“What were the secrets of the animal’s likeness with, and unlikeness from man?” John Berger pondered in his influential meditation on our relationship with animals. “The secrets whose existence man recognized as soon as he intercepted an animal’s look.” And yet for all the progress we’ve made, for all the advances afforded us by pioneering animal scientists like Jane Goodall, we still struggle to understand — or, in some cases, even acknowledge — the inner lives and emotional realities of our fellow non-human beings. Despite what every pet-parent sees with absolute clarity in watching, say, her dog whimper with agonizing anxiety or greet a friend with exquisite elation, the question of animal emotionality is still, perplexingly, something of a taboo.

In The Depths: The Evolutionary Origins of the Depression Epidemic (public library) — his fascinating exploration of how mood science illuminates “the unaddressed business of filling our souls” — psychologist Jonathan Rottenberg addresses this paradox:

Depression in animals has long been a hard sell. In the wake of René Descartes, an enormous gulf opened between humans and other species, and Cartesian thinkers ever since have argued that other animals are mere automata, furry robots. Skepticism about complex inner states in other species has endured even into the twenty-first century. The torch has been passed from behaviorists, who wanted to banish all notions of motivation from scientific purview, to contemporary neuroscientists, who accepted basic motivational drives but not anything as elusive as animal feelings, and finally to cultural psychologists, who have no place for animal depression, but for different reasons. For them, depression is a shared understanding, a historical artifact defined by human words and deeds.

Mood science seeks to refute these views… Our fellow mammals, be they rats, cats, or bats, provide the most compelling and dramatic evidence for depression in the animal kingdom. High and low moods equip these animals to track opportunities and resources in their environments; the capacity for mood is essential for guiding behavior in a changing world.

Much like the human version, Rottenberg argues that depression in animals spans the full spectrum of severity, from brief and shallow periods of low mood to long and intense stretches of depression. Animals also experience the same hormonal changes that depressed humans do, including higher secretion of steroid hormones and dampened immune system function. Perhaps most interestingly and indicatively, the body clocks of depressed animals — their circadian rhythms, which we already know are of tremendous importance to human well-being — are so disrupted that they produce the same irregularities in body temperature and sleep-wake cycle seen in depressed humans. Rottenberg adds:

Beyond the official symptoms of human depression, dogs and cats manifest numerous unofficial signs that are characteristic of depressed humans. Those who live with them know that reduced exploratory behavior, long hours hiding under the bed, and reduced interest in self-care and personal hygiene, reflected in less grooming or use of a litter box, are all signs that something is amiss.

In a heartbreaking illustration of my longtime lament that there is no nuance in news today, Rottenberg points out a particularly ungenerous and gratuitously one-note instance of how the popular media tends to treat what’s clearly a complex subject:

Psychiatric problems in small animals are often trivialized, so it is easy for pet depression to fly under the radar. Fortune Magazine mocked Eli Lilly’s decision to pursue FDA approval of a chewable Prozac for pets as the second dumbest moment in business of 2007, writing, “Thank God. We’ve been so worried since Lucky dyed his hair jet black and started listening to the Smiths.”

Understanding non-human depression, Rottenberg reminds us, isn’t just a matter of compassion but might also hold important keys to better understanding, and treating, human depression, which is what he explores further in the altogether fantastic The Depths. Sample it further here.