Chimpanzees enjoy close friendships too, and they tend to look for companions with similar personalities. 

Jorg Massen of the University of Vienna and Sonja Koski of the University of Zurich analyzed the personalities of chimpanzees from two zoos and observed how the different friendships developed, according to a news release.

The researchers made a log of which chimpanzees sat in direct body contact with each other regularly. 

"This is a clear sign of friendship among chimpanzees", Massen said.

They also identified whether or not the chimps had similar personality types.

"We found that, especially among unrelated friends, the most sociable and bold individuals preferred the company of other highly sociable and bold individuals, whereas shy and less sociable ones spent time with other similarly aloof and shy chimpanzees," Massen said.

The researchers believe this behavior is adaptive "because frequent cooperation becomes more reliable when both partners have similar [behavioral] tendencies and emotional states," the press release reported. 

The finding goes hand-in-hand with the observed "similarity effect" in humans. People often have the tendency to make friends with others who have similar traits. 

"It appears that what draws and keeps both chimpanzee and human friends together is similarity in gregariousness and boldness, suggesting that preference for self-like friends dates back to our last common ancestor," Massen said.

So why do some people make friends or even fall in love with somebody who has an opposite personality type?

"The attraction of opposites is also a very interesting phenomenon. From an evolutionary perspective this might be adaptive if these differences complement each other in complex cooperation which requires different 'expertises'. However, such complex cooperation does not exist in chimpanzees and probably also not in other animals, which might explain why such a heterophily is so far only found in humans," Massen said in an e-mail to Headlines and Global News.