Just Like the Joneses: Species' individuals subject to same aging process (Demography; Evolution)
Authors: Strauss Evelyn 
Published: August 2003

BOSTON
--In the endurance Olympics, humans kick the furry butts of their primate competitors. The most ancient baboons drop from the branches for good when they're about 30 years old, whereas the oldest known person smoked cigarettes for most of her 122 years. But not every human has the potential to take home a gold medal. New work presented at the AAAS meeting here on 16 February suggests some general rules that govern life-span within and between species.

To translate aging into a defined measure, demographers quantify the chance of dying--the mortality rate--as an individual grows older. After reproductive age, it increases exponentially. More than a decade ago, researchers noticed that the risk for different populations--prisoners of war (POWs) and U.S. females, for example--increases at the same rate even though the groups have different life expectancies. That is, at any age, a POW is more likely to die than is a female from the United States--but both face the same increase in their chance of dying after they age 10 years. Marc Tatar, an evolutionary biologist at Brown University in Providence, Rhode Island, and his colleagues wanted to find out whether these observations extend to other species. If so, even though different subpopulations have different life-spans due to genetic and environmental factors, the underlying process of aging--reflected by the increased susceptibility to death--is the same for all of a species' members.

The team compared death rates in three groups of baboons: animals at the Southwest Foundation for Biomedical Research (SFBR) in San Antonio, Texas, and two wild populations in East Africa--one in Gombe National Park, Tanzania, and one in Amboseli National Park, Kenya. Female adult mortality rates double in approximately the same amount of time in all three groups--every 3.5 to 4.8 years--even though each confronts different challenges. Amboseli National Park, for example, is a hostile, harsh environment, whereas Gombe National Park offers plentiful food. And SFBR boasts country club accommodations, complete with veterinary care. "This is about as good as it's going to get for a baboon," said Tatar. Like human groups, the baboon populations have different life expectancies even though the mortality doubling rates were similar. The average Amboseli inhabitant died by age 13, whereas the average SFBR animal was still screeching at age 21.

Baboons age about twice as fast as humans do, Tatar said; our doubling time for mortality rates is every 7 to 8.5 years. But because a particular rate of aging is characteristic of a species, "I want to suggest that we all share--within humans or within baboons--the same aging process," Tatar said. "What varies is frailty." This conclusion might alter the way researchers who study aging view their data, he suggested. Human genetic studies, for example, will reveal how individuals differ in their response to the aging process but will not expose secrets of the process itself. To uncover such machinations, scientists will need to compare species. In the primate Olympics, it turns out, analyzing a competitor's performance might provide insight into our own.

--Evelyn Strauss