Macaques suffer depression in much the same way as humans, and may be our best model for dealing with clinical depression.

An alert monkey and a depressed monkey. Reprinted from Biological Psychology, Vol. 69, Shively, C. et al., “Social stress-associated depression in adult female cynomolgus monkeys (Macaca fascicularis),” p. 67-84, ©2005, with permission from Elsevier

We humans once thought we were the only animals that could use tools, until Jane Goodall discovered that chimps could, too. We thought we were the only species that could use money, but then researchers trained capuchin monkeys to trade coins for food. Now we’re learning that lower primates can get the blues, too.

A new study in the journal Biological Psychology describes a behavior in female Crab-eating macaques that closely resembles human depression. Because the dejected monkeys exhibit many of the same symptoms as people with clinical depression, scientists hope the animals will serve as a useful model for understanding the disease and developing treatments.

“Depression accounts for more work days lost than any other disease,” said Carol Shively, the study’s lead author and a professor of pathology at Wake Forest University. “It’s an important disease for us to get a handle on, but we’ve never had an animal model for depression in females or in adult primates. Now we do.”

Veterinarians and scientists who work with the monkeys have long reported anecdotal evidence of depression-like behavior, but this paper is the first to rigorously describe their symptoms. The research team spent over two years documenting the behavior and taking physical measurements of the macaques. They then compared the data from active and depressed animals.

So, how do the scientists know when a monkey is depressed?

“It’s really not rocket science,” said Shively. “Depressed monkeys do not appear responsive to potential threats. It’s that slumped, collapsed body posture accompanied by a lack of responsiveness to environmental events.”

As in humans, monkey depression is deeper than just mopey behavior. The depressed macaques had less body fat, higher levels of stress hormones, lower bone density (consistent with osteoporosis), higher cholesterol concentrations and high heart rates. They were often subordinate in the community hierarchy and socially stressed by dominant animals, Shively said.

According to the study, the validity of current animal models of depression, which rely almost entirely on male rodents, has come under question. Male and female rodents react quite differently to drug treatments, and since—in humans—women are significantly more likely than men to suffer from depression, male rodents seem totally inadequate. Macaques have far more in common with humans, both neurologically and behaviorally, offering a more sophisticated model for scientists.

Shively plans to continue work on the immediate medical applications for the monkey model, but she’s equally intrigued by the evolutionary aspect of the macaques’ depression. The fact that the disease is present in other animal species suggests that it might, at least in some situations, be an advantageous response to stressful environments.

“If it’s there, and it hasn’t been selected out, then depression either has to be neutral with respect to reproductive success or it has to be good for it in some way,” she said. “So that brings up the question of whether it could possibly be adaptive.

“It’s interesting, because it would be a whole different way to approach what depression is.”

Originally published April 6, 2006

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