...starting 12/03/2012. You'll see Doc wearing a thick sweater/sweatshirt on in the 12/14/2012 strip. In fact, all of the cast have cold weather gear on in various levels including Pirta. She is seen wearing a sweater/sweatshirt and LONG pants in the 12/31/2012 strip.
You do see Doc wearing one of his trademark black t-shirts when the crew goes to Howie's bar after the snowball fight at the start of the infamous beer replicator storyline on 1/30/2013.
So even though Doc is a polar bear, he still needs extra insulation when out in the cold. Long sleeved shirts aren't that farfetched when out on a date or when going somewhere with his significant other during the cold weather months.