title: Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain.
abstract: The large variation in brain size that exists in the animal kingdom has been suggested to have evolved through the balance between selective advantages of greater cognitive ability and the prohibitively high energy demands of a larger brain (the "expensive-tissue hypothesis"). Despite over a century of research on the evolution of brain size, empirical support for the trade-off between cognitive ability and energetic costs is based exclusively on correlative evidence, and the theory remains controversial. Here we provide experimental evidence for costs and benefits of increased brain size. We used artificial selection for large and small brain size relative to body size in a live-bearing fish, the guppy (Poecilia reticulata), and found that relative brain size evolved rapidly in response to divergent selection in both sexes. Large-brained females outperformed small-brained females in a numerical learning assay designed to test cognitive ability. Moreover, large-brained lines, especially males, developed smaller guts, as predicted by the expensive-tissue hypothesis, and produced fewer offspring. We propose that the evolution of brain size is mediated by a functional trade-off between increased cognitive ability and reproductive performance and discuss the implications of these findings for vertebrate brain evolution.
authors.affiliation: Animal Ecology, Department of Ecology and Genetics, Uppsala University, Norbyvägen 18D, 752 36 Uppsala, Sweden.
authors.forename: Alexander
authors.initials: A
authors.lastname: Kotrschal
authors.forename: Björn
authors.initials: B
authors.lastname: Rogell
authors.forename: Andreas
authors.initials: A
authors.lastname: Bundsen
authors.forename: Beatrice
authors.initials: B
authors.lastname: Svensson
authors.forename: Susanne
authors.initials: S
authors.lastname: Zajitschek
authors.forename: Ioana
authors.initials: I
authors.lastname: Brännström
authors.forename: Simone
authors.initials: S
authors.lastname: Immler
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Maklakov
authors.forename: Niclas
authors.initials: N
authors.lastname: Kolm
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001921:Brain / Q000033:anatomy & histology*; D003071:Cognition / Q000502:physiology*; D005260:Female; D041981:Gastrointestinal Tract / Q000033:anatomy & histology; D007858:Learning; D008297:Male; D009929:Organ Size; D011036:Poecilia / Q000502:physiology*; D012098:Reproduction; D012641:Selection, Genetic