By Geoff Gallice
Bright colors in nature generally indicate danger. A brightly colored animal, for instance, typically uses its flashy hues to warn potential predators of a threat it might pose. Usually that threat is chemical: animals, ranging from insects and other invertebrates to frogs and snakes, have evolved a bewildering array of toxic poisons, venoms, and other chemical surprises that await would-be enemies. Bright colors help predators remember unpleasant experiences—they quickly learn to avoid the colorful meal that made them sick or stung them.
The passion-vine butterflies are a group of brightly colored or aposematic butterflies found throughout the rainforests of Central and South America. The most diverse genus is Heliconius and, as its common name suggests, this group of butterflies feeds on passion vines that are loaded with compounds known as cyanogenic glycosides—you don’t need to be a biochemist to guess that the cyan at the beginning of that name means it isn’t good to eat.
The glycosides are not harmful to Heliconiusbutterflies though. In fact, the butterfly larvae greedily devour vines that are high in these compounds. From them they are able to synthesize derivatives—similar chemical forms—that can be stored in the butterfly’s tissues.
Thanks to its diet of passion vines, a Heliconius butterfly is a mouthful of toxic, foul-tasting poison to a predator, which, in the day at least, is usually a bird. And since Heliconius are brightly colored, birds quickly learn to avoid them. Thus, the butterflies are chemically defended and, once the local birds have been educated, have essentially no diurnal enemies.
But what happens at night, when visual signals are less effective as a warning to nocturnal predators that hunt without good eyesight? Bats, for instance, hunt at night using echolocation or sonar, in which high-frequency sounds are produced and distance and direction to prey are judged largely by the time taken for the sounds to bounce, or echo, back to the bat’s ears. With echolocation, eyesight is unnecessary, and bats that echolocate are able to hunt very effectively even in total darkness. Many have monochromatic or otherwise very poor vision.
One peculiar aspect of Heliconius butterfly biology is their roosting behavior—that is, at night, they sleep collectively in aggregations. Roosts may contain as many as 10-15 individuals, although typically the number is less. The butterflies begin to gather at sunset and take as much as one hour to settle in for the night; they use tiny hooks at the ends of their feet to hang, upside down, from nearby small branches and the tips of dead twigs.
Many biologists believe that animals aggregate in order to dilute the effects of predation. Think of the chances of a buffalo being picked off by a lion alone versus in a large herd. The same should be true for aposematic butterflies in the daytime—considering that their bright colors most likely serve as a warning to birds, daytime Heliconius roosts would make a lot of sense. But the butterflies roost together at night, and it’s not clear why.
As a graduate student interested in tropical butterfly biology at the University of Florida, Christian Salcedo wanted to understand why Heliconius roost at night. Do the butterflies have any predators? Could they be using unseen signals to fend off attack, much in the way they use color in the day?
Salcedo decided to employ technology to study the question of Heliconius roosting at night. He created a camera system—he called it a “stand-alone nocturnal infrared camera system”—that would catch Heliconius predators in the act by activating video when an infrared beam was crossed.
After a number of nights the remote cameras had recorded several disturbance events, during which an animal—there was an agouti, an armadillo, a rabbit, and even a few stick insects—disturbed the butterflies, causing one or more of them to rouse momentarily from their sleep. In a jungle bumping with nocturnal creatures, this wasn’t very surprising. However, the most interesting event occurred when a bat was observed picking a Heliconius butterfly from its roost. Several moments later, the butterfly returned, apparently unharmed, to its roost-mates. Of course, Salcedo captured the event on video.
Watch videos of Heliconius roost disturbances here:
In some ways, these experiments shed light on the mysterious Heliconius night roosts. Based on the footage gathered with the remote camera system, we now know that Heliconius probably do have nocturnal predators. But the fact that the cameras recorded a butterfly returning to the roost after an attack, unharmed, actually raises many more questions.
Did the cameras capture an instance of predator education? That is, the bat clearly tried but did not eat the butterfly—in the video, do we witness the learning event that results in other Heliconius avoiding attack by at least this individual bat? If so, how can the bat recognize future roosting Heliconius and know to stay away?
Some bats are able to see ultraviolet light, which is reflected by parts of the wings of Heliconius. Could that be the key mechanism? Or perhaps predators like bats are able to smell pheromones or other chemical signals produced by the butterflies that act in a similar way as bright colors in the daytime, preventing a repeat bad-taste experience. Do bats typically capture prey and assess their palatability before deciding to devour them or let them go unharmed?
Video footage of a nocturnal attack is tantalizing, but we still have a lot to learn about the roosting behavior of Heliconius butterflies. The rarity of nocturnal attacks on the butterflies—Salcedo’s experimental roosts were attacked only several times in hundreds of hours of filming—make the matter of studying them more difficult. But, as is increasingly the case, technology coupled with a curious, motivated young graduate student willing to brave long, late hours and harsh tropical conditions can make great discoveries regarding the fascinating phenomenon of the roosting passion-vine butterflies.
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!