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Serotonin (the neurotransmitter which most anti-depressants act on) is found in all bilateral animals including worms and insects. Therefore I think it’s likely that virtually all of these organisms can experience some form of depression/serotonin imbalance.

There is research that shows that some lobsters will lose the courage to fight other lobsters, but when they are injected with serotonin they will regain the courage to fight. Many interpret this as evidence that even organisms as simple as lobsters can become depressed. (https://doi.org/10.1073/pnas.94.11.5939)

Gotta ask, why do you care about elk?

In theory yes but I don't think we have done the experiment. That said, some of the animal models we use for researching depression would likely apply to any animal. For example the mouse forced swim test is used as a depression test. The mouse is put in water and swims till it gives up, then give it an antidepressant and that mouse will swim longer before giving up. You might say "is this really a model of depression?" Well since we can't ask animals if they are depressed things like this are the best we can do to "depress" the mouse. It may well not be a great model but stuff like this is what we have. Note they do not drown the mice, just record how long the swim before giving up, then they are taken out. I would guess you can do the same experiment with an elk, and it is quite possible the antidepressants you give it might make it swim longer. Which as far as we know, that elk gets to depressed to continue and the drugs make him less depressed and continues longer. But again, we can't ask the elk, so we can't be certain just like the mice.

As to whether animals get depression like humans we can't really say again since we can't ask them. However a lot of animals have evolved to be very anxious due to predators. You may notice deer are very skittish. This evolved for a very good reason, a non anxious deer is likely to more easily to be killed by a predator. Now is this anxiety disorder in an animal? Probably not, but they certainly seem very anxious all the time. And I am betting if we gave them benzodiazepines in the wild they would be a lot less anxious (to their determent). If I were to guess it would not surprise me if animals get depression as a condition because we humans certainly did. We evolved with it and the question is why? There are theories that depression may have been an adaptation to maintain bodily energy in a limited food environment perhaps or a consequence of some immunological factors that we needed but resulted in the side effect of depression. None of those theories are proven but they do make some arguments why depression remained with humans when you would think it would be selected against over time.

I don't know honestly I feel down a hole about animals sharing affection with one another

Thank you for your in depth response :)

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Serotonin imbalance is not the cause of depression. Also serotonin does diffrent thinfs in different species, especially if they aren't closely related to one another.

Other than that I agree that probably most animals can get some kind of depression or depression like symptoms.

I mean, wouldn't that experiment potentially actually measure how long they can physically keep going? Or do we know they could swim for longer and just don't because they give up on the psychological level?

This reminds me of a point I've heard. Basically, the primal act of fleeing a predator is stressful, but that stress is a healthy response, and in some sense they get it out of their systems and therefore don't form generalized anxiety. Supposedly, humans only started getting generalized anxiety after we no longer feared predators. On the one hand, this rings true for me. On the other, it seems like an ad hoc explanation that just sounds really good.

I had to look this up since I don't do this test. It actually isn't exactly like I stated, they aren't kept in the tank till they stop, it is a fixed period and close observation is used to determine the amount of effort to swim and escape. And apparently mice float just fine without swimming so they do not need to swim to keep their heads above water. So it is a bit more subtle experiment than I let on. I am sure there are variations in the test but it looks like this is done for 6 minutes.

Depressive symptoms (8 of the 9 on the standardized questionnaire: hard to assess suicidal thoughts in animals) can be induced in animals. Would be an astounding coincidence if the same situation that caused those symptoms in humans, didn’t also cause depressed mood.

(Situation is helplessness/ unavoidable punishment.)

Thanks you, that makes a lot of sense.

And now I wonder what a "mice swim effort rating sheet" looks like

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