Sexual Selection, Natural Selection, and Body Size in Gammarus pulex (Amphipoda)

The breeding system of the freshwater amphipod crustacean Gammarus pulex (L.) contains a precopulatory phase during which a male carries a female. During this period, he defends her from other males, and the phase ends when she molts and the pair copulates, after which he soon abandons her. The intensities of episodes of sexual and natural selection for male and female body size are estimated from two years' field data from a stream population and a pond population. Sexual selection is significant for male size, but not for female size, and the reverse is true for natural selection. This pattern is predicted by standard theory, and the selection intensities involved are sufficient to account for the observed dimorphism. An alternative prediction, that the male-to-female size ratio in a pair has an important influence on mating behavior, was not upheld. The mean number of eggs fertilized by males per day of precopula was calculated for each sample. In the stream, no pair of means differed significantly, and in the pond, no pair of means from within the same breeding season differed significantly. Thus, males adjust the length of the guarding phase throughout the seasons to attain the same level of reproductive return.