Does neocortex size predict social grooming in bats?

The social brain hypothesis predicts that species with larger neocortex volume for their body size should possess more social complexity [1].

Does this apply to bats? It’s not really clear. Wilkinson [2] found that relative cortex volume did not correlate with colony size, but it was greater among those bats with stable social groups. Colony size is a poor measure of social complexity in bats, because a “colony” is not necessarily a measure of a social group or network.

Social grooming is a good measure of social complexity and bonding in primates, and it also occurs in bats [3-5]. Undergraduate Lauren Leffer and I did a study comparing social grooming rates of bats at the Organization for Bat Conservation in Bloomfield Hills, Michigan [3]. We scored the presence or absence of social grooming in 7,800 samples of an equal number of male and female bats of five species (rough average mass): Desmodus rotundus (30 g),  Artibeus jamaicensis (45 g), Carollia perspicillata (20 g), Eidolon helvum (300 g), and Rousettus aegyptiacus (150 g). One advantage of our approach is that all the bats were housed under similar captive conditions with no external parasites (so differences in social grooming are more likely to be driven by social factors rather than environmental ones). Even better, all the bats we watched had an equal and fixed association which also removes group stability as a factor.

I recently examined whether relative neocortex volume was correlated with observed rates. I first regressed previously published measures of body size (g) and neocortex volume (mm-cubed) [6-7], then extracted regression residuals for neocortex size, and tested whether these predicted social grooming rates using both log and rank transformations (social grooming rates were very zero-inflated).

Our tiny sample of five bats suggested that residual neocortex size is likely to predict social grooming rates (log-transformed: R2=0.75, F(1,3)=8.86, p=0.0588; rank: R2=0.77, F(1,3)=10.46, p=0.0481). The plot to the left shows the social grooming rates by rank.

This suggests to me that, even though social grooming is rare in bats (0.1–0.5% of time in the non-vampires), it may still be a useful measure of social complexity, as it is in primates. While we cannot draw strong conclusions from such a small sample, I think this provides some motivation for doing more comparative studies on social grooming in more bats.

References

1. Dunbar R, Schultz S. 2007. Evolution in the social brain. Science. (PDF)
2. Wilkinson GS. 2003. Social and vocal complexity in bats. In: de Waal FB, Tyack PL, editors. Animal Social Complexity: Intelligence, Culture and Individualized Societies. Cambridge, MA: Harvard University Press. p. 322-341. (PDF)
3. Carter and Leffer. In prep.
4. Carter G, Wilkinson G. 2013. Cooperation and conflict in the social lives of bats. In: Adams, R, Pedersen, S (eds). Bat Evolution, Ecology, and Conservation. Springer Science Press. pg 225-242. (PDF)
5. Carter G, Wilkinson G. 2013. Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment. Proceedings B. (link)
6. Baron G, Stephan H, Frahm HD. 1996. Comparative neurobiology in Chiroptera. Basel: Birkhäuser Verlag.
7. Pitnick S, Jones KE, Wilkinson GS. 2006. Mating system and brain size in bats. Proceedings B. (PDF)