3.1. Location on combs
Bees were classified according to age in the following way: 0–2 d, 3–7 d, 8–12 d and ≥13 d. The distribution of investigated drones on brood (open and sealed) and non-brood areas on the combs is shown in . The distribution of drones differed from that of worker bees investigated at the same time. Drones of all ages preferred the brood area significantly less often than workers of the same age. The maximum number of drones on brood areas was observed at an age of 3–7 days with 35.1%, and for workers at the age of 0–2 days with 72.6% (comparison between drones and workers, of all age classes on brood: χ2 = 182.63, on open brood: χ2 = 172.42, on sealed brood: χ2 = 50.09, threshold: 11.345, P < 0.01; comparison between age classes of drones and workers sorted for age: χ2 = 99.72, 7.66, 43.97, 40.59, threshold: 3.841, P < 0.05).
Comparison of the distribution of drones and worker bees of different age on different areas on the comb. The frequency is given in percent of total number of investigated bees (n in column).
In general, young drones were more often observed on the brood nest area than the old ones. The drones’ relative abundance on brood and non-brood areas was 1:2.7, 1:1.9, 1:3.9 and 1:11.8 from the youngest to the oldest drones. The relative abundance of workers on brood versus non-brood areas was higher than in drones and reversed with age (1:0.4, 1:0.7, 1:1.1 and 1:2.6 from the youngest to the oldest workers). With one exception the relative abundance of different age classes of both drones and workers on brood and non-brood areas was significantly different (). The number of drones, as well as of workers found on sealed brood decreased with increasing age of the insects. On open brood the same trend was visible for the worker bees. For the drones the trend was not so clear, because the 3-7 days old drones preferred the open brood nest disproportionately more frequently (; see for statistics). The relative abundance of drones of different ages on sealed versus open brood was 1:0.3, 1:1.2, 1:0.4 and 1:0.4 (youngest to oldest). By contrast, the relative abundance of workers on sealed versus open brood was higher than in drones (1:1.3, 1:1.9, 1:1.8 and 1:2.0).
Table I Chi2 values for the distribution of drones and workers of 4 different age classes on open and sealed brood, and non-brood areas (). Upper right half of tables: brood vs. non-brood, lower left half of the tables: open brood vs. sealed brood (*, threshold (more ...)
The frequency of drones on brood areas was 35.5% at 15 °C, 32.6% at 20 °C, 17.9% at 25 °C, 31.4% at 30 °C, and 35.6% at 34 °C experimental temperature. Significant differences were detected only between 25 °C and the other experimental temperatures (χ2 = 25.9–43.34 for significant and χ2 = 0.00–1.99 for insignificant comparisons; threshold: 7.879, P < 0.05).
3.2. Thorax temperature and thorax temperature excess
The median thorax temperatures of the drones decreased with increasing experimental temperature stress (range of medians: 28.5–38.3 °C, ). At experimental room temperatures of up to 25 °C younger drones had a warmer thorax than older ones. At room temperatures above 25 °C the thoracic temperatures of different age classes were quite similar. There, the warmest individuals were drones of the oldest group (≥ 13 days, median Tthorax = 38.3 °C) observed near the entrance running actively in preparation for leaving the hive. The variation (range) of the thorax temperatures increased with the age (especially at 15 °C, 20 °C and 25 °C experimental room temperature), but decreased with increasing experimental temperature. Also indicated in is the temperature of the ambient air (circles) and of the cell rim of the comb (squares) near the drones. Both temperatures were usually within the range of the body temperature. The temperature of the cell rims was always above the air temperature except in one age class at 20 °C.
Figure 3 Thorax surface temperature (A), and thorax surface temperature excess compared with abdomen (B), comb (C) and ambient air (D) of drones of different ages in dependence on thermal stress (Texp). Number of measurements with increasing age: 15 °C: (more ...)
The median thorax temperature excess against the abdomen may be used as a unit of measurement for the endothermy of insects (). From moderate to strong thermal stress (25-15 °C experimental room temperature), it was elevated above zero. The highest temperature excess of one individual was >12 °C at the highest thermal stress (15 °C). The median level of thorax temperature excess decreased with increasing experimental room temperature and was about zero at 30 and 34 °C. At 15 °C the two older age classes (8–12 d, ≥ 13 d) showed an obviously higher thorax temperature excess compared to younger drones. However, at the other experimental room temperatures this was not visible. The highest median temperature excess (1.4 °C) was observed in drones in preparation for leaving the hive (≥ 13 days, ). As the distribution of data changed strongly with experimental conditions, however, standard statistical methods were not very useful in evaluating the effect of thermal stress and age on the degree of endothermy. The variation was highest in the older and smallest in the youngest age classes, and decreased for all classes with increasing experimental temperature. The Kolmogorov-Smirnov test showed that distributions differed between age classes within the thermal treatments (P < 0.05), with the exception of the two youngest and two oldest age classes at 15 °C and the two youngest classes at 34 °C.
The median thorax temperature excess against the cell rim of the comb () showed no obviously uniform trend for age classes and experimental room temperatures. At 15 and 25 °C it increased discernibly with drone age from negative to positive values. However, at 20 °C the cell rim data of the youngest age class was missing and no trend could be observed. At lower thermal stress (30 °C, 34 °C) the temperature excess was around zero or only slightly elevated. Again, the variation decreased with increasing experimental room temperature.
