Neuroscientists are discovering that spending time with others may be a basic biological necessity, like need for food or water

To our human eyes, a mouse’s furred face doesn’t betray much emotion. But if you watch the body language of a mouse who’s reunited with one of her sisters after five days in a cage alone, you might suspect you know what she’s feeling.

The formerly isolated mouse chatters in squeaks too high for a human to hear. She follows her sister, crawling beneath the other mouse’s body as if trying to get a hug. She looks like she’s feeling what you or I feel when meeting a long-lost friend or a family member — maybe with more sniffing.

She looks like she’s been lonely.

Loneliness isn’t just for humans, and neither are its harms. Over the past decade or so, some researchers have come to believe that an animal’s craving for the company of others isn’t just a preference, but a basic, deeply held need. When we don’t socialize enough, we feel the lack like hunger or thirst, they say. When we’ve had our fill of togetherness, we feel satisfied or quenched.

The amount of socializing a creature needs may be particular to that species, and even to that individual. Scientists have found within-species social differences in birds, monkeys, fish and even cockroaches.

Among humans, “you can feel lonely at a party, or you can feel fine alone in your office,” says Kay Tye, a neuroscientist at the Salk Institute for Biological Studies in California. Whatever the ideal degree of togetherness, Tye and others think that an animal’s need to balance time alone and time with others represents a kind of homeostasis: an equilibrium that’s critical for survival. Today, they are on a hunt to find where, in the brain, this equilibrium is controlled — and hoping their work will hold dividends for lonely humans.

A range of socializing

Beavers live with their immediate families. Starlings flock in huge murmurations. Adult male orangutans roam solo until it’s time to find a mate. What determines an animal’s ideal amount of socializing?

Tim Clutton-Brock, an evolutionary biologist retired from the University of Cambridge, says several factors can push species to become more or less social as they evolve. One is the need to keep warm. Another is foraging: Does searching for food in a group make it easier for that animal to eat, or harder? What about predation — is there safety in numbers, or is it better to be alone and inconspicuous? Do females need help from others to raise their young?

Different species, and even individuals within species, have different social needs. Orangutans, for example, are the most solitary of the great apes.

“Dealing with the neighbors” is also important, Clutton-Brock says. For example, the meerkats he studies in the Kalahari Desert live in territorial groups, and constant conflict means it’s better to live in packs. A wild meerkat who’s separated from the group is visibly distressed and looks around constantly. “They very clearly get extremely worried,” he says.

Within each species, Clutton-Brock says evolution has probably allowed for a range of personality types around a certain species average. “There are costs to too much anxiety” about being alone, he says, “and costs to too little anxiety.” A species may do best with a mix of social styles.

Whatever an animal’s right amount of social activity, research suggests there can be dire consequences to mental and physical health when it’s not met. People who are socially isolated, or feel lonely, die sooner. Poor social connections are linked to heart disease and stroke. Certain female rats, when housed alone, are more likely to develop cancer.

Tye started investigating loneliness well before the pandemic brought the subject to the forefront. In 2016, she showed that certain neurons in the brainstem — the deepest, oldest part of the brain — are active in male mice who are isolated for a day and then meet another mouse. When scientists inhibited those neurons, the formerly isolated mice were more standoffish; when scientists activated the neurons, the mice were more eager to seek out company.

The researchers realized they might be getting a glimpse, Tye says, of “the cellular substrate of loneliness.”

In 2019, Tye and coauthor Gillian Matthews proposed that those brainstem neurons are part of a system of social homeostasis. Like a thermostat, they theorized, a mouse’s brain senses how much company the animal has been getting, and measures that against an ideal. This ideal can also be called a set point. In the human body, for instance, the set point for temperature is around 37 degrees Celsius (98.6 degrees Fahrenheit); when we deviate from that we’ll shiver or sweat. Likewise, the researchers suggested, the mouse’s brain drives its behaviors to maintain the right balance of social activity.

The scientists hypothesized that other animals, including humans, share this system. Though it’s not easy to test such a thing in people, Tye did team up with a research group at the Massachusetts Institute of Technology for an experiment in which people sat alone in a room for 10 hours.

Afterward, subjects reported craving social interaction. When they viewed pictures of people laughing together, their brains lit up in the same region as the brains of fasting subjects who viewed pictures of food: an area, also within the brainstem, packed with dopamine neurons that are involved in cravings.

Our sense of touch may be an important part of our social thermostat.

For more evidence that this craving is part of a true homeostatic system, Catherine Dulac, a neuroscientist at Harvard University and the Howard Hughes Medical Institute, looked in another part of the brain: the hypothalamus, a deep region just above the brainstem that houses control centers for hunger, thirst and our need for sleep. It calibrates each of these basic needs using a kind of neural thermostat — or, as Dulac likes to call it, a “bean counter.”

In the case of hunger, for example, scientists have found one set of neurons within the hypothalamus that drives appetite and tells an animal to eat. A separate set of neurons drives fullness — what biologists call satiety — and tells the animal to stop eating. Dulac guessed that she’d find a similar system in the hypothalamus for loneliness, comprising two sets of neurons: “one that encodes the need” for company, she says, “and one that encodes the satiety.”

In a study published in 2025, she and her colleagues isolated adult female mice for five days. On days one, three and five, each isolated mouse got to have a 10-minute visit with her sister. Peering inside the heads of the mice undergoing these separations and reunions, the researchers saw just what they were looking for: One cluster of neurons in the hypothalamus started firing when animals were isolated, and turned off when they were reunited. A second cluster of neurons did the opposite.