And then life happened. When Dexter was two years old, a family member of Rebecca’s who was having health issues needed some support, so Rebecca and Dexter left the farm and set down new roots. They settled into a little cabin in the woods near Asheville, North Carolina. After just three weeks in the new house, Rebecca found herself at the veterinarian’s office asking about the possibility of Prozac for a suddenly psycho dog, who had “gone completely bonkers, literally spending his entire day barking and scratching at my walls. What on earth is wrong with him?” She feared that the recent move had triggered some latent genetic abnormality, an obsessive-compulsive disorder that caused him to endlessly run along her walls—digging, yipping, and frothing at the mouth. She wondered about neurotic shadow-chasing behavior she had read about online; did Dexter have that condition?

Things were coming to a head very quickly, and the looming cloud over their once-perfect relationship showed no signs of clearing. Rebecca reported that Dexter hadn’t eaten in forty-eight hours, and neither of them had slept in days. Dexter had no interest in going for a walk other than to relieve himself, dragging her back to the house to scamper violently along the walls once again. He wouldn’t play ball. He refused her affection. The once-perfect pair was in deep trouble.

Rebecca asked her vet for a diagnosis and a prescription to help her friend. Instead, she was given my phone number. Her vet suggested she consult with a qualified dog behavior consultant before putting him on any medication, just to be sure they weren’t missing anything.

As I sat on Rebecca's sofa in her lovely new cabin and observed Dexter’s curious behavior, I had an instant suspicion. Though Dexter did look a little nuts while he was doing it, his behavior seemed perfectly appropriate to me—if my suspicion was correct.

“I don’t think Dexter is crazy at all,” I said. “I think you have mice in your walls. If I’m right, he’s just working very hard to do his terrier job. And he’s getting really frustrated because he can’t reach the little buggers.” Rebecca’s face reflected her total surprise, embarrassment, and relief. I crossed my fingers that I was right as I handed her a referral to another specialist—my trusted exterminator. Dexter might simply be following his instinct as a dog designed to destroy rodents. Rebecca thought terriers were cute, with their wiry hair and long beards; she hadn’t considered what a terrier actually was in the first place, how he might be different from all the other dogs she had known. Suddenly, all that time Dexter had spent running around behind the haystacks in the barn in Tennessee made a lot of sense, too.