Costs of intersexual conflict in the isopod Idotea baltica

In sexual reproduction one sex can increase its reproductive success at the cost of the other, a situation known as intersexual conflict. In the marin e isopod Idotea baltica, males guard females before copulation. The guardin g phase is preceded by struggles as females resist males' attempts to initi ate guarding. We determined whether the struggle and/or mate-guarding resul t in fitness costs in the form of decreasing fecundity and lower levels of the energy storage compounds, glycogen and lipids. Females that underwent t he period of struggles with males had decreased glycogen levels compared wi th females maintained alone. No such cost was found for males. Females guar ded by a male also had smaller eggs than females that were not guarded. Thu s the intersexual conflict, imposed by the fitness maximization strategy of the males, gave rise to both a fecundity cost and an energetic cost for fe males. The fecundity cost confirms the existence of intersexual conflict in I. baltica. This cost is shared by males, suggesting that the intersexual conflict restrains the reproductive output of both sexes.