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Endothermic heat production in honeybee winter clusters

Anton Stabentheiner^*, Helga Pressl, Thomas Papst, Norbert Hrassnigg and Karl Crailsheim

Institut für Zoologie, Universität Graz, Universitätsplatz 2, A-8010 Graz, Austria

View larger version (75K): [in a new window]   Fig. 1. (A) Infrared thermogram of the bees on the central comb of a broodless winter cluster (3.5 s after cage opening was started, FLIR ThermaCam SC2000). Note the warm thorax (yellow and white spots) of the endothermic bees with shivering thermogenesis. Two additional measurements in the same cluster confirmed this finding (not shown). The arrow points to the queen. Bees that visited the surface are visible at the lower left end of the cluster. (B) Temperature profile along the line marked in A; arrows mark the thoraces of endothermic bees hit by the line. (C) Infrared thermogram of the bees on the flat side of an outer comb (same cluster as in A); one (periphery) to four (centre) bee layers. Ambient temperature was 5.5°C 3 cm beneath the cluster and 3.7°C 1 m outside the open cluster cage.

View larger version (22K): [in a new window]   Fig. 2. Body surface temperatures of bees in a winter cluster along vertical scans on the surface of the flat side of an outer comb (A) and on the central comb (B). Lines at 9°C mark chill coma temperatures (Free and Spencer-Booth, 1960; Esch, 1988; Goller and Esch, 1990). Ta, air temperature 1 m outside the cluster cage; TaCl, air temperature 3 cm beside the cluster surface; O, outermost bee layer; I, intermediate bees; C, core bees.

View larger version (36K): [in a new window]   Fig. 3. Frequencies of bees with different temperature relationships between body parts in winter clusters on the central comb and on the cluster surface on the flat side of an outer comb. Hd, head; Th, thorax; Ab, abdomen. For cluster parts, compare Fig. 2. Body parts were considered as different if their temperatures differed by at least 0.2°C. Distributions significantly different within (A) and (B) at P<0.001, except intermediate and core in B (P<0.1; Bonferroni 2 statistics). (A) Sum of three measurements made with close-up instrumentation along 38 mm broad vertical scans (AGA 782 SW) within 10-15 s after the start of cage opening. For the central comb, frequencies were projected to the whole comb assuming a circular distribution of the bees (604 bees=100%; 132 bees measured). For the cluster surface, 78 bees=100%. (B) Simultaneous measurement of all bees on the central comb of a different cluster (Fig. 1A) 3.5 s after the start of cage opening (181 bees=100%).

View larger version (16K): [in a new window]   Fig. 4. Frequency distributions of the temperature difference between thorax (Tth) and abdomen (Tab) in different parts on the central comb of broodless winter clusters in endothermic bees (Class `a') and in bees with no or only minute endothermy (Class `c'; compare Fig. 3; four measurements). Broken arrows represent the means of Class `a': 1.8±1.26 (N=10) in (A), 2.1±1.95 (N=36) in (B) and 2.2±2.17 (N=68) in (C). Unbroken arrows represent the means of Class `c': 2.5±0.99 (N=37) in (A), 1.7±1.14 (N=27) in (B) and 1.0±0.70 (N=7) in (C). Insets show distributions with all classes included; means were 1.7±1.41 (N=65) in (A), 1.0±1.63 (N=135) in (B) and 1.4±2.04 (N=114) in (C). Medians in no part of the clusters were significantly different between Class `a' and Class `c'; medians were significantly different between the outermost bee layer and the intermediate and core bees in Class `c' and when all classes were pooled (insets) (P<0.005, U-test). Thd, head temperature.

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