Big brains decrease fertility.

More intelligent mammals, such as humans, whales, and dolphins, have decreased fertility. One ideas has been that the energetic cost of increased brain power has been meet by decreasing the size of the gut and decreasing reproductive function. Kotrschal et al. have tested this idea by selecting for brain size in guppies and obtaining populations of fish whose brains were larger or smaller than normal and differed from one another by about 10%. The cost of the increased brain power was a decrease in the size of the gut and a decrease in reproductive function. Here is their 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.