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Social Distancing Is Wearying — For Humans, And Maybe Even For Fish

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During this time of COVID-19 pandemic, some people claim to thoroughly enjoy the coronavirus-avoidance strategy of staying home and alone.

If that isn’t a universal feeling, that may be because, like all primates, humans are social animals. Group life sometimes has troubling costs — like conflict and even violence, just for starters. Even so, it has helped ensure the survival of the human species and that of many other primates, as well. Living in groups provides protection from predators (we can plot against them and gang up on them) and from starvation (we can share resources). Living socially eases the search for a mate. It allows us to learn from each other. It affords us the pleasure and comfort that we experience when a hormone rush results from a friendly or loving touch.

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For reasons like these, primates seem to have an almost biological need to be among friends and family, at least occasionally. Indeed, that need was made evident in research conducted in 2018 and 2019 at MIT’s McGovern Institute for Brain Research. Forty people who sat in a windowless room alone for ten hours reported to researchers that they had felt a desperation for social contact. In strength and number, those reports were very much like the ones they’d given the same research team about hankerings for food after a ten-hour day of fasting. What’s more, in this group of forty people, brain images showed a similar “craving signal” after both the social and nutritional deprivation experiences.

Maybe it’s no wonder that, for many people, enduring months of coronavirus-related social distancing has been difficult.

As it turns out, though, primates probably aren’t the only animals that need companionship. Even aquatic animals might. A recent study by an international research team led by Erin Schuman of Frankfurt’s Max Planck Institute for Brain Research shows that zebrafish, too, may hate to be alone long-term. The study was published in the December 2, 2020 issue of the journal Nature. Unlike humans, zebrafish can’t be asked how they feel when they’re alone. Instead, the researchers measured the concentration of a hormone that varies almost barometrically in zebrafish in response to the presence of fellow zebrafish. Like the production of some hormones in humans, the zebrafish hormone is triggered by physical sensation—though not by direct touch.

Zebrafish (Danio rerio) are freshwater fish from the minnow family. Schuman’s team raised some zebrafish alone in tanks and some zebrafish with others. Consistently, the researchers found that the level of the hormone called parathyroid hormone 2 (pth2) changed in fish as the number of fish present in its tank changed. When a fish was alone, no pth2 was apparent. If other fish were added to the tank, the first fish’s pth2 level rose. If a lot of fish were added, the pth2 level rose a lot. Pth2 levels could significantly rise in as few as thirty minutes

Intriguingly, when fish that were swimming alone could see other zebrafish through a barrier, their pth2 levels didn’t rise. Realizing that feeling the vibrations in the water from other fish swimming might be what triggers the rise in pth2, the researchers created a motor that moved water in the same way that swimming fish moved water. As suspected, the motor fooled the solo zebrafish. Pth2 levels rose. To make doubly sure that they were correct about the swimming motion, the researchers also ablated the lateral line, which is on the body of many aquatic animals and is a long row of small, hair-filled, tactile sense organs. The lateral line detects movements and pressure changes in surrounding water. The researchers found that zebrafish who’d been deprived of the use of those mechanosensory organs no longer responded with an increase in pth2 levels to the presence of fellow fish.

The hormone pth2 was first discovered during rodent studies at the National Institutes of Mental Health (NIMH) by Theodore Usdin, and was originally called TIP39. In a phone interview, Schuman explained, “One way to determine the importance of a hormone is to look at how many cells are affected by it. In zebrafish about 10% of cells have the receptor for pth2.”

Schuman also pointed out that there may be a common survival benefit to clanning or herding. “For fish, mammals, and even humans, one benefit may be that, to predators, a group looks bigger and more formidable than does one little individual.”

Should we take a hint from the fish? They know the clanning instinct serves them, and they probably feel no shame about it. In this time of COVID-19 social isolation, we may all do well to remember that our distress may not be without cause. When loneliness wears on us, it may be because we are biologically coded to be with friends and kin. We have evolved to feel safer with others.

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