Some orangutans can be very secretive, as a research team discovered when trying to follow a wild male Bornean orangutan (Pongo pygmaeus) in a remote forest in Borneo.
They managed to locate him by following the sound of his long, bellowing calls, which advertised his presence to others nearby.
The closer they came, the more apparent it was that he was not used to having people around him, says Benjamin Buckley of the University of Cambridge in the UK. "He was acting aggressively, throwing branches and shaking trees, and soon began to flee from us."
Eventually, the orangutan calmed down and sat quietly in a tree munching on some food. He turned his back each time the team tried to see what he was eating.
"After a short while I managed to find a spot to watch him clearly," says Buckley. "I saw that he had a squirrel in his hand, which he was tugging at with his teeth, and I could hear crunching noises as he chewed."
That came as a surprise, because Bornean orangutans are thought to be virtually vegetarian. They mainly eat fruit, but also occasionally munch on leaves, flowers, bark and small insects.
The team did not witness a chase or see any fresh blood. They think that the squirrel must already have been dead before the male started eating it.
The observation is published in the journal Primates. The researchers report: "The entrails of the squirrel were dropped but every other part of the carcass was chewed and swallowed, including bones, skin, fur and tail."
Much like humans, wild orangutans can develop their own dietary preferences
It is the first time a Bornean orangutan has been seen eating any kind of meat since a project looking into their lives started in 2003. These apes have been observed for 16,000 hours since the Tropical Peatland Project began.
Despite the new observation, the scientists do not think that meat is a normal part of the orangutans' diet. "It is unlikely that a large adult male orangutan would be capable of chasing and catching a creature as agile as a squirrel," says Buckley.
It is more likely that the orangutan scavenged the squirrel opportunistically, especially considering that there was plenty of fruit to eat.
There is a second species of orangutan, which lives on the island of Sumatra. They occasionally eat meat, having been observed preying on slow lorises on five occasions since 1999. However, they only did so when fruit was scarce, which was not the case with this latest observation.
It is now clear that, much like humans, wild orangutans can develop their own dietary preferences, says Adriano Lameira of Durham University in the UK, who was part of the team that observed Sumatran orangutans eating meat.
"Such individual food preference can be passed on through generations, or spread horizontally across populations, giving raise to diet cultures," says Lameira.
Orangutans and humans share a common ancestor, which lived over 12 million years ago. The fact that both orangutan species sometimes eat meat suggests that this common ancestor did so too, at least on occasion.
"This study puts us a step further in understanding what may have been the 'ingredients' of the first food cultures to emerge in our lineage," says Lameira.
Melissa Hogenboom is BBC Earth's feature writer, she is @melissasuzanneh on twitter.
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