Organisms can make different variations on membrane lipids at different times and in different places in molecules. Saturated lipids have kinks or bends in them due to the different pattern of chemical bonds - sort of like standing in a crowded train and putting out your arms. So this increases the distance between the individual molecules. So, the membrane is easier to bump through, side to side. If it is cold, membranes need to make it especially easy to move through.
Membrane lipids are different in main body and in the hooves of caribou.
The hooves are relatively cold and the membrane lipid composition is adjusted so that cell membranes have the right amount of flexibility at the cold temperature. But, in the warmer upper body, the membrane lipids are slighly different. There are many other examples. Fish like Salmon, which migrate from colder ocean water to warmer rivers, change the composition of the fatty acid ends of phospholipids. This requires the ability to rapidly replace lipids in the membranes.