Final answer:
Horses sleep under large trees on cold nights as the trees provide insulation and protection from the wind, helping the horses to conserve warmth and maintain their body temperature.
Step-by-step explanation:
On cold, clear nights, horses seek shelter under the cover of large trees to keep warm. This is because the trees act as an insulating layer, trapping heat rising from the ground and reducing the speed at which the ground cools down. Additionally, they provide protection from the cold wind which can increase the rate of heat loss from an animal's body. Since horses are endothermic, they rely on their metabolism to maintain body temperature, and by sleeping under trees, they are able to conserve warmth during the night.