[Stefan-A]

I admit that I might remember incorrectly, but I think they indeed found that it didn't result in any reproductive advantage. Courting, they didn't have a greater success rate.

The adders usually start leaving their dens before the snow has thawed. Outside the den, it could have some benefit, since the snakes could absorb heat more efficiently. As the angle of incidence changes during the year, they could theoretically become active earlier than the lighter colored ones.