Final answer:
Horses sleep under trees to reduce radiation and keep warm.
Step-by-step explanation:
On cold, clear nights, horses sleep under the cover of large trees to help them keep warm by reducing radiation.
When a horse is under a tree, the tree acts as a shield, blocking some of the heat loss through radiation. The tree absorbs the heat and prevents it from escaping into the cold night air, creating a warmer microclimate underneath.
This is an example of how animals use their environment to regulate their body temperature and conserve heat.