Paul J. Greenwood

Why are males bigger than females in pre-copula pairs of Gammarus pulex?

SummaryIn natural streams, males are bigger than females in precopula pairs of the freshwater amphipod Gammarus pulex. A set of experiments is described, the results of which are consistent with the hypothesis that males are larger as a result of a mechanical constraint and not intrasexual competition for mates. Where the male in a pair is relatively larger than the female the swimming performance is superior to those pairs in which the male and female are of similar sizes. This minimises the risk of being washed downstream by the current.

Evolutionary Aspects of Environmental Sex Determination

Summary The phenomenon of evironmentally cued sex determination (E.S.D.) is reviewed. E.S.D. is defined and its characteristics are described by comparison to other sex determining systems. The evidence for the presence of E.S.D. among Echiura, Nematoda and Crustacea is presented, with reference to the evidence for sex-specific size-related fitness in each group. A diversity of sex-determining cues, not all of them related to factors affecting fitness, are identified. The review considers current theoretical models for E.S.D. and sex-ratio manipulation in patchy environments, and discusses the relevance of these, models to the described examples. Past analysis has concentrated on; size in relation to nutrition but time may be an equally important factor in many cases.

Sex and size: investigating the roles of natural and sexual selection

Males are larger, and often showier, than females among most birds and mammals. This has been attributed to sexual selection, but it is not clear whether all sex differences are attributable to this rather than to natural selection. There is a misconception widely held, among biologists and elsewhere, that large male size is more common than is true in reality and that it is an inevitable consquence of sexual selection. A series of experiments designed to elucidate the factors responsible for the evolution of relatively large male size in the freshwater hoglouse Asellus are described. These include trials on male discrimination among differentsized females; the advantage of large size to males in gaining access to females; and the advantage of size in carrying a female in a stream. The results of these experiments are discussed in the context of increasing student understanding of sex differences and of the relative value of contrasting theories.