Trust, philosophers say, lies at the heart of human friendships. Now, scientists report that it’s also central to friendships between chimpanzees. To find out whether chimps trust their friends more than they do nonfriends, the researchers studied a group of 15 male and female chimpanzees at a sanctuary in Kenya for 5 months. They determined which chimps were friends by observing how often pairs spent time close together grooming, touching, and eating. They then tested pairs of friends and nonfriends for trust. Two chimpanzees entered separate but facing rooms. Between the two rooms, the scientists arranged a small vehicle with two compartments, each containing three pieces of bananas and three pieces of apples (a high-reward mix that chimpanzees prefer), on a track. The chimpanzee being tested had a choice of pulling one of two ropes. If she pulled the no-trust rope, she would immediately get two pieces of banana (a low-reward). But if she pulled the trust rope, the vehicle would move along the track to her partner. That chimpanzee could then eat the fruits from one of the compartments, but not the other. And if that chimp proved trustworthy, she could pull her rope, and send the vehicle back to her friend, who could then access the fruits in the second compartment. Each subject participated in 12 trials with her friend and nonfriend. The chimpanzees were much more likely to trust their friends than their nonfriends, the team reports today in Current Biology, choosing the riskier but potentially higher-reward option—and demonstrating that trust between friends isn’t unique to humans, but has an evolutionary past.