Both in Polistes biglumis and P. gallicus in most of the inhabited nests all types of brood were present: eggs, larvae and pupae (Table S1), with the exception of one foundress nest of P. biglumis with only one egg. The size of thermographed nests was quite variable in both species, the number of cells ranging from 18 to 99 in P. biglumis (mean: 61.6 cells), and from 19 to 381 in P. gallicus (mean: 101.7 cells) (Table S1). The mean number of wasps on the thermographed nests was higher in P. gallicus (12.6 wasps) than in P. biglumis (7.1 wasps). All nests of Polistes biglumis we observed in this study were built on stone substrate or walls (Figs. 1c, 2a). Only recently we found one nest built on a pile of wood. The choice of the nest substrate was more diverse in P. gallicus (Figs. 1d, 2b). They chose stone, concrete, walls, window grilles, and metal of fences or doorframes.
Examples of nests and fieldwork set-up in Obergail (a) and Sesto Fiorentino (b). 1 = thermocouple wire; 2 = global radiation sensor, 3 = Peltier-element IR reference source.
Daily nest temperature course
Polistes biglumis
Figure 3 shows a sequence of thermograms of a P. biglumis nest taken from dawn to dusk. Before sunrise the temperatures of the nest and of the wasps on it were quite low (mean ~ 15 °C) and uniform (~ 12 to 17.5 °C; Fig. 3a). The temperature of the stone substrate where the nest was built on was considerably higher (~ 20 °C). After sunrise (Fig. 3b,c) the nest temperature began to rise quickly. It only needed 13 min of sunshine (radiation) to heat the nest from ~ 17 to ~ 25 °C. Within one hour, temperature differences of almost 20 °C were measured within the nest. At 6:50, when the highest temperature on the nest was already at 36.2 °C, fast movements of the adults with inspections of the cells were observed (Fig. 3c). Soon afterwards the increasing temperature induced the wasps to start fanning (arrow in Fig. 3d). The wasps also began to gather water and spread it on and inside cells to cool the nest by evaporation (Fig. 3d,e). Towards late morning, some parts of the nest reached temperatures as high as 46 °C (Fig. 3e)! As soon as the nest was shaded by the substrate (~ 13:00) the nest temperature decreased according to the decrease in ambient temperature (Fig. 3f,g), reaching ~ 21 °C on average after dusk (Fig. 3h). At that time the substrate temperature (~ 25 °C) was still about 4 °C higher than the nest temperature.
Thermograms of a P. biglumis nest during a whole day (19.07.2017). (a) Before sunrise at 6:20; (b) during sunrise (06:33); (c) nest temperature increasing fast in sunshine; (d) with a fanner for convective nest cooling (arrow; see also Fig. S4); (e) with water drops for evaporative cooling when sunshine increased part of the nest to temperatures > 45 °C; (f,g) after sunset (nest now in shade) in the afternoon; (h) at dusk with wasps sitting motionless on the nest. Time = CEST = UTC + 2 h.
The nest and body temperatures of a complete 24 h cycle of a different nest are shown in Fig. 4a. At night the nest temperature and the wasps’ thorax temperature decreased slowly according to the decrease of the air temperature. The substrate temperature was always higher than the mean nest temperature, which surely helped to keep the nest temperature higher than the temperature of the surrounding air (Tanest). Variation of within-nest temperature (max–min) was low at night. As soon as solar radiation increased in early morning, the nest temperature and the body temperature of the wasps on it increased rapidly, and the variation of nest temperature (max–min) increased (see also Fig. 3b). Though the maximum nest temperature reached values as high as 46.9 °C, cooling measures of the wasps (fanning and spreading of water drops, see below) kept the mean nest temperature always below 38.5 °C. Cooling of the nest after sunset (at the nest) was much slower than the increase in the morning, following the decrease of ambient and substrate temperature (Fig. 4a,b).
Examples of daily temperature changes of nests and wasps of P. biglumis (a,b) and P. gallicus (c,d). Tthorax = mean thorax surface temperature of up to five adult individuals per time of measurement; gray ribbon: total range of nest temperatures (Tmax:Tmin) with mean; Tsubstrate = temperature beside the nest (see Fig. S1c,d); Tanest = ambient air temperature directly at the nest. Ta = ambient air temperature in shade 1–3 m away from nest; Radiation = global radiation hitting the nest; black bars = fanning events at the time of thermographic measurements: actually, many more fanning events were observed. (c) Fanning was never observed! See also Fig. S2 for another example of a P. gallicus nest in shade. Time = CEST = UTC + 2 h.
