AbstractGroups of wild animals can vary considerably in their composition, including in the proportion of group members who are male or female, that is the within‐group sex ratio. Variation in within‐group sex ratios can arise from active adjustment of litter sex ratios by mothers, from sex differences in mortality, dispersal and immigration, or from stochastic variation in recruitment. Variation in the within‐group sex ratio can have consequences for within‐group competition and can affect individual life histories throughout development. In this paper, we explore which processes may generate variation in within‐group sex ratios in wild Damaraland mole‐rats (Fukomys damarensis), a singular cooperative breeder. We investigate whether within‐group sex ratios predict the growth, body condition and philopatry of individuals. We show that although the population‐level sex ratio is balanced, skewed within‐group sex ratios are common, particularly among small groups. Our data suggests that stochastic variation in the sex of recruits explains natural variation in the sex ratio of wild groups. Non‐breeding individuals in groups with a sex ratio biased towards their own sex grow more slowly than individuals in groups biased towards the opposite sex, suggesting that intra‐sexual competition may decrease growth rates. We suggest that the costs of competition may contribute to the large variation in growth observed in social mole‐rat groups.