We can understand jet lag as a physiological shift in circadian rhythms. Neuroendocrinologists have developed a learned helplessness laboratory animal model of depression. Rats are submitted to a situation from which they can either escape, or that they can accept. Others give up and simply lie down. The latter animals have all the typical biochemical markers of depression.
Chital deer are from India, where their natural predator is the tiger.
When attacked, the animal has the choice to either stay and fight or run. Naturally the deer choose to run. But transport the Chital deer into a captive situation like a fenced-in farmyard, and because they can't flee from what they see as a danger, they stand still in a group in a corner of the yard. They appear calm, yet if the threat remains for 15 minutes, one or two of the deer in the group will simply lie down and die. Autopsy shows diffuse internal bleeding.