Moonlight avoidance in gerbils reveals a sophisticated interplay among time allocation, vigilance and state-dependent foraging

Abstract

Foraging animals have several tools for managing the risk of predation, and the foraging games between them and their predators. Among these, time allocation is foremost, followed by vigilance and apprehension. Together, their use influences a forager's time allocation and giving-up density (GUD) in depletable resource patches. We examined Allenby's gerbils (Gerbilus andersoni allenbyi) exploiting seed resource patches in a large vivarium under varying moon phases in the presence of a red fox (Vulpes vulpes). We measured time allocated to foraging patches electronically and GUDs from seeds left behind in resource patches. From these, we estimated handling times, attack rates and quitting harvest rates (QHRs). Gerbils displayed greater vigilance (lower attack rates) at brighter moon phases (full , wane , wax , new). Similarly, they displayed higher GUDs at brighter moon phases (wax . full . new . wane). Finally, gerbils displayed higher QHRs at new and waxing moon phases. Differences across moon phases not only reflect changing time allocation and vigilance, but changes in the state of the foragers and their marginal value of energy. Early in the lunar cycle, gerbils rely on vigilance and sacrifice state to avoid risk; later they defend state at the cost of increased time allocation; finally their state can recover as safe opportunities expand. In the predator-prey foraging game, foxes may contribute to these patterns of behaviours by modulating their own activity in response to the opportunities presented in each moon phase.

Survival and causes of mortality for pygmy rabbits (Brachylagus idahoensis) in Oregon and Nevada

“…For example, small mammals are known to forage during a new moon phase (i.e. darkness) rather than during a full moon phase (Kotler et al 2010), even in the absence of Figure 2. Relative intake of each choice treatment on the tradeoff risk trial for individual rabbits when rabbits were offered a choice to feed under low predation risk (dark concealment) that contained food with PSMs or high predation risk (transparent concealment) that contained PSM-free food.…”

The dilemma of foraging herbivores: dealing with food and fear

“…Hungry animals do not ignore the physiological feedback of fear but must respond to both risks. They may forage in risky places if they must (Berger-Tal et al 2010) or they may choose to deal with poor quality food in safe places. For browsing herbivores, the latter means dealing with plant defences including toxins.…”

Prey state shapes the effects of temporal variation in predation risk

“…Theoretical and empirical work suggests that prey may be more willing to accept increased predation risk while foraging if the risk of starvation is sufficiently high [17][18][19][20][21], thereby weakening the strength of non-consumptive predator effects on prey and emergent indirect effects on other species or ecological processes. Alternatively, because risk is inherently variable over space and time, prey may 'wait out' more dangerous periods, shifting all foraging activity to periods or places of relative safety [22][23][24][25]. However, both the quantity (duration or frequency) and quality (food availability) of intervening periods of safety can influence the capacity of prey to wait out periods of danger [26,27].…”