Operational sex ratios and behavioural sex differences in a pipefish population

In the pipefish Syngnathus typhle, only males brood embryos in specially developed brood pouches, supplying oxygen and nutrients. Laboratory studies have shown that this elaborate paternal care has led to sex-role reversal in this species: males limit female reproductive rate, females are the primary competitors for mates and males exercise greater selectivity in accepting mates. In the first field study of this pipefish, we describe mating behaviour in the wild and test the hypothesis that temporal variations in the operational sex ratio (OSR) determine sex differences in mating behaviour. Our study comprised two reproductive seasons of two sequential mating periods each, the latter separated by a lengthy interval of male brooding. During mating periods, females displayed to all males without wandering and males moved about searching for females, without reacting to all females. The OSR was least female-biased (or even male-biased) at the onset of the breeding season, when most pipefish were simultaneously available to mate, but became strikingly female-biased as males' pouches were filled. The OSR remained substantially female-biased during the second mating period, because few males became available to remate at any one time. As hypothesised, female-biased OSRs resulted in more female-female meetings. As well, females were above the eelgrass more often than brooding males, thus exposing themselves to conspecifics and/ or predators. In the second year, males arrived earlier than females on the breeding site and male pregnancies were shorter, because of higher water temperatures, so rematings occurred earlier. Males met more often during that year than the previous one, but male competitive interactions were still not observed. The field results support laboratory studies and demonstrate that behaviours associated with female-female competition are more prominent when the OSR is more female-biased. Correspondence to: A. Vincent