Hot Takes on Cool Nests

 

Hot Takes on Cool Nests: How Paper Wasps Thermoregulate Their Nests

Ainsley Yuknis, Rachel Bottum, Henry Liu; BIO 368

Polistes is the most common and widespread genus of paper wasps. They are known for creating open, comb-shaped nests, often on human structures. They are a eusocial species, with a foundress laying all eggs within a brood while other members help care for the larvae (Jeanne & Suryanarayanan., 2011). These social adaptations give them an advantage when it comes to thermoregulation: paper wasps can work together to stay warm. In most social insects, thermoregulation in nests is achieved through a mix of metabolic and behavior strategies. This usually means crowding together and vibrating their wings to heat up their nests. However, paper wasps create open nests, meaning metabolic thermoregulation is not as  efficient, especially in cooler climates, as most heat generated is lost to the air. For polistes in cold areas, selection plays a larger role in nest location than with species who build nests capable of internal thermoregulation. In addition to nest site choice, ‘immediate’ behavioral means like cooling by fanning and distribution of water droplets on the nest for evaporative cooling allow control of the nest temperature (Stabentheiner et al., 2022).

“Effect of climate on strategies of nest and body temperature regulation in paper wasps” explores our questions about wasp habitat selection. This study aimed to determine how thermoregulatory and site-selection behavior differentiated between two species of Polistinae in different climates. Two species were compared; one from the Mediterranean (P. gallicus) and one from the Alps (P. biglumis). The two species differed in how they maintained suitable brood temperature.  This study analyzed both substrate temperature (material on which the nest was built) and the thermal environment within nests (brood and body temperature) over the course of the day. Since the Alps species resides in conditions resembling that of Iowa, we include more detail on P. Biglumis than P, Gallicus results to create the hypotheses for our experiment.

For Polistes Biglumis, nest and wasp temperature were low before sunrise (mean ~ 15 C).  The substrate that the nest was built on was higher (~ 20C).  The nest temperatures rose very quickly after sunrise.  After 13 minutes of radiation the nest temperature went from ~17 C to ~25 C.  After one hour, the temperature had risen 20 C from before sunrise.  At peak heat, the wasps fanned to cool the nest down, they also gathered water and used evaporative cooling.  At ~13:00, when the nest was shaded, the nest temperature decreased (to ~21 C after dusk).  The nest did not cool down once shaded nearly as quickly as it warmed up once exposed to sunshine (Fig 1 a,b).  Overally, the substrate was always approximately 5 degrees warmer than the nest, which slowed the nest from cooling down as the air temperature cooled down. Strategies of maintaining appropriate brood temperature differed between the two species.  In cool climates (P. Biglumis), wasps increased the brood temperature simply by choosing nest locations that were facing South East (Figure 2).  On the other hand, P. gallicus nests were found in locations with little direct sunshine.

Figure 1: Daily temperature changes of nests and wasps of P. biglumis (c,d) T_thorax =mean thorax surface temperature of up to five adult individuals per time of measurement; gray ribbon: total range of nest temperatures (T_max:T_min) with mean; T_substrate=temperature beside the nest; T_anest=ambient air temperature directly at the nest. T_a=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.

 

Figure 2: Horizontal and vertical nest orientation of P. biglumis in Alpine climate, and of Polistes gallicus in Mediterranean climate. Mean values and Medians (thin bars) calculated according to the rules of circular statistics.

For our experiment, we did not look at individual species because we only analyzed one environment; Grinnell College. We hypothesized that paper wasps in Iowa would value warm and sunny areas (South East facing), much like P. biglumis. Iowa also has the factor of persistent winds due to the flat terrain. This may factor into paper wasps’ habitat selection, so we ranked the recession levels of windows and eaves hypothesizing that they would seek places protected from these winds. We took 5.5 meter transects of buildings on campus, noting whether nests were present, the floor they were on, the substrate temperature, and the building face.

            Since we analyzed campus buildings, the cardinal directions correspond exactly to the faces of buildings. This meant we did not have to factor in specific degrees in our measurements, simply going with the four categories of north, east, south, and west. We found that temperature was the largest factor in habitat selection. As hypothesized, wasps preferred warmer areas which also tended to be on the south and east faces of buildings. Wasps also preferred more recessed areas. This could both be due to the greater amount of space to build nests and protection from winds.

References

Jeanne, R. L., & Suryanarayanan, S. (2011). A new model for caste development in social wasps. Communicative & Integrative Biology, 4(4), 373-377.

Stabentheiner, A., Nagy, J. M., Kovac, H., Käfer, H., Petrocelli, I., & Turillazzi, S. (2022). Effect of climate on strategies of nest and body temperature regulation in paper wasps, Polistes biglumis and Polistes gallicus. Scientific Reports, 12(1), 3372.