Decreased Winter Appetite?

Hey all, I have yet another question. Is it normal for Tiger Salamanders to have a drastically decreased appetite during the colder months of the year? Over the past month or two I've noticed a significant drop in their food intake, and my animals have become increasingly picky and just generally uninterested.

I've tried all sorts of things to jump start their appetite (earthworms, cockroaches, crickets, waxworms) and now they're just barely interested in the roaches and crickets, and absolutely will not eat the earthworms or waxworms. I just don't get it!

A few more random facts you guys might need; I have three tigers. One still eats pretty regularly (one or two cockroaches every three days) and his weight is normal, if not a little chubby. The other two will only eat once a week if I'm lucky, and then only one or two cockroaches at a time. Their weight is still healthy, but thinner than I'd like. They all live together in a 20 gallon tank with an ambient temp of around 70F. I change the substrate monthly and keep the lid off the majority of the time for good ventilation.

One of my tigers is going on two weeks without eating and I'm getting a bit worried! If you need any more information just let me know. I'm hoping some of you guys might have some insights, or perhaps this is just normal behavior after all.

Appetite sure decreases with cold temps, but 70ºF is far from cold.....if this is the colder temp, at what temp were they before?

Sorry, I should have been more specific. The temperature has been dropping outside, but the room they are in has been staying pretty consistent from the summer months up until now. This just adds to the mystery I guess, because why would they be less hungry if they aren't directly experiencing the winter weather?

If the temps are the same, maybe they are just eating less because they are fat enough.
If they are next to a window i suposse it´s possible that they are sensing the change of seasons and behaving accordingly, i know my milksnake does.