The effects of sex ratio on sexual competition in the European lobster

During the breeding season an individual's access to mates may be affected by operational sex ratios, causing strong variation in mating success. We m anipulated adult sex ratios of the European lobster, Homarus gammarus, to t est the predictions of models that relate sexual competition to (1) the sex ratio, (2) the time that an individual is not available to mate and (3) 'c ollateral investment', whereby two males contribute to a single clutch. The model predictions proved to be relatively insensitive to collateral invest ment Male-male competition predominated in the male-biased but not in the f emale-biased sex ratio. This matches the predictions of one model that inco rporates an extended period of female receptivity because the time that a m ale was unavailable to mate was small compared to the time spent by females in cohabitation and parental care. Although females increased their compet itiveness when males were in the minority, male competition remained high. The insensitivity of male-male competition to sex ratios may be due to an u pper limit to the costs that males can afford when there is a serious risk of injury, preventing males from increasing their aggression when females a re in short supply. (C) 1999 The Association for the Study of Animal Behavi our.