The oldest rodents living in captivity are the naked mole-rats (Heterocephalus glaber), the oldest of which are now at least 26 years old. Because of their living conditions they are less at risk of being killed by predators or accidents in the wild than are other rodent species. Therefore their longer natural lifespans are predicted by the evolutionary theory of aging. There was no big selective pressure for them to be more vigorous when younger and instead the selective pressure on all the genes that affect aging was toward longer lasting compoents.
In nature, naked mole-rats are known to live at least 10 years. "We think they live longer in the laboratory than they do in the wild because they're safer here, but they're pretty safe in nature, too," Sherman says. One of the factors contributing to the evolution of longer life spans is reduced extrinsic mortality, which Sherman defines as causes of death that are outside an animal's control, such as drowning in a flash flood or being devoured by a snake. In nature, naked mole-rats are largely protected from sources of extrinsic mortality by inhabiting subterranean burrows in extremely hard soils. Protection is enhanced by cooperative defense against predators. As a result, naked mole-rats have evolved genetic traits that make them more resistant to senescence than similar-sized, solitary, surface-dwelling rodents. Indeed, the only rodent known to live as long as the naked mole-rat is the African porcupine Hystrix brachyura , which is protected by its large body size and quills.
The diminutive naked mole-rat has something else going for it: greater fecundity with advancing age. "A large, old breeding female mole-rat gives birth to an incredible number of young and continues to do so year after year," Sherman says. "Our record for a laboratory female is 28 pups in one litter and more than 900 pups in a lifetime." Fecundity seems related to body size, Sherman adds, noting that mole-rat queens, like queens in honeybees and termites, are considerably larger than workers in their colonies. Fecundity is important because if old individuals can make disproportionate reproductive contributions, there will be strong selection to postpone senescence.
While naked mole-rats are models that support senescence theory, they are not perfect role models for humans. Senescence occurs, simultaneously, on all aspects of any organism, which means there is no single gene for aging or for youth. "Senescence theory," says Sherman, "tells us why the fountain of youth still eludes us -- and probably always will."
It isn't clear exactly what argument Paul Sherman is making when he claims that the fountain of youth will always elude us. It will be very hard to redesign out bodies in a way that totally precludes aging from happening. But that is not what is necessary. We will be able to repair and improve our bodies to make them young again. What we really need are techniques for repairing the accumulating damage and to slow the rate of accumulation. When biotechnology advances far enough we can repair the damage with periodic medical treatments (eg organ replacements, gene therapy, cell therapy) then we will get effective rejuvenaton and be able to feel and be young again. Rejuvenation by repair and replacement will be achieveable within the next 20 to 40 years.
|Share |||Randall Parker, 2002 November 08 11:05 AM Aging Reversal|