In litter-bearing mammals, adult phenotype is influenced during prenatal life by the sex of neighbouring foetuses. This phenomenon, found so far only in laboratory studies, may have ecological importance in nature. We present the first evidence that litter sex ratio has consequences for lifetime reproductive success in mammals. Female Alpine marmots born in a male-biased litter, i.e. more likely to be located in utero next to male foetuses, are more likely to be dominant later in life – a prerequisite for reproduction in this species. We found no evidence for lower reproductive output of these females, a common cost of prenatal masculinization.