Abstract

Several hypotheses of the proximate control of protogynous (female-to-male) sex change propose that social group composition triggers sex change, but they do not address how proximate cues are altered by population density. I present three mutually exclusive encounter-rate threshold hypotheses that assume that population density determines rates of contact between social group members and that rates of contact are cues for sex change. Different densities are predicted to induce sex change, depending on the encounters assumed to be important in the sex change process (e.g., encounters with smaller and larger individuals). Tests of the models use a pomacanthid angelfish(Centropyge potten) to show that continued presence of a smaller (female) conspecific is needed for sex change, and that continued presence of a larger (male) conspecific can either inhibit sex change or prevent its behavioral stimulation. Using constant social group composition, sex change is prevented at higher density but not at a lower density. The absolute encounter-rate threshold hypothesis, which predicts sex change under intermediate-density conditions, is the most probable model of the social control of sex change in C.potteri

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