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

Abstract

A variety of field studies suggest that sex change in animals may be more complicated than originally depicted by the sizeadvantage hypothesis. A modification of the size-advantage hypothesis, the expected reproductive success threshold model, proposes that sperm competition and size-fecundity skew can strongly affect reproductive pay-offs. Size-fecundity skew occurs if a large female’s fecundity is markedly higher than the aggregate of the other members of her social group and, together with paternity dilution from sperm competition, can produce situations in which large females benefit by deferring sex change to smaller females. Deferral by large females can create sex-size distributions characterized by the presence of large females and small sex-changed males, and it is precisely these distributions that the traditional size-advantage model cannot explain. We tested the predictions of the new model with the bucktooth parrotfish, Sparisoma radians, on coral reefs in St. Croix, U.S. Virgin Islands. Collections and spawning observations determined that the local environmental regime of S. radians is characterized by pervasive sperm competition (accompanying 30% of spawns) and factors that can produce substantial size-fecundity skew in social groups. Dominant male removal experiments demonstrate that the largest females in social groups often do not change sex when provided an opportunity. Instead, smaller, lower-ranking females change sex when a harem vacancy arises. This pattern of sex change is in contrast to virtually all previous studies of social control of sex change in fishes, but provides strong support for the general predictions of the expected reproductive success threshold model. Key words: coral reefs, Labridae, protogyny, Scaridae, seagrass beds, size-fecundity skew, social control of sex change, sperm competition. [Behav Ecol 15:129–136 (2004)]