Bumblebees enter a state of rest called torpor, which is similar to hibernation. During torpor, bumblebees reduce their metabolic rate, lower their body temperature, and remain inactive until their energy reserves are replenished. Bumblebees usually enter torpor during periods of cold weather, darkness, or food scarcity. They often rest in nests or other sheltered locations, and some species have been observed to huddle together for warmth.