Final answer:
The structures in horses that lock their forelegs and hind legs in place so they can sleep standing up are called pasterns. Sleeping under the cover of large trees on cold, clear nights helps horses keep warm by providing them with shelter from the wind.
Step-by-step explanation:
The structures in horses that lock their forelegs and hind legs in place so they can sleep standing up are called pasterns. Pasterns are the part of the leg that connects the hoof to the fetlock joint. When horses lock their pasterns, it allows them to doze off without collapsing.
Sleeping under the cover of large trees on cold, clear nights helps horses keep warm by providing them with shelter from the wind. The trees act as natural windbreakers, reducing the amount of cold air blowing directly on the horses. This helps to maintain their body temperature and keep them warm.