Interactions shape cichlid brains

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Recent research provides evidence that the shape and size of cichlid brains is strongly influenced by environmental complexity and social interactions among conspecifics.

In a paper published in the latest issue of the journal Brain, Behavior and Evolution, Alexander Pollen, Adam Dobberfuhl, Justin Scace, Mathias Igulu, Susan Renn, Caroly Shumway and Hans Hofmann show that the size and shape of cichlid brains is strongly correlated with the heterogeneity of the environment in which the fishes live and the mode of social interactions with conspecifics.

The authors studied how brain shape and size change in seven species of ectodine cichlids from Lake Tanganyika: Asprotilapia leptura, Enantiopus melanogenys, Xenotilapia bathyphila, X. boulengeri, X. flavipinnis, X. ochrogenys and X. spiloptera.

They found that the sizes of the brain and the cerebellum (a region of the brain that plays an important role in the integration of sensory perception and motor output) are positively correlated to the number of species (one measure of habitat complexity) in a given habitat.

The sizes of the medulla (which relays nerve signals between the brain and the spinal cord) and the olfactory bulb (used in processing olfactory signals) are negatively correlated with habitat measures (number of species, depth, slope, surface roughness and rock size), while the size of the telencephalon (the part that is most involved in thought processes, motor control and sensory perception) increases with rock size.

The authors also found that social interactions play a lesser role in shaping the brain: only the sizes of the telencephalon and the hypothalamus (which secretes and releases neurohormones) are affected by social factors.

Telencephalic size is larger in monogamous species and in species living in greater densities, while hypothalamic size is smaller in monogamous species.

According to the authors: "The fact that the neural variation is correlated with differences in habitat preference and social organization strongly suggests that adaptive evolution has acted on cichlid brains."

For more information, see the paper: Pollen, AA, AP Dobberfuhl, J Scace, MM Igulu, SCP Renn, CA Shumway and HA Hofmann (2007) Environmental complexity and social organization sculpt the brain in Lake Tanganyika cichlid fish. Brains Behavior and Evolution 70, 21"39.