One hypothesized cost of foraging is an increased risk of mortality due to predation, which may result from greater conspicuousness and reduced vigilance for predators1-5. At high prey densities, a foraging animal must compromise vigilance to overcome the 'confusion effect'6,7 and achieve high feeding rates8,9. The optimal tradeoff between food intake rate and predator avoidance should depend on the forager's current hunger or energy deficit, as risk of starvation is assumed to be dependent on hunger state8,10-12. Therefore, the probability of a foraging animal being captured by a predator is expected to increase with its food deficit and food density, and thus be dependent on its current feeding rate1,8-12. But present evidence for an increased risk of predation associated with foraging is either circumstantial3,4,13-16, or indirect9. We report here the first direct evidence for a fitness-associated cost of foraging which is dependent on the forager's current feeding rate, its food (energy) deficit and local food density. Female guppies, Poecilia reticulata, foraging on zooplankton achieved higher feeding rates and were increasingly more likely to be captured by an ambush predator as their food deficit and prey density increased, and the pre-attack feeding rate of guppies which were captured was higher on average than that of survivors.

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Journal Nature
Godin, J.-G.J, & Smith, S.A. (Shelley A.). (1988). A fitness cost of foraging in the guppy. Nature, 333(6168), 69–71.