For those hoping to view wildlife, a visit to a tropical rainforest can be quite a frustrating experience. Unlike on the plains of Africa, rainforest animals can very easily conceal themselves among the dense vegetation, under a forest canopy that permits very little light to pass through. To make matters worse, many species avoid detection by resemblance to non-animal objects in their environment. Here, insects are the ultimate masters of disguise: stick insects and leaf-mimicking katydids imitate twigs and leaves—some even come complete with tiny spots meant to mimic diseased or chewed leaf bits. Still other insects sport patterns that allow them to blend in seamlessly on lichen-covered tropical tree trunks, some even with tiny frills and flourishes that bear an uncanny resemblance to mosses, fungi, and tree bark. By remaining unseen, small insects survive in a world teeming with hungry predators. Only one who is attuned to the jungle environment and the cryptic habits of its invertebrate denizens will appreciate the true, albeit hidden abundance of rainforest insects.
However, not all insects in the rainforest survive by hiding. Many biting and stinging insects—bees and wasps, mostly—invest little in camouflage, instead inviting hunters to attack with flashy colors and conspicuous behaviors. Those that do so are surprised by an unexpected counterattack and quickly learn that there are probably easier meals to be had. Importantly, predators are capable of remembering an unpleasant attempt to make a meal of an angry wasp, or an excited hive of stinging bees. So the rainforest, then, is full of predators, but those that have learned to avoid stinging insects—most predators probably learn this very early on—avoid insects that pack painful bites and stings. Given the extraordinary abundance of bees and wasps in the rainforest, this strategy appears to serve them well.
Below is a an example of just such a stinging insect: a wasp, right? Wrong. This is a picture of a katydid, a harmless relative of the crickets and grasshoppers. Biologists call such an animal a mimic: the katydid has escaped predation through protective resemblance—mimicry—of the much more noxious, stinging wasp, its model. In this case, the wasp must invest not only in a costly stinger and venom, but also in educating predators of its painful sting. The katydid knows nothing of these investments.
Amazingly, this katydid takes its trade one big step further than mere resemblance. Not only has this species foregone the typical, protective green coloration of most katydids, it has abandoned nearly every characteristic that makes it identifiable as a katydid at all. Instead of using its powerful hind legs for jumping the way katydids, crickets and grasshoppers tend to do, this individual gets where it needs to go by flying—in precisely the manner the wasp does. When it flies, the long hind legs trail behind, making the katydid nearly indistinguishable from its wasp model in flight. Even the antennae contribute to the deception, gesticulating back and forth, side to side, in a decidedly unkatydid-like, but wholly wasp-like manner. The katydid’s mimicry is exact to the finest detail.
Mimicry—the convergence, in this case, of not only the appearance but also the behavior of creatures as distantly related as a wasp and a katydid—is a potent testament to the transformative power of natural selection. And the wasp-mimicking katydid is but one example. The rainforest is overflowing with such wonder, if only we have the patience, and the eye, to look for it.
As you might intuitively guess, animals that are brightly colored are probably best avoided in the rainforest. One example that comes to mind are the showy dendrobatid frogs of the New World tropics. Known commonly as the ‘poison dart frogs,’ these amphibians are toxic and advertise that fact with flamboyant colors of bright reds, yellows, greens, and even striking blues. These colors warn would-be predators that ‘I taste bad at best, and at worst, I will kill you!’
Indeed, in nature many animals that are conspicuous in their coloration or behavior do not make a good meal. Most often, such animals are protected by toxins or poisons that they either manufacture de novo or acquire from their food. The famous dart frogs of American rainforests advertise their deadly batrachotoxins with gaudy and obvious colors, toxins which they acquire from the invertebrates—mostly ants and small beetles—that they eat.
Other animals, like an almost endless variety of colorful rainforest butterflies, feed on plants as caterpillars or flowers as adults that provide them with a wide range of noxious chemical compounds that they are able to store. These insects advertise their distastefulness with flashy colors, bold wing patterns, and slow, daring flight. Should a predator attack, it will quickly learn to avoid similar colors and patterns in the future; with these creatures, relatively few individuals bear the cost of educating predators of the toxicity of their species.
Butterflies, nearly without exception, fly during the day when they can use their flashy colors to warn their visually-oriented predators—birds, mostly—of their distastefulness. Moths, on the other hand, generally fly by night, when bright colors serve as a poor warning signal to nocturnal predators that generally hunt without the aid of good vision. In the rainforests of Madre de Dios, however, one group of moths stands as a striking exception to this rule. Here, a large number of species of clearwing moths have evolved a remarkable variety of garish colors, wing patterns, and strange forms; they fly boldly by day, practically daring prospective predators to attack them.
These clearwings belong to a subfamily of moths that entomologists have named the ‘Arctiinae.’ The name comes from the Greek αρκτος, which means ‘a bear’—this refers to the North American common name for their caterpillars: the wooly bears. Some species of wooly bear caterpillars feed on plants that provide them with toxic compounds that they can store in various parts of their bodies as larvae. As a result, the caterpillars are protected from attack by predators that have learned the hard way to avoid them. Other arctiine species acquire their chemicals as adults, often storing them in the integument—the entomological word for the insects’ skin or, more accurately, their exoskeleton.
The clearwing arctiine moths are a brilliant example of one an almost endless variety of incredible ways that rainforest animals protect themselves from the legions of predators that constantly patrol the forest floor, interior, and canopy looking for a meal. Whereas many animals—including most other moths—have opted to hide during the day, coming out cautiously only under the cover of darkness, these colorful moths fly by day, warning would-be attackers: “Eat me if you dare!
Another species of primate from Tambopata, this is the black-capped squirrel monkey (Saimiri boliviensis). Not counting the tiny tamarins, these are the smallest monkeys found in the rainforests of southeastern Peru and, as their common name suggests, are about the size of a squirrel. Squirrel monkeys forage in very large groups of up to one hundred animals or more, searching mostly for fruits and insects, although they will take small vertebrates like tree frogs or baby birds. Interestingly, here in Tambopata they can almost always be found foraging alongside the much larger brown capuchins (Cebus apella). Biologists have been trying to figure this out for decades: brown capuchins can be very aggressive, and animals the size of squirrel monkeys even make up an occasional part of their diet. So the question is this, Why do squirrel monkeys travel with the capuchins? Put another way, why do the capuchins tolerate the squirrel monkeys?
A recent study by Taal Levi et al. (2013) in northeastern South America showed that squirrel monkeys tended to be more abundant where brown capuchins were present, lending support to the long-standing hypothesis that the two species facilitate each others’ foraging. That is, more eyes on the forest might make it easier to find patchily distributed foods, such as fruiting trees or large caches of insects. Alternatively, larger groups might provide better protection from predators, as both species are food for a variety of species ranging from cats to snakes to even birds of prey. Teasing apart the importance of the various benefits associated with mixed-species groups has been difficult, and we still have much to learn.