I think its mostly photoperiod, but weather has to have something to do with it. For example, at the MN state fair, they have a duck pond with all kinds of wild north american duck species, all of them in their full breeding plumage. This is in the last part of august. Any duck hunter knows fully feathered drakes are few and far between even in the end of september. My dad and I asked how they get them to be feathered out, and the guy that is in charge of the ducks said they keep them inside and adjust the amount of light each day to get them to molt and grow their breeding feathers.
What he did say was that the ducks don't grow much of a down layer though since its like 80 degrees in the shed the keep em in. Now if it was light alone wouldn't they grow a thick layer of down like the do in the fall.
Now I know these are ducks and not furbearers. Now think about this, I can't imagine a furbearer getting a super thick prime coat if you decreased their photoperiod and kept their environment at 90 degrees. I have no doubt the fur would change but I can't imagine they would have the same amount of underfur they would in the wild in january in MN. Now imagine that you give an animal say 20 hrs of light but steadily decreased the temperature to very cold, say 0 degrees. I would have to think that that animal would grow a denser coat.