Social distancing can be hard for social animals like us, even when we know it's a matter of life and death.
There are countless logistical crises, of course, but avoiding human contact is also just lonely for a species that evolved to live around each other, and many people are struggling to stay vigilant as the coronavirus lockdown drags on.
Yet while it may feel unnatural to live like this, the sacrifices we're now making have deep roots in the animal kingdom. Social distancing occurs not only in species that always lead solitary lives, and thus avoid each other even when no one is sick, but also in some social species when circumstances call for it.
From ants and bees to mice, monkeys and apes, an array of social animals change their behavior to reduce the risk of spreading infections. Below is a closer look at how some other social species protect themselves and their communities from dangerous diseases. Many use strategies that wouldn't work for humans, but they still illustrate why isolating ourselves during an outbreak isn't as unnatural as it feels.
Black garden ants range across Europe, and parts of North America and Asia. (Photo: Lennart Tange [CC BY 2.0]/Flickr)
An ant colony is considered a "superorganism," in which hordes of individuals work together as part of a larger entity, sort of like neurons in a brain. Considering how well ants collaborate on common goals, it may come as little surprise that they excel at social distancing and other methods of disease control. Still, their methods and results are impressive, both in terms of identifying pathogens and neutralizing them.
Black garden ants (Lasius niger), for example, quickly adjust their normal routines when members of the colony develop a fungal infection. Their colonies naturally include both nurses and foragers, who either stay home to care for young ants or venture out to find food. The latter group sometimes picks up pathogens during their excursions, but when they do, both nurses and foragers swiftly respond.
That response begins before the infected ants even become sick, according to a study published in the journal Science, in which researchers exposed some foragers in a colony to spores from a fungus called Metarhizium brunneum. Within one day of exposure, the infected foragers started spending even more time outside the nest than usual, further limiting their contact with other members of the colony.
It isn't clear how the ants knew they were infected, but it's possible they can detect the spores on themselves, New Scientist reports. However they knew, isolating themselves so early could make a big difference in stemming an outbreak — an opportunity many human communities missed during the coronavirus pandemic. In fact, similar to what countless people are now doing to avoid the coronavirus, it isn't just infected ants who changed their behavior. Unexposed foragers also reduced their social contact after their colleagues picked up the spores, the researchers found, while nurse ants began moving the brood deeper into the nest.
Ants are the largest group of "eusocial" insects, who form complex societies with overlapping generations, cooperative brood care and reproductive division of labor. Nearly all ant species live this way, but so do several hundred species of bees and wasps, and they also must be vigilant to protect their tightly packed colonies.
That includes honey bees, one of the most famous of all eusocial insects, whose colonies can fall victim to a variety of bacteria, viruses, fungi and parasites. As with ants, the dense population of a honey bee hive means quick detection — and quick action — is needed to prevent a disease from running amok.
In a bacterial disease called American foulbrood, for instance, adult bees can smell certain chemicals emitted by infected larvae, namely a mixture of two pheromones that triggers a specific hygienic behavior. When bees smell this combo, they respond more consistently than they do to either pheromone alone, according to a study published in the journal Scientific Reports. Once the bees identify where this telltale smell is coming from, they'll remove any infected larvae from the hive.
Until the late 1990s, there was no evidence that nonhuman animals could recognize and reduce infection risk from other members of their species. That changed with research on American bullfrogs, whose tadpoles are impressively adept at dodging a dangerous fungal infection. Tadpoles are able to detect an infection of Candida humicola in other tadpoles, the researchers found, and can then use that information to proactively avoid other tadpoles harboring such an infection.
"Our understanding of predators and their prey has changed drastically since it was discovered that many kinds of prey animals can change their behavior and even their body shape when they smell nearby predators," study co-author and Yale University professor Skelly said in a statement at the time. "Responding to disease risk may be quite similar from an animal's perspective. In both cases animals appear to be able to use behavior to reduce the chance that they will be harmed or die."
Like us, the great apes are highly visual creatures. Even if they can't sniff out an infection like bees or tadpoles can, they may still use visual cues to stay healthy.
