Risk management in optimal foragers: The effect of sightlines on the vigilance and apprehension of foraging gerbils

Background/Question/Methods

Interactions between behaviourally responsive prey and predators are often games. In these games predators try to manage fear; prey individuals try to reduce predation risk while trying to maximise their food intake. One such foraging game exists between gerbils and their predators in the Negev Desert of southern Israel. Here, gerbils manage the trade-off between food and safety using behavioral tools that include time allocation (when, where, and how long to forage), apprehension, and vigilance. Theory tells us that the optimal level of a forager’s vigilance is affected by its encounter rate with predator and the effectiveness of its vigilance. We tested these predictions for gerbils at risk of predation from barn owls or foxes in a large vivarium. In particular, we reduced the effectiveness of vigilance by placing obstructions around feeding trays that blocked sight lines along either the vertical (vigilance against owls) or horizontal axis (vigilance against foxes). In addition, we manipulated the presence of foxes and owls.

Results/Conclusions

The giving-up density (GUD, food left behind in an assay tray) data revealed significantly higher GUDs (felt unsafe) in patches under high cover than in low cover. Horizontal sightline obstructions also had a significant effect on GUDs, with higher GUDs in trays surrounded by opaque fence compared to control. There was a significant effect of predator presence, with highest GUDs in the presence of barn owl. There was also a significant interaction between vertical sightline treatments and predator, with the low cover acting as a refuge. Our results strongly indicate the importance of sightlines in the forging decisions of gerbils.