If you've ever watched an owl dive-bombed by a squadron of angry birds or the prowling neighbourhood cat get its comeuppance from the sky, then you've seen "predator mobbing" in action. Researchers working in the forests of Borneo caught that same anti-predator tactic on camera recently, but with a few exciting differences: the cat in question was the rarely seen Sunda clouded leopard, and its adversaries belonged to two different primate species that joined forces against a common foe. 

The interaction was filmed last year by a field team from the Borneo Nature Foundation (BNF) in the Sabangau Forest of Central Kalimantan, a province in the Indonesian part of Borneo. The researchers have been running a long-term study of primate behaviour in the area, but the simian show of teamwork still came as a bit of a surprise.

When a group of white-bearded gibbons detected a Sunda clouded leopard lurking among the twisting vines of the forest canopy, they naturally raised the alarm – and the noisy warning brought help from an unexpected source: a nearby group of maroon langur monkeys.

"Despite collecting over 10,000 hours of behavioural data during 12 years of research on the two primate species in the Sabangau Forest, this was the first time we have witnessed an interaction like this," notes BNF co-director Dr Susan M Cheyne. "We normally see the gibbons, the more dominant species, chase the langurs away because occasionally they do compete for the same resources."

Pestering a potentially dangerous predator with loud calls and other aggressive behaviour is designed to advertise its location, and to convince it to move away from an area or abandon its hunt. 

In this case, the gibbon-langur offensive played out over the course of more than two hours, with the males hanging out only a few metres away from the cat, and one of the gibbons even risking a tug on its tail. The females and younger animals kept their distance but continued alarm-calling loudly from a nearby tree. 

The leopard, meanwhile, held its lofty perch, but it didn't retaliate.

So why all the harassment? The researchers offer two likely explanations. It's possible the mostly nocturnal cat was merely snoozing in the branches when the gibbons noticed it, setting off the anti-predator response. But the researchers also suggest the leopard may have done something to provoke the onslaught. An infant from the gibbon group had disappeared not long before this interaction played out, leading the team to suspect the leopard may have killed it. 

"We do not know for certain if this clouded leopard predated on the infant gibbon," says Cheyne. "But if the gibbons and langurs did not work to defend themselves against this predator then we may have seen more casualties."

Whatever the exact motives, the BNF researchers agree that this rarely seen show of cooperation between two species against a common enemy is a sighting for the books. 

"Scientists often look to our primate cousins to better understand humans and our own evolution. One thing we can learn from these primates is that strength in numbers and cooperation can be a very effective strategy against a dangerous threat," says the team.

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This wildlife interaction is described in a new paper published in the Asian Primates Journal.

Top header image: Andrea Schieber/Flickr