Dominance status and carcass availability affect the outcome of sperm competition in burying beetles

Intense intrasexual competition between males often results in the establishment of a dominance hierarchy, promoting differential access to females. Dominant males are expected to evolve more prudent sperm allocation strategies, whereas subordinate males are expected to counteract their mating disadvantage by increasing their sperm allocation. Burying beetles, insects that breed on small vertebrate carcasses, offer an ideal model for testing these predictions because aggressive interactions between males typically result in one dominant male that monopolizes access to the female, relegating subordinates to sneak matings. Males that have not found a carcass use pheromones to attract females and only obtain a small number of matings per day. We measured the mating frequency of males competing on a carcass and confirmed that dominant males mate significantly more often than subordinates. We then examined the paternity of competing males, with a 5:1 mating advantage for the dominant male, to determine whether relative mating frequency is sufficient to explain the severe reproductive skew in polyandrous broods in the field. When competing on a carcass, dominant males did not sire significantly more offspring than subordinates. However, when males mated without a carcass and did not interact, significantly more offspring were sired by the dominant male, but even here, the subordinate male’s paternity was greater than his share of matings. Our results show that when males compete on a carcass, dominant males have a clear mating advantage, whereas subordinates appear to transfer greater numbers of sperm per mating to compensate for their limited mating opportunities.