There are two famous, and extremely sad, accounts of dolphin suicide. The first involves Peter the bottlenose dolphin, who is said to have killed himself after being separated from the woman he "loved" -- animal researcher Margaret Howe Lovatt, who lived with him in a specially built house in the Virgin Islands for a few months in 1965 while trying to teach Peter English.

The NASA-funded project, led by an eccentric neuroscientist, took a lot of dark turns, as explored in a new BBC documentary called "The Girl Who Talked To Dolphins." The most tawdry is that Howe says that she gave an increasingly-amorous Peter -- there's no delicate way to put this -- hand jobs to keep him focused on their work.

"It was very precious. It was very gentle," Howe says in the film."It was sexual on his part. It was not sexual on mine. Sensuous, perhaps."

When the project, perhaps inevitably, lost its funding, Peter was moved from the Dolphin House to a dreary, isolated tank in Miami, where he became increasingly depressed. Lovatt says she was eventually told that Peter died after refusing to breathe.

A second dolphin named Kathy was one of several who played Flipper, and is said to have died under somewhat similar, if less salacious, circumstances. Also living in an isolated chamber at the end of her career as an entertainer, Kathy is said to have swum into the arms of her former trainer, Ric O'Barry, before she "ceased breathing, sinking to the bottom of the tank," as New York Magazine put it in an article describing O'Barry's evolution from dolphin wrangler to anti-captivity activist.

“She was really depressed… You have to understand dolphins and whales are not air breathers like we are. Every breath they take is a conscious effort. They can end their life whenever," O'Barry told Oprah. “She swam into my arms and looked me right in the eye, took a breath and didn’t take another one. I let her go and she sank straight down on her belly to the bottom of the tank."

Both these stories raise a host of important issues about animals in captivity. They also raise the question of what it means, exactly, for a dolphin to end its own life.

Brian Palmer in Slate traced the long history of examinations into animal suicides:

Victorian scientists were particularly interested in this question, according to historian Edmund Ramsden in a 2010 article. Humane societies were eager to prove that animals experienced humanlike emotions, and animal suicides offered proof. A series of such stories began to appear in periodicals in 1845. One involved a depressed Newfoundland dog that repeatedly leapt into the water, kept its limbs still, and held his “head determinedly under water for a few minutes.” Other dogs drowned or starved themselves after losing their owners. A deer jumped from a precipice to avoid capture by hunting dogs. A duck drowned itself after the death of its mate. Scorpions were thought to sting themselves when surrounded by fire. Researchers engaged in a fierce and ultimately inconclusive debate over whether any of these behaviors should be considered suicide. (Except for the scorpions, which clearly were not attempting suicide—they’re immune to their own venom.)

Palmer concludes that in the end, it's not really possible to tell if the animals are actually killing themselves in the sense that a human might, since "suicide involves a set of higher-order cognitive abilities. It requires an awareness of one’s own existence, an ability to speculate about the future, and the knowledge that an act will result in death" -- and we don't know if animals can do all of that.

Lori Marino, a behavioral neuroscientist, dolphin expert and founder of The Kimmela Center for Animal Advocacy, thinks we do know that some animals are aware of themselves as individuals, and can plan for their intentional deaths -- plus, she told HuffPost, dolphins also have the physiology to carry their plans through.

The soft case for dolphin suicide involves a condition called "failure to thrive," Marino explains, a state in which the animals stop eating and stop socializing, until eventually "they fade away."

Failure to thrive -- which may be brought on by the stress of captivity, a marine mammologist for the Humane Society told PBS -- can be thought of as a form of suicide, Marino says.

"There are plenty of examples of a loss of a will to live," she says. "Their immune system goes down. It crashes. They die."

But it's also possible that a dolphin could decide to kill him or herself in the even more dramatic ways that Peter and Kathy are said to have done. Marino argues this possibility in a white paper shared with HuffPost.

In "Suicide in Dolphins: A Possibility?," Marino -- who's pleased that the National Aquarium may retire its dolphin exhibits -- says that dolphins have the cognitive wherewithal to plan for and execute a suicide:

...(T)he totality of the behavioral evidence converges on the conclusion that dolphins possess a) a sense of self, b) the cognitive ability to think about oneself, c) and the ability to think about and plan for the future. All of these capacities underwrite the capacity for such complex intentional actions as suicide.

On top of this, Marino writes, dolphins have the physical ability to commit suicide by voluntarily holding their breaths until they die. And, she says, their brains are complex enough to allow for a "sophisticated capacity for emotion and the kinds of thinking processes that would be involved in complex motivational states, such as those that accompany thoughts of suicide."

But even if this theory is right, that doesn't mean dolphin suicide has actually happened; Marino says while Peter and Kathy's circumstances certainly caused them to suffer, we don't know for sure if either dolphin did, in fact, intentionally die.

"I don't think anyone knows the answer to that," she says. "I think what this does is take us back to the real question: Is it really ethical to put someone in that situation? [Peter and Kathy] should not have been living like that."

Speaking of living like that: In a strange coda to an entirely disquieting story, Lovatt ended up moving back into the house she'd shared with Peter, after marrying the photographer who documented her relationship with that unusual dolphin. She and her husband raised their three daughters in the Dolphin House, which was converted to suit the needs of an all-human household.