The greater the number of bees forming the cluster, the more …

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Correct answer: 3 (… its surface-to-volume ratio decreases)

The ability of the winter cluster to produce heat depends strongly on the number of bees composing it. Moreover, the smaller the cluster, the greater the heat losses according to the well-known mathematical surface-to-volume ratio described further in the appendices (Southwick, 1983). Consequently, small clusters with a low number of bees and a proportionally large heat-loss surface have little chance of surviving the winter. The overall metabolism of a cluster increases with the number of bees composing it, but not linearly; the figure of 17,000 bees represents the inflection point of this curve (see the definition of an inflection point in the appendices). At low temperatures, this phenomenon is less pronounced. As a result, a small cluster of less than ~1.7 kg (< 17,000 bees) will expend more energy to stay alive at 2 °C than at 15 °C, which is fairly easy to understand.

However, this relationship is reversed for a larger bee mass: a cluster weighing more than 1.7 kg will consume less energy at 2 °C than at 15 °C! At moderate ambient temperatures (10–14 °C), the cluster gradually dissociates, leading to a massive increase in the total surface area for heat loss and a concomitant sharp rise in metabolism (E. Southwick 1983).

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