Alternative Male Mating Tactics in a Thrips: Effects of Sex Ratio Variation and Body Size

Abstract

Field observations show that males of Elaphrothrips tuberculatus (Thysanoptera: Phlaeothripidae), a subsocial thrips that inhabits clusters of hanging dead oak leaves, engage in either of two mating tactics, fighting or sneaking that are associated with their body size. Large males fight and guard individual egg-defending females, relatively large nonguarding males fight and challenge guarding males, and relatively small nonguarding males attempt to mate with females while avoiding guarding males. Field collections conducted during 4 years showed that: (1) nonguarding males on the same leaf as guarding males and females (same-leaf' nonguarding males) were significantly smaller than nonguarding males on different leaves (different-leaf nonguarding males) in 3 of 4 years; (2) among years, the size difference between same-leaf and different-leaf nonguarding males was positively correlated with the sex ratio (percent males); and (3) within years, local populations in which at least one of the nonguarding males was a same-leaf male exhibited significantly higher sex ratios. By inference from the observation data, same-leaf males were regarded as sneaks, different-leaf males were temporarily unsuccessful fighters, and male choice of mating tactics depended upon their size and the local sex ratio. Both small body size and a high local sex ratio may be viewed as correlates of a male's inability to secure a female on eggs by searching or fighting. Alternative hypotheses for the data linking male size, location and sex ratio are discussed; relatively small nonguarding males may be intrinsically better at sneaking, or large nonguarding males may fight rather than sneak because of a strong selective association between prior guarding success and aggressiveness. In E. tuberculatus, male mating tactics may be classified as a condition-dependent, reversible strategy, for which the fitness returns of the two tactics are unequal; males sneak when they have an especially low probability of success at winning fights and guarding females.