Social Distancing and Animal Life

Lobsters, birds and primates use quarantine to avoid infections.

An interesting article published in the Scientific American August 2020 by Hawley and Buck.

Despite how unnatural social distancing may feel to the human population, it is very much a part of the natural world, practiced by mammals, fishes, insects and birds.  Social animals stay apart, they change behaviour such as grooming to stop the spread of disease that could kill them.  Strategies vary from shunning a sick animal to maintaining interactions with only the closest relatives. 

Although living in groups makes it easier for animals to capture prey, stay warm and avoid predators, it also leads to outbreaks of contagious diseases.  This heightened risk has favoured the evolution of behaviours that help animals to avoid infection.  Animals that social distance during an outbreak are the ones most likely to stay alive.  This, in turn, increases their chances of producing offspring that also practice social distancing when confronted with disease. 

These actions are what disease ecologists call behavioural immunity.  Wild animals do not have vaccines, but they can prevent disease by how they live and act.  However, immunity through behaviour comes with costs.  Social distancing from members of your species, even temporarily, means missing out on the numerous benefits that favoured social living in the first place.  Researchers have learnt that complete shunning is just one approach animals take.  Some social species stay together when members are infected but change certain grooming interactions whereas others such as ants limit encounters between individuals that play particular roles in the colony, all to lower the risk of infection.

The ability of spiny lobsters to detect and avoid infected group mates has been key to their persistence in the face of the Panulirus argus virus 1, which kills more than half of the juvenile lobsters it infects.  Young lobsters are easy prey for the virus because the animals are so social, at times found in groups of up to 20.  Safe homes in sponges, corals or rocky crevices along the ocean floor along with the snapping claws help the group of creatures defend against hungry predators such as triggerfish. 

However, it was noted in the early 2000s by Don Behringer of the University of Florida and his colleagues that some young lobsters were solo, even though it left them vulnerable.  Most of these lonely lobsters, the researchers found, were infected with the contagious virus.  The researchers felt that these lobsters did not choose to den alone but were being shunned.

To confirm their hunch, the investigators placed several lobsters in aquarium tanks, allowing healthy crustaceans to choose an empty artificial den or one occupied by either a healthy or a diseased compatriot.  The scientists reported that when disease was absent, the healthy lobsters preferred being social and chose dens with a healthy lobster over empty ones.  Lobsters strongly avoided the dens containing virus-infected lobsters, even though they had to be living alone. 

It turns out that infected lobsters have chemicals in their urine that serves as a danger signal to healthy group mates.  In a rather strange trial, scientists used super glue to block the urine-releasing organs of infected lobsters and in this trial healthy lobsters no longer avoided the sick ones.  In another trial by Mark Butler of Old Dominion University, he and his colleagues tethered a sick lobster to the home den of healthy lobsters in the Florida Keys. The healthy animals abandoned their safe havens for open waters, where of course they were at much higher risk of getting eaten.  However, when Butler’s team repeated the experiment with a tethered healthy lobster, there was no mass exodus.

Different members of different ant colonies have distinct roles to protect the colony’s survival.  In research by Stroeymeyt of the University of Bristol and published in Science in 2018, researchers used tiny digital tags to track the movements of common garden ant colonies during an outbreak of the lethal fungus, Metarhizium brunneum. 

The spores of this fungus are passed from ant to ant through physical contact – it takes one to two days for the spores to penetrate the ant’s body and cause sickness, which is often fatal.  The delay between this exposure and sickness allowed the trial to see whether ants changed their social behaviours in the 24 hours after they first detected fungal spores in their colony but before the fungus-exposed ants showed signs of sickness.  It was found that when the researchers applied fungal spores directly to a subset of forager ants that regularly leave the colony as compared with controlled colonies where foragers were dabbed with a harmless sterile solution, ants in the fungus exposed colonies started rapid and strategic social distancing after treatment. 

Within 24 hours these forager ants had self-isolated by spending more time away from the colony compared with the control group of foragers.  Uninfected foragers, which interact frequently with other foragers that might carry disease, kept their distance from the colony when disease was present. This prevented them from inadvertently putting the reproductively valuable colony members, the queen and nurses that care for the brood, at risk.  The nurses also took action by moving the brood further inside the nest and away from the foragers once the fungus was detected in the colony.  This strategic social distancing was so effective that all queens and most nurses from the study colonies were still alive at the end of the experimental outbreaks.

Birds generally avoid other birds which appear sick.  Research showed that finches surprisingly enough showed less avoidance dependant on the level of antibodies in their bloodstream and one other protein that may signal broader immune activation.  Birds with weaker levels of immunity avoided sick birds more strongly.

Similar patterns were also detected in guppies affected by a contagious and debilitating worm called Gyrodactylus turnbulli.  In work published in 2019 in Biology Letters, Jessica Stephenson of the University of Pittsburgh placed individual guppies that did not yet have worm infection in a central aquarium flanked by two tanks.  One was empty, and one contained a group of three guppies that represented potential contagion risk.  Many guppies preferred the side of the tank near other guppies, as expected for a social species, but some male guppies strongly avoided the side of the tank near the other fish, and these distancing guppies were later shown to be highly susceptible to worm infection. 

It was found in a report published in Science Advances by Clemence Poirotte and his colleagues that the daily grooming interactions of free-ranging mandrills in a park in Gabon avoided grooming those individuals with infection.  The mandrills could detect infection status based on smell alone.  Mandrills presented with two bamboo stalks rubbed in faeces strongly avoided a stalk rubbed with droppings from another mandrill that had lots of parasites.  However, in a follow-up study, mandrills continued to groom certain close relatives that had high levels of parasites, even while distancing from other parasitized group members. 

The researchers concluded that maintaining strong and unconditional alliances with certain relatives can have numerous long-term benefits in non-human primates.  In mandrills, females with the strongest social ties start breeding earlier and may have more offspring over their lifetimes.  Such evolutionary gains associated with maintaining some social ties may be worth the risk of potential infection.

Humans have a long evolutionary history with infectious diseases like other animals.  Many of our own forms of behavioural immunity, such as feelings of disgust in dirty or crowded environments are likely to be the result of this history, but modern humans, unlike other animals, have many advantages when plagues come to our doors.  We can communicate disease threats globally in an instant and this allows us to institute social distancing before disease appears in our local community. 

We have advanced digital communication platforms from emails to video that allow us to keep our physical distance while maintaining some social connections, but perhaps the biggest human advantage is the ability to develop sophisticated non-behavioural tools, such as vaccines, that prevent disease without the need for costly behavioural changes.  Vaccination allows us to maintain rich, interactive social lives despite contagious diseases such as polio and measles that would otherwise ravage us.

When it comes to stopping novel diseases like COVID-19, however, we are in much the same class as other animals.  Here, as in nature, tried-and-true behaviours such as social distancing are our best tools until vaccines or treatments can be developed, but just like other animals, we have to be strategic about it.  Like mandrills and ants, we can maintain the most essential social interactions and distance furthest from those who are most vulnerable and who we could infect by accident.  As unnatural as it may feel, we only need to follow nature’s lead.

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