On the lush, green Japanese island of Yakushima, two species of animals have struck up an unusual relationship seemingly out of a fable.
The resident Japanese macaque monkeys and sika deer have developed a close, mutually beneficial inter-species bond.
The macaques ride around the island on the deers’ backs like miniature jockeys, grooming their soft woolly fur as they go. In turn, the deer chow down on fruit dropped by the monkeys and head to their sleeping grounds at night to feast on the macaques' poo, effectively cleaning the monkeys' homes.
And far from being a mere coincidence, it seems the two species actively give signals to one another to initiate the behaviour.
“There is a kind of observation, a kind of thinking or insight – I think that they really try to interact and to have benefits of interacting together,” says Prof Cedric Sueur, an animal behavioural complexity expert based at the University of Strasbourg. “For me, they really understand [one other].”
It’s relatively common for animals to pick up behaviours from members of their own species. Studies have shown that bumblebees can learn how to unlock puzzle boxes from other members of the swarm. Baby meerkats are known to learn how to safely eat scorpions from their parents. And whales living in a particular group develop unique dialects and inflexions in their songs.
Researchers refer to this sort of social learning as a type of ‘culture’ as it creates certain trends of behaviours among specific animal populations. But Sueur and his colleagues theorise that the behaviour of the deer and monkeys suggests that social learning is also common across, not just among, species. They call this the rise of ‘co-culture’.
“I think there are a lot of species showing co-culture,” says Sueur. “But the thing is that we never thought in this way.”
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Studying these complex interactions offers a new lens through which to explore the intricacies of animal behaviour. As culture can serve as a kind of second inheritance – like genes, it's a way to pass on information from generation to generation – the collaboration between species likely helps behaviours to evolve quicker, says Sueur.
Observations of these shared cultural relationships are growing. In Yellowstone Park during the winter, ravens follow grey wolves around to find carcasses in the woods for them to eat. In the rainforests of Costa Rica, different species of bats share roosts, forage together and team up to defend their nests from predators. In the Indian Ocean, octopuses and grouper fish have been observed forming packs and hunting together.
“The idea that nonhuman animals could have culture was taboo for a long time, and now the examples are everywhere,” says Dr Jean-Baptiste Leca, an ethologist from the University of Lethbridge in Canada.
“It makes complete sense that different species would influence each other, in how they live and how they respond to the world.”
Leca has studied how long-tail macaques have learned to barter with tourists in Indonesia. In Bali, the cheeky monkeys have learned to steal valuable objects from humans such as sunglasses, hats and smartphones, and hold on to them until they can get food in return as a token of exchange.
However, Leca says, there’s still a lot of work to be done to figure out exactly how co-culture arises.
Dorothy Fragazsy, a primatologist at the University of Georgia who specialises in studying social learning and culture in the animal kingdom, agrees. She says while the idea of co-culture seems like a natural extension of what we know about social learning in animals, it doesn't mean it's going to be easy to demonstrate it in a compelling way.
So how sophisticated and advanced can co-culture in animals get? It’s a tough question to answer. Identifying social learning within a single species is already difficult enough, let alone trying to unpick the interactions across two or more species.
“I think this is an introduction to an idea, to get people to think about it,” says Dr Michael Huffman, a primatologist who developed the co-culture theory alongside Sueur. “We want to dig in deeper and see what's in there.”
Adding this new paradigm to how we think about the animal kingdom could open up whole new worlds of how we understand their behaviour. Ultimately, it could also remind us that human behaviour might not be as unique as we’ve long imagined.
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About our experts
Prof Cedric Suer is an animal behaviour researcher based at the University of Strasbourg. His research has been published in the journals Zoological Science, The Journal of Life and Environmental Sciences and The American Journal of Primatology.
Dr Jean-Baptiste Leca is an associate professor of psychology based at the University of Lethbridge. His work has been published in the journals Springer Nature, Animal Behaviour and Scientific Reports.
Prof Dorothy Fragaszy is a primatologist based at the University of Georgia. Her studies have been published in The International Journal of Primatology, Animal Welfare Science and Animal Cogintion.
Dr Michael Huffman is an associate professor at Kyoto University's Wildlife Research Centre. His work has been published in Springer Briefs in Biology, The American Journal of Physical Anthropology and The International Journal of Primatology.