Foraging casts in bumblebees: field conditions cause large individual differences

Here, we present results from two foraging studies with the bumblebee Bombu s terrestris L. We used the doubly labelled water technique which makes it possible to estimate energetic costs of free flight in a natural environmen t. One study was carried out in windy weather in the open, the second in a large greenhouse under controlled conditions. Individual mass-specific meta bolic rates from the open-air study varied widely (159.5 to 750.2 W kg(-1); n = 7) and differed significantly from expected values predicted from labo ratory data. Results from the greenhouse study were closer to expectations and the range of individual metabolic rates was much narrower (405.0 to 485 .5 W kg(-1); n = 8). The range of field metabolic rates was three to four t imes larger than that of laboratory studies (Heinrich, 1975; Ellington et a l., 1990; Cooper, 1993), which might reflect the fact that only a relativel y small percentage (25-30%) of bees are 'co-operative' in laboratory flight cost measurements (Hanauer-Thieser and Nachtigall, 1995; C.P. Ellington an d T.J. Wolf, pers. observations). However, all bees flew in the field studi es, even in strong winds. We conclude that estimates of energetic costs of free flight using laboratory data (time/activity/laboratory = TAL), an appr oach regularly used in tests of foraging models, might not be appropriate u nder many natural conditions. Whereas the TAL method provided a good estima te of mean metabolic rate it is a poor method for studies in which individu al variations are of interest.