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

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Summary

  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 modelled 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 be insensitive to social cues of future mating competition for both major and minor males. Major males allocated more to body condition when exposed to rivals, a response expected for a species in which the outcome of dyadic contests strongly affects male reproductive success. In contrast, minor male allocation to condition was insensitive to social cues.
  4. Our data illustrate how socially cued plasticity in pre- and post-copulatory traits can depend on the relative importance of these episodes of selection for individual male fitness. In O. taurus dung beetles, males strategically adjusted the amount of resources they allocated to winning pre-copulatory contests over access to females. Strategic allocation to pre-copulatory contest competition did not come at a cost to male investment in sperm competition, suggesting that males may trade investment into contest competition against some other life-history trait, such as longevity. The lack of plasticity in testes size suggests that selection from sperm competition may be a relatively constant feature of this species mating system.

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