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.