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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 question for the majority of species because little is known about the nature of the underlying mortality differences. 2. Examination of mortality data on approximately 600 000 medflies of each sex revealed a demographic paradox-male medflies possessed the higher life expectancy (average longevity) but female medflies were usually the last to die. 3. The underlying demographic cause of this incongruency was a male-female mortality crossover-females exhibited higher mortality than males to around 3 weeks, lower mortality than males from about 3-8 weeks, and mortality approximately equal to that of males thereafter. 4. The findings help explain the ambiguity of male-female longevity differences in the literature, suggest that relative male-female survival cannot be used as a proxy for sex mortality differences, shed light on sex biasing of older ages, and undercore the difficulties with comparative aspects of ageing. 5. We propose a general framework for sex-mortality differentials in which the underlying mortality factors are grouped into three interrelated categories: constitutional endowment, reproductive biology and behaviour. This framework provides conceptual structure as well as insights into how complex patterns in the sex-mortality ratio can arise. Inglés

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