People may unsuspectingly choose friends who have some DNA sequences in common with them, a new analysis finds.
Researchers compared gene variations between nearly 2,000 people who were not biologically related, and found that friends had more gene variations in common than strangers.
The study lends a possible scientific backing for the well-worn clichés, "We're just like family," or "Friends are the family you choose," the researchers said.
"Humans are unique in that we create long-term connections with people of our species," said Nicholas Christakis, a social scientist at Yale University involved in the study. "Why do we do that? Why do we make friends? Not only that, we prefer the company of people we resemble."
Humpback whales form lasting bonds, the first baleen whales known to do so. Individual female humpbacks reunite each summer to feed and swim alongside one another in the Gulf of St Lawrence, off Canada, scientists have found.
Toothed whales, such as sperm whales, associate with one another, but larger baleen whales, which filter their food, have been thought less social.