Carol Shively and her colleagues divided 36 monkeys into groups of four, and watched how the animals reacted as a social pecking order evolved. Lower-ranking females were more likely to become depressed.

The researchers found: the animals slouched around staring at the floor and lost all interest in their environment. They were also more likely to die prematurely. Five out of the nine most depressed animals died before the end of the experiment.

The most depressed monkeys had earlier lost body fat, developed higher heart rates and become less active. Their blood accumulated fatty components linked with heart disease, they had increased bone loss and their stress hormones were disrupted. Levels of oestrogen and progesterone both dipped, indicating impaired ovarian function. “That’s five or six major systems in the body that are dysregulated by depression,” Shively told New Scientist. We tend to think of depression as purely psychological, but there is more to it than that. “Depression really is a whole-body effect,” she says.

Shively thinks the swings in sex hormone levels could be a vital link between depression and heart disease. Low oestrogen levels have in the past been linked to increased risk of vascular disease in humans and monkeys.

Despite the hormonal changes, the animals’ menstrual cycles remained regular. “This suggests that depressed women may have low ovarian function that goes unnoticed because they still have menstrual periods,” Shively says. If this turns out to be true, it could help explain the known link between depression and coronary artery disease, she says.

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