Title
Adjustment in birth sex ratio in wedge-capped capuchin monkeys
Author(s)
Robinson J.G., O'Brien T.G.
Published
1991
Publisher
American Naturalist
Abstract
Fisher proposed an equilibrium model for the birth sex ratio in which expected reproductive success, E(RS), of sons and daughters, the birth sex ratio, and the relative cost of producing offspring of different sexes were inextricably linked. In a population of Cebus olivaceus, the observed intrinsic rate of natural increase, calculated from demographic life tables, was used as a measure of E(RS). Reproductive success of females varied with group size, and there was a quantitatively predictable variation in birth sex ratio. Groups with =15 individuals produced on average a birth sex ratio of 1:1.9, while groups <15 produced a birth sex ratio of 1:0.83. For the population as a whole, the birth sex ratio of 1:1.57 is strongly skewed toward daughters. On average, the E(RS) of sons is greater than that of daughters, which suggests that the cost of producing daughters is lower than for sons. Daughters might be cheaper because they "pay back' parents by not dispersing and thus help parents produce future offspring. -Authors
Keywords
capuchin; Fisher's theory; monkey; reproductive success; sex ratio; wedge-capped capuchin
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