Brain size affects female but not male survival under predation threat

Abstract

There is remarkable diversity in brain size among vertebrates, but surprisingly little is known about how ecological species interactions impact the evolution of brain size. Using guppies, artificially selected for large and small brains, we determined how brain size affects survival under predation threat in a naturalistic environment. We cohoused mixed groups of small- and large-brained individuals in six semi-natural streams with their natural predator, the pike cichlid, and monitored survival in weekly censuses over 5 months. We found that large-brained females had 13.5% higher survival compared to small-brained females, whereas the brain size had no discernible effect on male survival. We suggest that large-brained females have a cognitive advantage that allows them to better evade predation, whereas large-brained males are more colourful, which may counteract any potential benefits of brain size. Our study provides the first experimental evidence that trophic interactions can affect the evolution of brain size.

Keywords: Artificial selection; Crenicichla; Poecilia reticulata; brain size; guppy; pike cichlid; predation; semi natural; survival.

Figure 1

Experimental stream designed to test how brain size affects survival. Grey shadings indicate the depth profile, black arrow depicts a hose transporting water from the filter to the shallow area, white arrows indicate the direction of water flow.

Figure 2

Timeline of the experimental procedure to determine the relationship between brain size and survival in guppies. The small black arrows indicate whole-population censuses, the grey arrow indicates introduction of the guppy predator, a pike cichlid. The weekly censuses stopped after a predefined 50% survival criterion was met at week 20.

Figure 3

Survival curves of guppies selected for large and small relative brain size in experimental streams following introduction of a pike cichlid. (a) Shows variation in survival in the individual streams. (b) Shows the mean survival curves over all six streams. Note that in order to improve clarity, error bars are not presented. Solid lines: large-brained animals, broken lines: small-brained animals.