When people find out I study butterflies in the rainforest, one of several questions that I’m often asked is how long butterflies live for. My response: as long as it takes to get eaten.
Yes, the life of an adult butterfly is hard. If the purpose of the caterpillar is to eat and grow, then the purpose of the adult is to mate, and it is a struggle to mate before meeting the end. Indeed, with hordes of predators ready to make a meal of a juicy butterfly in the jungle, there is little time to waste.
For males, the struggle intensifies. Not only do they have to dodge attackers while seeking a female, but males must also compete with each other for the opportunity to mate with her. Since one competitive male can monopolize more than one female, the chance to pass on one’s genes isn’t guaranteed. So they must fight for that, too.
In a case of sexual… preemption, let’s call it, some male butterflies skip the whole seeking altogether. A male will hone in on a female pupa—where the caterpillar transforms into the adult—and wait around for her to emerge. As soon as she’s out, or perhaps even before, he mates with her. Biologists call it ‘pupal mating’.
The trouble is, all the males in the neighborhood are on to this trick, and they all want to be the first to mate with freshly-eclosed females. So what do they do? They all hang around her. They literally hang around, and even on, the female, waiting for her to emerge from her transformation. As soon as she’s out, they’re ready, but of course there can only be one in the end.
So what gives a male the advantage in this pupal mating strategy? A 1994 study by Erika Deinert et al. in the journal Nature set out to figure out just that. The authors figured there would be two forces—biologists call them selective forces—influencing the evolution of male morphology. One, larger males should be able to outcompete smaller males for a place on the female pupa. Second, and perhaps contrary to the first, males with smaller bodies should be able to mate more efficiently when the time comes. So which is it, larger or smaller males, that win the evolutionary contest?
When the researchers compared the ratio of wing length to body length in butterfly species that perform pupal mating to those that do not, they found that pupal mating species had larger wings relative to the length of their bodies. In those species, longer wings are used to shield the pupa from competitors once the male is in position and prevent others from landing, and smaller bodies are used to copulate more successfully once they’re on.
However, it’s important to note that there’s a limit to the benefits of large size—large wings might help a male secure his spot on the budding female, but they don’t do any good if he’s too big to mate. This is called ‘stabilizing selection’. And as we observe, males of pupal mating species aren’t monstrously large, only slightly so.
The observations by Deinert et al. provide strong evidence that the pupal mating strategy works, at least for male butterflies. From the point of the view of the female, it’s more difficult to see the benefit, although if being mated during or even before eclosion were very harmful it’s not hard to imagine females evolving to delay sexual maturity until fully emerged from the pupa. We need more experiments to better understand this remarkable behavior.
Going back to the initial question: How long does a butterfly live? Well, if you’re a female, it doesn’t have to be very long—you’ve likely got a male waiting to welcome you into the world, no need to waste precious time looking for him. And if you’re a male, it’s either as long as it takes to get eaten, or as long as it takes for a female to be born!
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.