Infectious disease and group living

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.

crowded room            not crowded room

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.

subgrouping

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.

sheep-human

Citations

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.

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Snails are Snowflakes

Have you ever heard someone say, or thought to yourself, “You are unique”? Of course you have. It’s a pretty common phrase. We’re all snowflakes. Among other sources, our personalities create this variation. No one behaves the same as someone else in all circumstances. Perhaps some individuals will behave similarly in some circumstances, but even then, their behavior would not be exactly the same. Such a statement likely surprises no one, but would it surprise you to hear that snails have personality?

They do! People who study animal behavior often define personality as consistent individual differences in behavior, meaning that individual A behaves in a consistent way that differs from the consistent behavior of individual B. For example, if one were to test the response of individuals to a new object (say, by measuring how close the individual would get to the object), over the course of repeated tests, one individual might consistently get closer to the object than another individual, who consistently stayed farther away. These individuals would be said to have personality.

In a recent study with a marine snail, the common periwinkle (Littorina littorea), and one of its parasites, the authors found that their snails displayed personality, which also varied with infection. Specifically, after removing a snail from the water and poking it, then placing the snail back in the water, the authors timed how long until the snail reemerged from the shell. Interestingly, the authors found that individual snails (each measured three times) differed consistently in how long they remained inside their shell. Snails, furthermore, differed by infection status, with infected snails remaining inside their shells for longer on average than uninfected snails. So all snails had a different and relatively consistent hiding time, but on average, infected snails remained inside their shells for longer periods. Snails are snowflakes too!

But what about those infection results? Did infection change the behavior of snails, or were uninfected snails with certain personalities more likely to acquire infection? Because the study design did not implement controlled infections, testing behavior before and after infection, the authors noted that they could not answer that question. However, either answer would be interesting, and relevant for humans. For example, if personality influences whether an individual acquires infection, then studying such processes in a snail could inform epidemiology of human diseases (studying disease transmission in snails, or the many other non-human animals that have been shown to display personalities, also circumvents the obvious ethical issues with controlled infections in humans). If infection influences personality, then investigating that process in a snail or other animal could likewise inform disease transmission in humans.

Citations

Seaman, B. and M. Briffa (2015). Parasites and personality in periwinkles (Littorina littorea): Infection status is associated with mean-level boldness but not repeatability. Behavioural Processes.