Advantages and a disadvantage of large size for male gammarus pulex (Crustacea: Amphipoda)

Summary

The breeding system of the freshwater amphipod Gammarus pulex includes a precopulatory guarding phase by a male. The length of this guarding phase is investigated with respect to a male's size and the number and size of his competitors. As the absolute number of competitors increases, so does the guarding time but as the absolute number of available females increases, the guarding time decreases. Takeovers of the females by unpaired males are more frequent in longer precopulas (Table 2). In contests for females, larger males have two advantages over smaller males; they are better able to make a takeover (Table 2) and better able to resist takeover attempts while paired (Table 3). Males increase the length of the guarding phase as the mean size of their competitors increases (Table 4). When not paired males are usually searching for available females, perhaps in the stream current. Females are unaffected by current speed but increasing current causes decreased male survivorship (Table 5) and increased precopula duration (Table 5). Searching in currents is more dangerous for larger males than smaller ones. It is proposed that the male size distribution observed is the result of selection pressure to increase size from male-male competition balanced by selection against large size while searching for females in the current.