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Demography study: Caring for the next generation will keep you living longer.

– How a species cares for the next generation is sometimes a more powerful determinant of lifespan than fertility, according to University of California, Berkeley, demography professor Ronald Lee.

"My extension of the classic theory says that reproductive fitness isn't just about bearing offspring," Lee said. "It's about investing in each offspring. Mortality and deterioration with age are shaped by the amount of future investing that an average organism would do."

It is this investment that causes the average age of an individual in a species to extend beyond fertility. On the other hand, an individual whose species does not invest in offspring will die closer to the decline of fertility.

In "Rethinking the Evolutionary Theory of Aging: Transfers, not Births, Shape Senescence in Social Species," appearing in the July 14 online edition of the Proceedings of the National Academy of Sciences, Lee asserts that the time and resources an individual puts into the next generation, what he refers to as intergenerational transfers, are important in determining the average lifespan of a species, and the way it deteriorates with age.

In his paper, Lee wrote, "The classic evolutionary theory of aging explains why mortality rises with age: As individuals grow older, less lifetime fertility remains so continued survival contributes less to reproductive fitness."

Evolutionary biologist William Hamilton, who was a proponent of the classic theory, thought expenditure on medical research to prolong life was wasteful.

"The old theory leads to a pessimistic view of medical gains," Lee said. "Those who believe in the classic theory are likely to think people are destined to die soon after they finish reproducing."

While Lee's new theory does pertain to humans, it is also applicable to a number of other species as well.

"This could mean a mouse breastfeeding its young, or a fish circulating water over its eggs," he said. "It can mean a bird risking its life to protect eggs in the nest from a predator."

He also explained that in some species of birds, older siblings remain to help younger offspring. "It may be beneficial to hang around another year," Lee said. "The bird will get stronger and be able to live on its own better, yet at the same time, it is contributing to the well-being of the younger siblings and helping the parents."

He also cited toothed whales and dolphins, which live in social groups. "Females can continue to lactate for extended periods," he explained. "Their last born may be suckled 10 to 15 years. Also, some grandmothers may suckle grandchildren, and tend them while the parents are out hunting."

Studies of contemporary hunter-gatherer societies have shown that children in these groups are not producing enough food for themselves until they are around 20 years of age. One possible reason is that hunting and gathering is difficult and demanding. It takes long training to become productive.

"They (hunter-gatherers) don't reach their peak until their 30s," Lee said. "The parents invest in their children over long periods of time, and they produce more than they eat. There is only a short period of time before they die that these people become dependent on younger members of the group."