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The Siberian tiger that killed Carlos Sousa Jr., 17, and mauled two other men, brothers Paul Dhaliwal, 19, and Kulbir Dhaliwal, 23, at the San Francisco Zoo on Christmas Day, has sparked a police investigation and much speculation as to who is to blame. Authorities are still investigating how the animal escaped — recent reports indicate she could have jumped or scaled the enclosure's wall, which is nearly 4 ft. lower than the recommended standard — and whether or not the victims taunted her before the attacks.

However uncertain the preceding circumstances, the facts of the assault are clearer: Just before the zoo's closing time, the 4-year-old tiger named Tatiana escaped her pen and attacked the older of the Dhaliwal brothers, then turned on and killed Sousa, who was apparently trying to save his friend by distracting the animal. She then made her way 300 yards to the zoo café, following a trail of blood left by the first injured man who had fled with his brother. It was there she attacked her third victim, the younger Dhaliwal, and was shot dead by police officers — 20 minutes after they had received the call that the tiger was loose.

So, what exactly was Tatiana's motive? It may well be that she, despite being born into captivity and identified with a human name, was simply being a tiger — acting as any other predator would in nature. It's no surprise that tigers can be aggressive. But is it possible that Tatiana may have remembered the three men — who may have taunted her — and set out for them specifically? Was she, in other words, holding a grudge?

"That tiger could have been surrounded by 10,000 people," says Dave Salmoni, the Animal Planet network's predator expert, who spent years training big cats; but if the animal has a mission, "it will avoid all of those people and just to go to those three people." Says Salmoni, "There's nothing more focused than a tiger who wants to kill something." The thing is, though, it's not easy to prompt such enmity: "To get a tiger to want to fight you is pretty hard," says Salmoni. "Tigers don't like to fight. They hunt to kill and eat. That's it." Unlike lions, which grow up in groups and are used to sparring, tigers are solitary animals, responsible for their own food and survival, Salmoni says. They will take the risk to fight only "if they feel they have to."

The gap between Tatiana's attacks on the men at the San Francisco Zoo was relatively brief, so the word "grudge," which implies ill will that persists over time, may not be appropriate in this situation. Perhaps Tatiana's behavior would more accurately be described as a crime of passion — no grudge necessary. Still, could years of captivity have led to harbored resentment against humans, and her eventual attack?

Citing Tatiana's so-called history of violence — her assault just over a year ago on a zookeeper during a feeding — Salmoni says, "It may hold what we call a grudge on people." Tatiana wasn't put down then because the zoo director had determined that the tiger was acting as a normal tiger does.

Captive animals have acted violently before. In 2006 an orca (a.k.a. killer whale) at SeaWorld in San Diego attacked its trainer, who survived. That summer an elephant killed its handler at the Elephant Sanctuary in Tennessee. In 2004 a gorilla at the Dallas Zoo went on a rampage, injuring four people. A white tiger critically hurt illusionist Roy Horn, half of the performing duo Siegfried & Roy, at the Mirage Hotel-Casino in Las Vegas in 2003. More recently, in February 2007, a jaguar at the Denver Zoo killed a keeper. Despite these, among other dramatic attacks, some people wonder why they don't happen more often. Salmoni suggests it's because animals are actually "very forgiving," and that the stories we hear are the exceptions. So, are those exceptions evidence that animals bear grudges?

It's controversial, but some experts believe it's possible. "There's a difference between what we know anecdotally and what we can prove," says Salmoni. Most people who work with animals, he says, would agree that they act on past experience. True, what we refer to as a grudge might more accurately be characterized, in the animal world, as conditional reinforcement. "Any animal that can be trained can remember, and if you can remember, you can hold a grudge," says Salmoni. If a 6-ft.-tall man once threw rocks at a puppy, that puppy could be conditioned to believe, later in life, that another 6-ft.-tall man is a threat, and may attack him.

Elephants, whose memory is often celebrated, have also been thought by some experts to hold grudges. But "grudge" may be the wrong word — and it's not exactly a scientific term. More tenable than the notion of animals bearing grudges is the theory that they suffer stress. A 2005 paper in the journal Nature examined what some scientists called an "elephant breakdown" in Africa, and argued that elephants that had randomly attacked rhinoceroses were behaving pathologically. They were, the scientists suggested, suffering from post-traumatic stress disorder — terminology usually reserved for humans — responding to years of hardship, inflicted by people. Their population and social order had been decimated by poaching, culls and habitat loss, and the elephants, in a sense, were striking back. Neuroscientist Allan N. Schore, one of the paper's authors and a professor of psychiatry and behavioral sciences at the UCLA Medical School, demurred at calling such actions "revenge" or evidence of a "grudge" — but says the fact that elephants act out under stress suggests that their psychology may not be so different from ours.

One of Schore's co-authors, Gay Bradshaw, professor of psychology and ecology at Oregon State University and director of the Kerulos Centre for the Study of Animal Psychology and Trauma Recovery, uses the term "trans-species psychology" in her work. She acknowledges that human and animal psychology are not the same, but says they hold more similarities than we tend to think. Like Schore, she's reluctant to use the word "grudge" when it comes to animals' motivations. But she believes that animals, like humans, can suffer from post-traumatic stress disorder, of which captivity is a key trigger, and can act abnormally.

Tatiana, a zoo tiger, was not acting under "natural" conditions, Bradshaw points out, and the animal's physical and social limitations ought to be taken into account when examining her violent behavior. This is not to say the tiger might not have attacked had she been in the wild, but Bradshaw says her history of captivity can't be ignored. Like the elephants in Africa, she might have been striking back.

The science of animal sentience is far from a firm one; there's no way of knowing exactly what any animal is feeling. But it's conceivable that something in Tatiana's life, beyond her instinct, could have impelled her to attack. She may have been simply behaving like a tiger — but, perhaps, behaving like a tiger is not so psychologically distinct from behaving like a human.

• Alexandra Silver
• Did Tatiana, the tiger that killed a zoo-goer on Christmas Day, attack for no reason? Did she have a bone to pick with men who taunted her? Or could it have been stress?