As I was editing David Stipp's article "A New Path to Longevity" for the January 2012 issue of Scientific American, Stipp told me about some intriguing research into unusually long-lived mammals. Investigators are studying such animals to better understand why the adult human body deteriorates with age and how to intervene in the process.

David examined that research in his 2010 book The Youth Pill: Scientists at the Brink of the Anti-Aging Revolution, and he gives a taste of it here, in an adaptation of one of the chapters. The piece focuses on the longevity secrets of naked mole rats. These odd creatures, which he describes as "grotesque, mouse-sized rodents," can survive more than five times as long as one would expect, based solely on their size.

Why are researchers concerned about animal size when considering longevity? As David explains in the book, larger mammals generally live longer than smaller ones (probably in part because they are better able to fend off predators). For a given mammal's longevity to be considered exceptional, then, it would have to be able to live much longer than is typical for its size. Those extended survivors are of particular interest because they are the animals most likely to have evolved biological mechanisms that slow aging, and that might thus hold clues for retarding human declines. In the case of naked mole rats, unusual levels of insulin in the blood and the activity of molecules known as chaperones seem to be involved.

Figuring out if a species is unusually long-lived for its size is not straightforward, but Steven Austad, of the Barshop Institute for Longevity and Aging Studies at the University of Texas Health Science Center at San Antonio, has developed a measure called the longevity quotient. He divides the greatest recorded longevity for a species by the life span that would be predicted on the basis of the species's average weight. In this scheme, a score of 1 is average; higher numbers reflect unusual longevity whereas fractions represent unusually short life spans. See rankings for selected animals, including the naked mole rat, here .

Naked mole-rats are unusually long-lived animals that are being studied for clues to slowing aging. Courtesy of Ltshears—Trisha M Shears/Wikimedia Commons

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A News Path to Longevity [Preview]

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Naked Mole-Rats Offer Clues to Living Longer

How Long-Lived Is This Mammal? [Slide Show]