Next, Fischer and Austad looked at gliding mammals and again found that they aged more slowly than expected. The results were not as pronounced as in bats, but most gliders still survived longer than their non-flying counterparts. North American flying squirrels, for example, survive almost twice as long as the similarly sized chipmunk. After working on the theory for over a decade, Austad can cite a litany of other examples. Indeed, clams in general are survivors, averaging as much as 14 years, while related molluscs that lack shells rarely live to see their fifth year.
Mammals sporting defensive spines also live longer than their less well defended counterparts.
Austad reasoned that if increased life spans really do go hand in hand with reduced risk of accidental death, you should be able to catch this happening. What's more, this logic should apply within species as well as between them. If one isolated population of animals finds itself with no predators, it should start to age more slowly than a population that is threatened by predation. Austad has found that this is the case with the Virgina opossums of Sapelo Island, five miles off the coast of Georgia, which have lived without predators for almost 4000 years. They survive up to 50 per cent longer than mainland possums, which rarely live past their second year.