How bees regulate hive temperature

Why 34-35 °C

Developing brood is extremely sensitive to temperature. Below about 32 °C development is slowed and adults emerge with malformed wings or impaired learning; above about 36 °C brood dies. The colony holds the brood nest within roughly one degree of 35 °C, which is comparable precision to a mammal regulating its own body.

Outside the brood nest the requirement relaxes considerably. A broodless winter cluster sits at 20-25 °C in the centre, and only rises to brood temperature when laying resumes.

Heating

Bees generate heat by decoupling their wings and shivering the flight muscles, which converts a great deal of energy into warmth. A heater bee presses her thorax against a capped brood cell, or inserts herself into an empty cell among the brood, and warms the surrounding cells directly — thermal imaging shows individual bees running thoracic temperatures above 40 °C while doing this.

At colony scale the mechanism is the winter cluster: an insulating mantle of tightly packed bees around a loose, actively shivering core.

Cooling

1. 1FanningWorkers position themselves at the entrance and inside and fan, driving a directional airflow through the nest.
2. 2Water collectionForagers switch from nectar to water when the colony is overheating. Water carrying is a distinct task and is triggered by demand from bees inside.
3. 3Evaporative coolingWater is spread in thin films on the comb and at cell mouths, and fanning evaporates it. This is the same principle as sweating and is the colony's most powerful cooling tool.
4. 4BeardingOn very hot days bees leave the nest and hang in a mass on the outside, reducing the heat generated inside and the crowding.

The Asian honey bee's extra trick

Apis cerana uses heat as a weapon. When a giant hornet scout enters the nest, hundreds of workers engulf it in a ball and shiver, raising the core temperature to around 46 °C. The hornet's lethal limit is a degree or two below the bees', and it dies while they survive.

The introduced western honey bee has no such behaviour, which is why Apis mellifera colonies in Japan can be destroyed by hornets in a way that native Apis cerana colonies are not. It is one of the clearest examples of what a co-evolved defence is worth.

What this costs

Thermoregulation is expensive. A substantial share of a colony's honey consumption goes on heating in winter and on water collection in summer, and a colony in a poorly insulated hive spends more of both. This is the practical argument for hive insulation, and for shade and water availability in hot climates.