In this review, I explore the effects of both social organization and the physical environment, specifically habitatcomplexity, on the brains and behavior of highly visual African cichlid fishes, drawing on examples from primates and birdswhere appropriate. In closely related fishes from the monophyletic Ectodinii clade of Lake Tanganyika, both forces influencecichlid brains and behavior. Considering social influences first, visual acuity differs with respect to social organization (monogamyversus polygyny). Both the telencephalon and amygdalar homologue, area Dm, are larger in monogamous species. Monogamousspecies are found to have more vasotocin-immunoreactive cells in the preoptic area of the brain. Habitat complexityalso influences brain and behavior in these fishes. Total brain size, telencephalic and cerebellar size are positively correlated withhabitat complexity. Visual acuity and spatial memory are enhanced in cichlids living in more complex environments. Howeverhabitat complexity and social forces affect cichlid brains differently. Taken together, our field data and plasticity data suggest thatsome of the species-specific neural effects of habitat complexity could be the consequence of the corresponding social correlates.Environmental forces, however, exert a broader effect on brain structures than social ones do, suggesting allometric expansion ofthe brain structures in concert with brain size and/or co-evolution of these
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