The operational sex ratio influences choosiness in a pipefish

Abstract

If more females than males are available for mating in the breeding population (i.e., the operational sex ratio, OSR, is female biased), males can afford to be choosy. In the pipefish (Syngnathus typhle) females compete for males, who are choosy. In nature OSRs are typically female biased, but may occasionally be male biased. In a series of experiments, males were allowed to choose between a large and a small female under a perceived excess of either males or females. Under female bias, males preferred the large female: they spent more time close to her than to the small female; they courted the large female sooner than the small; and they tended to copulate sooner and more often with the large female. Under male bias all these differences vanished and males mated at random with respect to female size. Males reproduced at a faster rate under male than under female bias because they received more eggs in their brood pouches. Thus, males switched from maximizing mate quality (i.e., being choosy) to minimizing the risk of not reproducing (i.e., being quick) as the OSR became male biased.