Which statement is correct?

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Correct answer: 1 (The more the temperature inside the hive decreases, the more the temperature of the core increases.)

The thermoregulation of the cluster is remarkable: the lower the outside temperature drops, the more the temperature of the endothermic core rises, while the temperature of the peripheral mantle decreases—though not below approximately ~6 °C, a threshold below which a bee dies.

The bees in the outer layer form an insulating mantle, preventing excessive heat loss by convection for the bees in the deeper layers. With their heads oriented toward the inside of the mass, this tightly packed layer insulates thanks to the numerous thoracic hairs that interlock like the wool threads of a sweater. This “mantle” protects a deeper layer of bees whose immobile wings are spread in a fan shape and allow a certain respiration of the warmer central core, which produces endothermic heat by isometric “shivering,” as described above. Not all bees at the heart of the “reactor” are active simultaneously. Only about ~15% of them are endothermic workers with a very high metabolism and maximal fuel consumption. The other bees at the center of the cluster contribute to the critical mass of individuals needed to optimize the cluster’s volume-to-surface ratio. The muscle contractions of the “heater” bees consume as much oxygen as a forager in flight! After 30 minutes of “heating,” the task is handed over to another worker that drastically increases its metabolism—similar to the relay changes among cyclists during a breakaway from a gruppetto.

To learn more:  Sense and nonsense of thermal insulation of beehives