Foragers often must contend with many predator individuals and many predator species, and this may challenge their ability to manage risk. We hypothesized that prey defensive responses would be less than additive as a function of encounter rate with a single predator type, because prey can “buy more safety” for the same defensive effort. More importantly, by controlling for predator behavior and encounter rate we were able test the hypotheses that: prey behavioral responses to multiple different predators would be greater than additive. Furthermore, we hypothesized that the additive responses would be diminished when predators were similar, because prey can use the same defensive behaviors to deal with like predators. We placed PIT tagged gerbils, of two different species in 2 large outdoor enclosures. Over the course of 80 experimental nights we exposed the gerbils to 5 predator treatments (none, owl, snake, feral cat, trained dog) at 4 different encounter rates (1 encounter per 0, 30, 60, or 90 min.). At the 60 min. encounter rate we also exposed the gerbils to all 6 pairwise predator combinations by alternating predator encounters. We measured the gerbils’ giving up densities (GUDs), time allocation to foraging, apprehension (vigilance), and resource handling strategy.
Results/Conclusions
We found that gerbils increased GUDs, decreased time allocation, and increased apprehension as a function of risk when comparing the no predator to predator treatments. However, there was not a consistent trend in responses to encounter rate. This may support our hypothesis concerning the value of defensive behaviors, but we cannot rule out the possibility our results derive from risk not changing linearly with encounter rate, or perceptual limitations of the gerbils. We supported our hypothesis that defensive behavioral responses to multiple different predators would be greater than additive. GUDs, time allocation, and apprehension were greater than additive in 4 of the 6 predator combinations, and in 2 of the combinations the responses were slightly greater than additive. In the instances where responses were slightly greater the gerbils seemed to match their behavior to just the riskier of the 2 predators. We did not support our hypothesis that responses to like predators would be reduced. Instead prey responses to multiple predators were proportionally stronger when the predators individually posed greater risk. We did not find an influence of predation on prey resource handling strategy, but additional data demonstrated a connection between resource handling strategy and competition between gerbils.