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Humans aren't the only species that forms friendships. Lots of animals prefer to spend their time only with certain individuals. 

"It's all very well showing that you do get social relationships and social bonds forming, but why on earth would you form them in the first place?"

University of Exeter animal behavior researcher Rob Heathcote. He and his team wanted to find out what benefit animals derive¹ from close social relationships. To do it, they set out for the Caribbean island of Trinidad, home of a small freshwater fish called the Trinidadian guppy.

"These guppies live in environments that have tons of predators³ around, so basically it really sucks to be a guppy. In some places they live, you'll be watching these shoals of guppies and a predator² is attacking them every twenty or thirty seconds or so." 

One common idea is that such animals form social groups to reduce the risk of being gobbled up. But Heathcote wanted to see whether the benefits of social living might come from the strength of individual social relationships rather than simple safety in numbers.

The researchers caught 240 female Trinidadian guppies and divided them into groups of fifteen, each group in its own small pool. While some of the fish were left alone, others were exposed to what looked like a predator—a doll version of a guppy-eating fish called a pike cichlid.

Those guppies exposed to the ersatz predator were more likely to establish stronger social bonds. But the groups formed by those guppies were on average smaller than the groups formed by the fish that were not exposed to predators. [Robert J. P. Heathcote et al., Fear of predation drives stable and differentiated⁴ social relationships in guppies, in Scientific Reports]

Heathcote thinks that the tighter-knit friendships that come from small groups allow guppies to more effectively cooperate in their predator avoidance activities.

“You can either hang out with a load of guppies, and every time these predators attack, you're unlikely to be the one that gets eaten. But obviously the first thing that happens when you form a big shoal is that this attracts every single predator in the vicinity… The alternative is you can be inconspicuous in a small social groups of three [or] four individuals. Everyone's kind of looking out for each other, they've got each other's back."

So spending time with a few close friends could outweigh⁵ the benefits of blending in with the crowd, particularly in dangerous situations. 

"Friendship," said Winnie the Pooh writer A. A. Milne through his character Christopher Robin⁶, "is a very comforting thing to have." Especially when there's a big scary fish trying to eat you.

—Jason G. Goldman