Testing a new version of the size-advantage hypothesis for sex change: sperm competition and size-skew effects in the bucktooth parrotfish, Sparisoma radians

A variety of field studies suggest that sex change in animalsmay be more complicated than originally depicted by the size-advantagehypothesis. A modification of the size-advantage hypothesis,the expected reproductive success threshold model, proposesthat sperm competition and size-fecundity skew can stronglyaffect reproductive pay-offs. Size-fecundity skew occurs ifa large female's fecundity is markedly higher than the aggregateof the other members of her social group and, together withpaternity dilution from sperm competition, can produce situationsin which large females benefit by deferring sex change to smallerfemales. Deferral by large females can create sex-size distributionscharacterized by the presence of large females and small sex-changedmales, and it is precisely these distributions that the traditionalsize-advantage model cannot explain. We tested the predictionsof the new model with the bucktooth parrotfish, Sparisoma radians,on coral reefs in St. Croix, U.S. Virgin Islands. Collectionsand spawning observations determined that the local environmentalregime of S. radians is characterized by pervasive sperm competition(accompanying 30% of spawns) and factors that can produce substantialsize-fecundity skew in social groups. Dominant male removalexperiments demonstrate that the largest females in social groupsoften do not change sex when provided an opportunity. Instead,smaller, lower-ranking females change sex when a harem vacancyarises. This pattern of sex change is in contrast to virtuallyall previous studies of social control of sex change in fishes,but provides strong support for the general predictions of theexpected reproductive success threshold model.