A mouse that chooses sugar water less often, and a fly that stops trying to cross a gap. Add a caption saying “They’ve lost their motivation” to footage of the experiment, and it might feel surprisingly convincing. But those words did not come from the animals.
So what clues lead researchers to describe a state as “depression-like”? What makes this work interesting is that they do more than watch whether an animal seems lively. They examine behaviors such as eating, moving, and trying something of its own accord separately. Two animals may stay in the same spot for very different reasons.
What Is Happening When a Mouse Stops Choosing Sugar Water?
One test used in depression research with rodents measures how much plain water and sugar water the animals choose to drink. A reduced preference for sugar water serves as an indicator that their response to reward may have weakened. The idea is to use behavior to study one aspect of a human symptom: no longer enjoying things that used to be enjoyable.[1]
But imagine leaving a favorite food untouched, even when it is right in front of you. Perhaps it has lost its appeal. Or perhaps you are full. With a drink, thirst and how easy it is to drink also matter. A single result leaves plenty of conditions to check.
That is why there is still room for investigation between the observation “The animal did not choose sugar water” and the interpretation “The animal no longer feels pleasure.” Researchers narrow the question by examining other behaviors, the animal’s physical condition, and the conditions being compared. It would be convenient if a choice of drink let us read an animal’s mind. Experiments begin by questioning that seemingly simple interpretation.
Can a Fly That Stops Trying to Climb Still Escape?
In a study published in 2017, Ries and colleagues repeatedly exposed male fruit flies to vibration stress. After three days, the flies made fewer attempts to climb at a gap that was difficult to cross. Yet their ability to respond to a stimulus with escape-like behavior remained intact. The authors called this a “depression-like state.”[2]
That combination matters. If all we know is that an animal “doesn’t move,” it is hard to tell whether it cannot move its legs, is tired, or is less likely to try something of its own accord. Comparing what decreases with what remains intact helps clarify the phenomenon under study.
The same study also examined the activity of neurons involved in serotonin signaling and behavioral changes after substances including lithium were given to the flies. One strength of working with these tiny animals is that researchers can experimentally investigate mechanisms linking stress and behavior.[2]
However, what was measured here was the behavior of flies under particular conditions. This does not justify concluding that human depression can be explained by a shortage of a single substance. Nor is it a reason for people to try medicines or supplements on their own. Producing a change in flies and identifying a safe, effective treatment for humans require different kinds of evidence.
Why the “-like” in “Depression-like” Matters
When assessing depression in humans, clinicians consider changes in mood and interest, along with sleep, appetite, patterns of thought, and effects on daily life. What a person says about their experience is also an important clue.[3] Animals cannot be interviewed in the same way, so researchers select observable features to compare.
Suppose, for example, that one animal stops approaching others. Does it avoid them even when the other animal changes? Have its eating and exploration changed too? Is it back to its usual behavior the next day? Questions like these turn the impression “It looks lonely” into things that can be investigated.
In a paper published online in 2022, von Mücke-Heim and colleagues proposed a framework for understanding states in mice by considering their duration, effects on everyday functioning, and multiple features. This is a proposal intended to make studies easier to compare, not an established set of diagnostic criteria for animals in general.[1] The term “depression-like” allows researchers to investigate similarities while leaving room for what is still unknown.
What Would You Ask First About a Mole With No Soil to Dig In?
“Does a mole become depressed if it lives somewhere it cannot dig?” It is a tempting question. But the studies of mice and flies discussed so far cannot tell us what state that mole is in. From here, the mole is a hypothetical example to help us think about how to observe an animal.
Before giving it an inner monologue about how “The mole next door has a better patch of land,” try asking different questions. Can it get food? Can it rest? Can it find somewhere to hide? This takes us beyond imagining what is in its mind and makes the living conditions we want to check more specific. This is about the order in which we consider explanations, not instructions for keeping a mole.
For the same reason, it is too soon to line up mammals, insects, and jellyfish and draw a boundary beyond which “Animals don’t get depressed.” Different species have different behaviors and require different ways of measuring them. We cannot go beyond the scope of these studies to rank species by whether they have feelings. It is important not to confuse lacking a way to investigate something with that phenomenon not existing.
How Useful Is the “Low-Power Mode” Metaphor?
Calling a decrease in self-initiated exploration or attempts to obtain something a “low-power mode” makes it easy to picture. Think of being able to launch a game but not to keep playing as you normally would. An example like this draws attention to differences that cannot be fully explained by asking only whether someone can move.
But this phrase is neither a diagnosis nor an explanation of the causes. Human depression can also involve intense agitation or restlessness.[3] Observing a reduction in activity does not, by itself, justify concluding that it is “an energy-saving function that evolved for survival” or that “It will get better if left alone.”
When people are struggling, they may add layers of words about themselves: “I couldn’t do it again,” or “It will always be like this.” Even so, describing depression as “an animal’s low-power mode plus overthinking” would leave out differences in physical condition, experience, and environment. Animal research gives us more ways to make comparisons. It is not a means of dismissing human suffering in a single line.
How to Explain That You Haven’t Been Feeling Like Yourself
If reading this article has made you concerned about how you are doing, putting actual changes into words is more useful when seeking help than trying to diagnose yourself from a fly experiment. Beyond saying “My mood is low,” examples might include “It’s hard to prepare meals,” “I wake up during the night,” or “I can’t enjoy things I used to like.” These are examples to help explain difficulties, not a diagnostic checklist.
If distress or difficulties with daily life persist, or if you are already struggling, it is reasonable to seek help from a healthcare professional. Treatments for depression include psychotherapy and medication, and the support people need varies.[3] You do not have to carry the burden of solving it alone by telling yourself to “Find a different patch of soil.”
Ask One More Question Before Deciding How an Animal Feels
Studies of mice and flies let us look more closely at what a broad phrase like “low spirits” might contain. Is the animal not drinking? Is it no longer trying things of its own accord? Can it still respond to stimuli? Does it return to its usual routine? The more features we observe, the more differences emerge that an initial impression could not reveal.
The next time you see a headline saying “Flies Get Depressed Too,” try asking one more question: “What did the researchers measure, and what changed?” There are fascinating mechanisms to explore without deciding that we already know what an animal feels inside.
References (3)
- [1] 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 (published online in 2022; assigned to a 2023 journal issue). A proposal for an assessment framework in mouse research. https://www.nature.com/articles/s41380-022-01762-w
- [2] Ries et al. (2017): Serotonin modulates a depression-like state in Drosophila responsive to lithium treatment. Original research using fruit flies. https://www.nature.com/articles/ncomms15738
- [3] NIMH: Depression. Official information on symptoms of human depression, seeking help, and treatment. https://www.nimh.nih.gov/health/publications/depression
