Sick Bats Observe Social Distancing

Changes in social behavior can alter how a disease spreads across a population. Transmission rates can decrease when healthy individuals avoid sick ones or increase, e.g., when parasites change their host’s behavior. Sick animals, for example, can self-isolate voluntarily or be excluded by their colony mates. Diseases can also induce changes in behavior directly, such as increased lethargy and sleep and reduced mobility. This can lead to sickness-induced “social distancing”.

Simon P. Ripperger, The Ohio State University, Columbus, USA, Smithsonian Tropical Research Institute, Ancón, Republic of Panama, and Leibniz-Institute for Evolution and Biodiversity Science, Berlin, Germany, Sebastian Stockmaier, Smithsonian Tropical Research Institute and University of Texas at Austin, USA, and Gerald G. Carter, The Ohio State University and Smithsonian Tropical Research Institute, have found that wild vampire bats that are sick spend less time near others from their community, which slows how quickly a disease will spread. The team had previously seen this behavior in the lab and devised a field experiment to confirm it in the wild.

The researchers captured 31 adult female vampire bats from a roost inside a hollow tree in Belize. Then, they simulated sickness in 16 of the bats by injecting them with the immune-challenging substance lipopolysaccharide. The control group of 15 bats received saline injections. The researchers glued proximity sensors to the bats, released them back into their hollow tree, and tracked changes over time in the associations among all bats under natural conditions.

Compared to control bats, “sick” bats associated with fewer groupmates, spent less time with others, and were less socially connected to healthy groupmates. A control bat had, on average, a 49 % chance of associating with each control bat, but only a 35 % chance of associating with each “sick” bat. “Sick” bats also spent 25 fewer minutes with each contact. This unintentional “social distancing” could slow the spread of pathogens across a social network, with the effects depending on the mode of transmission.