Testing the direct and genetic benefit hypotheses of polyandry in the wood tiger moth

Polyandry is widespread among many animal taxa, yet the benefits for females are still debated. The two main hypotheses to explain its evolution are the direct benefits and the genetic benefits hypotheses, which are not mutually exclusive. We tested both in the wood tiger moth Arctia plantaginis (Arctiidae) by comparing fitness components in single and multiple mated females. We measured female longevity and number of eggs laid (i.e. direct benefits), as well as offspring hatching success and survival (i.e. genetic benefits). Fitness components did not differ between single and multiple mated females; therefore, there was no evidence to support either direct or genetic benefits hypotheses, or any costs. After paternity analyses, we obtained qualitatively similar results by comparing clutches effectively sired by one male with clutches sired by two males, regardless of the number of times a female mated. We further investigated the proximate mechanisms driving the outcome of paternity patterns. First male precedence, last male precedence, and mixed paternity were present in equal proportions, although there was a trend towards last male sperm precedence in later clutches. Interestingly, in polyandric females, the age of the second male positively affected the number of eggs laid and the number of surviving offspring, indicating an advantage for older males, possibly due to a higher parental investment. We suggest in light of recent theoretical work that the acceptance of more partners in female A. plantaginis may have evolved to ensure fertilization and avoid the risk of virgin death. Significance statement Why do females mate with multiple males? Here, we investigate the effects of polyandry on female fitness components in the wood tiger moth Arctia plantaginis. We do not find any support for the direct or genetic benefits hypotheses, or any costs of polyandry. We do not find any clear paternity patterns as last male, first male, and mixed paternities are equally present. We suggest that in this species polyandry may have evolved to ensure fertilization and avoid the risk of virgin death.