Evol Ecol Res 18: 571-585 (2017)     Full PDF if your library subscribes.

Stress as an adaptation I: Stress hormones are correlated with optimal foraging behaviour of gerbils under the risk of predation

Justin R. St. Juliana1,2, Burt P. Kotler2, Nadja Wielebnowski3,4 and Jonathan G. Cox5

1Department of Biology, Indiana State University, Terre Haute, Indiana, USA, 2Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel, 3Conservation Science Department, Brookfield Zoo, Chicago Zoological Society, Brookfield, Illinois, USA, 4Department of Conservation and Research, Oregon Zoo, Portland, Oregon, USA and 5BokWerks, LLC, Austin, Texas, USA

Correspondence: J.R. St. Juliana, Department of Biology, Indiana State University, 200 North Seventh Street, Terre Haute, IN 47809, USA.
e-mail: jrs626@cornell.edu


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.

Keywords: desert rodents, faecal glucocorticoids, gerbils, moon phase, optimal foraging, predation risk, quitting harvest rate.

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