The olm, a foot-long salamander nicknamed "the human fish" because of its fleshy skin and tubular shape, is certainly a strange-looking animal. But beneath the surface, they're even weirder: Olms can live for 100 years, far longer than any other amphibian.

Scientists have no idea why.

"This species raises questions regarding aging processes," write researchers led by biologist Yann Voituron of France's Université Claude Bernard in a July 21 Biology Letters study.

The olms studied by Voituron's team are part of a population established 48 years ago to help conserve the rare amphibian, which is found in caves in Croatia and Slovenia.

When the project began, the olms were about 10 years old, making them nearly 60 now. Yet they "do not show any time of senescence," write the researchers, who estimate the olm's average lifespan to be 69 years, with an upper limit at the century mark.

Living in a stable environment without predators has made it possible for olms to have long lives, but the mechanisms underlying their longevity are unknown. In general, long life correlates with a large body size, but the half-pound salamanders are pipsqueaks compared to the next-longest-lived amphibian, the 50-pound Japanese salamander, which clocks in with a 55-year lifespan.

Voituron's team thought olms might have extremely slow metabolisms, but they proved metabolically similar to other amphibians, including African bullfrogs and European toads that live for about 40 years.

The researchers also wondered if olms might have special tricks for cleaning up oxygen-free radicals, the DNA-damaging molecules produced when cellular mitochondria turn nutrients into energy. Free-radical accumulation is linked to aging, but the olm's antioxidant activity is nothing special.

"The olm presents a paradox," wrote the researchers. "Neither its basal metabolic rate nor its antioxidant activity, the two most cited mechanisms that should be involved in enhancing lifespan, differ from species with a more reduced lifespan."

Voituron is now testing whether the olm might have extra-efficient mitochondria that emit fewer free radicals to begin with. "If you manage to produce more energy with less free-radical production, then you can avoid aging and increase lifespan," he said.

Image: Olivier Guillaume.

See Also:

Citation: "Extreme lifespan of the human fish (Proteus anguinus): a challenge for ageing mechanisms." By Yann Voituron, Michelle de Fraipont, Julien Issartel, Olivier Guillaume Jean Clobert. Biology Letters, online publication, July 21, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.