Depression in humans is diagnosed based on a list of subjective symptoms like feeling guilty all the time, loss of interest and pleasure derived from once pleasurable activities, or contemplation of one’s death. Many scientists believe non-human animals experience depression too, but since they can’t speak, it’s often very difficult to diagnose depression. Instead, they have to rely on observations of behavior and apparent mood. So, when talking about the ups and downs of animals, researchers don’t like to use the word ‘depression’ but rather ‘depression-like behavior’.

One of the core symptoms of depression is anhedonia, the decrease or loss of interest in pleasurable activities. Scientists often look for anhedonia in animals to spot depression patterns by measuring interest in the food they like a lot or their sex drive. Changes in sleep and wake patterns, how often animals interact with their social circle, or whether they readily give up when faced with a stressful situation are some of the other makers used to diagnose depression in animals.

Olivier Berton, an assistant professor of neuroscience in psychiatry at the University of Pennsylvania, reviewed studies of rodents, primates, and fish who lacked interest in, well, life. His own work with rodents suggests rats that are excluded from their social group stop exercising or eating.

While these rodents would have normally gone through just about anything to push a lever that displaced a sweet treat or solve a maze, depressed-like rodents couldn’t care less.

“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,” Berton said in an interview.

Depression-like behavior has been most extensively studied in cats and dogs, the most common pets. Nowadays, veterinarians seem to agree that animals can get depressed and even prescribe Prozac-like medication that improves their mood.

It wasn’t always like this, though. In fact, up until the 1980s, many veterinarians thought the idea that a dog could become depressed was silly. It wasn’t until a young doctor called Nicholas Dodman of the School of Veterinary Medicine at Tufts University got into the picture that this changed. A brilliant graduate of Scotland’s Veterinary School at Glasgow University, Dodman emigrated to the United States in 1981 and soon changed his interest from the general field of veterinary medicine to animal behavior and behavioral pharmacology.

His studies led him to believe that some of the dogs’ behavior was similar to human psychological states. Extrapolating from what he knew about human behavioral symptoms, Dodman concluded that dogs could become depressed and anxious. His ideas, however, were met with scrutiny from his fellow colleagues who argued: “Dogs don’t experience the same mental states and emotions that people do.”

This difference of opinion can be traced back to two trains of thought. René Descartes, a French philosopher, mathematician, and biologist claimed that only humans have feelings and conscious mental processes, and this sort of stuck for centuries. When Charles Darwin, the proponent of the theory of evolution, came into the picture he suggested the emotional experiences of animals are quite similar to those of humans.

Dodman clearly sided with Darwin. His logic told him that seeing how dogs and humans share similar brain structures, as well as brain chemistry, then there’s no reason to believe that dogs can’t get depressed. For instance, depression in humans is known to be partly caused by hormonal and chemical changes, which can be reverted to a degree with antidepressant drugs. So, Dodman gave dogs antidepressants and saw improvements in mood.

Slowly, veterinarians caught on, and drug manufacturers started making antidepressants specially designed for dogs. It’s a billion-dollar business now.

But while many veterinarians say cats and dogs can become depressed, scientists are more reserved claiming we can’t know for sure before extensive studies are performed.

What studies seem to suggest thus far, though, is that pets like dogs and cats help improve depression symptoms in humans. Having a dog for a pet promotes physical activity and meaningful emotional connections, lowers blood pressure, and makes us responsible. Most of all, dogs offer unconditional love which goes a long way for most people having a tough break in their lives.

“Pets offer an unconditional love that can be very helpful to people with depression,” says Ian Cook, MD, a psychiatrist and director of the Depression Research and Clinic Program at UCLA.