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The barbary macaque found itself in exactly the wrong place after escaping from another enclosure to climb over into the lions' den.
The big cats quickly spotted the intruder into their territory, and sprang into action.
Those present at the Blair Drummond Safari Park said that the screams of the monkey could be heard all around the park as the lions ruthlessly killed it in plain sight of the visitors.
An eyewitness told The Daily Record: "It was a large monkey and surprisingly fast - but it didn't stand a chance.
"The awful sound of the monkey screaming for its life will stay with us.
"We had hoped for a happy day out with our young child but ended up having to explain to a toddler what had happened."
Well, nature can be a bit like that out in the real world, to be fair.
After a while, one of the keepers from the park arrived to attempt to hide the body of the macaque from the public by parking a truck over it.
Undeterred, one of the lions grabbed it from underneath the vehicle and ran away with it in its mouth.
The staff eventually managed to get the monkey's body back.
A spokesperson for the park said: "We are sorry our visitors witnessed what must have been a distressing sight.
"The incident in the lion enclosure has never happened before and we share the shock and concern that this took place."
The Blair Drummond Safari Park is situated near Stirling and is one of the country's busiest and best-loved attractions.
Barbary macaques are currently listed as threatened and have been listed on the International Union for the Conservation of Nature's Red List since 2008.
The main threats to the monkeys - which are usually found in North African, although a population remains in Gibraltar - are degradation of habitat, poaching and the illegal pet trade.
Some barbary macaques have also been killed in retaliation for raiding the crops cultivated by farmers.
They are the only species of primate - apart from humans, obviously - to live freely in Europe, with around 230 living on the Rock of Gibraltar.
While that population is stable or even increasing, they are in decline across their traditional African habitats.
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