the crowd, singers avoid being assaulted by hordes of parasitic maggots, a strong incentive. Specialized hearing by parasites can generate regional variants in both the songs of hosts and the hearing preferences of the parasites. The diversity of rain forest trees here is explained, in part, by a similar process. Any tree species that becomes too common is cut back by fungi, herbivorous insect mouths, or viruses.

Keep Moving Forward

Rarity buys a measure of safety. Over time, this results in more diverse communities. Tachinid flies seek the sonic signature of a small number of species. Most other predators that hunt by ear, though, have ears and palates with more catholic tastes. Wolf spiders feel the tremors of singing insects both in the air and through vibrations in their legs. These generalist predators also shape the sounds of their prey. If you’ve ever tried to creep up on a singing tree frog or katydid, you’ve experienced the sudden silence that a passing shadow or rustle of vegetation can elicit. But prey do not only fall silent when danger swoops in, they often give alarm calls, a seemingly paradoxical response. But by calling, prey signal to the predator that it has been seen. With no possibility of a stealthy, unexpected attack, the predator is often better off leaving the area to seek less wary prey. Alarm calls are also part of the cooperative networks that bind animal societies. Calling animals warn others, benefiting their own progeny and kin, storing up social capital with neighbors, and helping their group thrive while others perish. The function of alarm calls is embedded in their acoustic structures. The structure of the see call conveys alarm to others while minimizing risk to the caller. High, pure tones with tapered starts and stops are cryptic, like camouflage for sound, giving the hunter little information about where the caller is located. The calls are hard to find because they lack abrupt onsets that provide binaural cues about position and are shrill enough to be at the edge of the hawk’s hearing. The high sounds also attenuate quickly in vegetation. If a predator lingers, the element of surprise gone, songbirds switch to repeated, lower pshht! The recipient of such mobbing behavior often moves on. Alarm calls are not generic. They do not merely convey the presence of danger. Some birds recognize the voices of mates and kin, responding more vigorously to alarm from these familiars than to strangers’ calls that, to human ears, sound identical. Alarm calls of birds and mammals can also contain information about the species and proximity of the predator. The signal for a distant predator is different from one whose strike is potentially imminent. Representation in sound of individual identity of predators demonstrates sophisticated cognitive capabilities. These animals recognize individuals, remember salient characteristics of each, then communicate this knowledge to others using sounds that carry information within their forms. In cleaving humans from all other animals, Descartes believed that non loquitur ergo non cogitat, he does not speak therefore he does not think. If the philosopher had opened his ears and imagination to the alarm calls of the birds outside his window, his logic might have been reversed, loquitur ergo cogitat.