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Friendship is an important part of human life. Friends can share our joys and sadnesses. We aren't alone. Many different animals also make friends.

Recently, a British research team finished an eight-year study of Indo-Pacific dolphins off the coast of Western Australia. These dolphins can use different tools to search for food. The researchers found that these dolphins are more willing to hang out with partners that like the same tool. Other factors, like family closeness or sex, have no impact on this relationship. It suggests that dolphins form social bonds based on shared interests.

Apart from dolphins, elephants and horses are also known to form friendships. Are friendships only limited to the same species?

Of course not, The Atlantic reported. In 2015,Timur, a goat, was left in the enclosure（围场） of a tiger, Amur, as a meal. But instead of eating Timur, Amur liked to play with him and got angry when others were close to his friend. That's because in captivity(圈养）, animals don't need to spend much time and energy marking their land or looking for mates the way they would in the wild. They are actually more likely to feel bored.

More interestingly, animals not only make friends but also try to keep lasting friendships. Take bats for example. In 2011, scientists found bats often hang out with a few particular friends, keeping loose ties to the rest of their group. Humans are different. "We do not work, play and live together with the same friends all the time," Swiss zoologist Gerald Kerth explained. "But nevertheless, we are able to maintain long-term despite our often highly dynamic（充满活力的）social lives."