Sci. Aging Knowl. Environ., 23 July 2003
Invest in the Future, Invest in Yourself
Model adds nurturing factor to evolutionary theory of aging
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/sageke;2003/29/nw102
Key Words: life-history theory
Everyone knows that parental nurturing bolsters a child's chance for long life, but a new mathematical model suggests that it also prompts the evolution of longevity in parents. The work updates a decades-old theory of how aging evolves and might explain why some organisms outlive their reproductive capabilities. Although some experts laud the effort, concerns remain about testing the model.
By standard evolutionary accounts, old age is a waste. Natural selection, the theory goes, wields power only as long as an organism is fertile, and thus it can't extend the length of time individuals live beyond their reproductive years. But many animals persist long after they cease making eggs or sperm. Some researchers postulate that this bonus time is beneficial (see Holmes Perspective). "Many species continue to care for their young after birth," says demographer Ronald Lee of the University of California, Berkeley. "Somehow we have to include that in the theory, but it hasn't been done."
Now, Lee has developed a mathematical theory that takes into account the investments parents and others make in rearing young. To do so, he used ideas from economics that describe intergenerational transfers: the flow of resources between individuals of different ages. In the new theory, natural selection's power at a given age is determined by a combination of remaining fertility (the "classic" component) and amount of energy invested in caring for progeny. The theory makes several predictions about life span that differ from the classic view and that match observations of human populations, says Lee. For instance, mortality should be high at birth, drop during childhood, then rise again as an individual ages; the classic theory holds that mortality rises from a minimum at birth. In addition, the model predicts that postreproductive survival should increase when parents invest more care in their offspring.
Some researchers applaud the new model. "It's a hugely important paper," says biodemographer James Carey of the University of California, Davis. "I view it as the most important addition to this theory since the mid-1960s. It's been a good sturdy theory, but it's time to develop new concepts." However, biologist Steven Austad of the University of Idaho in Moscow cautions that the theory "is absolutely untestable. You need some way of validating that it's correct or not, and [Lee] didn't provide any reasonable way of performing the tests." Evolutionary biologist Scott Pletcher of Baylor College of Medicine in Houston, Texas, adds that the theory's predictions don't hold true in all organisms. Some species, such as fruit flies, enjoy long life after fertility wanes even though they neglect their young, he says. Still, the updated model is valuable because it incorporates approaches that haven't previously been applied to biology, he says. Such disciplinary cross-pollination shows researchers innovative ways to deal with complications such as parental care. "That's one of the ways [the study of] the evolution of aging needs to move." Nurturing the theory to maturity might lead researchers to understand whether attentive care spurs longevity--of the parents.
--R. John Davenport
July 23, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150