Scientists are using monkeys to help explain depression in humans.
Researchers at Wake Forest university school of medicine say that depressed monkeys look and act similarly to humans, in addition to sharing a similar central nervous system.
Through brain scans using chemical trackers, scientists were able to follow the 5-HT1a receptor that binds serotonin, a chemical that is lower in depressed people.
Dr Carol Shively from the medical school said: "Brain scans of depressed female monkeys revealed the same underlying neurobiological changes that are found in the brains of depressed people.
"This is further evidence that these animal models can help researchers understand depression, develop new treatments and test their effectiveness."
Dr Shively added that monkeys had menstrual cycles, giving a "special opportunity" to study depression related to changes in hormone levels in women
Women are said to be twice more likely than men to suffer depression but Dr Shively said that this was the first time that a model of female depression in animals had been studied.
By using monkeys rather than humans, the team could also carry out brain scans on subjects that could not have been in contact with Prozac, the antidepressant drug, alcohol or illegal drugs that can alter neurotransmissions.