Thermoregulation and a Chilling Tale.

Introduction

I wrote this for BeeCraft’s Cyclopedia series but they already had an article lined up for “E” so this was adapted and used as a stand-alone piece for publication in a recent copy of their magazine.

Thermoregulation

E is for endothermy which is one strategy that honey bees have evolved to employ in keeping themselves warm and maintaining nest homeostasis.

This brings us immediately to a question often asked; are honey bees warm or cold-blooded? It is a complicated situation and not a surprise that some people incorrectly refer to honey bees as being cold-blooded animals. Firstly, it is probably best to dispense with the terms hot and cold-blooded as they are not really helpful to our understanding of the concept of homeostasis and thermoregulation (temperature control) in an organism. Let’s delve deeper to unravel the truth.

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 to the human 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 a small amount of metabolic heat which is not enough to have a significant effect their body temperature. They rely on ectothermy which is a behavioural strategy whereby the animal depends upon an external heat source 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 to 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. It relies upon stores of carbohydrate in honey to fuel this activity. Similar to homeotherms that must maintain a constant internal temperature, honey bees must maintain the brood nest temperature at a constant steady range of between 34.5 to 35.5 °C in order for the brood to develop normally. The brood nest temperature can be lower than this during broodless periods such as winter. temperature regulation and warming can be partially achieved by individual heater bees occupying vacant cells and isometrically contracting wing muscles without vibration. This activity can be seen on infrared film where the heater bee has a white red- hot thorax raised to nearly 38 °C before it enters the empty cell. The dark bee in Figure 1  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.

Foraging Trips

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. Because the tiny bees are like our human babies who have a larger surface area in relation to body mass, they have to deal with an enormous heat loss so they cool more easily than larger endothermic animals and birds.

Drowning?

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. In this situation, the bees have been immobilised through cold and an inability to regulate their temperature (ectothermy). They have chilled and been unable to raise their own temperature. Next time you find a freshly “drowned” bee, bring it indoors to dry and warm up and notice what happens. Mostly it will warm up in the room (ectothermy) and fly to the window having returned to life.

Observation Hive Blunder

I was able to use this knowledge recently to correct a big mistake with the observation hive bees. I should have checked the long-range weather forecast before transferring the wee colony from their overwintering nuc box to the hive inside my bee shed. I was too hasty and they chilled. That they survived over winter was pretty good as the 2021 queen was old. The white paint mark was nearly rubbed off her thorax.

Stressed

What happened was the bees got cold and stressed and couldn’t metabolise the sugar syrup so got “dysentery” and dwindled fast when the weather changed to bitterly cold days, and even colder nights. When I came in one morning and found the tiny patch of listless bees clinging to the frame in a torpid state, I acted fast to see if I could save them. I got help carrying the heavy glass- sided hive to a white sheet on the ground outside, and I removed the frame of bees. I thought that the queen might have been in the middle of the cluster but she fell off the back side of the frame onto the sheet. She appeared dead but moved slightly when I blew on her. So, I took her indoors and warmed her separately (and carefully) over a hot water bottle. I cut the small cluster of bees out of the frame with scissors and placed the comb in a sling made from a tea towel. I placed this next to an empty drawn comb, with wide insulated dummies on either side of them, in a well-insulated small nuc box. They remained tucked up in the tea towel for a week.

I placed fondant within easy reach and took the box inside to place on the warm stove while I resuscitated the queen. As she warmed up, she was able to cling onto my left index finger where she fed from a blob of honey. It was a relief when she extended her proboscis and sucked up the blob.

I placed the queen in the nuc box and watched as nurse bees immediately came up to pay attention and antennate her. I shut up the nuc and placed it over the hot water bottle which I changed a couple of times till I heard active bees moving about inside.

An hour or so later, I placed the nuc box where it had been all winter on a picnic table outside the entrance to the observation hive, and left it alone for a week. I didn’t have time to take photographs. Besides, I felt so bad about messing them up and probably killing them that I wanted to move on quickly.

Egg Laying Again.

A week later, imagine my surprise to find the queen busy laying eggs. What was confounding was finding multiple eggs in many of the cells. However, another week on and I find nicely sealed worker brood and the bees bringing in masses of pollen. We are having a big dandelion nectar flow just now and the fields are yellow with them and the apiary smells a little pungent like a cow’s byre. Some of the other colonies have nearly filled supers of dandelion honey.

I’ve always liked this queen, and her offspring, so my plan is not to put them back in the observation hive but to rear a new queen from this stock (in another strong colony) and see what happens next. The nuc is moving gradually to a new position in the back garden apiary which will be in the sun more than it is now.

As I said to someone the other day, nobody is ever really an expert in beekeeping. The bees and changing environment prevent that happening, and keep us all learning all the time.