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Background: Many organisms live in a world awash with predation risk. Optimal foragers trade off food and safety to maximize their fitness. A way for organisms to modify their behaviour, and appropriately trade off food and safety under changing conditions, is to do so as a function of stress hormone (glucocorticoid) concentration.

Hypothesis: As the risk of predation changes, stress hormones and an organism’s optimal foraging behaviour will change accordingly.

Methods: We evaluated connections between a stress hormone faecal glucocorticoid concentration (FGC) and optimal foraging behaviour (using the giving-up density technique) in two species of desert gerbils, Gerbillus andersoni allenbyi and Gerbillus nanus, in a large outdoor enclosure. Gerbils were subject to changing predation risk from barn owls (Tyto alba, present or absent) and moon phase (nights with a full moon being riskier).

Results: Gerbils had higher giving-up densities (foraged less) and were more apprehensive (a form of vigilance) on nights with a full moon and when owls were present. Also, gerbils showed elevated FGCs in response to the full moon. Owl presence or absence, however, was not related to FGC. Individual gerbils with higher FGC foraged longer, ate more food, and foraged later into the night. Hence, in this system there is a correlation between optimal foraging under the risk of predation and stress hormones. Stress hormone concentrations increase in response to FGC, an indicator of general predation risk.