Berlin ,Jan 15(AZINS) Friendship may not be unique to humans as chimpanzees also bond with each other based on trust, a new study suggests.

The findings suggest that friendship based on trust has evolved much earlier than previously thought.

Researchers observed the interactions of fifteen chimpanzees over a five-month period. Based on friendly interactions among chimp pairs, including grooming and eating together, the researchers identified each chimpanzee's closest 'friend' and a 'non-friend.'

The researchers from Max Planck Institute for Evolutionary Anthropology in Germany (MPG) then made the chimps to play a modified version of what is known as the human trust game, both with their friend and with their non-friend.

In the game, chimps had a choice between pulling a 'no-trust rope' and a 'trust rope.'

When the no-trust rope was pulled, the first chimp got immediate access to a food it did not like especially well.

When the trust rope was pulled instead, the other chimp got immediate access to a much more tempting food item and the option to send a treat back to the first chimp (or not).

The trust rope offered the potential for a win-win, but only if the first chimp trusted the other enough to send something back.

Each chimp played the game twelve times with its friend and another twelve times with its non-friend.

The results of those experimental interactions between the chimps showed much greater trust between friends than non-friends.

"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," said researchers.

The findings suggest that human friendship is not so unique, researchers said.

"Other animals, such as chimpanzees, form close and long-term emotional bonds with select individuals," said Jan Engelmann from MPG.

"These animal friendships show important parallels with close relationships in humans. One shared characteristic is the tendency to selectively trust friends in costly situations," he said.

The findings were published in the journal Current Biology.