In the study the researchers analysed data, which measured survival, and descriptions of breeding for both sexes in animals. Several explanations have been proposed for the lifespan difference between the sexes - it could be a result of the ageing effects of testosterone, or it could be evolutionary forces- having the men die early might ease pressure on valuable resources, for example, helping the overall success of the species. Or it could be something to do with mating behaviour.
Earlier observations had suggested that polygyny was a common characteristic among species in which males die younger than females, including red deer, lions and elephant seals. It was observed that elephant seals can have harems as large as 40 females but the males lived only 75 percent as long as the females.
The analysis of the study found that no consistent sex differences in breeding life spans, annual rates of mortality or rates of ageing in monogamous species. However, the team reported that the more polygynous a species was, the more short-lived the male was likely to be, and the shorter their duration of effective breeding.
"Fight hard, and not only die young but evolve to die young," Nature quoted Stephen Stearns of Yale University in New Haven, Connecticut, as saying. For example in polygynous red deer, males live 75 percent as long as females, and have an effective breeding period of less than half that of the females.
In swans, which almost always pair for life with only a 6 percent divorce rate, life spans and effective breeding periods were nearly equal.
Researchers said that the trait could be because in species in which males fight for mates, evolution favours the development of characteristics that help win fights, such as aggression or antler growth, at the expense of male longevity. "We need more long-term studies following individual animals through their lifespan to understand this better," Clutton-Brock said.
"The results can be simplified as fight hard, and not only die young but evolve to die young," Stearns said.