At Discover Magazine, Richard Pallardy offers an anecdote:

In April 1970, Ric O’Barry visited a dolphin named Kathy at the Miami Seaquarium, where she was languishing in “retirement” after three years as the title character on the television show Flipper. O’Barry, who had captured her from the wild and trained her to perform, remembers thinking that she seemed depressed. She was all alone in a concrete tank — not a good thing for a highly social animal like a dolphin. He claims the former cetacean starlet swam into his arms, sank to the bottom of the tank and refused to resurface, drowning herself.

Richard Pallardy, “Do Animals Commit Suicide?” at Discover Magazine (August 10, 2021)

There is no lack of stories along these lines. But, as a number of commentators have asked, are we really talking about “suicide” in the human sense?

For example, there was a spate of articles in the late 1800s about animal suicides, including dogs who dragged themselves to the graves of their masters to die, a cat that hung herself after her kittens died, and horses who would kill themselves after years of maltreatment. There is even a famous bridge in Scotland where dogs have been throwing themselves to their death for years.

The picture quickly gets complicated, though. For an act to be classified as a suicide, the agent must know that what it is doing will end its life. That kind of abstract thinking is probably out of the range of animals — even advanced ones.

Katherine Gammon , “Can Animals Commit Suicide?” at LiveScience (March 29, 2012)

These stories became very popular and were harnessed in support of a worthy cause:

At the time, these ideas were seized upon by animal rights groups, such as the UK-based Royal Society for the Prevention of Cruelty to Animals (RSPCA). Animal activists sought to humanise animal emotions, explains medical historian Duncan Wilson, of the University of Manchester, UK, who looked into historical cases of references to animal suicide in a 2014 research paper…

However, as medicine advanced in the 20th Century the human attitude to suicide became more clinical, and these type of “heroic portrays” of suicidal animals dwindled.

Melissa Hogenboom, “Many animals seem to kill themselves but it is not suicide” at BBC Earth (July 6, 2016) The paper referred to above is open access.

Pallardy, like the others, has some questions:

… while our knowledge of animal consciousness is ever-increasing, is the claim that certain animals not only suffer, but are aware of the concept of death and able to plan for their own sufficiently supported? We do know that animals possess varying degrees of self-awareness, experience depression, anxiety, and other mental illnesses, are capable of self-destructive behaviors, have some concept of death, at least in other animals of the same species, may even grieve, and are probably able to plan for the future in some cases.

Richard Pallardy, “Do Animals Commit Suicide?” at Discover Magazine (August 10, 2021)

Good questions. Death — as humans understand it — is an abstraction. Awareness of global reality tells us that

1) All humans (and all life forms) eventually die.

2) Therefore, I, as a human, will die someday.

3) People who die do not ever just come back and resume their former place in this world. That is true of me too.

It isn’t just a massive body of evidence that tells us this; it is abstract reasoning applied to the massive body of evidence.

Here’s a thought experiment: Suppose a pensioner has two dogs and one of them dies. He buries the lost pet in the back garden. Three days later, he comes home through the back way and finds the deceased pet running to greet him, as the other dog barks excitedly in the window. As between the dogs and the pensioner, which will be happy and excited and which will be profoundly shaken by the experience?

The elephants who try to prop up a deceased member of their clan probably don’t have anything like a human sense that their companion is dead. If they did, they would probably not be trying to prop the companion up.

Suffering animals who lose interest in living and attempt self-harm probably do not have a sense, as a human might, of “ending it all” because that idea is, in itself, an abstraction.

If animals could abstract, we humans would be in big trouble. That;s one way we know they can’t.

The grief portrayed in these very short vignettes is real but it is unclear that the animals understand what death means, in the way that humans do. Pallardy offers a useful illustration:

Another case cited in the animal suicide literature is equally subjective in its interpretation. In her 2013 book How Animals Grieve, Barbara King relates the story of a mother bear who, along with her cub, was subjected to the brutal practice of bile milking. In this procedure, which is largely carried out in Asian countries, catheters are inserted into bears in order to extract bile — traditionally believed to have medicinal properties. The mother bear supposedly broke free of her restraints, smothered her cub, and then smashed herself into a wall, ending her own life. That this animal intentionally killed her offspring and then herself in order to escape a miserable existence does not seem terribly plausible. More likely, she panicked or had been driven to a frenzied state due to the torturous conditions she had been subjected to.

Richard Pallardy, “Do Animals Commit Suicide?” at Discover Magazine (August 10, 2021)

It’s hard enough, when money and prestige are at stake, to require humane treatment of animals. Of course, deprived and mistreated animals become depressed and may engage in self-harm. That’s the problem to address. Folklore about animals that supposedly think like people does not help humane causes in the long run because sentimentality is no real help compared with practical interventions.

You may also wish to read:

In what ways are cats intelligent? Cats have nearly twice as many neurons as dogs and a bigger and more complex cerebral cortex.

In what ways are dogs intelligent? There is no human counterpart to some types of dog intelligence.

and

The real reason why only human beings speak. Language is a tool for abstract thinking—a necessary tool for abstraction—and humans are the only animals who think abstractly.