Thermal stability and muscle efficiency in hovering orchid bees (Apidae:Euglossini)

To test whether variation in muscle efficiency contributes to thermal stability during flight in the orchid bee, Euglossa imperialis, we measured CO2 production, heat loss and flight kinematics at different air temperatures (Ta). We also examined the relationship between wingbeat frequency (WBF) and Ta in five additional species of orchid bees. Mean thoracic temperature (Tth) for Eg. imperialis hovering in a screened insectary and in the field was 39.3+/-0.77 degrees C (mean +/- 95% C.I.), and the slope of Tth on Ta was 0.57. Head and abdominal temperature excess ratios declined with Ta, indicating that Eg. imperialis were not increasing heat dissipation from the thorax at high Ta. Elevation of Tth above Ta was correlated with WBF, but Tth alone was not. Estimates of heat production from both respirometry and heat loss experiments decreased 33% as Ta rose from 24 to 34 degrees C. Mean muscle efficiency over this temperature range was 18% assuming perfect elastic energy storage and 22% assuming zero elastic energy storage. Both efficiency estimates increased significantly as Ta rose from 24 to 34 degrees C. In all six species examined, WBF declined significantly with Ta. These data indicate that hovering orchid bees regulate heat production through changes in wingbeat kinematics and consequent changes in energy conversion by the flight motor. Temperature-dependent variation in elastic energy storage or muscle contraction efficiency or both may contribute to the observed trends.