Size may not matter to female insects, but shape certainly does. Researchers have found that genitalia are much more diverse in species in which the females couple with several males before choosing the sperm that fertilize her eggs. The findings, reported in the current Nature, suggest that the shape of male genitals helps females select the mate that fertilizes her eggs.
Much more complicated than the mammalian penis and vagina, insect genitalia are made up of numerous intricate parts--a male cockroach's genitals, for instance, includes a set of "Swiss army knife"-type gadgetry. Researchers have speculated that this complexity evolved to prevent individuals from separate species from reproducing--called the lock-and-key theory--or to signal a female to use one male's sperm over that from an earlier coupling.
To test these theories, Göran Arnqvist, an animal ecologist at the University of Umeå in Sweden, compared the precise shape of genitalia in 38 insect species. Arnqvist's group divided the insects into 19 pairs of closely related species; each of these pairs shared a common ancestor. In each pairing, one species partakes in monandry--in which the females mate only once before fertilization--while the other practices polyandry, in which females mate with many males.
Arnqvist reasoned that if the lock-and-key theory were true, the genitalia of monandrous species would vary more from their ancestors, because females who mate only once need to distinguish Mr. Right from the crowd before mating. If the sperm competition theory were true, polyandrous species would have changed more, because this would help females select Mr. Right after mating. Arnqvist scanned outlines from published taxonomic illustrations of genitalia for 3 to 28 males of each species into a computer, and calculated how much the genitalia of each pair of closely related species had diverged since their common ancestor. In 18 out of the 19 pairs, the genitalia from polyandrous species diverged more than had the monandrous species, indicating that sperm competition drives genital evolution. Yet Arnqvist says it's still unclear exactly how genital shape would signal a female to use that male's sperm.
Sandy Harcourt, an anthropologist at the University of California, Davis, says that in primate species in which females mate with many males, the males tend to have larger penises--placing their sperm closest to the eggs. He says that genital shapes in insects may similarly give sperm a leg up on the competition.