Western lowland gorillas, for example, live in social groups that females migrate to join, and as researchers reported in a 2019 study, disease avoidance can be a key factor when females are deciding to leave or join a group. The study looked at a bacterial disease known as yaws, which causes visible ulcers on the faces of infected animals. While studying nearly 600 gorillas over a decade, the researchers noticed females often leave males and heavily diseased groups to join healthier ones, avoiding other sick groups at all costs. This suggests gorillas have learned the disease is contagious, the researchers noted, and can recognize its symptoms in others.
Chimpanzees rely on visual cues, too, sometimes taking steps to limit infection that seem harsh to humans. As the famed primatologist Jane Goodall first reported in the 1960s, chimps may ostracize a member of their troop who has polio, a viral disease that can lead to paralysis. Healthy chimps have been known to shun or even attack chimps partially paralyzed by polio, although Goodall has noted some chimps eventually recovered and rejoined the social group.
In many social species, from insects to apes, animals recognize an infection in others and then take steps to avoid them. In house mice, however, the opposite also happens in at least some cases.
In a 2016 study, researchers examined how a disease outbreak might affect social dynamics of wild house mice living in a barn in Switzerland. To simulate an infection, mice were injected with lipopolysaccharides, a component of the bacterial cell wall, which results in an immune response and generalized disease symptoms, making the mice feel sick. All the mice were also identified and tracked with radio tags, letting the researchers learn how both sick and healthy mice responded.
Mice have the ability to detect illness in other mice, yet the researchers were surprised to discover healthy mice were not avoiding the sick mice, instead interacting with them as if nothing was different. "It was the sick mouse that removed itself from the group," lead author Patricia Lopes, a biologist at the University of Zurich, said in a statement. That behavioral change might not be intentional — maybe the sick mouse just felt lethargic — but it could still be evolutionarily adaptive, since it would help protect the sick mouse's relatives from the infection.
Although some of our fellow primates can be drastic with their disease avoidance — ejecting members from the social group, or abandoning the group themselves — the right solution depends largely on the species and the disease. In highly social mandrills, for example, a group member infected with parasites may not be ostracized entirely, but simply receive less grooming until healthy again.
Researchers discovered this as part of an ongoing research project on mandrills in Gabon. Following 25 mandrills over more than two years, they noticed grooming rates fell as individuals became infected with more parasites, but the infected monkeys were otherwise tolerated. The researchers collected fecal samples from the mandrills, too, noticing a different chemical signature in the poop of sick mandrills compared with healthy ones. Mandrills also showed more avoidance of poop with higher levels of parasites, suggesting they know when to cut back on someone's grooming at least partly based on the smell of their poop.
When the researchers treated the sick mandrills and freed them of parasites, other members of their social groups started grooming them regularly again.
Vampire bats live in colonies that can number in the hundreds or thousands, and they depend heavily on their social network for survival. That's because the bats support each other with mutually beneficial behaviors like reciprocal grooming and food sharing, which can be important for survival. Vampire bats need to drink about a tablespoon of blood per night, and three days without blood could kill them. To buffer the colony against that threat, bats who successfully find blood on a given night often regurgitate and share some with less fortunate bats back at the colony.
In a recent study, researchers hoped to learn how an infection can affect the social dynamics among vampire bats. Working with a small captive bat colony at the Smithsonian Tropical Research Institute in Panama, the researchers injected some bats with bacteria to stimulate their immune systems and make them feel sick. All the bats continued to socialize and share food, but the sick ones did make a few changes. Similar to humans, sick bats were more likely to withdraw from weaker social relationships — offering and receiving less grooming — but interacted more normally with close family members.
"Understanding how social interactions change in the face of illness is a key component in predicting the channels and speed at which a pathogen can spread across a population," said co-author and STRI researcher Rachel Page in a statement. "Close observation of vampire bat behavior sheds light on how social animals interact, and how these interactions change — and importantly, when they do not change but persist — as individuals become sick."