The common finding that juvenile male mammals show higher mortality than females1,2 is usually attributed either to the expression of deleterious recessive alleles on the X chromosome3 or to adaptive manipulation of the postnatal sex ratio by mothers who are unable to rear successful sons4,5. As a general explanation of differential juvenile mortality, the first of these two theories is unsatisfactory because increased male mortality is known to occur in several bird species where females are the heterogametic sex6,7. The second hypothesis was first proposed by Trivers and Willard4 and suggests that differential mortality should occur early in the period of parental investment8. We now show that, although several predictions of the latter explanation are fulfilled, the distribution of differential mortality in red deer and other mammals suggests that higher mortality rates among male juveniles are a consequence of a greater susceptibility of males to food shortage associated with their faster growth rates and increased nutritional requirements.