An estimated one in five people will suffer with depression in their lifetime. But scientists have now found that Macaque monkeys show similar signs of depression to humans, such as shying away from a group, losing their appetite and adopting a hunched posture. By studying creatures in a social situation, they hope to learn more about why humans may become depressed, which could improve treatment for the debilitating mental disorder. Scientists have found that Macaque monkeys show similar signs of depression to humans, such as shying away from a group (shown in E) and adopting a hunched posture (shown at A, B, D and E). Image C suggests bullying from dominant monkeys (marked in the pictures with red arrows) in the form of a cut to the monkey's ear. Intimidating behaviour (B and D) may explain the monkeys' depression . Previous studies have used rodents, which 'do not realistically resemble the etiopathological [causal] development of depression, which occurs naturally in a social context,’ according to the study, published in Nature. Researchers at Chongqing Medical University, China, and Wake Forest University in North Carolina, examined depression in cynomolgus macaque monkeys living in social colonies at Zhongke Experimental Animal Co. in Suzhou, eastern China. The cause of depression in some cases, appears to be down to bullying behaviour by dominant members of the colonies, such as intimidating and even attacking lower status monkeys. Each colony was made up of two males and approximately 20 females as well as their offspring, reflecting how they would live in the wild. Twenty females showing symptoms of depression, such as no interest in food, grooming or mating. Dominant behaviour is marked with red arrows, above and depressed monkeys with blue arrows. Feeding: The depressed monkeys spent less time eating and drinking than their healthy peers. Behaviour: They spent less time resting on elevated perches such as shelves and hanging chains, which are favourites with healthier and more dominant monkeys. Movement: Depressed subjects spent more time sitting on the floor and less time walking around. Grooming: The sad monkeys spent less time grooming each other and being groomed, which is a sign of sociability in healthy monkeys. They also spent less time taking care of themselves, by scratching, for example. Nursing: They also nursed their infants less than healthy monkeys. A total of 52 colonies were observed and 1,007 female monkeys screened for depression, Real Clear Science reported. Twenty females showing symptoms of depression, such as no interest in food, grooming or mating, were chosen and compared to 20 ‘happy’ monkeys and 10 that had been kept in isolation and were also thought to be depressed. The researchers found that the naturally depressed and isolated monkeys behaved in a similar way, such as ‘stereotypical prostrated and socially unresponsive posture as well as exacerbation of socially submissive behaviours,’ the study said. Naturally depressed monkeys  displayed a slumped posture, less interest in feeding and sex and diminished communication and reciprocal grooming with others. 'One to two subjects per stable social colony (17–22 subjects) manifested a depressive phenotype that may be attributed to psychosocial stress,’ the study said. The authors believe that their monkey model of depression mimics real life and is closer to how depression develops than humans (illustrated with a stock image) than rodent models . As well as behavioural changes, they also identified metabolic differences between depressed and healthy monkeys, which has also been witnessed in humans. The healthy monkeys spent longer feeding and drinking compared to their depressed peers who were not as interested in eating, for example. The authors believe that their monkey model of depression mimics real life and is closer to how depression develops in humans than rodent models. ‘This model's stable depression phenotype demonstrates superior validity to traditional rodent models of depression by better approximating the psychosocial stressors encountered by adult humans living in a resource-based competitive society and can aid further investigation into the mechanisms underlying depression,’ they wrote.
Experts from universities in China and North Carolina studied monkeys . Identified signs of depression, such as losing appetite and hunching over . Unhappiness may be caused by bullying behaviour by dominant monkeys . Studying primates in social situation may shed light on human depression .