CORRELATION OF SOUND GENERATION AND METABOLIC HEAT FLUX IN THE BUMBLEBEE BOMBUS LAPIDARIUS
Flying insects produce extreme amounts of heat as a by-product during the contractions of their thoracic flight muscles (Heinrich, 1989). Before flight, metabolic heat may serve to warm up the thoracic muscles until the minimum lift-off temperature is reached (Heinrich, 1974b; Stone and Willmer, 1989; Esch and Goller, 1991). Social bees and wasps are also able to use the heat produced in their flight muscles for brood incubation and for active regulation of nest temperatures (Heinrich, 1974a; Seeley and Heinrich, 1981; Schultze-Motel, 1991). In this study, we report simultaneous measurements of heat flux and sound generation by wing buzzing in individual bumblebee workers (Bombus lapidarius L.). Bumblebees used in the experiments were taken from colonies in observation nest boxes (Schultze-Motel, 1991) and placed into the cylindrical 100 ml stainless-steel vessel of a Calvet-type microcalorimeter (MS 70, Setaram, Lyon; Wadso, 1987). A small microphone had been installed below the screw cap of the calorimeter vessel. The sensitivity of the calorimeter under these conditions was 41.7 mV W-1. Both the calorimeter and the microphone signals were amplified and recorded on a dual-channel chart recorder. In 32 out of a total of 36 measurements, the bumblebees showed prolonged periods of sound generation, most frequently at the beginning of experiments. We assume that the sound was not produced in an alarm reaction, but by flight movements of the wings when the animals attempted to lift off inside the calorimeter vessel. The buzzing sounds produced by bumblebees are caused by oscillations of the flight muscles inside the metathorax (Schneider, 1975). Previous endoscopic observations of bumblebees sitting on the bottom of our calorimeter vessel had shown that there was a one-to-one correlation between episodes of wing movements and sound production. The microphone recordings thus allowed an easy way of measuring locomotor activity inside the calorimeter. The simultaneous recordings of calorimeter and microphone signals showed a very good agreement between periods of sound generation and increased metabolic heat flux from the animals. This was most conspicuous in some experiments without continuous wing buzzing activity but with distinct episodes of intense sound generation that were always coupled to a simultaneous increase of heat flux (Fig. 1). Between episodes of sound generation, heat flux typically returned from maxima exceeding 200 W kg-1 to basal values around 10 W kg-1. During one experiment, we recorded a deviation from the usually observed synchronism between metabolic heat flux and sound generation (Fig. 2). The acoustic activity of the animal started about 45 min after the beginning of the experiment. Remarkably, the heat flux signal showed a steep increase as early as 5 min before the first sound generation was observed. During the period of continuous wing buzzing, heat fluxes of more than 350 W kg-1 were measured. These fluxes correspond to metabolic rates during free flight in other Hymenoptera: about 300 W kg-1 in the carpenter bee Xylocopa capitata (Nicolson and Louw, 1982), 300-500 W kg-1 in honeybees, Apis mellifera (Nachtigall et al. 1989) and about 350 W kg-1 in the bumblebees Bombus lucorum and B. pascuorum flying in a wind tunnel (Ellington et al. 1990). We suggest that the sharp increase in heat flux before the beginning of sound generation represents a preflight endothermic warm-up event. Apparently, the warm-up of flight muscles proceeded without any wing buzzing.