Contrasting responses of pre- and post-copulatory traits to variation in mating competition

1. Original sperm competition theory assumed that males trade expenditure on searching for mates for expenditure on the ejaculate, and predicted that males should increase their expenditure on the ejaculate in response to increased risk of competition. A recent extension of this theory has modeled pre-copulatory expenditure in terms of direct contest competition, and predicts that when the gains from marginal investment in weaponry are large, males might be expected to allocate resources to armaments even at the expense of the ejaculate. 2. Here we examine socially cued plasticity in allocation to pre- (body condition) and post-copulatory (testes mass) traits in a male dimorphic beetle, Onthophagus taurus, where major males fight for access to females and minor males obtain reproductive success via sperm competition. Both male morphs were either reared in social isolation or exposed to rivals during the period of sexual maturation following adult emergence. 3. Testes mass was found to b...