Natural selection and sex differences in morbidity and mortality in early life

Both morbidity and mortality are consistently reported to be higher in male s than in females in early life, but no explanation for these findings has been offered. This paper argues that the sex difference in early vulnerabil ity can be attributed to the natural selection of optimal maternal strategi es for maximizing lifetime reproductive success, as modelled previously by Trivers and Willard. These authors theorized that males and females offer d ifferent returns on parental investment depending on the state of the envir onment. Natural selection has therefore favoured maternal ability to manipu late offspring sex in response to environmental conditions in early life, a s shown in variation in the sex ratio at birth. This argument can be extend ed to the whole period of parental investment until weaning. Male vulnerabi lity in response to environmental stress in early life is predicted to have been favoured by natural selection. This vulnerability is most evident in the harsh conditions resulting from pre-term birth, but can also be seen in term infants, and manifests as greater morbidity and mortality persisting into early childhood. Malnutrition, interacting with infection after birth, is suggested as the fundamental trigger mechanism. The model suggests that whatever improvements are made in medical care, any environmental stress w ill always affect males more severely than females in early life. (C) 2000 Academic Press.