Living in big groups has its disadvantages: diseases spread quickly, large crowds may be more obvious to predators, and members have to compete for food, mates, and territories. In species where recognizing specific individuals is important, it’s also much harder for members to identify each other when living among lots of group members. Fortunately, there's a solution to this last problem: according to a new study in Current Biology, members of species that live in large groups tend to be more "unique" than members of species that live in small groups.

To examine this phenomenon, the authors studied ground-dwelling sciurid rodents, a family that includes squirrels, chipmunks, and prairie dogs. These rodents, like many other gregarious animals, benefit greatly from being able to identify each other. Knowing your companion from an intruder, or recognizing individuals of different social ranks, is a huge advantage in these societies.

But the task of recognizing each other gets much more complicated as group size increases. There are not only more targets to consider, but there are also more distractions getting in the way. The authors hypothesized that individuality should increase with the average group size of the species, which would make recognition easier.

In this study, the rodents’ alarm calls were used as measures of individuality. Alarm calls in sciurid rodents are especially telling because the animals not only pay attention to the call, but consider the identity of the animal that made the call when they decide how to react. So, being able to identify specific individuals becomes especially important. The authors recorded 20 calls from 10 different individuals of each species for a total of 1600 vocalizations. The calls were then analyzed for length, frequency, and amplitude.

Overall, social group size explained over 88 percent of the individual variation observed. Species that live in large congregations, such as Belding’s ground squirrel, exhibited more unique calls than species such as the black-tailed prairie dog, which live in smaller groups. Interestingly, social complexity was not a significant predictor of individuality. The authors calculated three different measures of complexity, but none had any bearing on the uniqueness of the calls. (The authors used phylogenetic independent contrasts, so evolutionary relationships between species were controlled for in these comparisons.)

It’s likely that the relationship between group size and individuality holds true in many taxa, as long as there are advantages to being able to indentify particular individuals. On the other hand, species that live in fluid groups in which membership changes frequently—such as schools of fish—may not be subject to the same types of selection, because being able to distinguish each other isn’t vital to their success and survival.

Current Biology, 2010. DOI: 10.1016/j.cub.2011.01.051 (About DOIs).

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