Well, this is nothing short of terrifying. Late last week, footage was released of an incident at the Henry Doorly Zoo & Aquarium in the US of a male silverback gorilla being accidentally provoked by a little girl engaging in some classic 'chest-beating' behaviour. The gorilla proceeds to rush at the crowd of spectators and use the full force of its weight to crack the window of its glass enclosure.

Named Kijito, this 20-year-old male western lowland gorilla (Gorilla gorilla gorilla), weighs an impressive 170 kg (375 pounds).

Hopefully the little girl wasn't too traumatised by the incident, because it can actually teach us a couple of really fascinating and valuable things about captive gorillas. Firstly, no one was ever actually in any real danger, because that glass window is three half-inch layers thick, and Kijito was only able to break through one of them.

"The zoo said the glass on the exhibit is engineered to account for the size, strength and speed of a large male gorilla," local news station KETV Omaha reports. "The glass has three layers, and each layer alone can withstand a gorilla's force. While one layer of glass did crack, leaving two layers untouched, the exhibit remains open."

Secondly, while it's easy to assume that the gorilla was retaliating against the little girl, it's more likely that it was showing off to the other gorilla you can see in the footage, says Henry Doorly Zoo & Aquarium zookeeper, Dan Cassidy. "Although the gorilla may have been engaging with the public to receive a reaction, the gorilla was most likely exhibiting typical gorilla behaviour and likely posturing toward another male gorilla," he told KETV. "It is common for male gorillas to display to each other, and occasionally they use the glass because of the noise it makes on their side."

Cassidy adds that the gorillas aren't forced to be on display and be exposed to the public and their insensitive chest-beating all the time - they have the choice to go somewhere more private in their enclosure if they wish.

Chest-beating might be the most well-known aggressive gorilla behaviour, but it's certainly not the most commonly used. Gorilla intraspecies communication is incredibly complex, involving 25 distinct vocalisations, ranging from grunts, belches and barks to ear-splitting screams and roars, and and a nine-step threat display that aims to settle territorial disputes in a non-violent manner.

According to the book, Where War Lives: A Journey Into the Heart of War, by Canadian-born photojournalist Paul Watson, this ritual plays out like so:

"A mountain gorilla normally goes through nine escalating steps before violence becomes a final option. He starts by hooting, slowly, then faster. The he performs a feeding ritual. If he needs to say more, he rises up on two feet, throws vegetation around, and pounds his chest with cupped hands. If the matter is still unresolved, he kicks the air with one leg, then runs sideways, thrashes at the vegetation, and finally, thumps the ground with his palms."

You'd really want to back yourself if you were on the receiving end of all that from 170 kilograms of sheer muscle. 

Source: KETV