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Parental investment Offspring sex Dominance rank Primates
Introduction
Parental investment theory predicts that parents should invest more in the offspring whose sex is less costly to rear and/or provides the higher fitness returns [1]. Differences in maternal dominance rank can, under certain circumstances, result in sex-biased investment. Trivers and Willard [2] argued that when variance in reproductive success (RS) is higher for males than for females and maternal investment has a higher impact on male RS than on female RS, high-ranking mothers should invest more in sons, and low-ranking mothers should invest more in daughters (male-quality hypothesis, or MQ). However, Clark [3] and Silk [4] suggested that, when females are philopatric and compete for resource, high-ranking mothers should invest more in daughters and low-ranking mothers should invest more in sons (local resource competition hypothesis, or LRC).
Although in some mammals there is good evidence of sex-biased investment in relation to maternal dominance rank [1], most studies of birth sex ratios and rank in cercopithecine monkeys produced no significant differences or contradictory results [5]. This pattern of findings may suggest that sex ratios in cercopithecines are subject to counteracting selective pressures, with either one prevailing on the other depending on the environmental circumstances [6]. The contradictory findings, however, could also be explained by the offspring survival hypothesis: whenever offspring mortality is significantly sex-biased, mothers should adjust birth sex ratios so as to maximize sex-- specific offspring survival regardless of their dominance rank [7].
This study was aimed at investigating sex-biased maternal investment in a large population of rhesus macaques (Macaca mulatta). The offspring survival hypothesis and the following predictions of the LRC hypothesis were tested: (1) offspring sex ratios at birth should be female biased for high-ranking mothers and male biased for low-- ranking mothers; (2) interbirth intervals should be longer following daughters than following sons, especially for low-ranking mothers; (3) mortality should be female biased, and (4) daughter mortality should be higher for low-ranking than for high-ranking mothers. Although no predictions of...