Behavioral Ecology Blog

Parasitism has long been thought of as a significant cost of sociality- as their transmission is typically density defendant. Basically- as the density of hosts increases (as it does in social species) so does the rate of transmission, which is parasites is equivalent to reproduction. More recently however, the ubiquity of this relationship has been challenged- mostly as studies emerge that fail to find significant correlation between the two variables.

Most recently, a short piece was published in Biology Letters by Bordes et. al., entitled “Rodent sociality and parasite diversity” Here, they put another small nail in the coffin in finding a negative relationship between parasite richness and a measure of sociality. Weird, huh…

They come up with 2 different hypotheses- I’ll first give you the one I think is more plausible…

1. That social species have evolved behavioral defenses to mitigate the effect of parasites. Basically here they are thinking about allogrooming and related behaviors. I judge this to be a very likely explanation, especially as grooming is very well documented in many mammalian taxa…
2. This one is a bit more complicated, and has to do with intraspecific competition between parasite species themselves. Basically sociality creates a homogeneous environment (from the perspective of the parasite), co-infection is more likely– at least initially. This co-infection sets the stage for competition, and thinking of the situation where one species outcompetes the other- a reduction in parasite richness might be the outcome.

While the study itself is neat- I think for me the take home message is this: Biology at the level of intraspecific interactions is really really complex- and while theory is great for making broad generalizations- it is absolutely true that as we continue to study specific systems many exceptions will be found to the theory driven rules.

It is exactly for this reason (I feel) that it is important to continue to study animals in the wild… Surely effects exactly like the ones here would have been missed if it were not for good field studies..