Pupal mating Heliconius butterflies don’t wait ’til she’s older
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!