DEFERRED REPRODUCTION AS A RESPONSE TO SEXUAL SELECTION IN A CORAL REEF FISH: A TEST OF THE LIFE HISTORICAL CONSEQUENCES

Ideally, individuals that initiate reproduction early in their lifetime gain the advantage of short generation time and reduced juvenile mortality. But in many species, males enter into reproduction much later than do females. This pattern is most common in species where a few larger or older males monopolize matings, and thus sexual selection is intense (Trivers, 1972; Warner, 1980). Theory in behavioral ecology suggests that in this situation younger males have little to gain from reproductive activity, since their probability of being successful is vanishingly small. Instead, they should avoid reproduction, maximize their foraging time, and channel their energy into growth, survival, or other factors likely to elevate their status (Caughley, 1966; Orians, 1969; Geist, 1971; Selander, 1972; Trivers, 1972; Wiley, 1974; Wittenberger, 1979). Life-history theory makes a similar prediction: when the returns in reproductive success are low over a broad range of reproductive effort, an organism should skip reproduction and allocate energy to growth and survival until a more favorable situation occurs (Williams, 1966; Gadgil and Bossert, 1970; Schaffer, 1974; Pianka and Parker, 1975; other references in Stearns, 1976). These explanations of deferred reproduction assume that younger males have less competitive ability than older males. As Wiley (1981) points out, the fact that younger males avoid reproduction is not proof that they are competitively inferior, especially in animals with determinate growth. Other factors, such as the need for experience, could certainly contribute to selection for deferred reproduction. When older males are much larger, however, the potential trade-off between current reproduction and future growth assumes importance because larger individuals tend to win in contests (e.g., Rand, 1967; LeBoeuf, 1974; Howard, 1978; Warner and Hoffman, 1980b). This suggests a general test of the supposed adaptiveness of deferred male reproduction: when male competitive ability depends on size, the reproductive activity ofyounger males should vary inversely with the intensity of sexual selection. When sexual selection is less intense, the current potential rewards for young males are increased and future prospects are decreased; they should therefore direct a larger proportion of their time and energy to reproduction. This should be true as long as mating success increases appreciably with investment in reproductive activities, and should be reflected in slower growth or increased mortality. In contrast, females should have time budgets and life histories that are much more insensitive to the intensity of sexual selection among males, because their reproductive success depends more on absolute size rather than on relative size (Schoener, 1971; Trivers, 1972). These predictions have generally been investigated through comparisons between species that appear to differ in the intensity of sexual selection, but such comparisons are fraught with problems of uncontrolled environmental variables and differing phylogenetic histories (Stearns, 1977; Warner, 1980), Instead, I test the predictions within a single species by measuring the time budgets