Publication Alert: How hot is the sun for a desert bee? (Answer: Very)
Image of fighting Centris pallida large males (metanders) by Bruce Taubert.
Solar radiation alters heat balance and thermoregulation in a flying desert bee (Johnson et al. 2025; Journal of Experimental Biology)
Acknowledgement of Funding and Conflicts of Interest: This work was supported by Friends of the Sonoran Desert, The Social Insect Research Group, United States Department of Agriculture (award 2017-68004-26322). Open Access funding provided by North Dakota State University. Deposited in PMC for immediate release. The authors declare no conflicts of interest.
Animals around the world are facing increasing temperatures as a result of climate change; desert animals may be especially vulnerable, given that they live in climates that already subject them to very extreme temperatures. Investigating how animals gain and lose heat allows us to understand their likely body temperature in a given set of climatic circumstances - and try to see what mechanisms, behavioral or physiological, they might use to adapt and survive in their climate.
In this study, Meredith Johnson created heat budgets (essentially, all the thermal inputs and outputs for an animal) and analyzed bee body temperatures to understand how a variety of individuals of the bee Centris pallida, a solitary digger bee that lives in the Sonoran Desert, might be impacted by solar radiation while flying. How MUCH does solar radiation actually contribute to the bees’ body temperature, compared to being in the shade?
“Solar radiation was responsible for 43 to 54% of mean total heat gain. Bees flying in the sun had thorax temperatures 1.7°C warmer than bees flying in the shade, storing a very small fraction of incident radiation in body tissues.”
So the sun plays a very large role in what might heat a flying desert bee! But, bees flying in the sun were only a bit warmer than those flying in the shade. Do any other methods of heat gain or loss change in response to increased heat load from the sun?
C. pallida is an amazing bee species for studying this question as the species has alternative reproductive tactics: males come in two ‘morphs’, large and small, which vary in their morphology, as well as reproductive and flight behavior. So, there is high intraspecific diversity - allowing us to see if different individuals from the same species might adjust their thermoregulation differently in the sun v. shade.
As covered by Jarren Kay of the JEB team, different types of C. pallida (females, small males, and large males) use different strategies to balance the heat ‘budget’ in the sun v. the shade:
“As it turns out, each group deals with the warm weather differently. The large males decreased their metabolism when in the sun, which makes them produce less body heat. These large males are also a lighter colour than the smaller males, helping them reflect the sun’s rays, keeping them cooler in the sunlight. But these solutions only cool them a bit. The large males and females could potentially move the fluids around between their wing muscles and abdomen to help shed the excess heat in a process called convective cooling. The small males don’t seem to do this and the way they keep cool remains a mystery. ”
Largely, C. pallida bees of all sexes/morphs responded to solar radiation by increasing convection - or, heat loss to the air around them. However, this may present a problem under climate change as convective heat transfer is only ‘cooling’ if air temperatures are cooler than the animal’s body temperatures. As air temperatures continue to get warmer, flying bees may have difficulty using convective cooling as a thermoregulatory strategy when in the sun, ultimately reaching a tipping point where non-sustainable evaporative cooling is necessary for short-term survival. However, as water is not plentiful in the desert, the short-term use of evaporative cooling to avoid lethal heat stress may result in long-term survival or fitness declines as bees have to find water and re-fuel.
These results build on some prior results from my thesis research (Barrett and O’Donnell 2024) that shows the differences in coloration between large and small morph males matters for reducing the amount of energy they absorb from the sun. Importantly, these data show that animals’ strategies for balancing the heat budget vary significantly based on microclimate (sun v. shade) and that there’s room for plenty of intraspecific diversity!
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