The Trivers and Willard model (TWM) predicts that for polygynous ungulates, females of high phenotypic quality should produce more sons than daughters, whereas females of low phenotypic quality should produce more daughters. Kruuk et al. showed that in red deer the TWM only applied when the population was below carrying capacity, and they suggested that published examples supported their interpretation. More recently, Saltz proposed that mothers' age rather than condition could account for skewed sex ratios observed in ungulate populations. We tested these predictions by using data on maternal age, mass, kidney fat reserves (KFI), and fetal sex ratio in an invading population of Himalayan tahr (n=252), a highly sexually dimorphic and polygynous ungulate introduced to New Zealand. Differences in the body mass and KFI of female tahr supported the prediction that the populations in areas colonized for less than 15 years were below carrying capacity, whereas those in areas colonized for more than 30 years were at, or near, carrying capacity. There was no trend for mothers either of larger mass or with greater KFI to produce more sons than daughters. There was also no evidence of a quadratic relationship between maternal age and the proportion of male fetuses. However, the proportion of male fetuses declined with increasing sampling date independent of maternal attributes. Among 1193 females checked for pregnancy, pregnancy rates increased to a maxima in mid-July. Thereafter, the proportion of females pregnant declined among the three age classes (1, 2, and 3 or more years). Our results therefore provide support for the idea that males experience greater mortality in utero. The role of differential fetal mortality in determining ungulate birth sex ratios deserves further investigation.