Abstract and Summary
The breeding system of the freshwater amphipod, Gammarus pulex, has a precopulatory female guarding phase by which a male ensures his presence when the female moults and he can fertilize her brood and also prevent other males gaining access to her. The duration of the guarding phase is influenced by the sizes of the animals in a pair; larger males guard for longer than do smaller males and larger females are guarded for longer than are smaller females. Also, larger males guard more tenaciously than do smaller males and females are guarded more tenaciously as their moult approaches. Finally, larger females are found to be more active in precopula than smaller females.