For males belonging to some species of the Dasyuridae family of marsupials, sex is a fatal, frenzied final act. After intercourse, the immune systems of these palm-sized, mouselike creatures collapse and they die soon afterward. Scientists have long argued about the reason for this sexual suicide, which is unique among mammals. Is it an act of altruism, so that males will not compete with their offspring for food? Or is it the result of dangerous genetic mutations triggered after breeding? Neither, according to a new study. By comparing 52 different species of Dasyuridae from diverse habitats across Australia, New Guinea, and South America, a team has found that frantic competition for choosy females appears to be the culprit. At higher latitudes, insects are abundant only for brief periods, and mating is largely timed to the availability of food. For some species, this means that the entire breeding period can be as short as a few days per year. Females mate promiscuously, forcing males to compete even more fiercely. For males determined to get their genes into the next generation, reckless sexual abandon appears to provide the best chance at fatherhood, even if they die in the process, the researchers say. Just how reckless? Compared with a roughly 4-hour average copulation time among species with longer, more laid-back breeding seasons, Dasyuridae males spend an average of 9 hours in the act before collapsing from exhaustion , the authors report online today in the Proceedings of the National Academy of Sciences. To sustain such a feat, evolution has endowed these doomed males with a gift: larger-than-average testicles.