Sanctuary. Volume 10 (1992), Number 4: 1–2.
Bees and Heat
By John P. Roche
When watching a bumblebee forage among a bed of flowers on a cool morning in late summer, we may wonder: How can the bee stay warm enough to fly? Bumblebees don't only warm themselves up so that they can fly at low ambient temperatures, they also warm their broods to speed the development of their offspring, much as birds do. Bumblebees even have a device for getting rid of excess heat on warm days to prevent overheating. For their size, bumblebees are astonishing thermoregulators.
In conventional biological parlance, birds and mammals are considered endothermic, meaning that they maintain high body temperatures independent of environmental temperatures. All other animals are considered to be poikilothermic, meaning that they are dependent on heat from the environment to determine their body temperatures. In actuality, this distinction is not so clear. "Endothermic" animals sometimes partly abandon temperature control, and "poikilothermic" animals sometimes display sophisticated mechanisms of manipulating their temperatures, not just behaviorally, but also physiologically. As we will see, bumblebees, which at first glance may appear to be totally at the whim of environmental temperatures, are capable of physiological thermoregulation.
The mysteries of bumblebee thermoregulation were ingeniously discovered in a series of experiments by zoologist Bernd Heinrich. A bumblebee's flight muscles are only capable of producing flight within a narrow temperature range: between 30°C and about 44°C. Below this range, the enzymes necessary for converting chemical energy into mechanical energy are inefficient. Above this temperature range, the bee dies. Without a mechanism for warming its flight muscles on a cold morning, a bumblebee would be unable to fly and forage and would eventually starve. What the bee does to solve this problem is to shiver its flight muscles, which produces heat.
Like all insects, the bumblebee's body consists of a head, a thorax, and an abdomen. The flight muscles are in the thorax, and this is the body segment that bumblebees warm up with their shivering flight muscles on a cold day before flight. The circulatory system of a bumblebee is an open one, consisting of one blood vessel running from the head to the abdomen. This vessel, or "heart," is open at both ends and blood freely flows through the rest of the body outside of the vessel. The bee's circulatory system assists it in warming its thorax when outside temperatures are low. As blood is pumped through the heart from the thorax into the abdomen, it passes through a narrow region called the petiole that connects the two body segments. In cold conditions, the warm thoracic blood is pumped in a slow, steady flow through the petiole. As it passes through the petiole, it passes cooler blood flowing outside of the heart from the abdomen back to the thorax. As the cooler blood passes the warmer blood, it picks up heat from the blood being pumped through the heart. Then it carries the heat back into the thorax. This elegant physiological mechanism is called countercurrent heat exchange, and it is employed by a range of animals, including tuna and seals. Countercurrent exchange allows the bumblebee's flight muscles to be warmed up quickly, and it prevents unnecessary loss of heat from the abdomen when the bee is foraging on cold days.
The bee must raise its thorax to at least 30°C for its flight muscles to work. But on a warm day, it can quickly produce too much heat while flying. If the bee did not have a way to eliminate the excess heat, it would quickly overheat and die or be forced to be inactive and risk starvation. How the bumblebee solves this problem in thermoregulation is by changing the pattern of its blood flow.
When flying at high temperatures, the bee pumps its heart more strongly, but it pumps it only about half as often as it does at lower temperatures. This changes the movement of blood from a steady flow into a series of pulses. Under this pattern of flow, cooler blood from the abdomen only passes through the petiole back into the thorax between pulses of the heart. As a result, warm blood in the heart that is headed to the abdomen does not pass the cooler blood returning from the abdomen, and the heat in the blood in the heart is allowed to flow out into the abdomen. From the abdomen, this heat is "dumped" out into the environment from the abdomen's hairless undersurface. (The rest of the bee is covered with hairs that act as insulation in cool temperatures, but the abdomen's underside lacks hair to facilitate heat transfer.)
This same mechanisms of passing heat from the thorax to the abdomen is used by the queen bee when she is incubating her brood. She places the smooth underside of her abdomen over her brood clump and dumps heat into her offspring. With the temperature of the brood raised, the cycle from eggs to larvae to pupae to adult bees goes more quickly and more bees can be produced in a given amount of time. In the short summer season of temperate regions, quick production of bees for the colony is vital. In arctic areas, incubation of broods is even more important and arctic bumblebees have been observed to have higher incubation temperatures than temperate bumblebees.
Naturalists have long marveled at the importance of bees as pollinators of plants, and at the astounding efficiency with which they collect food. The mechanisms that bumblebees use to control the temperatures of themselves and their young provide further reasons to show wonder and appreciation for these sophisticated little creatures.
[A wealth of additional information on thermoregulation in insects is available in Bernd Heinrich's book, The Hot-Blooded Insects: Strategies and Mechanisms of Thermoregulation. Springer Science & Business Media.]
(Reprinted from Sanctuary, a publication of the White Memorial Conservation Foundation.)