Number of mates and timing of mating affect offspring growth in the small marsupial Antechinus agilis

Many fitness benefits of polyandry have been proposed. We addressed four hypotheses that are relevant to the agile antechinus, Antechinus agilis, a highly promiscuous marsupial: polyandry (1) involves females 'trading up' to obtain good genes for offspring; (2) promotes sperm competition/sperm choice to obtain good genes; (3) enables females to avoid genetically incompatible gene combinations (specifically inbreeding); or (4) enables females to avoid infertility. We tested the predictions of these hypotheses using female choice trials, manipulative mating experiments and microsatellite analysis of paternity. Because timing of mating affects fertility and prenatal abnormality rates in this species, we also tested whether it affects offspring growth rates. We found support for a genetic benefit of polyandry: offspring of polyandrous females grew faster than offspring of monandrous females, and this effect was more pronounced for females that mated close to ovulation. However, although larger males sire more offspring in the wild in this species, females did not initially choose large mates and did not 'trade up' based on male size. We found no evidence of genetic incompatibility effects. Monandrous females were not less likely to conceive than polyandrous females, although females in the wild are more likely to encounter infertile and subfertile males than in this study. Females that mated closer to ovulation had slower-growing litters. We conclude that female antechinuses benefit through increased offspring growth rates by mating with multiple males, and by mating several days before ovulation.