Learning more about depression in animals could one day benefit humans, say scientists who believe that mammals share the same basic wiring in their brain for emotions as humans do. (Although not every scientist agrees with that premise.)

In the October 5 issue of Science, Assistant Professor of Neuroscience Olivier Berton and his colleagues at the University of Pennsylvania reviewed recent studies of rodents, primates, and fish who lacked interest in their environment and their fellow animals.

We spoke with Berton about what we do—and don't—know about animal depression.

Do animals get depressed?

Depression is diagnosed in humans based on a list of symptoms that are all very subjective. Common core symptoms include feelings of guilt, thoughts of death, and loss of pleasure. Because animals can't communicate even if they have these kinds of experiences, strictly the answer is: We can't say.

What signs may indicate if an animal is depressed?

There are certain aspects of the disease that may be measured in animals. One of the core symptoms of depression is anhedonia, the decrease and loss of interest in pleasurable activities. We measure interest in food that animals like a lot or in motivation for sexual activity. We also measure how they are interacting socially with other animals in the group, and changes in sleep patterns and daytime activities. Another behavior that has been used frequently to measure animal depression is whether they readily give up when exposed to a stressful situation.

What animals seem to exhibit signs of depression?

Definitely the most convincing observations derive from nonhuman primates. Based on behavioral observation, trained observers can say a monkey looks depressed. Because their emotional behaviors are similar to that of humans, just by looking at their facial expressions or the way their gaze is directed, we can get an indication of whether an animal may be experiencing sadness.

Can you really study animals in this environment?

One problem is that many lab studies in primates and rodents are conducted in captive animals that are raised in relatively impoverished conditions compared to their natural habitat. This can cause depression-like changes. Currently there is not a lot of data available that compares animal emotional behaviors in the wild versus in laboratory setting.

How would animals deal with depression in nature?

I don't know. There are very few systematic studies of this kind. It is possible that behavioral disorders in animals in the wild may impair their chances of survival. Maybe there is a point where they cannot deal and are more easily preyed upon.

Could domestic animals be depressed?