Humans, whales and dolphins have evolved to live well beyond child-bearing age because this helps raise the survival chances of their descendants, argues a new theory of ageing in social animals.

The classic theory of ageing asserts that fertility is the sole determining factor in lifespan. But in many species, grandparents - and especially grandmothers - can influence natural selection long after their own reproductive years have ended, said Dr Ronald Lee of the University of California at Berkeley, in this week's issue of the journal, Proceedings of the National Academy of Sciences.

"In some species, post-reproductive females make substantial contributions to their descendants, either through direct parental care or through grandparental care," Lee said. "Such contributions continue after birth in all mammals - most notably primates - all birds, many insects, and some fish.

"Post-reproductive bottlenose dolphins and pilot whales babysit, guard, and even breastfeed their grandchildren," he said. Lee's theory may also shed light on why women go through menopause.

He argues that in trying to explain why mortality rises with advancing age, evolutionary theorists have overemphasized the part played by natural selection, which has little or no effect on lifespan once individuals are no longer fertile, because they can no longer pass on their genes.

If a species makes no post-birth investment in raising its offspring, then natural selection for lifespan is indeed likely to depend entirely on fertility, Lee said: butterflies lay many eggs, then die.

But in species where parents have few offspring and invest time, energy and other resources into promoting their children's survival, natural selection should logically favour a longer lifespan, he said. Evidence in favour of that idea can be found in humans, as well as their close relatives among the primates, where the gender that provides most parental care tends to have a longer lifespan.

So, if grandparents promote their children's success as parents in their own right - effectively an economic process involving an inter-generational 'transfer of resources' - even more selective pressure should favour living longer, he argues.

Lifespan depends heavily on the transfer effect, he said, noting that mortality for human hunter-gatherers and other social species where inter-generational transfers occur accords well with his new theory: "The average infant in an Efe huntergatherer group is cared for by 11 people in addition to its parents. Co-operative breeding occurs in some mammals, many insects and 200 species of birds."

In an accompanying commentary, Dr Alan Rogers of the department of anthropology at the University of Utah questioned why evolutionary processes would have promoted early menopause.

"If selection favours production of children, how could it ever favour an early end to fertility? Why do women not continue producing babies into old age? Or to look at the problem from the other direction, how does selection weed out harmful mutations that increase mortality late in life?" he asked.

"The force of selection affecting genes expressed in 50-year-old women should depend on the contribution that such women make to future generations. But if these women have stopped reproducing, this contribution would seem to be nil. Thus, harmful mutations acting late in life should accumulate and death should follow soon after reproduction stops. In most species, this is exactly what does happen. But there are exceptions."

Anthropologists have found clear evidence that older women have a beneficial effect on children and grandchildren in traditional societies, and comparisons with other primates have shown that birth rates are higher in humans than in other apes: "This is all consistent with the 'grandmother hypothesis', which holds that the labour of older women accelerates the rate of childbearing in humans," Rogers added.

He said Lee's complex economic calculations on the transfer of resources between individuals and between generations in a species have reconciled a number of problems in previous attempts to explain early menopause and mortality rates: "This is the most comprehensive evolutionary theory of ageing that we have seen to date," he said.

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