Thermoregulation: Are Honey Bees Warm-blooded?

The unsettled weather over the last few days of the year ranges from still, clear, frosty, and sunny to lashing rain, floods, and gales. Upturned spare hive stands stored on roofs blew off the home apiary hives today, but the hive are standing and survived the storm which blew at 80 mph across some parts of Scotland.

Apiary Visit.

My 20-year-old Subaru sat and failed its recent MOT. So badly, in fact, that it is no longer viable to repair and retest it. Sometimes decisions are taken out of our hands and it makes sense environmentally and economically to share a vehicle. However, it happens to be a truck and I need to adapt and handle it remembering that it is nothing like my car on corners. No time like the present, and, in brilliant sunshine yesterday I drove to the out apiary in the walled garden near the river Nairn to count Varroa. Before setting off, we sat for ages in our driveway in almost freezing temperatures waiting for the ice to melt on the windscreen. Why is it that most of us fill our garages with things other than cars which we leave outside to freeze up?

Derek Mitchell.

I listen with my ear up close to the brood box of each colony and am reassured to hear a low hum. At present, all 11 colonies are alive and going into the new year. I’ve been thinking about the winter cluster and thermoregulation, especially in light of engineer and beekeeper Derek Mitchell’s recent suggestion¹ that tightly clustering is stressful for a colony, and that our wooden hives are mostly not well enough insulated.

Poly Hives are Popular.

Some of you will be familiar with Mitchell’s studies and work on insulation and the use of poly hives. Interestingly, a beekeeping equipment supplier friend of mine tells me that his hive sales last year were mostly of poly hives. Despite the plea to use less plastic, the demand for cedar hives has apparently dropped. Is this due to the rising cost of wood, or rising awareness for the need to better insulate our colonies in winter?

Warm or Cold-blooded?

This brings us to the question often asked; are honey bees warm or cold-blooded? It is a complicated state and not a surprise that some people mistakenly refer to honey bees as being cold-blooded animals. Firstly, let’s dispense with the term cold-blooded because it is not scientifically correct or helpful to our understanding of the concept of homeostasis and thermoregulation (temperature control) in an organism. Let’s delve deeper to unravel the answer.

Humans, like many other mammals and birds, are homeothermic which means that they are able to maintain a constant body temperature independent of the outside environment. In humans the normal body temperature varies for everyone, and alters throughout the day, but generally ranges between 36.2 – 37.2 °C. Deviation much above or below this range is harmful and so endothermic activity relies on the maintenance of certain levels to sustain life.  A constant body temperature is achieved through endothermic activity. Endothermy is a bioenergetic strategy used mostly by birds and mammals and is achieved through cell metabolism from the burning of calories obtained from food which create energy and heat. It is balanced with heat and energy loss though respiration, urine, and faeces. Endothermy is a high-energy strategy that allows the animal to sustain intense activity over long periods.

On the other hand, most amphibians, reptiles, fish, and invertebrates are poikilothermic which means that they cannot regulate their body temperature except by ectothermic activity. They can only produce small amounts of metabolic heat which are not enough to significantly affect their body temperature. They rely on ectothermy which is a behavioural strategy whereby the animal depends upon external heat sources such as sunlight, or a warm rock, to where they must move. Or, they must burrow to keep cool when the ambient temperature gets too hot.

Poikilotherms don’t require a fixed body temperature to sustain life and their temperature can greatly fluctuate with little to no adverse effects on their overall health. Most terrestrial ectotherms such as snakes, and many lizards, are poikilotherms. Interestingly, the naked mole rat is considered to be the only poikilothermic mammal. It takes a lot less energy to heat or cool an ectotherm but these animals are not generally capable of intense activity over long periods.

So where do honey bees fit in? Ah well, they use both endothermic and ectothermic strategies so, along with wasps, they are called heterotherms. A colony can generate heat by endothermy in the winter cluster by isometrically contracting uncoupled wing muscles. This method relies upon stores of carbohydrate from honey to fuel this activity. Similar to homeotherms who must maintain a constant internal temperature, honey bees must maintain the brood nest temperature at a constantly steady range of between 34.5 to 35.5 °C in order for the brood to develop normally. This can be partially achieved by individual heater bees occupying vacant cells after heating thoraxes up from isometrically contracting their wing muscles without vibration. This activity can be seen on infrared film in the image above where the heater bee (arrow) has a glowing hot thorax and temperature raised to nearly 38 °C . It will enter an empty cell and heat up the nearby brood. The dark bee in the photograph is maintaining its temperature close to that of the nest environment by using the ectothermic strategy of absorbing heat from its surroundings rather than producing it through metabolism.

High Energy Foraging.

On foraging trips, the energy cost for the worker must be balanced with the gain and it is harder for the individual bee to maintain its temperature on cloudy days. On sunny days solar energy (ectothermy) can be harnessed by a worker to reduce its own endothermic effort. It can use the sun to raise its thoracic temperature by nearly 3°C which helps it increase the nectar sucking speed. Tiny honey bees, like our human babies who have a larger surface area in relation to body mass, have to deal with an enormous heat loss so they cool more easily than larger endothermic animals and birds. Honey bees are therefore considerably vulnerable when working outside the hive.

Honey bees don’t often drown in water. Their waxy exoskeleton acts like a life-vest and they are usually found bobbing about on the water. If you were to add detergent to the water they would certainly drown as it dissolves the exoskeleton and water floods the respiratory system. Bees in water are immobilised by the effects of cold and unable to raise and regulate their temperature. Next time you find a freshly “drowned” bee, bring it indoors to dry and warm up and see what happens. Mostly it will warm up in the room (ectothermy) and fly to the window having returned to life. However, a drop of honey on a teaspoon will supply the individual bee with some energy to generate heat by the endothermic strategy.

Happy New Year.

I wish you all a Happy New Year with good health and successful beekeeping in 2024. Thank you for following the blog and I look forward to creating more posts in the coming year. However, I shall be so busy during the early part of 2024 that I might not create a blog post every week. If anyone would like to share their knowledge and experiences in a guest blog I shall be very pleased to publish them.

Reference:

¹Mitchell D. 2023 Honeybee cluster—not insulation but stressful heat sink. J. R. Soc. Interface 20: 20230488. https://doi.org/10.1098/rsif.2023.048