CARPENTER BEE (XYLOCOPA MICANS) RISK INDIFFERENCE AND A REVIEW OF NECTARIVORE RISK-SENSITIVITY STUDIES

This paper presents new results of risk-sensitive foraging studies of the carpenter bee, Xylocopa micans, and reviews the work to date on ri sk sensitivity in nectarivores. In the field, nectarivores choose amon g alternative food sources (flowers) that differ in the variabilities of their nectar rewards. In the lab, the foraging situation for carpen ter bees was experimentally simplified by offering the bees a choice b etween either ''low variance'' or ''high variance'' artificial flowers . The two flower types differed in their variabilities but offered the same expected shortterm rates of net energy gain to test the predicti ons of the short-term rate maximization mechanism. Foragers were teste d under two energy budget conditions, hungry and well-fed, to test the predictions of the z-score model. Individual carpenter bees were indi fferent to variability in both nectar volume and nectar sugar concentr ations, and their risk-indifference was unaffected by energy budget. T hese findings of risk indifference support neither the variance discou nting nor the z-score model of risk sensitivity. Since the low and hig h variance flower types are equivalent for carpenter bees in short-ter m rate of energy gain, there can be no selection on carpenter bees to be sensitive to variability based on differences in rate of gain. Stud ies of risk sensitivity in honey bees and bumble bees using variance i n nectar concentration support this contention. These findings are com pared with other nectarivore risk sensitivity studies in order to high light the most likely mechanisms underlying aversion to variation in n ectar rewards (short-term rate maximizing, the Weber-Fechner law of pe rception and learning non-empty flowers) and to suggest future researc h in the interplay of these three mechanisms.