No evidence for larger brains in cooperatively breeding cichlid fishes

The social brain hypothesis posits that frequent social interactions, characteristic of group living species, select for greater socio-cognitive abilities and the requisite neural machinery. An extension of the social brains hypothesis, known as the cooperative breeding brain hypothesis, postulates that cooperatively breeding species, which live in stable social groups and provide allocare, face particularly pronounced cognitive demands because they must recognize, remember, and differentially respond to multiple group members. These socio-cognitive challenges are thought to have selected for increased cognitive capacity, supported by a bigger brain. To test the prediction that cooperative breeders have larger brains, we performed a phylogenetically controlled comparison of the whole brain masses of adult fish from 16 closely related species of cooperatively and independently breeding lamprologine cichlid species from Lake Tanganyika. We collected data on brain mass from males of eight species of lamprologine cichlids and added this to brain mass data from eight more species found in the published literature. Controlling for body size and phylogeny, we found that cooperative breeding species did not have larger brains, and this was true of for both our field-collected data set and the expanded data set including published values. This study adds to a growing body of literature from other taxa that cast doubt on the cooperative breeding brain hypothesis.