OPERATIONAL SEX-RATIOS AND BEHAVIORAL SEX-DIFFERENCES IN A PIPEFISH POPULATION

In the pipefish Syngnathus typhle, only males brood embryos in special ly developed brood pouches, supplying oxygen and nutrients. Laboratory studies have shown that this elaborate paternal care has led to sex-r ole reversal in this species: males limit female reproductive rate, fe males are the primary competitors for mates and males exercise greater selectivity in accepting mates. In the first field study of this pipe fish, 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 reproduc tive seasons of two sequential mating periods each, the latter separat ed by a lengthy interval of male brooding. During mating periods, fema les displayed to ail males without wandering and males moved about sea rching for females, without reacting to all females. The OSR was least female-biased (or even male-biased) at the onset of the breeding seas on, when most pipefish were simultaneously available to mate, but beca me strikingly female-biased as males' pouches were filled. The OSR rem ained substantially female-biased during the second mating period, bec ause few males became available to remate at any one time. As hypothes ised, female-biased OSRs resulted in more female-female meetings. As w ell, females were above the eelgrass more often than brooding males, t hus exposing themselves to conspecifics and/or predators. In the secon d year, males arrived earlier than females on the breeding site and ma le 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 ob served. The field results support laboratory studies and demonstrate t hat behaviours associated with female-female competition are more prom inent when the OSR is more female-biased.