Abstract
This paper examines the energetic consequences of queen bumble bees (Bombus appositus) using a relatively simple threshold volume rule to allocate time among multiflowered Delphinium nelsonii plants. Threshold volumes that maximize a forager's rate of net energy intake (optimal threshold volumes) were calculated, using an estimate of nectar availability in naturally occurring plants. Optimal threshold volumes are near median standing—crop volumes in flowers, and increase both with standing—crop volumes and with the degree that standing—crop volumes are correlated among flowers within plants. Estimates of threshold volumes used by naturally foraging bumble bees are very similar to optimal threshold volumes. Moreover, bees efficiently realign threshold volumes to near—optimal levels as nectar availability varies seasonally. Occasional violations of the threshold departure rule prevent bees from maximizing rates of net energy intake. Nonetheless, average rates of return adjusted for the violations were found to be higher than rates from any of three random plant—departure rules. The hypothesis that foragers violate the threshold departure rule to reduce variability in the rate of net energy intake was not supported. It is argued that violations of the departure rule result, in part, from foragers having to sample flowers occasionally to update estimates of threshold volumes as standing—crop volumes vary. Sampling is most frequent when the need for accurate estimates of threshold volumes is highest and the energetic cost of obtaining this information is lowest.
Published Version
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