Sex Ratio and Maternal Investment in Ungulates

Differential maternal investment in sons and daughters can be accomplished by adjusting the sex ratio of offspring or by investing differentially in individual sons and daughters. In polygynous mammals, such as most ungulates, sons are larger than daughters at the end of the lactation period, and the raising of a son can be more expensive than raising a daughter. After weaning, daughters will provide extra maternal investment by sharing their mothers' home range, and local resource competition probably increases the costs associated with producing a daughter. In some species sons seem to benefit from their mothers' phenotypical superiority through heavier maternal investment before weaning, while daughters can share their mothers' high social rank also after weaning. I suggest from the reviewed literature that local resource competition shapes offspring sex ratios at the population level most clearly when the diet type is browse that is a more defendable resource than graze. It is also probable that differential costs, for example in the form of the subsequent reproductive success and the survival of the mother, are associated with producing sons and daughters are an important selective force modifying sex ratios born to superior and weak females within populations, in addition to a stronger correlation between mothers' quality and the reproductive success of the offspring of one sex. This correlation can be associated also with fitness costs, because the rearing of a weakly competitive offspring may result in the wastage of reproductive effort.