- Animal welfare researchers reviewed the scientific literature to ask if animals can develop diagnosable clinical depression.
- They found that animals such as chronically stressed lab rats and intensively kept pigs demonstrate many symptoms of depression.
- However, no studies were able to demonstrate the co-occurrence of sufficient criteria to make a diagnosis of depression.
Across different areas of research, certain animal behaviors have been described as “depression-like.” These behaviors typically involve unusually low activity (such as the slumped postures of research primates) or reduced interest in pleasurable activities, also known as anhedonia (for instance, stressed pigs that lose interest in sweet foods).
Aileen MacLellan, a doctoral student at the University of Guelph, noticed the term “depression-like” popping up more and more in the scientific literature.
“I wanted to investigate what it means to say an animal is exhibiting ‘depression-like’ behavior,” she says. “Does it mean they are actually experiencing something consistent with clinical depression in humans?”
Diagnosing Depression in Animals
In a new paper published in the journal Behaviour, MacLellan and her colleagues tackle this question—and its ethical implications.
The starting point for the researchers was the wealth of information that decades of research on depression in humans has yielded.
Verbal self-report is necessary for some of the diagnostic criteria for depression, such as feelings of worthlessness or guilt and thoughts of suicide. However, research shows that depressed people also exhibit behavioral, physiological, and cognitive changes. These changes can be objectively assessed in other species, opening the door to comparative investigations.
A vast amount of research also supports the contention that many animals are capable of experiencing emotions and longer-term moods, and that stressors affect them in ways similar to humans. Thus, it follows that animals may have the capacity to experience depression.
But how to confirm a diagnosis? MacLellan and her colleagues looked for evidence, in animals, of the same diagnostic criteria used for humans: five or more signs of depression for a period of two weeks or longer.
Searching for Symptoms of Depression
MacLellan identified two research areas on which to focus. First, the researchers reviewed the evidence for depression in laboratory rats deliberately subjected to chronic stress, commonly used in biomedical research to "model" human depression.
Over all the studies reviewed, chronically stressed lab rats demonstrated all measurable diagnostic criteria, as well as additional signs and biomarkers of depression. In addition, some of these symptoms were reversed through treatment with antidepressants.
“We found evidence that chronically stressed rats do show signs of depression, such as anhedonia, changes in weight, and cognitive deficits,” says MacLellan.
The second area that the researchers reviewed was animals not being stressed on purpose, but still experiencing stress from living in sub-optimal conditions. This included intensively kept pigs and laboratory mice housed in barren cages. Again, MacLellan and her colleagues found signs of depression across numerous studies.
However, in both the rat models of depression and the poor welfare cases, no diagnosis could be confirmed. It is the co-occurrence of multiple symptoms that is crucial for diagnosis in humans, and no single research study has looked at enough signs of depression in any one population of animals. MacLellan and her colleagues conclude that in species other than humans, the co-occurrence of sufficient diagnostic criteria for depression remains to be demonstrated.
MacLellan would like to see other researchers investigate all of the measurable criteria for depression in one population of animals, particularly in populations that are already showing worrying evidence of depression, such as mice in barren cages or pigs in restrictive environments. Such evidence could be important for several reasons.
First, testing for the co-occurrence of signs of depression could improve the validity and specificity of animal models. Better research models are likely to help scientists learn more about human depression.
In addition, there are considerable ethical implications if conventional housing or husbandry practices are found to induce depression. MacLellan hopes that convincingly demonstrating that animals under these conditions experience depression could motivate improvements to alleviate animal suffering.
“Often, we are aware of the changes that could be made to improve animal welfare,” she says. “It’s a matter of where the tipping point is to move the marker forward. I hope that if the hypothesis that animals can experience depression is supported, it could improve the welfare of more animals.”
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MacLellan, A., Fureix, C., Polanco, A., & Mason, G. (2021). Can animals develop depression? An overview and assessment of ‘depression-like’ states, Behaviour (published online ahead of print 07 October 2021). doi: 10.1163/1568539X-bja10132.