Zebra longwing butterflies (Heliconius charithonia) can be found flitting about the southern United States through Central and South Americas. Like several other species of the Heliconius genus, male zebra longwings often find a mate before she has emerged from the pupal stage of life, guarding her until she becomes an adult and ready for mating. (Only one other species outside this genus is known to perform pupal guarding, as scientists have named this behavior.) But when this species of butterfly is in the pupal stage, males and females look alike, and researchers have wondered how the males know which ones to guard.
Biologists from Texas and Germany, reporting in the Proceedings of the Royal Society B, found that the male and female pupae emit different chemicals when they near the end of this stage of life. Males emit linalool and females linalool oxide. Adult males likely use visual clues to find pupae and then use these short-range olfactory cues (that is, they smell these two chemicals) to determine whether they've found a male or a female.
The male adult butterflies, however, aren't entirely successful in their identification; in the biologists' experiments, nearly a third of male pupae were under guard. The technique, however, is usually a good one that assures a male gets to mate.
The females, meanwhile, seem to have gotten the short end of this deal. They have no choice in their mates. But could they also benefit from this strategy? Yes, the scientists say. The adult males have to compete for the opportunity to guard a female pupa and, thus, only the bigger, stronger males would win that spot. These larger males will likely gift the female with a spermataphore that has more nutrients and chemical defence. So even if she doesn't get a choice in the matter, the female butterfly—and perhaps more importantly, her offspring—still gets an advantage in life.