- Many adaptive theories have been proposed for explaining variation in progeny sex ratio (PSR) of vertebrates (Hardy 1997). Of these, two opposing hypotheses are commonly used for ungulates: the Trivers and Willard hypothesis (TWH, Trivers and Willard 1973) and the local resource competition hypothesis (LRC, Silk 1983, Caley and Nudds 1987). Trivers and Willard (1973) proposed that when adaptive variance in reproductive success differs between the sexes, mothers should invest more in the sex with the higher variance and, therefore, opt to produce this sex when conditions enable the extra investment. Because most ungulates are polygynous with reproductive success varying considerably between males, this hypothesis is invoked if a male-biased PSR is associated with favorable conditions (e.g. Clutton-Brock et al. 1988 and others mentioned in Table 1). LRC predicts that the gender less apt to disperse imposes overall greater competition on the mother. To minimize this competition, mothers should produce the non-dispersing gender when conditions are favorable. Because in ungulates sons tend to disperse earlier than daughters, the LRC hypothesis is invoked if favorable conditions are associated with a female biased PSR (e.g. Hewison and Gaillard 1996 and others mentioned in Table 1). A separate adaptive theory for each of two opposing situations makes any results explainable on an ad hoc basis. This has caused several critics to question the findings (Williams 1979, Festa-Bianchet 1996, and others) and attribute them to random occurrences. However, as long as the pattern remains consistent within a species, the findings cannot be challenged and could be explained by differences in the biology of the various species (Kojola 1998). Nonetheless, I documented nine cases where the pattern was inconsistent within a species (Table 1). As pointed out by Hewison and Gaillard (1999), the abundance of intraspecific inconsistencies requires some explanation. While Type-I-error (Festa-Bianchet 1996) or heterogeneity within species (Hewison et al. 1999) are possible explanations for these inconsistencies, they appear improbable to me. It is my opinion that in many cases PSR is determined by the age of the mother and if the age of the mother is unaccounted for spurious correlations may be produced between PSR and other factors that have an age-dependent impact on the population. I demonstrate this effect using a Leslie matrix model (Appendix I) based on the dynamics of a reintroduced population of Asiatic wild ass exhibiting PSR that is dependent on maternal age (Saltz and Rubenstein 1995).