New paper: When sickness changes a social network, different kinds of social ties respond in different ways

This photo, taken under a bridge in Panama, shows two vampire bats. The bat on the left that looks clean and healthy; the one on the right is covered with bat flies and guano and looks to be having a bad day. Do vampire bats avoid groupmates that seem sick? It probably depends on the disease and the type of social relationship. 

We have a new paper on this topic in the Journal of Animal Ecology, led by PhD student Basti Stockmaier and co-authored by Dan Bolnick, Rachel Page, and myself [1]. The topic of how pathogens spread through social networks is timely, because we are currently on the verge of the next pandemic (COVID-19). Governments are doing everything they can to stop the spread by changing how often people interact.

Some amount of social distancing can happen passively just by individuals getting sick.

Sickness behaviors (like lethargy and reduced movement) can reshape a social network as a pathogen or parasite is spreading across it, because, as we all know, more socially connected individuals are more likely to transmit a pathogen to each other. Individuals that are sick are often less socially connected, which will tend to slow down how a pathogen spreads [2].

What we showed in this study is that the type of social relationship also matters. By that, I mean the type of social connection (like parent-offspring vs unrelated) and the type of behavior (like grooming vs food sharing). When a vampire bat (or a person) is sick, some types of social connections are weakened, but others are not (like family relationships). For example, a female vampire bat is less likely to groom with a typical roost-mate that is sick, but she won’t reduce her grooming towards her own offspring that is sick.

The same thing was recently reported in mandrills just a few days ago. Mandrills avoid grooming those groupmates that are parasitized with some nasty protozoa, but they continue to groom their close kin [3]. We showed this difference can be found even when there is no pathogen.

Basti did this by injecting vampire bats with a substance (lipopolysaccharide) that challenges their immune system without a live pathogen. So the vampire bats felt and acted sick for hours, but the injections did not harm their health. Then he looked at how their social behavior changed; and specifically, he looked at differences between various types of social interactions.

This plot shows how different kinds of social grooming relationships shifted when one of the two bats was sick. Grooming rates on the right of the dashed line are positive changes where the mean grooming rate tended to increase more often. Rates on the left tended to decrease. The error bars show the 95% confidence interval.

There are some general lessons here. For example, COVID-19 in China was spreading and clustering mainly within families, because these social connections would not be reduced by sickness behaviors, or by the closure of schools and public events. In other words, a sick individual might be less socially connected in general, but they might spend the same time (or even more) with their family.

The type of behavior also matters: sick vampire bats reduced their social grooming, but not their food sharing. This difference makes sense because food sharing is really important for vampire bats. Likewise, a sick person might completely stop shaking hands, but they might still engage in more vital social interactions.

When individuals are sick, social networks change, but the most important social behaviors and relationships won’t change as much.

When modeling pathogen transmission, it can be useful to also consider what defines a social connection and also the category of relationship (e.g. a parent or spouse versus a co-worker). By considering how different social connections change in response to sickness, we can better understand how social networks will change as a pathogen spreads. 

We have other ongoing related studies. We did a field experiment using wild vampire bats tracked with high-resolution proximity loggers in Belize (paper coming soon).

The disease that is most relevant when talking about vampire bats is rabies. Transmission of rabies by vampire bats poses a real problem for agricultural development and public health in Latin America, and control is difficult. The study above does not at all mimic the behavioral effects of rabies, which might make vampire bats more likely to get in fights with other bats.

Elsa Cardenas Canales is a PhD student with a vet degree who is working at the USGS National Wildlife Health Center. She is testing the effectiveness of oral vaccines that can be applied to the fur of vampire bats to be socially spread through the colony. Typically, vampire bats are controlled by simply poisoning them (which is actually ineffective for controlling the rabies virus [4]). Using an oral vaccine instead of a poison, treated bats would instead be vaccinated against rabies (which actually could reduce rabies outbreaks). The basic idea has already been tested using fluorescent powder [5]. Elsa did an experiment to test her procedure, and she also took many hours of video of how rabid vampire bats behave.

In the video below, Daniel Streicker talks about these methods for controlling rabies and how rabies might even help us better understand how to anticipate the emergence and spread of other viruses like SARS-CoV-2 the virus that causes COVID-19.

References

  1. Stockmaier S, Bolnick D, Page R, Carter G. 2020. Sickness effects on social interactions depend on the type of behaviour and relationship. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.13193 (PDF) Press: Smithsonian MagazineOSUPhys.orgUConn
  2. Lopes, P. C., Block, P., & König, B. (2016). Infection-induced behavioural changes reduce connectivity and the potential for disease spread in wild mice contact networks. Scientific Reports6, 31790.
  3. Poirotte, C., & Charpentier, M. J. (2020). Unconditional care from close maternal kin in the face of parasites. Biology Letters, 16(2), 20190869.
  4. Streicker, D. G., Recuenco, S., Valderrama, W., Gomez Benavides, J., Vargas, I., Pacheco, V., … & Altizer, S. (2012). Ecological and anthropogenic drivers of rabies exposure in vampire bats: implications for transmission and control. Proceedings of the Royal Society B: Biological Sciences, 279(1742), 3384-3392.
  5. Bakker, K. M., Rocke, T. E., Osorio, J. E., Abbott, R. C., Tello, C., Carrera, J. E., … & Streicker, D. G. (2019). Fluorescent biomarkers demonstrate prospects for spreadable vaccines to control disease transmission in wild bats. Nature Ecology & Evolution, 3(12), 1697-1704.

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