Fake Sperm Keeps Males in the Running

Butterfly sex is tricky. After mating, females hold the males' sperm in a special storage organ for a few days, perhaps in order to permit fertilization at the optimum time. In return, males produce a whole bunch of junk sperm that can't fertilize an egg, but loads up the storage organ. Why butterflies indulge in this seeming double negative has remained a mystery for the past 100 years. A new study may solve the riddle of the subterfuge.

Male butterflies produce two different kinds of sperm: fertile and non-fertile. Non-fertile sperm doesn't develop incorrectly; it's actually a different kind of sperm that lacks a head and nucleus. As much as 90% of the sperm that a male butterfly donates to a female can be of this dud variety. The new study found that the more the junk sperm fills up a female's storage organ, the longer the female is likely to wait before mating again. She may not even mate again at all. This would increase the male butterfly's chances of producing offspring, since his sperm wouldn't have to compete with other butterflies' sperm.

While the theory has been around for a long time, this is the first proof to support it. Zoologists Penny Cook and Nina Wedell tested green-veined white butterflies during their mating period. They mated female butterflies either to virgin or to previously mated males. Then they allowed the female butterflies to either mate again or not.

The virgin males produced larger loads of sperm, with more duds, than mated males. But the mated males produced more fertile sperm than the virgins. So the packet of sperm that mated males produced was smaller than that of virgin males, but packed more punch.

If sperm fertility were all that determined whether a female mated again, then the females should have re-mated less frequently after mating with previously mated males, because they received more fertile sperm. However, the reverse was true. Females whose sperm storage banks filled up with dud sperm were much less likely to seek a new mate. Females whose organs were less full, even if the sperm was more fertile, went out and bred again.

The researchers speculate that infertile sperm bears two benefits: it keeps females from mating with other males, thereby improving the success rate of each male; and it probably takes less energy to make than fertile sperm, which eases the demands on a male butterfly's body. Adult butterflies get most of their sustenance from protein they stored during their larval stage. The nectar they eat simply acts as a sort of "popcorn diet" to keep them going. Hence, energy is a precious commodity to adult butterflies. The more they expend, the shorter their lifespan. Since filling up a female with dud sperm satisfies her better than releasing a few high-quality sperm, the male doesn't have to waste energy producing a lot of fertile sperm.

Sources

"Of Mice and Men." Bob Holmes. New Scientist, February 13, 1999, page 4.

"Non-Fertile Sperm Delay Female Remating." Penny Cook and Nina Wedell. Nature, February 11, 1999, page 486.