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  • In evolutionary biology and evolutionary psychology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers and Dan Willard, suggests that female mammals are able to adjust offspring sex ratio in response to their maternal condition, for example it may predict greater parental investment in males by parents in "good conditions" and greater investment in females by parents in "poor conditions". The reasoning for this prediction is as follows: assume that parents have information on the sex of their offspring and can influence their survival differentially. While pressures exist to maintain sex ratios at 50%, evolution will favor local deviations from this if one sex has a likely greater reproductive payoff than is usual. Trivers and Willard also identified a circumstance in which reproducing individuals might experience deviations from expected offspring reproductive value—namely, varying maternal condition. In polygynous species males may mate with multiple females and low-condition males will achieve fewer or no matings.

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