Poor Tubby is going bald in his old age!
Don't worry I didn't let Samantha see the photos.
Like Phred and KK mentioned above, Samantha gets an occasional fur mat on her lower back too.
I really need to get her a new slicker brush like you have since her regular brush is starting to wear out from so many years of brushing.
I really hope that it isn't serious and that Tubby gets better soon.
If you would like I could save some of Samantha's black hairs from her brushing and mail it to you to make a hair piece for Tubby!
It sounds like it could be Stud Tail, after all, Tubby is a real Stud!