Guppies with big brains outlive others by outmaneuvering their predators—but this only works in female guppies, not males. 

Brains come in all sizes, even within the same species. And surprisingly, we don’t know that much about how interactions between species impact the evolution of brain size. Previous work revealed that for the tropical guppy, Poecilia reticulate, brain size seems to matter: Females with bigger brains outperformed smaller-brained ones in a learning task. But these precious organs are quite energetically costly, and for selection to favor increased brain size, the cognitive benefits must outweigh these costs. 

So to see how brain size affects survival under predation threat in a naturalistic environment, Stockholm University’s Alexander Kotrschal and colleagues bred Trinidadian guppies, artificially selecting them for both large and small brains. Then they housed mixed groups of small- and large-brained fish in half a dozen semi-natural, experimental stream setups with their predator from the wild, the pike cichlid (Crenicichla alta). The team monitored guppy survival every week for five months. 

Large-brained females, they found, enjoyed 13.5 percent higher survival compared to small-brained females. Brain size had no noticeable effect on male survival.

The team thinks that females with mighty noggins have some cognitive advantage when it comes to evading predation. Similarly endowed males, on the other hand, were also more colorful. Being so conspicuous to predators likely counteracted any benefit or edge they might gain from brain size. These males, however, were also faster swimmers, New Scientist reports, and better at learning and remembering the location of females.

"This is the first time anyone has tested whether a larger brain confers a survival benefit," Kotrschal says. "The fact that large-brained females survived better in a naturalistic setting is the first experimental proof that a larger brain is beneficial for the fitness of its bearer. This is like watching evolution happen and shows how brain size evolves."

The findings were published in Ecology Letters earlier this month.