Which statement best describes the thermoregulation of the winter cluster when the temperature drops in the hive?
- The more the temperature drops, the more the outer mantle of the cluster warms up.
- The more the temperature drops, the more heat the core produces, while the mantle cools down.
- The more the temperature drops, the lower the outer mantle can fall, down to about 2 °C.
Show answer
Correct answer: 2.
The more the temperature drops, the more heat the core produces, while the mantle cools down.
Why?
The winter cluster works with two complementary zones: a warmer core, where certain bees actively produce heat, and a more compact peripheral mantle, which limits heat losses.
When the temperature drops in the hive, the bees of the core increase their heat production through muscular contractions. The outer mantle, for its part, becomes colder and denser, which improves the insulation of the cluster.
The statement referring to a mantle at 2 °C is too low and therefore misleading. In practice, the peripheral mantle can cool considerably, but it normally remains above a critical threshold close to 6 °C.
What to understand
The winter cluster is not heated uniformly. The bees at the centre do not seek to keep the whole hive at a high temperature: they maintain above all the cohesion and the survival of the cluster.
The peripheral mantle plays an insulating role. The bees there are more tightly packed, less active and colder than those of the core. This organisation reduces the heat losses to the cold air of the hive.
Thermoregulation also depends on the size of the colony, the stores, the presence of brood, the humidity and the duration of the cold period. A small cluster is more vulnerable, because it loses proportionally more heat.
Key points
When the temperature drops, the core of the cluster increases its heat production.
The outer mantle cools and becomes denser, but in a viable cluster it should not fall towards 2 °C.
The winter cluster is therefore a dynamic collective structure: it heats at the centre and insulates at the periphery.
Further reading
► Sense and nonsense of thermal insulation of hives
► Overwintering in the honey bee
► Insulation of hives put to the test of the collective thermoregulation of bees