The amorous Gammarus: size assortative mating in G. pulex

Abstract

The hypothesis that size assortative mating in Gammarus pulex is due to male-male competition was examined in three experiments. When males were given a choice of two mates they preferred to enter into precopula with the female that offered the greater utility in terms of eggs fertilized per reproductive effort. When a receptive female was simultaneously encountered by two males it was usually the larger male that entered into precopulatory amplexus. These was more male-male conflict in the presence of a receptive female than in that of an unreceptive female. Few take-overs occurred but those that did were by males larger than the male in precopula. These data are consistent with the hypothesis that competition results in size assortative mating; however, it is suggested that the optimal time (relative to female moult) of entry into precopula varies as a function of both male and female size. Thus large males enter into precopula earlier than small males and prefer to take a large female to a small female. Male-male competition may determine some of the costs and benefits associated with size assortative mating but it is not the mechanism by which it occurs.