The Reference Desk/Understanding a running gag in Doctor Whoah!

I was hoping somebody could shed some light on a particular reoccurring visual joke in the Doctor Whoah! strips.

For the former half of the strip’s run, the Eighth Doctor, whenever he showed up, was depicted as having cat-like features; cat ears, whiskers, and a tail. The gag was seemingly retired around the time of his Big Finish makeover.