Striped field mice pair posing

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CAMBRIDGE, Mass. — Social distancing is as common a practice these days as washing hands, but apparently humans aren’t the only ones who adhere to the health strategy. It turns out that mice also engage in social distancing when their mates get sick, according to a new study.

Neuroscientists from MIT say mice have a brain circuit that stops them from mating with other rodents that appear to be unwell.

The research, published in the journal Nature, shows that when male mice encounter a female mouse showing signs of illness, they practically ignore the rodent and make no attempts to mate with them as they normally would. The behavior is controlled by a circuit in the amygdala which detects distinctive smells from sick animals and triggers a warning signal to stay away.

“As a community, it’s very important for animals to be able to socially distance themselves from sick individualism,” says study author Dr. Gloria Choi, an associate professor of brain and cognitive sciences at MIT, in a statement. “Especially in species like mice, where mating is instinctively driven, it’s imperative to be able to have a mechanism that can shut it down when the risk is high.”

Dr. Choi’s lab has previously studied how illness influences behavior and neurological development in mice, including the development of autism-like behaviors after mothers are unwell during pregnancy. This latest work is her first to reveal how illness can affect healthy people’s interactions with others who are sick. Certain behaviors such as mating and fighting are innately programmed in mice and many other animals, meaning they automatically engage in them when triggered.

“We wanted to see whether there’s a brain mechanism that would be engaged when an animal encounters a sick member of the same species that would modulate these innate, automatic social behaviors,” she explains.

How a certain scent tells mice when others are ill

Previous studies have shown that mice can distinguish between healthy mice and those which have been injected with LPS, a bacteria which causes mild inflammation. The studies suggested that mice use body odor to suss out sick individuals.

In the new study, the researchers placed male mice in the same cage with either a healthy female or one that was showing signs of illness, from LPS. They found that the males engaged much less with the sick females and made no effort to mount them.

The team then tried to find the brain circuit behind this behavior. They say the vomeronasal organ, which processes pheromones, feeds into a part of the amygdala. This region is activated by the presence of LPS-injected animals. Further experiments revealed that activity in this part of the brain is necessary to suppress the males’ mating behavior in the presence of sick females. When this activity was turned off, males would try to mate with sick females.

In addition, artificially stimulating the brain suppressed mating behavior in males even when they were around healthy females. The findings show that this part of the amygdala communicates with another part called the medial amygdala, carried by a hormone. This communication is necessary to suppress mating behavior.

Dr. Choi says the link to this hormone is intriguing because thyroid dysfunction has been implicated in depression and social withdrawal in humans. She now plans to explore the possibility that internal factors such as mental state can alter these hormone levels in brain circuits to control social behavior.

“This is something we are trying to probe in the future – whether there’s a link between thyroid dysfunction and modulation of this amygdala circuit that controls social behavior,” she says.

SWNS writer Laura Sharman contributed to this report.

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