Lizards are sometimes called cold-blooded because they depend on their surrounding temperatures to keep them warm.
Just going to follow up on your thought here.

The fluidity of the cell membrane does vary with composition.  Off the top of my head there are four main components of the animal cell membrane:

1) proteins.  membrane proteins are responsible for the import and export of nutrients, waste, information etc from the cell, so lets assume that this needs to be fairly constant part of the composition while the seasons change.

2) cholesterol and similar compounds.  These are flat an inflexible compounds which will tend to lower the fluidity of the membrane. 

3) unsaturated fatty acids and lipids.  These are long carbon chained compounds with some double bonds in them.  Because the double bonds cause the chains to be less mobile, but they don't pack well as they are not as flexible, so tend to make the membrane a bit more fluid.  Think about how butter (mostly saturated fats) is solid at room temperature while olive oil (monounsaturated oil) is liquid. 

4) saturated fatty acids and lipids.  These are completely mobile carbon chains.  Without any double bonds, these tend to be the more flexible and can create a more tightly packed membrane, resulting in a stiffer membraine.

You can imagine for the cell to maintain the same flexibility at the lower winter temperatures (4) and (2) would tend to decrease and (3) might tend to decrease.  

Does this make sense?