SYMMETRICAL ASSESSMENT OF FEMALE QUALITY BY MALE GAMMARUS-PULEX (AMPHIPODA) DURING STRUGGLES OVER PRECOPULA FEMALES

Single male Gammarus pulex often attempt to take over females held in precopula by other males. Two experiments were conducted to investigate the role of assessment by males of relative resourceholding potential and resource value (female quality) in dettermining decisions during struggles. In the first, single males were presented with precopula pairs, the females of which varied considerably in fecundity and proximity to moult (and hence time to copulation). In the second, variations in female reproductive quality was reduced by staging struggles over recently moulted females. During encounters, intruding males assessed female quality rapidly and accurately; the persistence of take-over attempts increased as time to female moult decreased (experiment 1) and as female size increased 2). The relative size of contestants did not influence the structure or duration of struggles, indicating that males do not assess relatvie resource-holding potential directly. Rather, as the absolute size of intruding males increassed. take-over attempts became more vigorous in terms of the time spoent in struggles and the number of tugs at females. Take-overs occurred when defending males were small relative to intruding males and small relative to their female partners. These results are discussed in the context of assessment strategies in general and Gammarus mating decisions in particular. © 1990 The Association for the Study of Animal Behaviour.