Male traits under cryptic female choice in the spotted cucumber beetle (Coleoptera: Chrysomelidae)

Abstract

Males of the spotted cucumber beetle (Diabrotica undecimpunctata howardi) rhythmically stroke females with their antennae during copulation. Males that stroke quickly have a higher probability of being accepted as a mate. We determined (1) the mechanism by which females prevent unattractive males from passing spermatophores, (2) whether antennal stroking signals to females the likelihood of receiving a nuptial gift, and (3) if other male traits in addition to stroking are subjected to sexual selection from female preference. Dissections of pairs flash-frozen in copula during and after antennal stroking showed musculature that, when contracted, folded the vaginal duct leading to the female's bursa copulatrix in a way that prevented complete penetration by the aedeagus. These muscles were always contracted while males were stroking and always relaxed after stroking had ceased. Males accepted as mates did not differ from males that failed to pass a spermatophore in either absolute or relative body weight, aedeagus length, or the amount of cucurbitacins (potential nuptial gifts) sequestered in their spermatophores. Although 99% of the beetles that came to cucurbitacin-rich Cucurbita fruits in the field were males, males that had sequestered cucurbitacins did not stroke females faster than males with no cucurbitacins, and fast-stroking males were not more likely to find and sequester cucurbitacins than were males that stroked more slowly. Males with a cucurbitacin slurry painted on their antennae had no mating advantage over controls. We conclude that females discriminate among males after copulation has begun on the basis of antennal stroking displays (or some trait correlated with stroking speed) that males perform to entice females to relax their bursal sphincter.