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

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

If that is not a universal feeling, that may be because, like all primates, humans are social animals. Group life sometimes has worrying costs, such as conflict and even violence, just to begin with. Even so, it has helped ensure the survival of the human species and that of many other primates, too. Living in groups provides protection against predators (we can plot against them and attack them) and against hunger (we can share resources). Living socially facilitates the search for a partner. It allows us to learn from each other. It gives us the pleasure and comfort we experience when a hormonal fever results from a friendly or loving touch.

Three chimpanzees seem to be having a meeting.

Three chimpanzees seem to be having a meeting. More information

For reasons like these, primates seem to have an almost biological need to be among friends and family, at least occasionally. In fact, that need became evident in research conducted in 2018 and 2019 at the McGovern Institute for Brain Research at MIT. Forty people who sat in a windowless room alone for ten hours informed the researchers that they had felt a desperation for social contact. In strength and number, those reports were very similar to those given to the same research team about food cravings after a ten-hour day of fasting. In addition, in this group of forty people, brain images showed a similar "craving signal" after experiences of social and nutritional deprivation.

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

However, it turns out that primates are probably not the only animals that need companionship. Even aquatic animals could. 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, can hate being alone in the long run. The study was published in the December 2, 2020 issue of the journal Nature. Unlike humans, zebrafish cannot be asked how they feel when they are alone. Instead, the researchers measured the concentration of a hormone that varies almost barometrically in zebrafish in response to the presence of other zebrafish. Like the production of some hormones in humans, the zebrafish hormone is triggered by physical sensation, though not by direct contact.

Zebrafish (Danio rerio)

Zebrafish (Danio rerio) GETTY

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The zebrafish (Danio rerio) is a freshwater fish of the minnow family. Schuman's team bred some zebrafish only in tanks and some zebrafish with others. Constantly, researchers found that the level of the hormone called parathyroid hormone 2 (pth2) changed in fish as the number of fish present in their tank changed. When a fish was alone, no pth2 was evident. If other fish were added to the tank, the pth2 level of the first fish increased. If a lot of fish were added, the pth2 level increased a lot. Pth2 levels could increase significantly in as little as thirty minutes

Interestingly, when the fish that were swimming alone could see other zebrafish through a barrier, their pth2 levels did not increase. Realizing that feeling the vibrations in the water of other swimming fish could be what triggers the increase in pth2, the researchers created an engine that moved the water in the same way that swimming fish moved the water. As suspected, the engine deceived the solo zebrafish. Pth2 levels increased. To doubly make sure they were right about the swimming movement, the researchers also ablated the lateral line, which is in the body of many aquatic animals and is a long row of small tactile sense organs full of hair. The lateral line detects movements and pressure changes in the surrounding water. The researchers found that zebrafish that had been deprived of the use of those mechanosensory organs no longer responded with an increase in pth2 levels to the presence of companion 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 telephone 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 pth2 receptor.”

zebrafish

GETTY zebrafish

Schuman also noted that there may be a common survival benefit for the clan or grazing. "For fish, mammals and even humans, one benefit may be that, for predators, a group looks larger and more formidable than a small individual.”

Should we take a hint of the fish? They know that clan instinct serves them, and they are probably not ashamed about it. In this time of COVID-19 social isolation, we can all do well to remember that our distress may not be causeless. When loneliness takes us, it may be because we are biologically coded to be with friends and relatives. We have evolved to feel safer with others.