Evolution of brain size in class-based societies of fungus-growing ants (Attini)
Highlights
► We examined sociality and brain size in nine genera of fungus-growing ants (Attini). ► Fungus-growing ants showed broad variation in colony size and social organization. ► Brain size varied among taxa in larger colonies and in more tightly knit societies. ► Larger colony size was associated with differential investment in olfactory brain centres. ► Analyses of social complexity using a scalogram yielded results similar to those using group size as a proxy.
Section snippets
Data Sources and Study Taxa
We obtained data on relative brain mass for 18 species across nine genera in the ant tribe Attini from Seid et al. (2011). The glomeruli of the antennal lobes are the brain regions primarily responsible for processing olfactory inputs. We obtained the relative sizes of the antennal lobes (antennal lobe/optic lobe) and number of glomeruli for 25 species in nine attine genera from Kelber et al. (2009). Since each glomerulus receives projections from neurons expressing a single receptor gene (
Results and discussion
Among attine species there was a significant quadratic association between relative brain size and colony size (R2 = 0.56, N = 18, P = 0.005; after PIC: R2 = 0.52, P = 0.006; Fig. 1a, Table 1), which provided a better fit than a linear equation (R2 = 0.24, N = 18, P = 0.039), but was marginally nonsignificant after PIC correction (R2 = 0.19, P = 0.07). This association was strongly affected by two species, Acromyrmex echinatior and Atta colombica, which are leafcutter species with huge colonies that are commonly
Acknowledgments
We thank Simon Tierney and anonymous referees for helpful comments of earlier versions of this manuscript. A.J.R. was supported by a Latin American Fellowship from the Smithsonian Tropical Research Institute (STRI), and by the Smithsonian Institution’s Scholarly Studies Program (to W. Eberhard, A. J. Riveros and W. Wcislo). We are also grateful for the generous support from the F.H. Levinson Fund to STRI’s Laboratory of Behavior & Evolutionary Biology, and for general research funds from STRI.
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E-mail address: [email protected] (A. J. Riveros).