An Australian animal behaviour expert says a silverback gorilla shot at a US zoo on the weekend would have perceived the screaming adults in the crowd above its enclosure, rather than the little boy who fell in, as the real threat.

Key points: Gorilla would have immediately ruled boy out as a threat, Professor Gisela Kaplan says

Gorilla would have immediately ruled boy out as a threat, Professor Gisela Kaplan says Professor described nature of male silverback gorillas as "conflict negotiators"

Professor described nature of male silverback gorillas as "conflict negotiators" Still a risk the animal may have harmed the boy, Professor says

Harambe, a 17-year-old western lowland silverback gorilla, was shot at Cincinnati Zoo on Saturday after a four-year-old boy crawled through a barrier and tumbled into its enclosure.

Animal Behaviour Professor Gisela Kaplan, from the Centre for Neuroscience and Animal Behaviour at the University of New England, said the gorilla would have immediately ruled out the boy as a threat.

"He would have known by the sheer size and demeanour of the little boy that he was not a threat, because size difference does matter in the natural world," she said.

"It would have been very obvious to the gorilla at once that it [the boy] was not a threat, so there was no need to react aggressively to it.

"Some people have argued that 'well, he dragged the boy through the water,' but I have to say that they do that with their own young to get them out of danger quickly and the danger in this case, ironically, were the screaming adults.

"There was pandemonium there and total chaos and the gorilla very likely responded to the screaming and took the boy away, around the corner, away from the screaming [to save him]."

Animal lovers responded to the killing with anger, but the director of the Cincinnati Zoo has stood by the decision to shoot Harambe, saying the ape was not simply endangering the child who fell into the enclosure, but was actually hurting him.

'They don't attack willy-nilly'

Professor Kaplan described male silverback gorillas as "gentle giants" and "conflict negotiators".

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"What makes the incident so controversial and tragic is that it's been known now, for at least 30 years, by various incidences in American and English zoos, that gorillas are very gentle and the males have one role: to keep the peace in the group," she said.

"So he is a conflict negotiator and peace maker."

Male silverback gorillas are "not aggressive" but instead "their entire personality is for peace-keeping," Professor Kaplan said.

"They are not carnivores, they don't kill other things, they don't kill other people. They don't attack willy-nilly," she said.

"What they do in the wild is, if there is a real threat perceived by the silverback male, they will charge forward as a warning and sometimes he will use branches to make a bit of a percussion sound as well. [He will] rattle the tree a bit and flay his arms around and that usually convinces [the opponent] to backtrack really quickly."

Boy falling in spelt 'doom and tragedy' for gorilla

In looking at what happened at Cincinnati Zoo, Professor Kaplan said she was reminded of a similar incident at Jersey Zoo.

"There was a boy, about the same age, who fell into the enclosure and, of course people don't have guns in England, so they didn't shoot the animal, and so the story unfolded," she said.

"The gorilla saved the boy from drowning and then cuddled and cuddled [him] and calmed the individual down … the important part of that was that it changed the view of gorillas completely. It was not the King Kong image of the dangerous, fierce animal but a gentle giant."

Professor Kaplan said that there was still a risk, even though it was small, that the animal may have caused harm to the boy.

"It's clear there is always the possibility, a 1-2 per cent possibility, that in a moment of panic and pandemonium and chaos, animals and people make decisions they would not otherwise make," she said.

"So, there remains this 1 or 2 per cent risk for the boy and, as the zookeeper said, 'we cannot take that risk' and legally they would be right. Even the 1 per cent risk would have been regarded as a risk they should have responded to.

"But the tragedy is that this gorilla probably came to help, and was shot for it.

"And if there had been any harm to the boy, in any shape or form, the silverback would have had to have been shot, so the fact that the boy fell in was doom and tragedy for the silverback and for its entire family."