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Cincinnati Zoo: Shooting gorilla only choice, says primatologist

"I doubt that 20 adult human males, or an entire football team, would be able to hold down a male gorilla against his will. They will be pulverized," says Frans de Waal.

Public outrage has spilled across the internet after a large male gorilla was shot and killed at the Cincinnati Zoo over the weekend, when a toddler fell into the animal's enclosure.

Cincinnati police are investigating the family of the child, and a petition on Change.org called for "Justice for Harambe," the deceased ape. In its defense, the zoo has said that the situation was so dangerous for the child that it had no choice but to kill the gorilla.

Many people are throwing out opinions about how dangerous the situation was for the toddler. Even Frans de Waal, perhaps one of the world's best-known primatologists, wrote in an op-ed shortly after the incident that it was "hard to decide" whether the gorilla needed to be shot.

But in an email to CNBC, de Waal said that the zoo did what it had to do with the information available, considering the imminent risk to the child's life.

"The more I think about it, and the more evidence surfaces, the fewer viable options I see that they had," de Waal said. "The internet is very busy trying to pin blame on someone, but in the end saving the life of the child did seem to tolerate no other course of action than the one the zoo has taken. The alternatives had too many uncertainties. It is sad, but it is one of those cases where a quick decision needed to be made and there was not the luxury of wait-and-see. A gorilla is so immensely strong that even with the best of intentions (and we are not sure that Harambe had those) the child's death was a probable outcome."

So how strong are male gorillas? Thane Maynard, the director of the Cincinnati Zoo, said the primates have enough strength to crush a coconut with one hand.

Harambe weighed about 450 pounds, which is slightly larger than typical estimates for male western lowland gorillas.

Measuring animal strength, especially in human terms, is not easy, but estimates have said that Harambe's strength was anywhere between six and 15 times that of a human.

De Waal puts it this way:

"We generally say that a chimp male has 5 x human male arm strength," he wrote to CNBC. "This has proven to be hard to test, and is based on a 1920s study. But I can assure you, from what I have seen, that an adult chimpanzee can break or bend things that human strength has no effect on. Stainless steel is bent out of shape, well-attached bars are rattled loose, and so on. A gorilla male, given its much larger build, larger hands, and larger muscles, is just incomparable to us.

"I doubt that 20 adult human males, or an entire football team, would be able to hold down a male gorilla against his will," he continued. "They will be pulverized."

This isn't to say gorillas are predatory or aggressive toward humans — they are not. De Waal noted in his earlier op-ed that they are typically "peaceful vegetarians."

Others have supported the idea that the situation could have been resolved without a shooting, or have played down the danger faced by the toddler.

Julia Gallucci, a primatologist with the animal-activism group PETA, told NBC News that the gorilla's behavior suggested the animal might be trying to nurture the child or protect it.

"Gorillas have shown that they can be protective of smaller living beings and react the same way any human would to a child in danger," Gallucci said.