Mac, the miniature donkey, can be kind of a jerk. He bats his eyelashes, angles his long furred ears toward you, flatteringly, like TV antennas, and pushes his belly up against your thighs. Then, just as you’ve grown comfortable with his small, stocky presence, his burro smell of sagebrush and sweet alfalfa, something dark and confusing stirs within him. He stiffens, whips his head back, and bites down hard on the bony part of your shin and doesn’t let go. Or he rears to stamp his hooves on your toes, or he kicks his back legs like sharp springs in the direction of your kneecaps or into your actual kneecaps. If this wasn’t painful, it would be funny. Mac is, after all, the size of a goat. But because you can’t predict when it will happen, he is also a little scary. Mac shifts so suddenly from being affectionate and needy to violent and aggressive, transformations that don’t seem to be triggered by anything in particular, that some people have taken to calling him “schizo donkey.”

I am not one of these people. But I believe that he’s disturbed. This, however, is not Mac’s fault. Not entirely anyway. His mother, a stoic Sardinian miniature donkey, lived on the ranch where I grew up. She died within days of giving birth to Mac, and he was given to me to raise. I was twelve years old and saw this tiny donkey as a living stuffed toy. I spent hours bottle-feeding him and playing with him, until I got distracted by Anne of Green Gables books and my seventh-grade crush, a tan boy who skateboarded behind the local McDonald’s. Mac was weaned too quickly, exiled to a corral without a donkey mother to show him the ropes—a small, unself-confident creature among indifferent adults. Another donkey may have been fine, but Mac wasn’t another donkey. Eventually he began to turn his attacks on himself, biting his own fur off in chunks when he became frustrated or erupting in violent outbursts against people and other animals, outbursts that kept him from receiving the affection he also seemed to crave. Now, more than twenty years later, I know that Mac’s experience and the disturbing behavior that resulted from it, is far from unique.

Humans aren’t the only animals to suffer from emotional Thunderstorms that make our lives more difficult, and sometimes impossible. Like Charles Darwin, who came to this realization more than a century ago, I believe that nonhuman animals can suffer from mental illnesses that are quite similar to human disorders. I was convinced by the experiences of many creatures I came to know, from Mac to a series of Asian elephants, but none more persuasively than a Bernese Mountain Dog named Oliver that my husband and I adopted. Oliver’s extreme fear, anxiety, and compulsions cracked open my world and prompted me to investigate whether other animals could be mentally ill. This book is the tale of what I found: the story of my own struggle to help Oliver and the journey it inspired, a search to understand what identifying insanity in other animals might tell us about ourselves.