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06/08/1998 07:00:00

Parent that takes care of offspring tends to outlive the other parent, study shows

PASADENA--The parent who stays home to take care of the kids may be getting a good deal healthwise. New primate research from the California Institute of Technology shows that a primary caregiver tends to live longer than the other parent.

In a statistical study of 10 primate species, including humans, apes, and various Old and New World monkeys, the Caltech researchers show that the parent that cares for the offspring is significantly longer lived than the mate, regardless of gender. Titi monkey males of South America, for example, which take care of the baby after the mother has given birth, outlive their mates by 20 percent.

"The numbers show that if there is a difference in role, the sex doing the bulk of the care is likely to survive longer," says John Allman, a Caltech biology professor who is lead author of the study.

"This follows from the fact that it takes a lot of energy to raise a big-brained offspring like a human or an ape or a monkey," he adds. "The sex not caring for the infants will not be as crucial for the survival of the species."

The size of the brain is the key, Allman says. Species with big brains mature slowly and have only one baby at time, and these babies depend on their parents for a long time.

"Big brains are very expensive," Allman says. "They are costly in terms of time, energy and anatomical complexity. This reduces the reproductive potential of the parents because extra-special care must be provided to insure that this reduced number survive to reproductive age."

In an article appearing in the current issue of the journal Proceedings of the National Academy of Science, Allman and his coauthors outline their data from the 10 species of primates. To determine the lifespans of males and females that have borne offspring, the researchers analyzed the data from zoo populations, field studies, laboratory research, and human historical and demographic documents.

The researchers were especially interested in reviewing field studies as well as zoo data to ensure that artificial effects were not skewing the data. However there is also data from natural populations of primates that supports this hypothesis.

"Female gorillas, orangutans, and chimpanzees have a proportionally larger survival advantage than human females," Allman says. But the advantage of female gorillas is not so pronounced, and this could very well have to do with the fact that male gorillas play with their offspring and take on certain other nurturing duties.

In fact, the Caltech hypothesis is not only that the caregiver who takes care of the offspring tends to outlive his/her mate, but also that the effect disappears when parents share in caregiving more or less equally.

Perhaps for this reason human males and females also have lifespans that are fairly similar in length. The current figure is about 8 percent, but does not take into consideration the fact that medical care has significantly reduced the death rate from childbirth. Swedish demographic data from the late 1700s, by contrast, shows that females lived about 5 percent longer than their mates in those days, when childbirth was a leading cause of death in women.

The demographic data of the 10 primate species shows remarkable conformity to the hypothesis. In all of the primates studied in which females are the primary caregivers-spider monkeys, gibbons, orangutans and gorillas-females live significantly longer than males. Human female live longer than males, but the difference is smaller than in these primates and the male role is larger although less than the female role in childrearing.

In the two primates studied in which females and males share caregiving more or less equally, there is no difference between the survival rates of the sexes. In the two primate studies in which males have a larger role in caring for offspring, the owl monkey and the titi monkey, males live longer than females. The effect is significant for the owl monkeys, but not for titi monkeys because of the smaller sample available for these animals.

Allman acknowledges that the results are somewhat counterintuitive: many people think that child raising is quite stressful, and if anything should shorten the life of a harried parent. But just the opposite is true.

"There's probably not one single reason that the caregiver outlives the other parent," he says. "Risk taking in males and estrogen in females are probably factors, and there may even be a beneficial hormonal or chemical change that occurs through extending care to another.

"There's evidence that greater longevity can also coincide with the taking care of an elderly parent or even a pet," he concludes.

"So it could be that taking care of others is just good for you."

The other authors of the paper are Andrea Hasenstaub, a junior majoring in mathematics and engineering at Caltech; Aaron Rosin, a former Caltech student who graduated with a degree in biology; and Roshan Kumar, another Caltech graduate, who is now a researcher at the Scripps Research Institute in La Jolla, California.

Written by Robert Tindol