MALE CHOICE AND MALE-MALE COMPETITION IN IDOTEA-BALTICA (CRUSTACEA, ISOPODA)

Sexual selection in mate-guarding Crustacea may involve several proces ses: male choice, male-male competition, and female choice. To evaluat e the relative importance of the different processes in mate choice of the aquatic isopod I. baltica we studied 1) the mate-choice criteria of males, 2) effects of sex ratio on the outcome of the mating contest , and 3) the role of size in male-male interactions. When given a choi ce between a small and a large female, males most often chose the one that matured earlier for parturial ecdysis. Maturity was a more import ant choice criterion than female size, but these also correlated posit ively. Large males had a mating advantage in both male- and female-bia sed sex ratios; pairing was size-assortative only in the male-biased r atio where guarding was also longer. If an extra male was placed with a precopulatory pair, 30 % take-overs occurred, large males surpassing . Present and earlier work suggests that male size is an asset in both intra- and intersexual interactions. There is little or no direct phe notypic sexual selection on female size: sexual selection for large ma les presumably contributes to the evolution of sexual size dimorphism in I. baltica.