A MALE-FEMALE LONGEVITY PARADOX IN MEDFLY COHORTS

1. A long-standing question in biology is whether longevity is greater in females or in males for most non-human species. This is an open qu estion for the majority of species because little is known about the n ature of the underlying mortality differences. 2. Examination of morta lity data on approximately 600 000 medflies of each sex revealed a dem ographic paradox-male medflies possessed the higher life expectancy (a verage longevity) but female medflies were usually the last to die. 3. The underlying demographic cause of this incongruency was a male-fema le mortality crossover-females exhibited higher mortality than males t o around 3 weeks, lower mortality than males from about 3-8 weeks, and mortality approximately equal to that of males thereafter. 4. The fin dings help explain the ambiguity of male-female longevity differences in the literature, suggest that relative male-female survival cannot b e used as a proxy for sex mortality differences, shed light on sex bia sing of older ages, and undercore the difficulties with comparative as pects of ageing. 5. We propose a general framework for sex-mortality d ifferentials in which the underlying mortality factors are grouped int o three interrelated categories: constitutional endowment, reproductiv e biology and behaviour. This framework provides conceptual structure as well as insights into how complex patterns in the sex-mortality rat io can arise.