Parameters of variable reward distributions that affect risk sensitivity of honey bees

We investigated risk sensitivity with harnessed honey bees in a proboscis-extension conditioning paradigm. We conditioned each subject to turn its head and extend its proboscis towards one of two presented odors; one odor was associated with a constant reward and the other with a variable reward that was either low or high, with probabilities P and (1-P), respectively. Reward values and probabilities were set so that the expected value of the variable alternative was equal to that of the constant one. We performed six experimental conditions in which variability was in reward volume and three conditions in which variability was in reward concentration. The experiments were designed to systematically test the effect of various parameters that describe the reward distributions on levels of risk sensitivity. Risk aversion was greatest when variability was in reward volume, and the variable distribution included zero rewards and had a high coefficient of variation (CV=s.d./mean). The variance itself did not affect risk sensitivity. Subjects were risk indifferent when the variable distribution did not include zero rewards, however these distributions were positively skewed. The independent effects of zero rewards and distribution skew remain to be tested. Subjects were risk indifferent in conditions where variability was in reward concentration, but concentration range was limited and these distributions did not include zero rewards and were skewed. We conclude that risk aversion to variability in reward amount is a robust phenomenon for some reward distributions. A systematic evaluation of the effect of various reward distribution parameters on choice behavior should complement functional and mechanistic approaches.