For a long time, it didn’t occur to me that horses had friends. I’d notice that my horse was often standing near another particular horse when I’d fetch him for a ride, and I’d notice that they’d often whinny to each other as we walked off. But my play with my horse was never on his terms. I never lingered in his pasture, or played with his friends. I was very me-centric.

It is often easy to see who your horse likes most in his herd: they’ll be standing near each other whenever you look. They’ll be dining or having a drink or relaxing under a tree together. Friendships in horses may be assessed by their typical proximity to each other, or the frequency with which they groom each other, or the frequency with which they play with each other.

Researchers have learned much about the friendship choices that horses make. Best friends are familiar, and bonds develop over time. In humans and horses, familiarity rarely breeds contempt. It usually breeds comfort and friendship.

Best friends in humans and horses are also usually of the same social rank. If your horse is aggressive, his best friends in the pasture will likely also be aggressive; if he is submissive, his best friends will likely be equally submissive. The most aggressive horses of a herd will have the fewest friends, a trade-off for being first at the gate or getting the best spot at the feed.

Within a band of horses, friendships are usually strongest among family members, but kinship seems less important than familiarity in determining friendship. These principles match up with the principles of human friendship and friendships in other animals: familiarity, family, and matching on temperament and rank.

The glue that makes your horse so gregarious and sociable derives from two underlying hormones: oxytocin and cortisol. When we get together, our oxytocin levels go up, and our cortisol levels go down. When your horse is taken from his herd for a trailer or trail ride, cortisol goes up, oxytocin down. Proximity is rewarding, and separation is punishing. Evolution might have created the herding impulse with these two chemicals.

Oxytocin makes us feel good, and gives a feeling of friendship, love, deep trust. Oxytocin likely contributes to a mare’s love of her foal, your fondness of your horse, and your horse’s fondness of you. It is is released during bonding, kissing, hugging, sexual reproduction, both during and after giving birth, and during nursing. When two horse friends groom each other, or even stand side by side swatting flies, it is produced. Friendship is pleasurable. Friendships run on oxytocin.

You’ll know that love is in the air when you take your time grooming him. Never mind that mud on his legs – it doesn’t bother him. Focus on what it is that makes him lower his neck, close his eyes, tremble his chin, and groan with happiness. You’ll know it when he tries to reciprocate by burying his nose in your armpit or licking you. To have his love, love him, and let him love you back. A friendship within a species is very special. Between two species it is an extraordinary thing.

I hope you thrill your horse with your arrival each day, and break his heart when you must leave. And I hope he does the same to you.