Abstract
Shifts to new ecological settings can drive evolutionary changes in animal sensory systems and in the brain structures that process sensory information. We took advantage of the diverse habitat ecology of Neotropical army ants to test whether evolutionary transitions from below- to above-ground activity were associated with changes in brain structure. Our estimates of genus-typical frequencies of above-ground activity suggested a high degree of evolutionary plasticity in habitat use among Neotropical army ants. Brain structure consistently corresponded to degree of above-ground activity among genera and among species within genera. The most above-ground genera (and species) invested relatively more in visual processing brain tissues; the most subterranean species invested relatively less in central processing higher-brain centers (mushroom body calyces). These patterns suggest a strong role of sensory ecology (e.g., light levels) in selecting for army ant brain investment evolution and further suggest that the subterranean environment poses reduced cognitive challenges to workers. The highly above-ground active genus Eciton was exceptional in having relatively large brains and particularly large and structurally complex optic lobes. These patterns suggest that the transition to above-ground activity from ancestors that were largely subterranean for approximately 60 million years was followed by re-emergence of enhanced visual function in workers.
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Acknowledgments
Phil Ward and Sean Brady shared insights into the evolutionary history of doryline army ants. Thanks to three anonymous reviewers for making valuable comments particularly related to statistical analyses. Thanks to Michael Kaspari and David Donoso for providing C. andicola specimens. Kaitlin Baudier, Marie Clifford, Yamile Molina, Karina O’Donnell, Siobhan O’Donnell, Tom Soare, Sean Tully, and William Wood assisted with specimen collections and field research. Ants were collected in Costa Rica and Ecuador under research permits from those countries. Research was funded by NSF grant IOS 1209072 and Drexel University startup funds to S.O’D.
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Communicated by: Alain Dejean
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Bulova, S., Purce, K., Khodak, P. et al. Into the black and back: the ecology of brain investment in Neotropical army ants (Formicidae: Dorylinae). Sci Nat 103, 31 (2016). https://doi.org/10.1007/s00114-016-1353-4
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DOI: https://doi.org/10.1007/s00114-016-1353-4