Simultaneous manipulations of two potential mates by male Gammarus pulex (Amphipoda) : a mechanism of active mate choice

Gammarus pulex (Linnaeus, 1758) is a common European freshwater amphipod that has been the subject of considerable research effort by ecologists and ethologists (see Dick & Elwood, 1989a, b; Elwood & Dick, 1990; Dick, in press and references therein). The behavioural ecology of mate choice, size assortative mating and precopulatory mate guarding behaviour in this and other Gammarus species has received particular attention, both theoretically and empirically (Elwood & Dick, 1990). Males are known to select precopula partners on the basis of size, and hence fecundity, and moult stage, and thus proximity to the short period (approx. 12 hours) of female receptivity to copulation following moulting (Dick & Elwood, 1989a, b). The purpose of this note is to illustrate, describe and discuss implications of the simultaneous manipulations of two females performed by males.