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Many studies assume that an increase in brain size is beneficial. However, the costs of producing and maintaining a brain are high, and we argue that brain size should be secondarily reduced by natural selection whenever the costs outweigh the benefits. Our results confirm this by showing that brain size is subject to bidirectional selection. Relative to the ancestral state, brain size in bats has been reduced in fast flyers, while it has increased in manoeuvrable flyers adapted to flight in complex habitats. This study emphasizes that brain reduction and enlargement are equally important, and they should both be considered when investigating brain size evolution.

Behavioral development in the worker caste of many adult ants follows a pattern of task transitions that contribute to the division of labor within colonies. In the ant Pheidole dentata, the number of tasks that minor workers attend to increases as they progress from brood-care activities within the nest to acts outside the nest such as foraging and defense. In this study we investigated synapse maturation in the lip region of mushroom bodies in young and old minor workers because of its potentially crucial role in behavioral development, task performance, and repertoire expansion. As minor workers aged, individual presynaptic boutons enlarged and acquired more synapses and vesicles, but the total number of synapses in the lip region did not change significantly. Glial cell processes occupied less of the synaptic neuropil as ants matured. These findings indicate an expansion and enhancement of efficacy at specific sets of synaptic connections between the projection interneurons and Kenyon cell dendrites and a commensurate loss of other connections as minor workers age and expand their behavioral repertoire.

Extensive studies of vertebrates have shown that brain size scales to body size following power law functions. Most animals are substantially smaller than vertebrates, and extremely small animals face significant challenges relating to nervous system design and function, yet little is known about their brain allometry. Within a well-defined monophyletic taxon, Formicidae (ants), we analyzed how brain size scales to body size. An analysis of brain allometry for individuals of a highly polymorphic leaf-cutter ant, Atta colombica, shows that allometric coefficients differ significantly for small (<1.4 mg body mass) versus large individuals (b = 0.6003 and 0.2919, respectively). Interspecifically, allometric patterns differ for small (<0.9 mg body mass) versus large species (n = 70 species). Using mean values for species, the allometric coefficient for smaller species (b = 0.7961) is significantly greater than that for larger ones (b = 0.669). The smallest ants had brains that constitute ∼15% of their body mass, yet their brains were relatively smaller than predicted by an overall allometric coefficient of brain to body size. Our comparative and intraspecific studies show the extent to which nervous systems can be miniaturized in taxa exhibiting behavior that is apparently comparable to that of larger species or individuals.

Changes in the relative size of brain regions are often dependent on experience and environmental stimulation, which includes an animal's social environment. Some studies suggest that social interactions are cognitively demanding, and have examined predictions that the evolution of sociality led to the evolution of larger brains. Previous studies have compared species with different social organizations or different groups within obligately social species. Here, we report the first intraspecific study to examine how social experience shapes brain volume using a species with facultatively eusocial or solitary behaviour, the sweat bee Megalopta genalis. Serial histological sections were used to reconstruct and measure the volume of brain areas of bees behaving as social reproductives, social workers, solitary reproductives or 1-day-old bees that are undifferentiated with respect to the social phenotype. Social reproductives showed increased development of the mushroom body (an area of the insect brain associated with sensory integration and learning) relative to social workers and solitary reproductives. The gross neuroanatomy of young bees is developmentally similar to the advanced eusocial species previously studied, despite vast differences in colony size and social organization. Our results suggest that the transition from solitary to social behaviour is associated with modified brain development, and that maintaining dominance, rather than sociality per se, leads to increased mushroom body development, even in the smallest social groups possible (i.e. groups with two bees). Such results suggest that capabilities to navigate the complexities of social life may be a factor shaping brain evolution in some social insects, as for some vertebrates.

The behavioral development of minor workers of the ant Pheidole dentata involves a progression of tasks beginning with brood care and culminating in foraging as individuals age. To understand the role of brain neurochemistry in age-related division of labor, we measured the levels of serotonin, dopamine and octopamine in individual brains of minor workers of different age. Serotonin and dopamine levels were significantly correlated with worker age: both increased as minor workers matured, and serotonin rose significantly in the oldest ants. In addition, the serotonin:dopamine ratio was significantly higher in the oldest workers. Octopamine levels did not change with age, although the ratios of octopamine:serotonin and octopamine:dopamine were significantly higher in the youngest workers. These age-associated changes in biogenic amine levels suggest an involvement of neuromodulators in minor worker behavioral ontogeny and temporal polyethism in P. dentata.

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