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

Typically, tests of risk-sensitive foraging involve observinga subject's choices of alternative prey types differing in somecombination of mean and variance of expected foraging gain.Here, we consider the problem of risk-sensitive foraging whenthere is a single prey type. We observed worker bumble bees(Bombus occidentalis) foraging in an array of artificial 2-flowerinflorescences. After visiting the bottom flower in an inflorescenceand obtaining a reward of some size, the bee decides whetherto visit the top flower or to move to a new inflorescence (apatch departure). Here, risk-sensitive behavior is expressedas the forager's choice of patch departure threshold (PDT) ofreward obtained in the bottom flower. We measured the PDTs ofbees whose colony energy stores (and therefore energy requirements)had been manipulated (Enhanced or Depleted). Simulations ledus to predict that shortfall-minimizing bees should decreasetheir PDTs when their colony energy reserves were depleted,relative to when the reserves were enhanced. Bees did not usea strict patch departure threshold, but instead the probabilityof departure varied with nectar volume in the bottom flower.Colony energy stores did affect patch departure behavior, butthis effect was confounded by the order in which manipulationof colony reserves was applied. Further, simulations of observedbee patch departure decisions did not produce behavior expectedif the decisions were based on shortfall-minimization. We concludethat a bee's decision of when to leave an inflorescence is notpredicted by a static shortfall-minimizing model. Our resultsalso implicate an important interaction between learning andforaging requirements. We review risk-sensitivity in bees, anddiscuss why risk-sensitive foraging may be adaptive for bumblebees.