Procreation Penalty? Scientists challenge prevailing theory of reproduction and longevity (Reproduction; Evolution)
Authors: Davenport R. John 
Published: August 2003

"You've taken 20 years off my life," the mother complains to her rebellious teenage daughter. Many scientists believe that's no exaggeration: According to a theory of aging, living things must choose whether to perpetuate their genes or preserve themselves. Now, a new study claims to have overturned the idea. But some researchers say the data support the conventional wisdom.

Studies of fruit flies bolster the notion that creatures face a tradeoff between reproduction and survival: Flies that delay reproduction outlive earlier breeders. A 1996 study of British royalty suggested that the same holds true for humans: Blue bloods who bore children earlier died younger than those who reproduced later. But statistician Hans-Georg Müller of the University of California, Davis, and colleagues questioned the conclusions from the study on humans, suspecting that changes in reproductive patterns skewed the analysis.

Müller and colleagues analyzed demographic data on French-Canadian women from the 17th and 18th centuries, selected because of the short time period during which they lived and because of their high fertility rates--more than half of the women gave birth to at least eight children. The group exhibits human reproductive potential unchecked by birth control, according to Müller. He and his colleagues examined data that indicated how many children the women had, how old they were when they gave birth, and when the women died.

The researchers looked for mortality trends associated with number of children. They selected only women who died at age 50 or older to exclude mothers who perished during childbirth, a significant killer at that time, says Müller. Using several statistical analyses, they found that women with more children outlived women with fewer offspring. They also discovered that, among 50-year-old women, those with the youngest children lived the longest. Additional number crunching revealed that the age of a 50-year-old mother's youngest child better predicted her longevity than did her total number of children.

The study suggests that the presence of young children later in life fosters longevity, says Müller, but it doesn't explain why the link exists. Nevertheless, he says, it shows that the "cost-of-reproduction theory is just plain wrong." Other researchers disagree. "It may not be the offspring themselves that are contributing to longevity," says demographer S. Jay Olshansky of the University of Illinois, Chicago. Instead, he says that the potential to produce those offspring is a more likely influence on life-span. "And that's precisely what you'd predict from evolutionary theory: Females that are capable of producing offspring later will live longer." By choosing women who had reached age 50, he adds, the researchers might have selected a group of hardy individuals with an unusual capacity for surviving childbirth and other dangers. Still, he says the authors make a valuable contribution: "They've added to the evidence that already existed linking reproduction and mortality; they did an elegant job of demonstrating that [link] for humans." As scientists debate the interpretation, parents will likely continue to ponder whether their own offspring shrink their lives or just their bank accounts.

--R. John Davenport; suggested by Donna Holmes

H.-G. Müller, J.-M. Chiou, J. R. Carey, J.-L. Wang, Fertility and life span: late children enhance female longevity.
J. Gerontol. A Biol. Sci. Med. Sci.
57
, B202-B206 (2002).
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