Final answer:
Cholesterol regulates the fluidity of animal cell membranes by stiffening the membrane at higher temperatures and preventing the membrane from freezing at lower temperatures.
Step-by-step explanation:
The major chemical that regulates the fluidity of animal cell membranes by stiffening the membrane at higher temperatures and preventing the membrane from freezing at lower temperatures is cholesterol. Cholesterol acts as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. It extends the range of temperature in which the membrane is appropriately fluid and functional.