Stress as an adaptation II: Does experimental cortisol supplementation affect predation risk assessment in foraging gerbils?

Background: Animals are well known for trading off food and safety, and we have previously shown for Allenby's gerbil (Gerbillus andersoni allenbyi) that the intensity of this trade-off changes with energetic state. Furthermore, we have shown that concentrations of corticosteroid metabolites in desert rodents' faeces change in response to changing levels of food availability, competitor density, and moonlight. This suggests that stress hormones play an important role in mediating the trade-off of food and safety and in managing foraging. Hypotheses: Higher concentrations of the exogenous stress hormone cortisol will increase gerbils' marginal valuation of energy (MVE) and their vigilance. In general, cortisol will mediate responses to slowly changing factors associated with food and safety, but not to rapidly changing ones. Methods: In order to test these hypotheses, we manipulated cortisol concentrations in a set of gerbils by injecting each subcutaneously with 21-day slow-release 0.01 mg cortisol pellets and compared their foraging behaviour with that of a control group. The experiment was conducted in a large outdoor vivarium where we could simulate features of the gerbils' desert environment, manipulate the presence of a predatory owl (i.e. a rapidly changing factor), and quantify patch use over the course of a lunar cycle from new moon to full moon (i.e. a slowly changing factor). Foraging behaviour was quantified by giving-up densities (GUDs; the amount of food left in a resource patch after foraging), time allocated to foraging, and harvest rate curves in artificial foraging patches (seed trays). Results: Giving-up densities were affected by the interaction of cortisol treatment and moon phase, but not the interaction of cortisol treatment and owl presence. Gerbils implanted with cortisol foraged longer, but harvested food more slowly (suggesting greater vigilance and apprehension), than placebo-treated gerbils. This reaffirms that glucocorticoids affect energy acquisition, and provides a physiological context to explain how foragers manage risk and the trade-off between food and safety.