New Theory Of Aging Offered

Thinking of trying to get lucky with that special someone? Classical aging theory might make you think twice: you're more likely to die once you've reproduced. Luckily, UC Berkeley demographics professor Ronald Lee's new theory offers a more optimistic outlook on your life expectancy-provided that you don't skip town the next morning.

Classic theory argues that genetic conditions that strike the young tend not to get passed on to the next generation because the carrier generally dies before reproducing.

Instead, mutations that cause death late in life are the ones that run unabated through the gene pool.

However, Lee points out a few flaws in the theory. For one, many individuals remain alive even after raising offspring.

"You would expect mortality to rise very rapidly after the end of reproduction," Lee said. "But of course humans and quite a number of other species go on living after they've done reproducing, so that's a puzzle for that theory."

Furthermore, classic theory would also lend itself to an equal mortality rate among population members not of sexual maturity. Instead, mortality rates are highest for newborns and decrease steadily for several years.

Lee's theory is an attempt to address the inconsistencies he found in the classic hypothesis.

"When I read that theory it just didn't seem right to me," Lee said. "The first thing that struck me was that I would expect the force of natural selection against mortality to be much higher for a human child at age 12 than for an infant, and the reason is a great deal more has been invested in the 12-year old."

Despite how cold the main premise may sound, since more time, effort and care has been expended on the older child over the span of several years, the loss of a newborn is a significantly smaller loss.

"If the newborn dies then the couple can just go ahead and have another kid afterwards," Lee said. "There are offsetting effects when the kid dies early, but when the kid dies at age 12, these offsetting effects are much less and it's mostly a total loss."

Lee's new proposal offers that in addition to reproductive fitness, parental involvement in rearing young can be a key factor in determining life expectancy.

Hence, the longer the offspring is dependent on its parents, the longer the parents' life expectancy.

"In the case of humans, kids remain economically dependent up until age 20," Lee said. If a mother has her last kid at age 45, that kid might be dependent on her until her sixties. So there should be natural selection against mortality at those post-reproductive ages."

Lee used mathematical means to help support his theory. Measuring mortality rates against length of parental involvement, Lee established a proportional relationship in hunter-gatherer groups of many different species.

One of the key groups studied was primates.

Amongst groups where the males were primary caregivers, the average life expectancy was shown to be significantly longer compared to groups where the males took on little to no child rearing responsibilities.

Toothed whales also were shown to be a prime example of Lee's new theory in practice.

Toothed whales have been known to breastfeed their last offspring for more than a decade, and continue to breastfeed and care for the next generation as well.

"There's a continuing investment in the next generation even after reproduction is done," Lee said. "The toothed whales live considerably longer than the other whales."

Despite all the new insights, a perfect theory still seems a long ways off as some species continue to baffle researchers and stray from theoretical predictions.

"There are cases where it's just hard to see any connection," Lee said.

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