Polistes gallicus
Most P. gallicus nests were built in locations with no or only little direct sunshine (Figs. 2b, 4c, Fig. S2). In their habitats temperatures in midsummer are often already quite high in the morning, and may increase to values higher than 40 °C during the day (Fig. 4d). Mean temperatures of the nest and of the imagines on it were usually higher than the air temperature close to the nest (Tanest). In most nests variation of within-nest temperature (max–min) remained small throughout the day. On hot days (Tanest > 40 °C), however, maximum temperatures of empty cells in the nest margin sometimes reached values as high as 49.9 °C even in shade. Body temperature of the adults was mostly similar to the mean nest temperature (Fig. 4c, Fig. S2). At night, the nest temperature decreased according to the decrease of Tanest, similar to P. biglumis but at a higher level (Fig. 4d).
The situation was different in one large nest which had been built in a location exposed to the morning sun (Figs. 4d, 5). On a hot day when Tanest increased to values higher than 42 °C, the body temperature of the adults increased to values up to 5 °C higher than the mean nest temperature. Nevertheless, though the combined effects of high air temperature and intense insolation increased part of the nest to a temperature of ~ 58 °C (Fig. 4d), mean nest temperature was kept below 41 °C. This was accomplished by cooling with many water droplets in the cells (dark spots in Fig. 5), and by the occurrence of fanning during the period when the sun was shining on the nest (Fig. 4d; see arrows in Fig. 5c). Fanning, however, was quite rare in all the other observed nests, even during the hottest time of the day! Water droplets were carried onto this nest until evening (Fig. 5h), as at that time the nest temperature was still at about 35–38 °C.
Thermograms of a large P. gallicus nest during a whole day (01.08.2017). Thermograms are rotated 90° clockwise (the upper part is on the right). (a) Before sunrise (6:36); (b) during sunrise (06:46) with the first water drops visible (dark spots); (c) with two fanners for convective nest cooling (arrows, see also Fig. 4d); (d) with more cooling drops; (e) after sunset at the nest site (nest now in shade); (f–h) after sunset in the afternoon and evening. Time = CEST = UTC + 2 h. For temperature evaluation see Fig. 4d.
Body and nest temperatures
Figure 6 shows a comparison of the dependence of body and nest temperatures on ambient air temperature and insolation between the two species. In the lower ranges of air temperature, usually at night, body temperature followed Tanest closely in both species. The exposition of the P. biglumis nests to the morning sun at ESE (Fig. 7) increased the wasp body temperature to values of often more than 15 °C higher than the surrounding air. However, body temperatures remained always below 40 °C (Fig. 6a). In P. gallicus, by contrast, the body temperature of the wasps increased considerably above 40 °C, to maximum values of about 46 °C, especially (but not exclusively) during intense insolation in the nest exposed to the morning sun (Fig. 6b).
Surface temperature of the thorax of adult wasps, of different stages of brood and of water drops of P. biglumis (left) and P. gallicus (right), in dependence on ambient air temperature close to the nest (Tanest) and global radiation (color scale). Egg f.n. = single egg on a foundress nest; diagonal lines = isolines. Regressions were calculated for shaded conditions (Radiation = 0–100 W/m2; black or gray solid lines) and sunshine (Radiation > 100 W/m2; pink broken lines); P <<< 0.0001 for all except for linear regression in (f) (P = 0.021, pink line). For regression functions and detailed statistics see Table S2. Part of Tthorax values in P. gallicus included from Kovac et al.25.
Horizontal and vertical nest orientation of Polistes biglumis in Alpine climate, and of Polistes gallicus in Mediterranean climate. Mean values and Medians (thin bars) calculated according to the rules of circular statistics46. Note the bifurcated distribution of nest orientation in P. gallicus.
Multiple linear model regression proved a significant influence of both ambient temperature close to the nest (Tanest) and global radiation on the temperature of thorax, head and abdomen in both P. biglumis and P. gallicus (P << 0.0001, ANOVA; Table S3). The same relationship was found for the temperature of the larvae and pupae (Table S3). For the mean nest temperature, including the brood, empty cells and the wasps on it (see “Poly” in Fig. S1c), the relation can be described by the multiple linear model equation Tnest(mean) = 2.38935 + 1.0085 × Tanest + 0.00696722 × Radiation in P. biglumis, and Tnest(mean) = 6.58421 + 0.821208 × Tanest + 0.00265919 × Radiation in P. gallicus, with R2 (adj. for df, %) = 82.3 and 83.5, respectively (P <<< 0.0001, ANOVA; for details and more model regressions see Table S3).
An extended model including the substrate temperature (where the nests were built on; see Fig. S1c,d) explained an even greater part of the total variation, with R2 (%) = 85.1 in P. biglumis and 93.7 in P. gallicus. The model equations were: Tnest(mean) = − 2.14936 + 0.510637 × Tanest + 0.00475538 × Radiation + 0.611022 × Tsubstrate in P. biglumis, and Tnest(mean) = 8.01929 + 0.0986386 × Tanest + 0.00318776 × Radiation + 0.620503 × Tsubstrate in P. gallicus (see Table S4 for more details).