The median thorax temperature excess against the ambient air was always elevated above the zero level (). The median of the thorax was typically more than 1 °C higher than the surrounding air in the hive. It was highest at 15 °C and increased with increasing age at this temperature. However, this trend was not visible in the other trials (20–34 °C) where it was quite similar between age classes. The variation decreased at higher experimental room temperatures.
3.3. Frequency of heat production (endothermy)
Another possibility to describe the drones’ contribution to colony heat production is to count the number of drones heating their thorax above a certain level. We classified the drones according to the following scheme (numbers are °C):
- (a) ectothermic: Thead ± 0.19 = Tthorax = Tabdomen ± 0.19 (= difference to 100% in ),
Figure 4 Percentage of heating drones of different ages in dependence on thermal stress (Texp) at different levels of endothermy: (A) Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 0.2; (B) Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 0.5; (C) Thead (more ...)
- three levels of endothermy: weak: Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 0.2 (), moderate: Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 0.5 (), sizable: Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 1.0 ().
The first level contains drones with a minimal to strong endothermic reaction, whereas the third level includes only drones with a sizable heat production (). The number of actively heat producing drones decreased in most cases in all age classes with increasing experimental temperature. This trend was visible at all 3 levels of endothermy. The very strong temperature stress at Texp = 15 °C demonstrated that the ability of endothermy significantly depends on the age of the investigated drones. The oldest drones (≥ 13 d) were at all 3 levels more frequently endothermic than the youngest ones (0–2 days) (P < 0.05; χ2 = 5.67; χ2 = 9.54; χ2 = 7.02; threshold = 3.841, df = 1). At higher experimental room temperatures an age dependence was less clearly noticeable. The last two columns represent old drones (≥ 13 d) inside (X) and outside (Y) the hive in preparation for leaving the colony and flight. More than 60% and 100% of them were still endothermic at the third (‘sizable’) level, respectively ().
The inserts in show the portion of heating drones of all age classes on brood and non-brood areas and their mean ambient hive air temperature (Tair). On the non-brood area a continuous increase of the frequency of heating drones with decreasing experimental room temperature was detected. This trend was also present in the brood area but at 15 °C the number of heating drones decreased strongly. The mean Tair near the measured drones, as shown in the inserts of , decreased nearly linearly with decreasing experimental room temperature in both brood (squares) and non-brood areas (circles). The one exception from this observation in the brood area at 15 °C (increased Tair) concurred with the observed decrease of heating drones there.
In trials with medium and high experimental room temperatures (20–34 °C) the portion of heating drones did not differ between brood and non-brood areas (P > 0.05; χ2 < 3.18; threshold = 3.841, df = 1) except at 34 °C (; P < 0.05; χ2 = 8.82; threshold = 3.841, df = 1). At a very low experimental room temperature of 15 °C significantly less heating drones were observed on the brood area at all 3 levels of endothermy (P < 0.01; χ2 = 49.23, 60.75, 54.22, respectively; threshold = 6.635, df = 1). presents the frequency of heating drones of 3 age classes (: 0–2 d, B: 3–7 d, C: ≥ 8 d) at two different levels of endothermy, sizeable and weak (insert), on brood and non-brood areas. With decreasing Texp the frequency of heating drones increased in the 3–7 day and older (≥ 8 days) drones especially in the non-brood area. Differences between the two areas were highly significant for the two older classes (P < 0.001; 3-7 d: χ2 = 20.89, 14.66; ≥ 8 d: χ2 = 29.05, 24.61; threshold = 10.83, df = 1) at a Texp of 15 °C.
Figure 5 Percentage of sizably (Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 1.0) and weakly (Thead + 0.2 ≤ Tthorax ≥ Tabdomen + 0.2, inserts) heating drones of different ages (A: 0–2 d, B: 3–7 d, C: ≥ 8 d) on brood (more ...)
With an ANCOVA we tested whether location on brood or non-brood area, temperature of ambient air (Tair) or comb surface (Tcell rim) had an influence on the frequency of heating drones (inserts in , data for Tcell rim not shown). Results clearly revealed that the location on the comb (brood or non-brood) had no influence on the frequency of heating (P = 0.097, P = 0.866, P = 0.912 for the weak, moderate and sizable level of endothermy, respectively). Further analysis with the brood and non-brood data combined showed that the frequency of heating drones was negatively correlated with Tair (P < 0.01; R2 = 0.7609, 0.7521, 0.5967 at the 3 levels, respectively; n = 10), and it was also negatively correlated with Tcell rim (P < 0.05; R2 = 0.76973, 0.68097, 0.56665; n = 10).
The temperature of the comb surface correlated significantly with the temperature of the ambient air beside the ectothermic drones (Tcell rim = 0.7388 × Tair + 9.6975, R2 = 0.6073, P < 0.0001, n = 1291), and the heating drones (at least weakly endothermic; Tcell rim = 0.8550 × Tair + 5.9413, R2 = 0.7293, P < 0.0001, n = 463).
In conclusion our results suggest that ambient air and comb temperature near the measured drones had most influence on their frequency of heating and level of endothermy, and no influence of location (brood or non-brood areas) was found.