Longevity differs across species; tortoises can live for over a hundred years while most rodents live for under a decade. Now, researchers have discovered a possible mechanism to explain these differences in longevity.

In order to find out what causes different species to live for longer or shorter periods of time, the scientists examined life history data of nearly 1,400 bird species. Avian lifespan varies considerably across the globe, and this variation can be explained through species' body mass, clutch size, and by the local diversity of predator species.

The classical evolutionary theory of aging predicts that high mortality rates in adult animals due to predation, exposure to parasites and other events will be associated with shorter maximum life-spans. This is because under high external mortality, most individuals will already be dead before natural selection can act on rare mutations that cause healthier aging. Yet contradictory results have caused scientists to doubt this particular theory.

Now, scientists have tested this theory by studying bird species. They found that the more predator species present in the same habitat, the more evenly they are distributed, and the lower the lifespan of the bird species. This relationship actually supports the classical theory of aging.

"With our results of a negative relationship between predation pressure and longevity that is largely independent of other key life history traits we were able to confirm the universality of the 50 year old evolutionary theory of aging on a bread geographical scale," said Mihai Valcu, one of the researchers, in a news release.

The findings reveal a little bit more about why lifespans differ among various species-at least when it comes to birds. The study could help further the understanding of evolution in general, and could allow researchers to better understand why humans possess the lifespan that they do.

The findings are published in the journal Ecography.