Monkeys use researchers 'as human shields' to avoid leopards and big cats in the wild

Scientists found that South African Samango monkeys were happier eating on the ground if humans were around

The Samango monkeys of South Africa usually have a good reason not to stray too far from the forest. Although they spend much of their time loping through the trees they know to keep within a certain range: climb too high and they're targets for eagles, too low and they could be a big cat's lunch.

However, it seems there is an exception to this behaviour - and that’s when people are around. A new study from the journal of Behavioural Ecology reports that Samango monkeys under observation by scientists use the researchers  as “human shields”, counting on their presence to avoid being picked off by leopards.

Scientists found that when they set up feeding stations at different levels in the forests (some on the ground, some mid-way up trees, and others higher up) the monkeys not only ate more when they were being watched but also favoured the ground-based stations, suggesting that they did not see humans as a reliable defence against airborne threats.

Katarzyna Nowak, a junior fellow at Durham University and the lead author on the paper, told The Independent that when she started studying the monkeys they showed a ‘vertical axis of fear’: "The amount of food monkeys depleted from buckets over the course of the experiment varied with height, with the most food left uneaten at ground level, where there is a risk of predation by leopards and caracals."

A leopard in Yala National Park in Colombo. Image credit: Reuters

But, she explains, "when a human observer was following monkeys, they ate more food at every height, with the most notable differences at the bottom two levels. Animals are expected to deplete more food where and when they feel safe [but] we had not expected human followers to have such strong effect!"

Nowak explains that the monkeys most likely viewed the researchers “as human shields” with their presence passively deterring leopards and other terrestrial predators from entering the area. Big cats are generally hunted in the region where the researchers were studying as they are thought to prey on livestock.

For Nowak, a research associate at Qwaqwa's University of the Free State, the experiment doesn’t only show the intelligence of the monkeys, it also calls into question the concept of ‘habituated’ wildlife – the idea that some animals are so used to human presence that they act as if they weren’t around.

The research builds upon work field studies by Joel Berger, which suggests that just as with quantum mechanics there's an observer effect in play: the act of watching something changes the thing itself.

"Many studies of behaviour rely on the habituation process and we often assume that animals' behaviour becomes independent of our presence," says Nowak. "If observer presence can alter the way in which animals forage and use space this has obvious implications for observer-led studies of vigilance, trade-offs and predator-prey interactions."

She adds that this observer effect isn't always beneficial for animals. In studies carried out in Gombe National Park researchers found that carnivorous chimpanzees took advantage of the fact that a prey species of theirs - the red colobus monkey - was easily panicked by scientists and used this knowledge to 'flush' out and kill the smaller monkeys.

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