What is torpor in general?
Torpor, is generally scientifically defined as follows;
Torpor, a state of lowered body temperature and metabolic activity assumed by many animals in response to adverse environmental conditions, especially cold and heat. The torpid state may last overnight, as in temperate-zone hummingbirds and some insects and reptiles; or it may last for months, in the case of true hibernation and the winter torpor of many cold-blooded vertebrates.
"Torpor". Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2018. Web. 05 Jan. 2018 <https://www.britannica.com/science/torpor>
Basically “torpor” is a physical response to changes in temperature in a number of animals.
We’ve talked about torpor before in a previous article but not quite in as much detail. Here I’d like to get a bit more specific.
What is torpor in regards to honeybees?
Torpor is not effected identically across the board between mammals, lizards, birds and insects Unfortunately, all too many times, people will try to apply the understanding of torpor to honey bees as it applies to mammals. That is to our disadvantage.
In regard to bees, torpor is a point in which the temperature has the effect of causing bees to be unable to sustain an inner temperature that allows bees to stay mobile and active.
I’d like to quote directly from the well respected book, “Honey Bee Democracy” by Dr. Tom Seeley in the description of bee swarm/cluster temperature regulation. He cites Dr. Bernd Heinrich research in about 1980.
Heinrich discovered many marvelous things about temperature regulation in honeybee swarms, all of which are key to understanding how a swarm prepares to fly to its new home. First, he found that a swarm does indeed precisely control the temperature of the cluster’s core so that it stays at 34– 36 ° C (93– 97 ° F) regardless of the ambient temperature. He also found that a swarm allows the temperature of the cluster’s mantle (outer layer) to vary with the ambient temperature, but that it keeps the mantle temperature above 17 ° C (63 ° F) even if the ambient temperature falls to freezing (0 ° C or 32 ° F). This means that the outermost bees, which are the coolest, keep themselves warm enough to stay active on the swarm. If they were to cool below 15 ° C (59 ° F) they would enter “chill torpor” and easily fall from the swarm. They would also be too cold to warm themselves back up by shivering.
Seeley, Thomas D.. Honeybee Democracy (Kindle Locations 2025-2032). Princeton University Press. Kindle Edition.
I highlighted some pertinent information as to the effect of torpor on honey bees above on the effect of torpor on bees in a cluster.
Bees resist Torpor
In effect, torpor is something honey bee colonies actively work to prevent. Bees in the cluster work to generate heat that warms not only themselves but is trapped within the cluster by the outermost layer of bees (the “mantle”. Those bees in the mantle don’t just take it as a given that being exposed to the cold, they must enter torpor. They will work to retain their own effective body temperature and move to the inside of the cluster to preserve that temperature if possible. Honey bees actively resist torpor.
…when the ambient temperature falls below 17 ° C, and the mantle bees start to feel too cool, they crowd inward, causing the swarm cluster to shrink, its porosity to decrease, and its heat loss to diminish (figs. 7.2 and 7.3). In this way the mantle bees skillfully trap inside the swarm cluster the metabolic heat generated by the thousands of resting, immobile bees, and they also keep themselves sufficiently warm. It is only when the air temperature falls below about 10 ° C (50 ° F) that the mantle bees must take the extra step of raising their metabolic rate by shivering.
Seeley, Thomas D.. Honeybee Democracy (Kindle Locations 2037-2041). Princeton University Press. Kindle Edition.
Honey bees have figured out how to maintain over the Winter
Thus Heinrich discovered that the bees in a honeybee swarm have an effective means of conserving their energy reserves. The mantle bees, those most exposed to low temperatures, minimize their need for active metabolism by doing two things when the air becomes cool: (1) letting their body temperatures drop to just above the chill-torpor temperature rather than working to maintain a higher body temperature, and (2) keeping their body temperatures above the chill-torpor temperature mainly by huddling rather than shivering. Of course, these energy conservation measures mean that most of the time the outermost bees in a swarm are too cold to fly, something that is easily demonstrated by skimming a spoonful of mantle bees from a swarm and shaking them into the air. The bees tumble to the ground rather than fly away. So before a swarm can take off to fly to its new home, the cool bees in the mantle must warm their flight muscles to the flight-ready temperature of 35 ° C. And not just in theory! When Heinrich made continuous recordings of the temperatures at various locations in a swarm cluster from when the bees settled to when they departed, he found that during the last hour or so before takeoff, the temperature in the mantle did indeed rise to match the 35 ° C of the core.
Seeley, Thomas D.. Honeybee Democracy (Kindle Locations 2045-2050). Princeton University Press. Kindle Edition.
Again, I’ve highlighted pertinent text that affirms that honey bees avoid or resist torpor because for honey bees, it does not have the same effect as it does in mammals. Topor in mammals is beneficial to their survival for short term temperature changes in conserving heat and energy. For mammals. It’s a short term solution whereas hibernation is for long term survival in extreme changes of temperature.
Not so for bees. For bees, torpor is a state that can lead to death due to failure to maintain just enough self generated heat.
How is torpor important in honey bee colony management?
For many apiculturists, Winter is a nervous time. We try to send bees into Winter with plenty of stores to sustain energy specifically so that they can continue to generate enough heat to prevent torpor.
We also use mechanical, environmental and other methods to make the hive more efficient in heat retention. By making hives more heat efficient, we allow bees to expend less energy to maintain sustainable temperatures in the cluster. Conserving energy also helps the colony to retain their food stores for a longer period of time, preventing starvation.
Causes for concern in beekeepers
In recent years, we have seen an increase in concern and interest in methods being used to check the conditions of over-wintering colonies. Unfortunately in many articles and videos torpor and it’s effect on bee colonies is not accurately explained or understood. Sometimes it is even suggested to be a normal or “good” thing that bees enter a state of torpor in cold weather. These misunderstandings have caused colonies to be lost unnecessarily.
- We can use efficient hives that are insulated naturally, or if necessary, adding extra insulation.
- Ensure that hives are well ventilated. Keeping hives dry can’t be overstated.
- Use effective pest presence restrictors. Entrance guards or different sized bottom boards perhaps. When we try to do whatever keeps predators and scavengers from getting into hives. Colonies can be depleted or have their resources consumed before they can access them.