Picture two scenes: 1) a small room, crowded with people; 2) the same room, but with many fewer people. Now, in both of your rooms, imagine that one person starts coughing. If we assume that the coughing indicates infection with something, perhaps pneumonia, in which room would you be most worried about transmission of the pneumonia, the crowded or the relatively less dense room? Probably, you are most concerned about the crowded room. Because it is more crowded, you are more likely to be in contact with someone, and then more likely to spread the infection yourself if you acquire it. This enhanced risk of parasite, or infectious disease, transmission is often considered a major cost of group living. But, fortunately for us and some other group living animals, the room analogy inaccurately represents reality.
In the original rooms, I imagined the people standing around shoulder to shoulder, randomly moving. But people don’t move that way. They associate with some more than others. Family, friends, co-workers. We each have our own social spheres, which are much smaller than the 7ish billion people on the planet, or even the 100 other people who happen to work in my building. Many non-human animals that live in groups also follow this pattern as well. In a recent paper that analyzed previously published work across 43 species to find more general patterns, a meta-analysis, Nunn et al. (2015) found that as group size increased, the number of subgroups within the larger group increased (where subgroup refers to smaller groups within the larger group). These subgroups limit contact among individuals; individuals primarily contact only the limited number of individuals in their own subgroup, not everyone else in the larger group.
Via this effect on contact rates, subgrouping may affect parasite transmission. Nunn et al. (2015) tested this hypothesis with a model simulating disease transmission within groups with and without subgrouping. According to the model, as group size increased, the percentage of the group that was infected, or prevalence, increased. But, subgrouping reduced this increase in prevalence. An empirical study of bighorn sheep lambs (Ovis canadensis) and pneumonia epidemics supports the general theme of the model. Manlove et al. (2014) found that sheep populations were organized into subgroups, and that during pneumonia epidemics, not all subgroups became infected. Thus, because subgroups had limited contact rates with one another, their presence may have reduced the transmission of pneumonia in comparison to a situation without subgroups, which could not be directly tested in this system. However, infection prevalence did not show a direct relationship with group size, in line with the predictions of Nunn et al.
Manlove, K.R., Cassirer, E.F., Cross, P.C., Plowright, R.K. & Hudson, P.J. (2014). Costs and benefits of group living with disease: a case study of pneumonia in bighorn lambs (Ovis canadensis). Proceedings of the Royal Society Biological Sciences Series B, 281, Article No.: 20142331.
Nunn, C.L., Jordan, F., McCabe, C.M., Verdolin, J.L. & Fewell, J.H. (2015). Infectious disease and group size: more than just a numbers game. Philosophical Transactions of the Royal Society of London B Biological Sciences, 370.