TUESDAY, Feb. 1, 2005 (HealthDayNews) -- Two new studies suggest that our simian cousins may have more in common with us than we realize.
Female monkeys suffer from depression, especially when they're isolated, while the innate sense of fairness in chimpanzees seems to be pegged to interpersonal relationships, researchers have found.
The findings could give experts more insight into how monkeys and chimpanzees evolved separately from humans. And in the case of one monkey study, "we can study these animals and learn things that will help us understand human depression better," said co-author Carol Shively, a professor of pathology at Wake Forest Baptist School of Medicine, in Winston-Salem, N.C.
While some animals appear to suffer from depression, researchers have so far only used rats as a model for human behavior, Shively said. For example, researchers have given Prozac to rats to see if it helps motivate them to swim longer.
In her study, Shively and her colleagues studied 36 adult female cynomolgus monkeys who lived with other monkeys in groups of four. The findings appear in the April issue of the Journal of Biological Psychology.
The researchers noted that some of the monkeys were lethargic, had higher heart rates and underwent hormone disruptions -- all considered definite or possible signs of depression in humans. The depressed monkeys were also more likely to occupy a subordinate role within the monkey community hierarchy, meaning they were often isolated and subject to aggression.
Why might animals, including humans, get depressed? "We think that what happens is when the individual is faced with circumstances that they can't stand and can't change immediately, one way to deal with that is just withdraw from it," Shively said. "Eventually, those circumstances might change, and if they did, you saved yourself a lot of energy and angst."
As for humans, Shively said her research provides more evidence that depression is a "whole-body" disorder, affecting a variety of body systems. Indeed, scientists think research into the menstrual cycles of monkeys will help them understand depression in women, which often coincides with events such as menstruation, pregnancy and menopause.
In another new study, researchers studied chimp concepts of fairness by watching what chimpanzees did when their counterparts got better treatment.
Researchers at the Yerkes National Primate Research Center in Atlanta looked at two groups of chimpanzees -- one whose members had known each other for more than eight years. Researchers gave grapes to some members of the groups as a reward for work, while others got cucumbers, which chimps consider less appetizing.
The idea was to see how the chimps reacted when their counterparts got treated better for doing the exact same thing. The findings appear in the Jan. 26 online issue of the Proceedings of the Royal Academy of Sciences, Series B.
The monkeys who had known each other the longest were less likely to protest about the inequity. This appears to be similar to behavior in humans.
When you know someone very well, said study co-author Sarah Brosnan, "you don't count every favor given. You're much less likely to respond to an unequal situation. Because, in the grand scheme of things, it may not be that big of a deal." But with a stranger or acquaintance, "you're much more likely to count every favor given and received," said Brosnan, a researcher at Emory University.
The chimps in the study weren't a perfect reflection of humans, however. Among humans, someone may decide to boycott a reward -- say cookies given to half the kids in a class -- if others aren't being treated equally. The chimps, however, kept eating even if their friends got the short end of the stick.
More information
Learn more about chimps from the preservation group Chimps Inc.
