Sexual selection and the fitness consequences of male body size in the seed beetle Stator limbatus

We examined sexual selection on male body size in a laboratory population of the seed beetle, Stator limbatus, and the fitness consequences to females of mating with larger males. Large males produced larger ejaculates than small males. Both males and females lost body weight as a consequence of breeding, and large males lost more weight than small males. The amount of weight lost by males correlated as highly with female fecundity as did the amount of weight lost by females. Similarly, male and female body weight correlated equally highly with female fecundity. These results indicate that males make substantial contributions to female fecundity, probably through nutrients transferred in their ejaculate. As a consequence, fecundity selection should favour large body size in both males and females. We found no preference for large males when virgin females were presented with only one male, but when presented with two males simultaneously, females were more likely to mate with the larger male. This result is consistent with relative female choice or male-male competition, although no indications of male-male competition were observed. Females that mated with small males re-mated sooner than females that first mated with large males. Females that first mated with a non-virgin male were also more likely to re-mate than females that first mated with a virgin male, suggesting that females re-mate to obtain additional sperm or nutrients and not just as a form of mate choice. In addition to the possible benefits from mate choice and male-male competition, large males gain a mating advantage through reduced sperm competition. This large male advantage, combined with fecundity selection on males as well as females, may account for males being larger than females in this species.