A large morph male Centris pallida (top) flies away with a female (bottom). Photo credit: Bruce Taubert.
With the sun beating down on the dry ground and dull-coloured plants, many deserts seem like the last place you'd expect to find bees – but they're there. These bees have evolved a number of different ways to beat the heat, but as climate change causes the world to warm up, it's also making some deserts even drier than before. In the Sonoran Desert of the southwestern USA and northwestern Mexico lives a solitary, ground-dwelling bee (Centris pallida) which might already be equipped to handle the hotter and drier conditions that are coming their way. To find out what might happen to these bees when the world warms even further, Meredith Johnson and Jon Harrison of Arizona State University, USA, teamed up with Meghan Barrett of Indiana University, USA, to test how hot the bees get while flying around in the scorching heat of the desert sun or in the shade of a grove of trees.
In April 2022, the trio headed out into the desert near the Salt River in Arizona, USA, and collected bees and separated them into three groups (large males, small males and females) before measuring how much carbon dioxide and water they exhaled while flying in the sun or the shade. The bees breathed out more carbon dioxide when they were flying in the sun than they did when in the shade, regardless of the temperature. Interestingly, as the air temperature rose in the shade of the trees, the small males breathed out less carbon dioxide. A similar phenomenon happened with the large males while in the sun. This suggests that the different types of males cool themselves down in different ways.
After the team flew the bees, they measured how hot the head, flight muscles and abdomen of the bees were to see if the bees’ body temperature was getting close to the temperature of the air and if the scorching sun was making them even hotter. Although the bees’ head temperature was unaffected by the sun, their middle section (which contains the wings, legs and important flight muscles) was hotter than that of the bees in the shade, and got closer to the air temperature as the air got hotter. Johnson and colleagues saw a similar rise in the temperature of the abdomen in bees that were flying in the sun compared with the ones under the shade of the trees. In fact, their abdomen temperature was 3°C higher in the sun than it was in the shade even though the temperature of the air was the same. So, the sun is making the bees warmer; is there anything they can do to combat this increase in body temperature?
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. Regardless of how they keep cool, the reality is that the scorching sun is making it harder for the bees to stay cool while going about their business. If the temperature continues to rise, these bees – which are already near the limit of the temperatures they can handle – won't be able to take the heat.
