26 Cheverud J.M.

Falk D.

Vannier M.

Konigsberg L.

Helmkamp R.C.

Hildebolt C. Heritability of brain size and surface features in rhesus macaques (Macaca mulatta).

14 Sol D. Revisiting the cognitive buffer hypothesis for the evolution of large brains.

27 Chittka L.

Niven J. Are bigger brains better?.

2 Healy S.D.

Rowe C. A critique of comparative studies of brain size.

7 Striedter G.F. Principles of Brain Evolution.

28 Gahr M. Brain structure: Causes and consequences of brain sex.

29 Laland K.N.

Reader S.M. Foraging innovation is inversely related to competitive ability in male but not in female guppies.

30 Houde A. Sex, Color, and Mate Choice in Guppies.

31 Brooks R.

Caithness N. Female choice in a feral guppy population: are there multiple cues?.

Our results show that the evolution of relative brain size in vertebrates can be a fast process when under strong directional selection. The realized heritability of relative brain size was also substantial in both sexes, matching those detected in mother-offspring studies []. Furthermore, our demonstration of a direct association between brain size and cognition suggests that selection for increased cognitive ability can be mediated through rapid evolution of brain size. Because cognitive abilities are important to facilitate behaviors such as finding food, avoiding predation, and obtaining a mate, individuals with increased cognitive abilities are likely to have higher reproductive success in the wild []. However, the link between a larger brain and cognitive abilities has recently been challenged because of the high cognitive capacity of some small-bodied and small-brained invertebrates such as bees and ants []. Moreover, the field of cognitive evolution has recently shifted toward emphasizing fine-scale structural differences in the brain as the main feature linking brain morphology and cognitive ability []. Our results now show that larger brains really can be better, at least on the within-population level, and that variation in a relatively crude measure of brain morphology, relative brain size, is directly associated with variation in cognitive ability. Interestingly, the effect of relative brain size on cognitive ability was only evident in females. We offer two explanations for the sex-specific response in our experiment. First, relative brain size may not reflect cognitive ability in males to the same extent as in females. We find this explanation unlikely because in most species, general brain functions are usually shared between the sexes []. Second, the design of our cognitive test may have been more suitable for testing female cognitive ability. In the guppy, females are more active and innovative while foraging [], most likely reflecting the fact that female reproductive success is mainly food limited whereas males are limited by their access to females []. Because females feed more, they may thus have had more time to associate the cue with food in our experimental design. Moreover, in some populations, female guppies choose their partner based on male melanin spot coloration []. The female visual system may thus be preadapted for more efficient processing of the black symbols used in this experiment.