The COVID-19 pandemic has taught millions of people across the world at least one valuable lesson: the single most important thing that one can do to stave off the spread of an infectious disease is to practice social distancing. As long as infected hosts are in limited contact with healthy hosts, a pathogen can’t spread too much. In some cases, an epidemic can be entirely eradicated if a pathogen runs out of hosts.
Remarkably, social distancing as a response to disease isn’t a behavior exclusive to humans. Many social animals change their behaviors such as grooming in an active effort to stop the spread of disease that could kill them.
For instance, Caribbean spiny lobsters (Panulirus argus) normally live in groups, but healthy lobsters avoid members of their own species if they are infected with a deadly virus. Other animals as diverse as monkeys, insects, and birds are capable of detecting and avoiding sick members of their species. And, according to a new study published today in the journal of Behavioral Ecology, vampire bats (Desmodontinae) could also be added to the list.
Previously, a team of researchers led by Gerald Carter, assistant professor of behavioral ecology at Ohio State University, found that vampire bats that had been treated with lipopolysaccharide (LPS), an immune-challenging substance, became lethargic. These bats moved less and engaged in fewer social interactions such as grooming.
“However, studying bats in captivity usually means forcing them into unnatural situations,” Simon Ripperger, a postdoctoral researcher at Ohio State University and postdoc fellow at the Smithsonian Tropical Research Institute in Panama, told me.
With what they learned from captive studies, Ripperger and colleagues traveled to Lamanai, Belize, to tag and track vampire bats in their native habitat. They captured 31 adult females from a roost inside a hollow tree and injected a random batch of half of them with the LPS substance. The other half acted as the control group and received saline injections.
“We worked within an archaeological site at Lamanai, Belize, in the rainforest, surrounded by Mayan temples. We knew from experience during past years, that this particular hollow tree had a colony of vampire bats. So we set up nets in front at sunset. However, it was not before 2 a.m. that the first female emerged from the tree. So it was literally 8 hours of standing still in the dark next to the net, being eaten by mosquitoes, until we finally knew that things seemed to go our way. Three hours later we had our required sample, which is always a very exciting and rewarding moment,” Ripperger told ZME Science.
Each bat was fitted with tiny proximity sensors that weigh less than a penny and then released back into the hollow tree, which enabled the researchers to track behavioral changes over time in the associations made by the 16 ‘sick’ bats and the 15 controls.
“This new technology automatically sensed who is near whom and sent the data to a base station, which allows 24/7 monitoring without disturbing the bats. This technology was a huge step forward in studying social networks in wild animals and only allowed us to bring the studies from captivity into the wild. Back then, it was only the third time that we deployed these brand-new sensors and it was always very exciting to see whether everything works as planned. Downloading the first batch of data and seeing that everything works and most of the bats stayed at the colony always gives one goosebumps,” Ripperger said.
According to the results of the study, the sick bats spent less time with others and were generally less social with their healthy groupmates.
Healthy bats had a 49% chance of associating with other control bats, but only a 35% chance of associating with each sick bat. On average, sick bats spent 25 fewer minutes of socializing per partner. These differences declined after the treatment period, as well as when the bats were sleeping or foraging outside the roots.
These findings suggest that induced sickness behavior dramatically changed the social network in the vampire bat colony. However, Ripperger says these observed changes in the social networks “are most likely not the product of ‘sick’ individuals isolating themselves and actively avoiding others so that healthy members don’t get sick.”
Instead, the social distancing seems to be more likely due to lethargy and increased sleep, which reduced movement and the number of opportunities for social encounters.
“Imagine you have the flu and are exhausted and don’t feel like getting out of bed at all. In consequence, you will meet fewer people in your daily routine,” Ripperger said.
For many social animals, physical distancing seems to have an evolutionary advantage since it increases survivability. Some species, such as lobsters, detect the presence of infected individuals from their odor. When a lobster is infected, it releases chemicals in the urine that serve as a warning signal to healthy group members, which quickly isolate diseased individuals.
But it is ants that probably have the most sophisticated social distancing protocol. When a contagious disease such as a fungus sweeps through their society, both sick and healthy insects quickly alter their behavior. For instance, the sick ants self-isolate while healthy ants reduce their interactions with their peers when they sense disease is present in the colony. What’s more, healthy ants are known to close ranks around the most vulnerable members of the colony, such as the queens and nurses, keeping them isolated from forager ants that are most likely to become sick.
Vampire bats don’t seem to be this diligent about preventing disease from spreading through their community, as the findings suggest that their behavior is more likely a byproduct of lethargy. However, their behavior might be different when they encounter a real pathogen that they may be familiar with.
“The differences are that the bats are not really sick, because there is no real pathogen, and COVID is caused by a virus and the molecule we used is bacterial. However, a parallel is that social distancing, which is now a big aspect of our daily routines, is not exclusive to humans, it can also be found in social animals. Even such a simple mechanism as lethargy causing reduced movement and in consequence reduced social encounter rates (we also call this ‘passive social distancing’) reduces the contact rates between ‘sick’ and healthy individuals and would also help to slow the spread of a pathogen,” Ripperger said.
In the future, the researchers plan to extend their fieldwork and monitoring of vampire bat social networks using proximity sensors that will also include cows, the preferred prey of the bats.
“One great opportunity of our sensors is not only that we can track the social encounters among all members of an entire social group at high resolution. The data are of such high resolution that we can use it to redefine social networks in a way to match the properties of a particular pathogen. Some pathogens might require close contact among two individuals, others might already be contagious if two individuals come within a distance of one meter. Some pathogens might require longer time periods to spread, others might be likely to spread within a minute. Our data allow us to filter for close encounters or for longer ones, re-define the social networks and we can use these data to model the spread of a particular pathogen. This advance in tracking technology will hopefully help researchers in the future to better understand the spread of pathogens in wild animal populations in general,” Ripperger said.