Saturated fats have long straight tails of fatty acids and can pack or clump tightly together in cells and animal bodies. Unsaturated fats have kinks in their tails due to double bonds, which prevents them from packing together as tightly. Animals that are ectothermic (their body temperature fluctuates with the environment) need to keep their membranes fluid at cooler temperatures and thus use in their membranes. Think about needing fats to be liquid.