The Effect of Activity Level and Ambient Temperature on Thermoregulation in Isolated Honeybees (Hymenoptera: Apidae)

Abstract

Thermoregulation of isolated honeybee workers was studied at 25° and 35°C ambient temperature by means of infrared thermography. Thoracic temperatures of bees free to move within small boxes were measured repeatedly and analyzed according to the bees' activity level: standing motionless, walking slowly, walking fast, etc. Also, thermographic measurements were made on bees experimentally immobilized but unharmed. The artificially immobilized bees were thermoregulating to an unpredictable extent, with a thoracic temperature excess of 0°-9°C above ambient temperature, which leads to the conclusion that research on energy turnover has to be done in animals which are free to move if conclusions relevant to natural situations are intended. The thoracic surface temperatures of the bees that could move freely also varied within wide limits, in this case following a bimodal distribution. One peak was near ambient temperature and the second peak around 35°-42°C at 25°C and around 38°-43°C at 35°C ambient temperature. Furthermore, considering the bees' level of activity led to a clear reduction of variability of the data. Spontaneously motionless bees exhibited the smallest temperature excess, amounting to 2.3 and 0.3°C at 25° and 35°C, respectively. The most active bees regulated thoracic temperature at an increased level (up to 42. 8°C) to be able to move fast and to be prepared for immediate takeoff. In order for bees to regulate thoracic temperatures at a constant level independent of ambient temperature, energetic investment was much higher at lower ambient temperature: moderately active bees showed a thoracic temperature excess of 9.3 °C at an ambient temperature of 25 °C, but only 3.5°C at 35°C. Fast walking bees showed an even increased temperature excess of 14.3° and 5.6°C, respectively. Since, for the same behavior, the difference in ambient temperature did not change the bees' speed of motion as estimated by eye, it can be concluded that the greater part of the thoracic temperature excess originates from thermoregulation via the thoracic flight muscles, and was only in small part the result of walking activity.