Risk-Sensitive Foraging in a Patch Departure Context: A Test with Worker Bumble Bees

Typically, tests of risk-sensitive foraging involve observing a subject's choices of alternative prey types differing in some combination of mean and variance of expected foraging gain. Here, we consider the problem 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 flower in an inflorescence and obtaining a reward of some size, the bee decides whether to visit the top flower or to move to a new inflorescence (a patch departure). Here, risk-sensitive behavior is expressed as the forager's choice of patch departure threshold (PDT) of reward obtained in the bottom flower. We measured the PDTs of bees whose colony energy stores (and therefore energy requirements) had been manipulated (Enhanced or Depleted). Simulations led us to predict that shortfall-minimizing bees should decrease their PDTs when their colony energy reserves were depleted, relative to when the reserves were enhanced. Bees did not use a strict patch departure threshold, but instead the probability of departure varied with nectar volume in the bottom flower. Colony 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 decisions 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-minimizing 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 bees.