Size Matters

The disproportionately endowed carabid beetle reveals that the size of female—and not just male—genitalia influences insemination success.

Tracy Vence
Jun 30, 2014

LOCK AND KEY: Size-matching between male (left) and female (right) genitalia aids mating success in a beetle species. (Scale bar, 2 mm)TEIJI SOTA AND YUTAKA OKUZAKI


The paper
Y. Okuzaki, T. Sota, “How the length of genital parts affects copulation performance in a carabid beetle: Implications for correlated genital evolution between the sexes,” J Evol Biol, 27:565-74, 2014.

The subject
Studies of various species have shown that the size of male genitalia can affect reproductive success. The male carabid beetle Carabus subg. Ohomopterus maiyasanus has a notoriously long copulatory piece—a hook on the root end of its penis. The female, too, can boast an elongated vaginal appendix into which this hook is inserted, and Teiji Sota of Kyoto University in Japan wanted to determine why such size matching may have evolved.

The experiment
Sota and a colleague evaluated the copulation performance of size-matched and -mismatched carabid beetle couples. Insemination success was not necessarily affected by genital size matching initially. But when females mated a second time, size made a difference. When the females’ appendices were relatively long, males with shorter copulatory pieces—making for a closer size match—were more likely to replace spermatophores from the males that had come before them with their own.

The interpretation
Sperm competition is a known evolutionary driver of exaggerated male genitalia. But as these double-mating experiments have shown, bigger is not always better, suggesting that sperm competition can affect both males and females in complex ways. Other forces, such as sexual conflict or reproductive interference from other species, may have helped select for the outsize genital parts sported by these beetles.

The significance
That the researchers considered both the male and female genitalia sets this study apart from most, which have tended to focus on male parts only, says David Hosken, an evolutionary biologist at the University of Exeter, U.K. “That can tell you some interesting things, but until you can see the female element, you’re never going to get the full picture.”