Why Cats Sniff Each Other’s Butts

Anyone who lives with more than one member of Felis catus knows that our beloved felines love to smell each other’s anal regions. Now a research team from the Department of Evolution and Ecology and Genome Center University of California, Davis, explains why, with their cataloging of the microbiomes of domestic cat anal glands. The bacterial members of the microbiome produce and release organic compounds that affect the behavior of another cat. The findings are published in Scientific Reports.

A microbiome is the collection of microbes that live in or on an organism. The microbiome accounts for 90 percent of a person’s cells, packed in because bacterial cells are so much smaller than ours. These microscopic residents live under our arms, between our toes and butt cheeks, in our guts and noses and spleens and eyebrows and, well, everywhere.

The new cat study compared the DNA sequences of a gene commonly used in evolutionary investigations, to identify bacterial species residing in domestic feline anal glands. The investigators also identified the “volatile organic compounds” (VOCs) that the anal glands emit, thanks to those microbes. The study evaluated anal gland emissions of several other mammals, including dogs, hyenas, foxes, pandas, and of course humans.

An Explanation from Microbiology

The Internet is full of helpful information on why, exactly, cats sniff each other’s hindquarters.

Veterinarians Ryan Llera and Lynn Buzhardt, in Why Cats Sniff Rear Ends, compare the feline practice to two people who meet, assess physical features and body language, and then quickly greet each other with an expression of recognition or affection – or ignore each other.

Cats begin a potential social encounter with hesitant head sniffing, which may progress to light head bumping as pheromones waft from facial glands. Pheromones are chemicals that trigger a social response in members of the same species. Hormones, in contrast, act within an individual. So overall, the powerful sense of smell guides a feline’s social life.

Drs. Llera and Buzhardt flesh out the response: “The aromas produced provide information to one cat what her new-found friend likes to eat and what sort of mood she is in. By simply smelling a companion, a cat can determine whether they are male or female, happy or aggressive, or healthy or ill.”

But for further intel, a cat must also check out another’s butt odors. These emanate from the openings of the paired anal glands tucked inside the rectum, and are emitted during defecation. When a relieved feline rockets around with the zoomies from a satisfying dump, we humans are usually too preoccupied laughing to notice the odoriferous microbiome-laced secretions. And even if we are paying attention, the pungent poop masks the smells from the anal glands.

The info embedded in anal scents supplements that from mere head rubbing. Will the pair of felines approach and touch, or one jump back and hiss? The scents are so personal that a pair of cats can tell if they’re already acquainted, a little like recognizing someone in a gym who routinely emits a distinctive odor of dirty socks.

Not surprisingly, a dominant cat takes the lead in sniffing the other’s butt and may hiss if displeased. A shy cat may back off and even sit to squelch the emissions, like a shy human at a party retreating to a back room. Among our four felines, it’s pretty clear who the aggressor is – Milton.

Exploring the Components of Cat Anal Gland Secretions

For those who fondly remember the functional groups of organic chemistry, the volatile organic compounds – VOCs – that spew from cat rears are alcohols, aldehydes, esters, and ketones. We can’t smell these emissions, but in cats they influence mating, aggression, and marking territory, and probably some behaviors of which we humans aren’t aware.

In the new study, Connie Rojas and colleagues used the tools of genetics (DNA sequencing), protein chemistry (mass spectrometry), and microbiology (culturing) to identify the components of the anal secretions. The material came from 23 domestic cats being cajoled and dragged to the UC Davis Veterinary Medical Teaching Hospital for innocuous, elective procedures like cleaning teeth, for which cats are sedated. Owners gave written permission for their cats to take part in the study, but the cats were not consulted. I know that Milton would not have consented to have his anal glands interrogated.

Comparing anal gases of various species revealed, perhaps not surprisingly, that dietary differences underlie the distinctions.

The domestic cat anal gland microbiome has a signature of four types (genera) of bacteria: Corynebacterium, Bacteroides, Proteus, and Lactobacillus, with a few scant others.

A dog’s anal glands share species of Bacteroides and Proteus with domestic cats, but also harbor Enterococcus.

The wild spotted hyena, with its meaty diet, houses Anaerococcus, Eubacterium, Porphyromonas, and Proprionibacterium, but shares Corynebacterium with cats. At the other end of the dietary spectrum from carnivores is the bamboo-loving giant panda. It’s anal gland microbiome shares Corynebacterium with cats, but in addition houses Pseudomonas, Porphyromonas, Psychrobacter, and Anaerococcus.

The comparative contributions of the bacterial brew also vary among cats. It’s a little like comparing soups that have the same components, but in differing amounts – say, a tomato-rich minestrone versus a more bean-based concoction or onion soup with a smidge of tomato.

Age and body size influence the nature of the domestic cat anal gland microbiome. The obese cats among the group had slightly different compositions, but the sample was too small to tease any meaning from this observation. The study also didn’t consider participants’ health other than bad teeth, diet, and the “overall living environment.”

The genetic analysis indeed showed that the bacteria living in the anal glands could be responsible for making the hundreds of released organic compounds. Presumably, the aerosol of organic compounds affects the host animal’s behavior by binding to receptors on specific cell types, such as neurons, eliciting the characteristic response – similar to the actions of hormones and pheromones.

The researchers hope to continue and expand the study to include more domestic cats as well as other feline species. It’s intriguing to think that a bunch of bacteria can control such traits as mating and mood in a large, willful, multicellular creature.

I love when chemistry explains biology.