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Ant Brains

Ants may be tiny, but they still have brains. In fact, a recent study by Amador-Vargas et al. has shown that studying these tiny brains can yield interesting discoveries about colony processes and, potentially, the evolution of division of labor in insect societies.

A Beautiful Brain (Figure 1 in Amador-Vargas et al. 2015)

A Beautiful Brain (Figure 1 in Amador-Vargas et al. 2015)

The researchers set out to test two hypotheses, the “task specialization hypothesis” (TSH) and the “social brain hypothesis” (SBH). In the context of ants, the TSH predicts that growth in colony size will favor unequal growth in brain size, with ants in different roles exhibiting growth in different regions of the brain (corresponding to their different tasks). In other words, large colony sizes promote task specialization, which in turn drives cognitive differentiation between individuals. Alternatively, the SBH predicts that due to an overall increased requirement for cognitive function (e.g. nest mate recognition) in larger social environments, total brain volume should increase throughout the colony, regardless of task, as colony size increases. Here, Amador-Vargas et al. particularly focus on mushroom bodies (MB), the “integration centres of the brain”, in the acacia ant Pseudomyrmex spinicola.

The study produced several interesting findings, but the key discovery is that, at least in these ants, an increase in colony size does seem to drive some cognitive differentiation (in accordance with the TSH). That is, larger colonies tended to include ants that more consistently stuck to one task, and also exhibited increased differences in brain sizes between ants dedicated to defense and those tasked with foraging for food.

The exciting thing about this finding is that it may be one step towards understanding how a division of labor evolves in insect societies. Potentially, a growth in average colony size for a species may increase the likelihood that it will evolve castes – that is, workers that are morphologically and (often) functionally distinct. The species studied here contains workers that are “monomorphic” (i.e. without physically distinct castes), as opposed to “polymorphic” (i.e. with physically distinct castes). However, future comparisons of the brains in different species spanning a full range of polymorphism would help to elucidate whether or not the trends observed in this study represent one case of a general phenomenon in which an increase in colony size drives not just behavioral but also cognitive differentiation between individuals. Such cognitive differentiation may provide a link between monomorphism and polymorphism in ant societies.

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