Wild vampire bats socially distance when they get ill

When disease strikes, wild vampire bats self isolate voluntarily, or are forced to do so by their colony, thereby slowing the rate an illness spreads, according to a new field study.

The behaviour had previously been observed in bats in a lab, but the experiment is the first to document such behaviour in the wild.

Changes in social behaviour such as this can alter how a pathogen spreads across a population, and transmission rates can significantly decrease when healthy individuals avoid sick ones.

It is known that among some social insects, unhealthy individuals either self isolate or are excluded, and various other species exhibit changes in behaviour when they fall ill, resulting in increased lethargy and lower levels of sociality - all of which induces social distancing and does not require action from others.

In order to see how vampire bats responded to illness, US researchers in Belize captured 31 adult female bats from a roost inside a hollow tree and simulated illness by injecting half of the bats with endotoxins which stimulate an immune response.

Meanwhile a control group of bats in the same colony received saline injections.

Over the next three days, 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 16 "sick" bats and 15 control 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 when considering both direct and indirect connections.

During the six hours of the treatment period, a “sick” bat associated on average with four fewer associates than a control bat.

A control bat had, on average, a 49 per cent chance of associating with each control bat, but only a 35 per cent chance of associating with each apparently sick bat.

During the treatment period, “sick” bats spent 25 fewer minutes associating per partner.

These differences declined after the treatment period and when the bats were sleeping or foraging outside the roost.

“The sensors gave us an amazing new window into how the social behavior of these bats changed from hour to hour and even minute to minute during the course of the day and night, even while they are hidden in the darkness of a hollow tree,” said the study's lead author, Simon Ripperger from Ohio State University.

“We've gone from collecting data every day to every few seconds.”

The research is published in the journal Behavioural Ecology.