The first comparison of the brain sizes of social and non-social individuals of the same species provides more support for the social brain hypothesis (we evolved our big brains to deal with social groups). The tropical sweat bee species, Megalopta genalis, have two sorts of queen: solitary ones, who themselves go out from the nest to forage for food, or social ones — who stay at home and sends out her daughters. Although even the social queens don't have bigger brains overall, the area associated with learning and memory (the mushroom body) was more developed in the social queens than in the solitary bees (and also the social daughters — suggesting dominance is also a factor).
(2010). Socially induced brain development in a facultatively eusocial sweat bee Megalopta genalis (Halictidae).
Proceedings of the Royal Society B: Biological Sciences.