Managing the risk of predation: The use of time allocation and vigilance by gerbils

Foraging animals have numerous tools for managing risk of predation.  Foremost among these are time allocation and vigilance.  Together, their use determines a forager’s giving-up density (GUD) in a depletable resource patch.  We examined Allenby’s gerbils (Gerbillus andersoni allenbyi) exploiting depletable seed resource patches in a large vivarium.  The gerbils foraged under varying microhabitats and moon phases while subjected to the presence of a red fox.  We measured time allocation electronically and GUDs from the amount of seeds left behind in resource patches.  From these, we estimated handling times, attack rates, and quitting harvest rates (QHRs) of the gerbils.  Gerbils displayed greater vigilance (lower attack rates) at bright moon phases (full<wane<wax<new).  Interestingly, gerbils were more vigilant in the bush than the open microhabitat, suggesting that vigilance is less effective in the open.  At the same time, gerbils displayed higher GUDs in the open than the bush and in brighter moon phases (wax>full>new>wane).  Finally, gerbils displayed higher QHRs in the open than the bush and at new and waxing moon phases than at waning and full moon phases.  Differences in foraging behavior between microhabitats heavily reflect time allocation, while differences across moon phases reflect changing time allocation and vigilance, as well as changes in the marginal value of energy to the foragers.  In this manner, gerbils apply multiple tools to manage risk in an environment of constantly changing opportunities.