article_snippet: 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.
article_snippet: Intimidating behaviour (B and D) may explain the monkeys' depression .
article_snippet: 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.
article_snippet: Dominant behaviour is marked with red arrows, above and depressed monkeys with blue arrows.
article_snippet: They spent less time resting on elevated perches such as shelves and hanging chains, which are favourites with healthier and more dominant monkeys.
article_snippet: The sad monkeys spent less time grooming each other and being groomed, which is a sign of sociability in healthy monkeys.
article_snippet: The sad monkeys also spent less time taking care of The sad monkeys, by scratching, for example.
article_snippet: The sad monkeys also nursed The sad monkeys infants less than healthy monkeys.
highlights_snippet: Experts from universities in China and North Carolina studied monkeys .
  
highlights_snippet: Unhappiness may be caused by bullying behaviour by dominant monkeys .
  