Not even PETA objects to cutting the testicles off a cat. For decades, pet owners have been taking their companion animals for genital snips and other forms of ungendering, with the laudable goal of reducing the number of unwanted offspring and staving off unpleasant behavior around the house. Animal welfare organizations concur, neutered pets are less likely to escape and roam the neighborhood, get hit by a car, or scent mark the furniture. Spayed females don’t go into yowling heat or bleeding estrous, and have a reduced risk of breast cancer. A male without gonads has zero risk of the various diseases that afflict them. There’s hardly any controversy over the unsexing of America’s cats and dogs: According to an epidemiological study published in April, something like four-fifths of the former and two-thirds of the latter have been spayed or neutered. But how does it feel for the animals? Could losing its genitals make your cat a little blue?
Studies show if you remove a woman’s ovaries for medical reasons, you increase her risk of anxiety and depression. The gradual decreases in hormone levels that come with aging can cause mood swings, but going from youthful levels to neuter levels overnight seems to be even worse. The Mayo Clinic studied more than 600 Minnesotan ladies who had both ovaries surgically removed before menopause, and found they had an increased risk of being diagnosed with depression or anxiety in later life. Men whose testicles are amputated, or who receive another form of androgen deprivation therapy for the treatment of cancer, may also be at increased risk for mood disorders.
None of that should be surprising. Sex hormones are known to interact with the brain in complex ways, and estrogen and androgen receptors in the hippocampus and amygdala seem to regulate mood. Since spaying and neutering can change a pet’s behavior—that’s a big part of why we do it—it’s fair to wonder whether the surgery might also affect an animal’s mental or emotional state. Neutered people are prone to depression. Why wouldn’t a spayed cat feel the same?
The best available evidence on this question comes from mice and rats. Cheryl Frye at SUNY-Albany has been investigating sex hormone effects on the rodent brain for almost two decades. As a stand-in for humans, her group has either spayed female mice or let them age to mouse menopause, then asked, essentially, how do they “feel.” According to Frye, in the absence of sex hormones there is “a modest but consistent” increase in anxious and depressive behavior.
How do you measure anxiety in a rodent? Mice like dark spaces and corners but are also foragers, driven to explore. What’s called “anxiety-like behavior” is measured by exploiting the balance between these drives. If you put a mouse in a plus-shaped maze with two of the arms covered, how long will she spend exploring the open arms? There are other tests to show whether a mouse has "depression-like" symptoms. The gold standard, called the “forced swim,” test was developed in France in 1977, and measures how long it takes a mouse to stop trying to escape from a beaker of water. The more time it takes for the mouse to go limp, the less “depressed” she is. In a dry version called the tail suspension test, a mouse is held upside down by the tail and researchers see how long it takes for her to stop struggling. The mood-boosting effects of most commercial antidepressants were first discovered using these behavioral assays of squirming mice.
So how does unsexing surgery affect a mouse's mental state? Frye’s lab, and others, have shown that mice without ovaries often refuse to explore the plus maze, and fail the sink-or-swim test. Similar results were shown in neutered male rats, and a few studies found a biological correlate of rat and mouse depression—reduced neuronal growth in the hippocampus—in the brains of gonadectomized rodents.
Related work has been done in non-human primates. For a small study in Brain and Behavioral Research, researchers removed the ovaries of some Japanese macaques at a National Primate Research Center in Beaverton, Ore., where hundreds of females live in close contact with their mothers, grandmothers, sisters, and female cousins. Researchers reasoned these snow monkeys could model mood changes due to ovariectomy without confounding variables like the social stigma of barrenness that might affect women. The center picked 10 females of equivalent rank in the dominance hierarchies and removed the ovaries and uterus from five of them. The other five had their “tubes tied,” so were sterile but still had intact ovaries. Since the monkeys wouldn’t understand the biological ramifications of surgery, and would have similar social lives, any difference between the two groups could be attributed to ovarian hormones.
During an annual corralling of the monkeys three years after surgery, the authors noted that spayed monkeys ate and drank more, and groomed and had sex less—suggesting social impairment and stress. When researchers confronted the monkeys with a black rubber snake, four out of five ovariectomized monkeys backed away and closed their eyes. (The others touched the snake and played with it.) The scientists deemed this odd behavior a “non-adaptive” response to threat and novelty, and concluded that the presence of ovarian hormones keeps female macaques calm and socially engaged. Other research on the same Oregon snow monkey troop suggests spaying impacts serotonin levels.
Injecting estrogen or testosterone into lab animals can also affect their apparent mood or emotional state. Brief, small doses of each hormone seem to have antidepressive and anxiolytic effects in rats and aged mice, for example. If having more hormones makes a rodent act like it is on Prozac, then it stands to reason that taking them away altogether might be a downer.
Few researchers have tested this systematically in dogs and cats. In 2006, a small study of German shepherds on a Korean Air Force base did show that female dogs without ovaries were more “reactive” than sexually intact ones, meaning they were more likely to bark and growl when a test dog was walked past their kennels. Other studies report an increase in separation anxiety and noise phobias (e.g., fear of thunder or fireworks) in some dogs and shyness in cats after spaying or neutering, particularly if done at an early age.