Brain size correlates roughly with intelligence. So, assuming that more intelligence gives a selective advantage, what limits the size of our heads? To see if bigger brains are better and what the tradeoffs might be, Kotrschal et al. experimentally addressed the effects of selection for brain size in guppies (Poecilia reticulata). After only two generations of selection, the authors obtained populations of fish whose brains were larger or smaller than normal and differed from one another by about 10%. The big-brained female fish (but not the males, for some unknown reason) were better than those with smaller brains at a task where the fish associated the number of symbols (two or four) with a food reward. The cost of the increased brain power was a decrease in the size of the gut and a decrease in reproductive function. The brain is very active metabolically, and thus its growth must be balanced against the cost of maintaining other processes in the organism. The offsetting effect on reproductive function is consistent with interspecies comparisons in which more intelligent mammals, such as humans, whales, and dolphins, have decreased fertility.
Curr. Biol. 23, 168 (2013).
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