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Why do some cats seem to get along with other cats? Their hormones offer a clue.

Cats can live solitary lives, but some don't seem to mind roommates. Researchers think hormones and the gut microbiome offer clues to feline duality.
really cute photo of a cat named Theo
Researchers studied a group of cats to try to better understand why some seem to tolerate cohabitation better than others.Elise Wrabetz / NBC News

Despite a reputation for preferring to share space on their own terms, cats often live in groups. This behavior may seem contradictory, but new research suggests it’s a curious consequence of domestication — and biological factors can help explain why some felines may embrace their fellow cats more than others.

In a study published Wednesday in the journal PLOS ONE, scientists link displays of cat behavior to hormones and the gut microbiome. These elements shape interactions among cats and may explain how they tolerate cohabitation despite their solitary nature. 

For example, the study found that cats with lower cortisol and testosterone levels were more tolerant of other cats and more willing to share food. Cats with higher levels of these hormones were less likely to interact with their fellow study participants. Meanwhile, cats with higher testosterone levels were also more likely to try to escape the room where they were observed. 

Researchers at Azabu University in Japan conducted the study. 

Although the study team hypothesized these results — cortisol and testosterone are known to influence aggressiveness — they did not expect to find that high oxytocin levels were not associated with a greater chance of being friendly or peaceful. 

“We were surprised at the results,” Hikari Koyasu, a postdoctoral fellow at Azabu University, said. “Even though a positive correlation between oxytocin and affiliative behavior has been reported in animals living in groups, results in [these] cats were the opposite. Cats with high oxytocin had less affiliative behavior with other cats.” 

In other animals, like wild chimpanzees and domesticated dogs, oxytocin surges bond groups together. However, while cats may spend time together in the same space, the study team proposed that “they might not be able to form tightly connected groups since each cat might consider the other cats out-group individuals.” 

Most wild cats live solitary lives, with the notable exception of lions. Researchers think that when the ancestors of pet cats self-domesticated, they concurrently developed the ability to live in groups without too much conflict — it was better to put up with other cats than lose the opportunity to be fed by humans. This change is reflected in hormone levels observed in this study and beyond; other researchers have found that cortisol concentrations are higher in European wildcats than in feral cats. 

This study’s oxytocin discovery shows that hormones do not necessarily act the same way across different species, said Maren Huck, a senior lecturer at the University of Derby. While an expert in cats, Huck was not a part of this new study and said she would like to see this result confirmed in further research on solitary animals. 

These findings underscore the fact that domestic cats, which descended from a solitary species, “are still not as fully social as other domestic animals who descended from group-living species,” Huck said. 

However, this doesn’t mean domestic cats always view each other as enemies. Huck described cats as “semi-social” animals that can tolerate the presence of some cats and enjoy the company of others — though they may not enjoy the company to the same degree as highly social animals, like horses. It often depends on the cat’s nature. 

Cat owners should feel free to view their cats as friends — if they accept domestic cats as having a unique way of showing friendship, said Carlo Siracusa, an associate professor of clinical animal behavior and welfare at the University of Pennsylvania who was not involved in the study. 

“Cats use proximity, but not necessarily physical contact, to show how much they like another individual — the closer, the friendlier,” Siracusa said. “It would have been interesting to know if cats [in the study] with a higher level of oxytocin spent more time in proximity of other cats, but not necessarily physically interacting with other cats.” 

The study team did note that a longer study with more cats could result in observing oxytocin levels linked to close relationships. In this case, the scientists recruited 10 male and five female cats from a shelter. The cats lived together in the same room at the shelter before the experiment. 

All of the cats were neutered, and because neutering causes testosterone levels to decline, there was no difference in levels of this hormone between the two sexes. 

The cats were divided into three groups, each housed in a room for two weeks. The rooms had more than five kitty beds, five litter boxes and constantly available food and water. During this process, the study team observed the cats’ behavior and collected urine and feces samples. These yielded hormones and gut microbes, respectively — though the team was only able to collect fecal samples from eight cats. (Koyasu said the cats were not very cooperative participants and would not use the bathroom if there were people nearby.) 

Analysis of the fecal samples they did collect suggests cats who frequently interacted with each other had more similar gut microbiomes, a finding in line with the understanding that environment and food influence the gut’s makeup. Although the researchers did not determine a relationship between specific bacterial species and examples of behavior, they reasoned that gut microbiomes must play a role in the social lives of cats because of the gut-brain axis — the two-way relationship between the brain and the digestive tract. 

If gut microbes related to friendship are identified, “we may be able to feed them to cats and form a friendly group,” Koyasu said.