Complex
brains and behaviors have occurred repeatedly within vertebrate classes throughout evolution. What adaptive pressures drive such changes? Both environmental and social features have been implicated in the expansion of select
brain structures, particularly the
telencephalon. East African cichlid fishes provide a superb opportunity to analyze the social and ecological
correlates of
neural phenotypes and their evolution. As a result of rapid, recent, and repeated
radiations, there are hundreds of closely-related species available for study, with an astonishing diversity in
habitat preferences and social behaviors. In this study, we present quantitative ecological, social, and
neuroanatomical data for closely-related species from the (
monophyletic) Ectodini
clade of Lake Tanganyikan cichlid fish. The species differed either in
habitat preference or social organization. After accounting for
phylogeny with independent contrasts, we find that environmental and social factors differentially affect
the brain, with
environmental factors showing a broader effect on a range of
brain structures compared to social factors. Five out of seven of
the brain measures show a relationship with
habitat measures.
Brain size and
cerebellar size are positively
correlated with species number (which is
correlated with
habitat complexity); the medulla and
olfactory bulb are negatively
correlated with
habitat measures. The
telencephalon shows a trend toward a
positive correlation with
rock size. In contrast, only two
brain structures, the
telencephalon and
hypothalamus, are
correlated with social factors.
Telencephalic size is larger in monogamous species compared to polygamous species, as well as with increased numbers of individuals; monogamy is also associated with smaller
hypothalamic size. Our results suggest that
selection or drift can act independently on different
brain regions as the species diverge into different
habitats and social systems.
Copyright 2007 S. Karger AG, Basel.
