Today I noticed black stripes just next to the frontal teeth on one of my boars. It was difficult getting a good picture showing his entire mouth, but there appears to be two very similar lines on his upper teeth, just as the lower ones (see pictures). I was just curious about what it is, since I've never really been able to take a good look at them on other guinea pigs before, (this one likes getting petted on his lower neck). I thought perhaps it was just hairs that had gotten stuck in his gums during grooming or washing, but I couldn't really get them out, or even move them around just by poking his teeth with my finger. Again, it doesn't have to be hairs. I have no real knowledge about guinea pigs' teeth. Have any one of you ever seen this before?