RISK-SENSITIVE FORAGING IN A PATCH DEPARTURE CONTEXT - A TEST WITH WORKER BUMBLE BEES

Typically, tests of risk-sensitive foraging involve observing a subjec t's choices of alternative prey types differing in some combination of mean and variance of expected foraging gain. Here, we consider the pr oblem of risk-sensitive foraging when there is a single prey type. We observed worker bumble bees (Bombus occidentalis) foraging in an array of artificial 2-flower inflorescences. After visiting the bottom flow er in an inflorescence and obtaining a reward of some size, the bee de cides whether to visit the top flower or to move to a new inflorescenc e (a patch departure). Here, risk-sensitive behavior is expressed as t he forager's choice of patch departure threshold (PDT) of reward obtai ned in the bottom flower. We measured the PDTs of bees whose colony en ergy stores (and therefore energy requirements) had been manipulated ( Enhanced or Depleted). Simulations led us to predict that shortfall-mi nimizing bees should decrease their PDTs when their colony energy rese rves were depleted, relative to when the reserves were enhanced. Bees did not use a strict patch departure threshold, but instead the probab ility of departure varied with nectar volume in the bottom flower. Col ony energy stores did affect patch departure behavior, but this effect was confounded by the order in which manipulation of colony reserves was applied, Further, simulations of observed bee patch departure deci sions did not produce behavior expected if the decisions were based on shortfall-minimization. We conclude that a bee's decision of when to leave an inflorescence is not predicted by a static shortfall-minimizi ng model. Our results also implicate an important interaction between learning and foraging requirements. We review risk-sensitivity in bees , and discuss why risk-sensitive foraging may be adaptive for bumble b ees.