They've been using the same bedding, eating the same brand of food, drinking from the same bowl, and doing everything else the same since they day they were born. (Well, the day they came home... you know.)
Anyway, story time. Several years ago, I had a hooded rat, name of Hammersmith. He was an old rat, pretty nice, kinda crotechtey sometimes but a cool rat on the whole. One day I noticed a growth in the area of one of his back legs, which slowly grew larger and larger as the months passed. Didn't seem to be bothering him at all, but it was freaking me out a bit. It's not often one sees a two-assed rat.
So eventually, I took him to the vet. The vet, who seemed to think I was a bit weird for bringing him a rat in the first place, gave him a look, poked at him a few times, and then asked me a couple questions. Is he moving around alright? Is he eating? Is he drinking? Has his behaviour changed at all? My answers were yes, yes, yes, and no. And so, the vet proclaimed Hammersmith the Rat to be healthy and happy, and sent me on my way.
And on the way out the door, he said, "That'll be 35 dollars, please."
So today, I look upon my little degu, who lives well and happy with his brother in their gigantic ferret cage, and I ask myself the same questions, and I give myself the same answers. He bounces off the walls, he eats, he drinks, and he has his thrice-daily fights with his brother to see who gets to be on top this time. And so a trip to the vet just isn't in the cards for him right now. The moment he starts exhibiting any sort of symptoms of illness, I'll have him there in a heartbeat, but as long as he's just going bald, then... he's just going bald.