Chimpanzees share the same instinct as humans to form close friendships based on trust, scientists have found.
A study of 15 chimps at a Kenyan sanctuary had shown a shared trait of "selectively trusting friends in costly situations", researchers said.
The findings suggest the characteristics of human friendships have a "long evolutionary history and extend to primate social bonds", they added.
The report, published in the journal Current Biology, examined the interactions of chimpanzees at Sweetwaters Sanctuary in Kenya over five months.
Scientists identified each chimp's closest "friend" by those who ate together and groomed each other and one of their "non-friends".
The researchers then played a game with each chimp, first with their "friend" and later with their "non" friend, in which they were given the option of pulling one of two ropes.
One rope allowed the chimp to eat a food that he or she was not keen on, while the other rope gave their partners access to a more tempting food and the option to share it.
The interactions between the chimps showed "much greater trust between friends than non-friends", according to the study.
The researchers said: "Chimpanzees were significantly more likely to voluntarily place resources at the disposal of a partner, and thus to choose a risky but potentially high-payoff option, when they interacted with a friend as compared to a non-friend."
Jan Engelmann, one of the scientists behind the study, said: "Human friendships do not represent an anomaly in the animal kingdom.
"Other animals, such as chimpanzees, form close and long-term emotional bonds with select individuals. These animal friendships show important parallels with close relationships in humans.
"One shared characteristic is the tendency to selectively trust friends in costly situations."
The scientists plan to investigate more possible similar characteristics between humans and chimpanzees, including whether chimps are more likely to offer help to their friends.