Behavioral games between predators and prey: Little Egrets and Gold Fish in test environment

Background/Question/Methods

Optimal behavior of a predator depends on what its prey are doing and vice versa. Yet empirical tests of optimal foraging generally take place with only one of the two species free to adjust its behavior. Do predators and prey follow predictions of optimality when they are interacting and each is free to adjust its behavior?

We used common goldfish (Carassius auratus) and wild-caught little egrets (Egretta garzetta) in experimental arenas consisting of circular enclosures (radius 3.5 m), each with three artificial pools (radius 0.76 m). Each pool had a habitat where the goldfish could hide, and an open habitat where they could collect their food but were exposed to predation. An egret was allowed to forage in an arena with either (experiment 1 = homogeneous environment with 15fish/pool) or (experiment 2) in arena with heterogeneous environment (10, 15, 20 fish/pool, respectively). Consumed fish were replaced daily. We measured the time goldfish spent in the open and under cover; the time it took for them to emerge from cover after an egret visit (i.e. the refractory time, rf); the number of fish eaten; and the time an egret took to return to a pool after leaving it (i.e. the return time, rt).

Results/Conclusions

Predictions: A) during an egret’s visit to a pool, fish should spend a greater proportion of time hiding and thus become less vulnerable to predation. B. Fish in larger schools enjoy safety in numbers. C) An egret will maximize its chance to encounter exposed fish by adopting a return time equal to fish refractory time (rt = rf ). D) The behavioral response of each player to each other will be fish density dependent. An egret should maximize its catch of fish if it will take into account the fish density dependent anti predatory behavior. Indeed, in the presence of an egret, goldfish significantly reduced the time they spent in the open and it was fish density dependent. The results suggest that the egret and the fish are playing a behavioral game. Egrets that responded by adjusting their foraging time in a pool and their return time to a pool according to the density and the weariness of the fish were the most successful in capturing fish. The fish respond adaptively to the egrets foraging strategies by adjusting their emergence to the risky and productive open microhabitat according to the frequency of visitation by the egret.