Figure 1.
Close-up thermogram of honeybees.
Left and right: ectothermic bees (light blue: Tthorax = 25.2°C and 24.9°C). Top and bottom: endothermic bees (white: Tthorax = 35.6°C; orange: Tthorax = 32.4°C). Ta = 25°C, Tcell rim ∼24°C. Measurement performed in a peripheral area of an observation hive during high cold stress (Texp = 20°C).
Figure 2.
Worker honeybee thorax temperature on different locations in the observation colonies in dependence on cold stress (Texp).
Box plots: median Tthorax with 1st and 3rd quartile, maximum and minimum. Notches which do not overlap indicate significant differences (P<0.05). Tcell rim and Ta: medians. Numbers = n (Tthorax). * are two bees during intense examination by hivemates [45], [46].
Figure 3.
Frequency of honeybees of different ages on brood (br) and non-brood (n-br) areas.
Shading of bars = thermal stress (Texp). (A) Marked bees on combs; 100% = 12732 bees (measurements) of all age classes. Insert: detailed analysis of 0–2 d old bees. (B) Bees in cells; 100% = 350 bees of all age classes. Percentage of bees in brood nest cells per age class (young to old): 92.5 (184 of 199), 87.5 (70 of 80), 92.3 (36 of 39), 76.2 (16 of 21), 100.0 (5 of 5), 100.0 (6 of 6); no significant differences between age classes (P>0.05, Chi2 = 0.00–6.08). Insert: distribution of unmarked bees on brood and non-brood areas (‘nn’ in Figs 7, 8); n = bees = 100% (per Texp). (A, B) Significant differences between brood and non-brood: * P<0.05, *** P<0.001, Chi2 test.
Figure 4.
Body temperature and temperature excess of workers of different ages on brood in dependence on thermal stress (Texp).
Box plots: median with 1st and 3rd quartile, maximum and minimum of (A) Tthorax, (B) Tthorax-Tabdomen, (C) Tthorax-Ta, and (D) Tthorax-Tcell rim. n = measurements (bees) (range in parenthesis).
Figure 5.
Percentage of endothermic bees in dependence on thermal stress (Texp).
Shading of bars = age. (A-B) Percentage of actively heating workers on brood (main graphs) and non-brood areas (inserts); (C-D) percentage of actively heating workers in open brood nest cells. Parts (A, C) and (B, D) display different levels of endothermy (Tthorax-Tabdomen). Number of investigated workers of each age class ( = 100%) written above each column in (C) and (D). Number of measurements for non-brood areas (inserts) with increasing age: Texp = 15°C: 14, 47, 140, 971; Texp = 20°C: 12, 127, 157, 651; Texp = 25°C: 85, 227, 125, 698; Texp = 30°C: 72, 178, 172, 562; Texp = 34°C: 68, 189, 163, 776. Statistics: *, differences significant (P<0.05, chi23.841 >3.94) between 0–2 d old and >2 d old bees at a certain Texp (at Texp = 20°C, P<0.1, chi22.706 = 3.484). #, differences not significant (P>0.05) between Texp = 15°C and Texp >15°C in each age class; otherwise P<0.05 (chi26.239 >6.94 in A, B and chi25.731 >7.39 in C, D).
Figure 6.
Intensity of endothermy (Tthorax-Tabdomen) of bees on combs in relation to individual bees' local ambient temperatures (Tcell rim and Ta).
See Fig. S1 for correlations between Tcell base and Tcell rim, and between Ta and Texp.
Figure 7.
Thermal relations in open brood nest cells visited by bees.
Box plots: median with 1st and 3rd quartile, maximum and minimum. Notches which do not overlap indicate significant differences (P<0.05). nn = unmarked workers; red crosses = medians of all age groups of marked bees (not significantly different from nn bees, except Tthorax-Tabdomen at Texp = 25 and 30°C, P<0.05, U test).Tcell 30% is the cell wall temperature at the estimated mean contact position of the thorax with the cell wall at 30% cell depth as measured from the base, calculated by linear interpolation between Tcell base and Tcell rim. Numbers = measurements (bees).
Table 1.
Regressions between hive temperatures and experimental temperature.
Figure 8.
Heat transfer between bees in cells and brood combs.
Percentage of workers of different ages in brood nest cells with (A) higher Tthorax than Tcell 30%, (B) approximately equal Tthorax and Tcell 30%, and (C) lower Tthorax than Tcell 30%, in dependence on thermal stress (Texp = shading of bars). Sample numbers in (C) represent 100%. nn = naturally emerged, unmarked bees. nn bees did not differ from the averaged age classes of marked bees (P>0.1; Chi22.706 = 0.00–2.143) with exception of bees at Texp = 30°C in (B) (P<0.05; Chi23.841 = 4.163).