Final answer:
Cholesterol is the substance that maintains animal membrane fluidity at low temperatures, acting as a temperature buffer and playing a role in organizing membrane structure.
Step-by-step explanation:
The substance that aids in maintaining the fluidity of animal membranes at low temperatures is cholesterol. Cholesterol lies alongside phospholipids in the membrane, functioning as a buffer to maintain membrane fluidity across a range of temperatures. It prevents the membrane from becoming too rigid in cold temperatures and too fluid in high temperatures, thereby extending the functional range for cell membranes. Additionally, cholesterol has roles such as organizing transmembrane proteins into lipid rafts, further supporting membrane structure and function.