Twice a week for five weeks, researchers at the Southwest National Primate Research Center carried marmosets away from their mates for four hours at a time. The monkeys had no way of knowing if, or when, they were coming back.
Common marmosets are small, tree-dwelling monkeys from Brazil that form long-term pair bonds and raise their young together, much like humans do. That makes them one of the best animal models for studying what happens when a social bond is broken. Behavioral endocrinologist Aaryn Mustoe and his team at the Texas Biomedical Research Institute tracked the stress hormone cortisol in 14 bonded marmoset pairs during repeated separations. About half showed the expected pattern: a stress spike during isolation, then a return to normal after reunion. The other half didn't react at all, and that turned out to be the worse outcome, tied to inflammation and blood chemistry associated with poor long-term health.
What mattered most for recovery wasn't just having a partner. It was the quality of the reunion with one, and it didn't seem to matter which monkey made the first move. A stressed marmoset that couldn't reach out to its mate still recovered faster if its partner came to it.
Mustoe's findings echo what former U.S. Surgeon General Vivek Murthy concluded when he declared loneliness a public health epidemic in 2023, linking it to higher risks of depression, heart disease, dementia, and early death. UC San Diego psychiatrist Dr. Ellen Lee co-authored one of the first major reviews of how loneliness affects the human brain, pointing to changes in the prefrontal cortex, which is the region linked to empathy and the social skills people need to reconnect in the first place.
Bonnie Petrie talks to both researchers about what's going on inside a lonely body, in this episode of Petrie Dish.
Guests:
Aaryn Mustoe, Ph.D., Texas Biomedical Research Institute
Ellen Lee, M.D., UC San Diego
