Based on the temperature of their environment.
Too cold - add more cholesterol. Cholesterol prevents too tight packing of the membrane. You can also change the amount of saturated and unsaturated fats. Add more cis unsaturated fats to make it more fluid.
Too hot - also add cholesterol. It adds firmness, since it can't move as much to the membrane. If the membrane was too fluid it would break apart and everything would be able to pass through.
