We kept them in a large enclosed metal barn in a very humid climate. It was hot as heck in the summer and cold in the winter. Some of them were in there 7+ years and stilled look good. Direct sunlight or rainfall I would think would be the worst thing for them. Some of the ones that were about the same age that were hung up on a windmill tower in the heat, rain, sun and high humidity looked horrible. Really chalky and cracked. They looked like they had been there 20 yrs when they had only been there 7 or less.
Your darn right they will deteriorate! I had some in my garage for about 3 years. These south Texas temperatures will make the skull very brittle and dry the heck out of them. If the skull hits the concrete, you will be sweeping up a thousand mini pieces of skull.