Risk management in optimal foragers: the effect of sightlines and predator type on patch use, time allocation, and vigilance in gerbils

In the foraging game between gerbils and their predators, gerbils manage risk of predation using the tools of time allocation (where, when and for how long to forage) and vigilance. The optimal level of a forager's vigilance should be affected by its encounter rate with predators and the effectiveness of its vigilance in reducing mortality risk. The physical structure of the environment can alter the effectiveness of its vigilance and therefore alter its foraging behaviour. We tested this 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 directed against owls) or horizontal axis (vigilance directed against foxes), thereby changing the physical structure of the environment. In addition, we manipulated the presence of foxes and owls. In general, gerbils harvested fewer seeds, allocated less time to foraging in dangerous patches, and used more vigilance while foraging where and when risks were higher (i.e. in the presence of predators and in bright moonlight). Vertical and horizontal sightline treatments interacted synergistically to further raise perceived risk. These results imply that blocking sight lines reduces the effectiveness of vigilance, causing gerbils to use it less. Moreover, in the presence of a predator, the gerbils 'response to the blocked sightlines was more severe - harvesting less food and spending less time and vigilance - in the patches with the increased risk. This was especially so in the presence of the predator that was expected to most benefit from blocking that particular type of sight line: cover that blocked vertical sight lines was riskiest in the presence of owls, and cover that blocked horizontal sight lines was riskiest in the presence of foxes. These results strongly indicate the importance of sightlines and landscape features such as bushes in the risk management and forging decisions of gerbils, demonstrating that bush cover provides mixed blessing to gerbils by providing cover, but making vigilance ineffective.