Size-assortative pairing in the lotic amphipod <Emphasis Type="Italic">Gammarus zaddachi</Emphasis>, an examination of hypotheses and the influence of current speed

Three main hypotheses have been put forward to explain size-assortative pairing in gammarid amphipods: microhabitat separation, sexual selection and loading constraint. In order to determine which hypothesis best explains this phenomenon in the estuarine species Gammarus zaddachi, I first measured the body lengths and dry weights of precopula pairs collected from two field sites with substantially different current speeds. Second, I performed three laboratory experiments in order to estimate the importance of the following processes: (1) male choice; (2) male–male competition and (3) male–female acceptability. The loading constraint hypothesis seemed best supported by the data in that field-collected male G. zaddachi size correlated well with female size in precopula pairing in both fast and slow flowing water. In the laboratory, males preferred females of their same size group (large versus small), and ‘won’ them in the male–male competition experiments. Size-assortative pairing is thus likely a consequence of the loading constraints imposed upon these males by virtue of them having to carry and manoeuvre their partners through flowing water, while attempting to maintain station in an optimal microhabitat. Males may therefore forego the largest, most fecund females, in favour of a practicable payload (small male–large female pairings were rare). However, there seems to be a lower limit to this selection, indicated by the high degree of cannibalism on small females by large males.