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Facultative Sex-Ratio Manipulation

Several conditions favor the evolution of parental control over offspring sex. (1) When deviations from the equilibrium sex ratio are common, individuals may benefit from adjusting their production to favor the rare sex. (2) The equilibrium sex ratio itself may vary spatially or temporally, particularly as a result of variability in the extent of kin interactions and fluctuations in the intensity of local mate competition. (3) Parental resources may vary, making some individuals superior son producers and others superior producers of daughters. These resources include: (a) the quantity of parental investment available; (b) the quality of parental investment available; (c) environmental suitability, that is, the impact of a particular environment on the relative fitness of the two sexes; (d) genetic suitability, that is, variation in individual propensity to produce superior offspring of one sex for genetic reasons; and (e) the composition of family groups, that is, the sex of siblings produced simultaneously or sequentially. Conservatism (failure to produce extreme ratios) by facultative manipulators may be expected if more than one of the above potentially conflicting factors affects optimal allocation, and if manuipulation involves as additional expense. Conservatism by facultative compensators may also be expected if the sex composition produced by others in the population is somewhat unpredictable, or if others are likely to practice compensation as well. A graphic model is presented for facultative compensation of the sex ratio by females of iteroparous, polygynous species. The model assumes that sex-ratio adjustment entails some cost, and it predicts compensatory patterns based on how individuals absorb these costs and on the existence of specialized production of the sexes. The model assumes a continuum of types ranging from superior to inferior resource allocators. Superior allocators place relatively large amounts of energy into each reproductive attempt and therefore achieve their lifetime reproductive success in fewer litters than inferior individuals. Superior allocators are assumed to be superior predisposed and facultative producers of sons. Depending on the relative costs of facultative female production, a series of curves is generated corresponding to specialization on females by (1) inferior allocators, (2) superior allocators, or (3) no female class. An experiment on facultative compensation by female mice (Mus musculus) involved the repeated rearing of unisexual litters. One group (F) reared only daughters, a second (M) reared only sons, and a third (C) reared litters of the same composition as born. The overall production by F and M females did not differ significantly, but the distribution of reproduction varied among classes. Results were in accordance with the model for the case in which facultative production of females is more expensive than nonfacultative production of males and in which inferior allocators are female specialists. Weaning weights of offspring of C females support a hypothesis of sex specialization. Relatively superior allocators wean males at about 120% the weight of females. Inferior allocators wean the sexes at about equal weight. Several important questions remain unresolved; principal among them are the cause and nature of specialization on females, and the overall importance of compensatory reproduction as a mouse reproductive tactic. For theoretical reasons and because of a tendency among mouse geneticists to differentially discard male offspring at birth, murine sex-ratio manipulation deserves further study.