Biases in the ratio of males to females occur in many polygynous mammal species. According to the mother's condition, investment in sons or daughters may have different fitness benefits in terms of the quality of offspring and hence quantity of grandoffspring produced. In many cases, such as red deer in Scotland, mothers in good condition differentially invest in sons, because males are more costly to rear. However, the reverse may sometimes be true. Kruger et al. studied sex-ratio variation over 30 years in a population of springbok in the southern Kalahari region of South Africa. Females in better condition produced more daughters than sons. It seems that the faster onset of sexual maturity in females will produce greater fitness returns in the unpredictable Kalahari environment. Rainfall may be an important controlling factor: Daughters were differentially produced earlier in the wet season, giving them a greater chance of reaching maturity in good condition themselves. The mechanism of sex-ratio adjustment probably lies either in an ability on the mother's part to discriminate between X- and Y-bearing sperm or condition-dependent selective implantation of male or female embryos. — AMS
Proc R. Soc. Lond. B 272, 375 (2005).