Pollen provides most of protein in a bees diet and bumblebees prefer pollen with high protein content. Bumblebees also regulate the temperature of their thorax for flight efficiency. Between 25 C and 38 C, flight efficiency increases with increasing thoracic temperature. A higher temperature requires more energy, but makes flight more efficient. A group of scientists* found that bumblebees react to pollen quality by increasing their body heat. Scientists provided bumblebees with pure pollen or pollen cut with cellulose to lower protein content. The bumblebees returning to the hive after collecting the high protein pollen had a higher thoracic temperature than bees feeding on pollen cut 3:1 with cellulose to lower protein content. This suggests that bumblebees respond to high quality pollen by elevating body temperature. This could allow the bumblebees to collect more pollen when high quality pollen is available.
*Katherine S. Mapalad, Daniel Leu & James C. Nieh. Bumble bees heat up for high quality pollen. Journal of Experimental Biology 2008 211: 2239-2242.