The widely held scientific belief that females of all species generally outlive their male counterparts may not hold true, and is certainly not valid for Mediterranean fruit flies, according to the findings of an entomologist at the University of California, Davis. In a longevity study of 1.2 million Medflies, entomology professor James R. Carey found that young male Medflies were at a lower risk of dying than were females. But female death rates began to decline during mid-life and by the advanced age of 60 days -- equivalent to 85 years in humans -- the chance of dying was equal for both males and females. Among the extremely old Medflies, females had lower death rates. Carey reports this mortality "crossover" phenomenon in the cover article of the January issue of the Journal of Animal Ecology. "These results indicate that there is not a universal law in male/female longevity," said Carey, an authority on insect demography who uses the fast-breeding Medflies as models for studying lifespan theories. "Furthermore, it is impossible from this study to classify either sex as more robust or longer lived, since longevity varied according to the flies' environment and treatment." While women generally outlive men by a margin of 4 years to 10 years in industrialized nations, a long-standing question in biology is whether greater female longevity has a fundamental biological basis. Scientists have long believed that the female longevity advantage is a universal "law" of nature that holds true for all species. They attribute the female advantage to riskier male behavior and the fact that females have two X chromosomes, while males have one X chromosome and one Y chromosome, which is significantly smaller and contains much less genetic information. Carey suggests that the Medfly longevity study has two key implications for humans. If differences in human longevity between males and females are as complex as they are for Medflies, future gains in longevity due to improved medical treatment or environmental conditions probably will be uneven between the sexes. Secondly, by studying aging differences between the sexes at certain stages of human life, researchers may be able to identify biological variances that influence an individual's life expectancy at that stage. Carey proposes that differences in male-female mortality be viewed as complex products of physiological, biological and behavioral traits, rather than simple behavioral or chromosomal differences. Fluctuations in factors affecting these traits at various stages in life may temporarily give one sex an advantage over the other. In humans, all three types of traits influence mortality differences between the sexes. "For example, men often behave in ways more damaging to their health in terms of smoking, alcohol consumption, hazardous occupations and violence," said Carey. "Yet women are more vulnerable to disease and clinical problems associated with childbearing during the reproductive ages." Carey's analysis is a follow-up on the Oldest Old project, funded by the National Institute on Aging. In 1992, the project used Mediterranean fruit flies to produce the largest mortality study of any non-human species. It was conducted at the Moscamed Medfly rearing facility in Metapa, Chiapas, Mexico.