Birds of a feather flock together.

Opposites attract.

These are two proverbs we are all familiar with. When it comes to , we are keen on the former, but when it comes to love, it is the latter we hold dear. This got me thinking: Do animals make a similar distinction depending on whether they are looking for a best friend or for that special someone?

The answers tell us a lot about how we make these judgments as well.

First, let’s establish that animals do form friendships with each other. Like humans, sometimes these friendships are fleeting, and other times they are stable over many years. Although there are many benefits to having friends, it takes time and energy to maintain these relationships. If you are human, that may involve late-night phone calls comforting a friend after a breakup. If you are a chimpanzee, you may spend time digging bugs and dirt out of your pal’s hair.

But, human or chimp, how does one actually decide who to be friends with? How alike you are may be the key, whether it is age, , social status (high school, anyone?), or even . A recent study looking at what factors predicted friendships between pairs of chimpanzees revealed that individuals with similar personalities were more likely to be friends. Massen and colleagues report (in Evolution and Human Behaviour) that chimpanzees were buddies with each other, while shyer types also stuck together. For chacma baboons, touchy-feely types tended to pair up while the more aloof become friends—without all the touching and feeling, of course.

Regardless of whether you are a human, chimpanzee, baboon, or even a meerkat, one thing is for certain: We become friends with others similar to ourselves.

Surely, then, when it comes to love opposites attract, right?

Wrong.

Barnacle geese prefer to mate with individuals that are similar in size, while in great tit birds, having a similar personality makes all the difference. But this trend toward finding someone like you isn’t just for the birds—it's for us, too. Despite the common belief that opposites attract, the data prove otherwise and show that married couples are tend to be similar to each other on a variety of traits.

Until recently though, scientist weren’t sure if couples start out that way or simply become more alike as time goes on; however, a study published last month in the Journal of Evolutionary Biology is shedding new light on this chicken-and-egg question. In it, researchers found that people in a rural Senegalese village choose to marry someone who is as cooperative and helpful to others as they themselves are.

Maybe, as in the animal world, you can’t go wrong pairing up with people more like yourself, whether it’s in size, shape, personality, or some other trait that matters to you, such as being thrifty, patient, or funny.

When Plato wrote, “Similarity begets friendship,” did he forget about love? I don’t think so. Perhaps that explains why friendship often is the best place to start when looking for a romantic partner.