- In a paper published recently, Saltz (2001) demonstrated, with the aid of a simple demographic model, that when progeny sex ratio (PSR) is a function of maternal age, spurious temporal patterns of PSR at the population level arise which may appear correlated to various environmental conditions. An example with Asiatic wild ass was provided (Saltz 2001, pp. 377-379). Based on a literature review Saltz (2001) concluded that maternal age appears to be the predominant factor determining PSR in ungulates and, therefore, studies that have linked PSR with environmental conditions (such as habitat quality or population density) must be treated with caution. In the wake of this paper, Hewison et al.(2002) claimed that, based on theory and empirical data, maternal age is not and could not be a predictor of PSR in ungulates.