A mouse that no longer shows much preference for sweet water. A fly that stops trying to climb a ledge in front of it. Seeing either, you might wonder: is this animal feeling down?
Animal studies do show that responses to rewards and self-initiated behavior can change after stress. What makes this interesting is that researchers can go beyond imagining how an animal feels. They can distinguish between what it has stopped doing and what it can still do. Those differences offer clues relevant to depression in humans.
What Can We Learn When a Mouse Stops Choosing Sweet Water?
In some rodent studies, researchers offer plain water and sugar water, then measure how much the animals drink of each. If their preference for sugar water falls after chronic stress, their response to rewards may have changed. This gives researchers a way to investigate aspects of the human experience of no longer enjoying something that used to be pleasurable.
But thirst, appetite, responses to taste, and physical health also affect how much an animal drinks. Drinking less sugar water could mean that sweetness has become less appealing, or that eating and drinking have become difficult in the first place. Those are different situations. Researchers therefore interpret the results alongside other behaviors and measures of physical condition.
This is part of both the challenge and the fascination of animal research. A person can say, “I don’t enjoy things anymore.” With a mouse, researchers have to explore that possibility cautiously through several observations. One choice of drink is not a diagnosis.
The Flies Stopped Climbing. But They Could Still Escape.
In a fruit fly study reported by Ries and colleagues in 2017, repeated vibration stress reduced self-initiated behaviors, including attempts to climb at a gap that was difficult to cross. Reactive behaviors, such as escape responses, were reportedly preserved.
Lumping all of this together as “not moving” misses an important distinction. Is the animal unable to move its body, or is it making fewer attempts on its own? That difference changes what an experiment is actually measuring.
The researchers also investigated the activity of neurons involved in serotonin signaling and changes in behavior when substances such as lithium were given. They described the condition as a “depression-like state.” Fruit flies allow researchers to examine the neural mechanisms linking stress and behavior in detail. That strength needs to be considered separately from how much the findings can explain about human illness.
The experiment does not prove that human depression is simply caused by a shortage of serotonin. Nor does it offer treatment advice suggesting that people should take the same substances. Reading it as a study of the conditions that alter fly behavior makes its findings clearer.
Why Do Researchers Say “Depression-Like”?
In human depression, assessment covers changes in mood and interest, as well as sleep, appetite, thinking, and effects on everyday life. How the person feels is also important. Animals cannot give the same kind of account, so researchers focus on particular features that they can observe.
Suppose, for example, that an animal starts avoiding others of its species. How long does that last? Have its eating and exploration changed too? Is it afraid of a particular individual? As observations accumulate, an initial impression that the animal “seems depressed” turns into more specific questions.
A paper by von Mücke-Heim and colleagues also proposes a framework for understanding a mouse’s condition in terms of its duration, impact on daily functioning, and multiple features. This is a research proposal, not an established set of diagnostic criteria that can be applied to animals in general.
What Can We Really Say About a Mole Without Soil?
What would happen to a mole in a place where it could not dig? It is an intriguing question. But the rodent and fly studies discussed here cannot, on their own, tell us whether a mole is in a depression-like state.
A useful starting point is to consider the conditions that let that animal live as it normally would. Can it get food, rest, and find somewhere to hide? Rather than looking only at reduced movement, we can ask what it can no longer do and how it responds when its environment changes. That gives us clearer questions to investigate than putting a human thought into its head, such as “I wish I had the life of the mole next door.”
For the same reason, it is difficult to line up mammals, insects, and jellyfish and draw a single boundary beyond which “animals do not get depressed.” Behaviors and ways of studying them differ across species. Having little evidence for a state is not the same as showing that it does not exist.
When Is “Low-Power Mode” a Useful Metaphor?
Calling stress-related reductions in exploration and reward-seeking a “low-power mode” makes them easier to picture. Trying things takes effort, and the animal reaches out or makes attempts less often. Focusing on this physical side can reveal things that are harder to see when we think only in terms of feelings.
But this is only a metaphor. Depression has varied causes and symptoms, and it can also involve intense agitation or restlessness. We cannot conclude that it is an energy-saving function that helps survival and should therefore be left alone.
People can also suffer from blaming themselves or repeatedly dwelling on past events. Yet describing depression as “animal low-power mode with overthinking added” would miss the complex relationships among the body, daily life, and experience. If distress or difficulties in everyday life persist, it is okay to seek help from a healthcare provider rather than trying to handle everything by changing how you think.
We Can Learn Something Without Deciding How a Fly Feels
Mice and flies do show behavioral changes after stress that researchers call “depression-like.” Those changes allow us to study shifts in responses to rewards and self-initiated behavior.
A mouse that does not choose sweet water. A fly that does not try to climb. Looking beyond first impressions raises more questions about each. Can it eat? Can it move? Does it try things on its own? Do the changes persist?
Rather than rushing to decide whether animals feel the same way humans do, there is value in testing these distinctions one by one. A simple description like “it has no energy” can conceal several mechanisms that we can investigate.
References (4)
- NIMH: Depression — https://www.nimh.nih.gov/health/publications/depression
- Krishnan & Nestler: Animal Models of Depression: Molecular Perspectives — https://pmc.ncbi.nlm.nih.gov/articles/PMC3270071/
- Ries et al.: Serotonin modulates a depression-like state in Drosophila responsive to lithium treatment — https://www.nature.com/articles/ncomms15738
- von Mücke-Heim et al.: Introducing a depression-like syndrome for translational neuropsychiatry: a plea for taxonomical validity and improved comparability between humans and mice — https://www.nature.com/articles/s41380-022-01762-w