A multifactor ANOVA comparison between the two species, compensating for the identified main environmental effects on nest and body temperature (Tanest, Radiation, Tsubstrate) uncovered interesting similarities between the two species (Table S5), despite the differences in climate. Mean compensated nest temperature (Tnest(mean)), and compensated temperatures of empty cells and cells containing eggs or larvae did not differ between species (‘Mean T’ in Table S5). In both P. biglumis and P. gallicus the mean temperature of empty nests, being without the thermal control by the adults, increased in parallel with ambient temperature (Fig. S3a,b). Maximum temperatures of empty nests reached quite high values in sunshine, even in the cool alpine climate experienced by P. biglumis (> 45 °C; Fig. S3a).
Cooling mechanisms and nest orientation
In contrast to the adult wasps’ body temperature, brood temperature in both species usually did not exceed 42.5 °C (Fig. 6c,d). However, according to the differences in climate, strategies of brood temperature control were partially different between the two species. In P. biglumis, living in cool climate, the adults were able to increase the brood temperature considerably above the ambient level on sunny days (Fig. 6c) by building their nests exposed to the morning sun, facing ESE on average (Fig. 7). Nevertheless, they started cooling measures soon after sunrise to prevent overheating of the brood. First, they started heavy fanning to cool the nest by convection (Fig. 4a,b; see below). In addition, they flew out to bring water drops to the nest (Fig. 3d,e). The temperature of the water drops remained below 38 °C even in bright sunshine (Fig. 6e).
In most nests of P. gallicus the foundresses had built their nests preferably in locations with little direct sunshine on the nest (Fig. 4c, Fig. S2). Nest orientation was more variable, showing double-peaked distributions both in the horizontal and in the vertical direction (Fig. 7). Evaporative cooling with water droplets was the predominant acute measure of thermoregulation. Despite ambient temperatures up to 45 °C, droplet temperature nearly always remained below 38 °C (Fig. 6f), which mostly kept the brood temperature below 42.5 °C (Fig. 6d).
On warm sunny days, fanning was observed to be frequent in P. biglumis. It was rare in P. gallicus, occurring only when the nest was exposed to the sun (Fig. 4d). Therefore, its effect on nest cooling was investigated in P. biglumis in detail. Wasps engaged in fanning were often patrolling hectically across the nest, sometimes flying a small loop around the nest and again patrolling across the nest after landing (Fig. S4f, see Video S1). They frequently were putting their head inside cells for short periods, probably for temperature measurement with their antennal thermosensors (topmost wasp in Fig. S4b). Soon after start of fanning cell temperatures decreased around their position (Figs. 3d, 8, Figs. S4d, S5, S6).
Cooling effect of fanning in a Polistes biglumis nest. Temperatures measured on cell rims and centers of cells close to and distant of the fanner, and of the body of the fanner. Also shown are maximum, mean and minimum nest temperature, air temperature close to the nest (Tanest), and in (d) the change of the total distribution of nest temperatures during different times of fanning (percent of nest at a certain temperature estimated from number of pixels in “Poly” in Fig. S2c). Time = CEST = UTC + 2 h. Gray bars = duration of fanning. For more samples see Figs. S5 and S6.
In the example shown in Fig. 8 the temperature of cells close to the fanner experienced a sharp temperature drop within a second after start of fanning. The temperature decrease was higher at the cell rims (up to ~ 7–13 °C) than inside of the cells (up to ~ 6–9 °C) (Fig. 8a), and mostly much smaller in distant cells on the opposite part of the nest (Fig. 8b). Fanning also decreased the body temperature of the fanners by 1–2 °C on average, despite the activation of their flight muscles (Fig. 8c). After stop or interruption of fanning the cooling effect vanished quickly, and temperatures returned to the initial values within seconds (Fig. 8, Figs. S5, S6). Fanning shifted the temperatures of the whole nest by several degrees to lower values (Fig. 8d, Figs. S5, S6).
In an evaluation of 9 fanning events the mean cooling effect amounted to ~ − 2.4 °C and − 1.9 °C for maximum and mean nest temperature, respectively (Table 1). Rims and centers of cells close to the fanner decreased by − 5.7 °C and − 4.4 °C, whereas in cells distant to the fanner (on the opposite end of the nest) temperatures decreased by only − 0.7 °C and − 0.4 °C, respectively. It has to be kept in mind, however, that quite often more than one wasp was fanning at a time (see Video S1). The cooling effect on the nest may therefore often be spacially more extensive than shown in Figs. 3d, S4d and 8, and larger than reported in Table 1.
Source: Ecology - nature.com
