I can't dispute what you are saying that rat tails only show up on crossed angus cows, because I can't confirm that my angus were pure back in the day when we had a few Char bulls.  All I know is the rats and the skunks were frustrating because they always got picked out.  The rest of the char cross calves sold well and gained well, but the sort off's were the main reason we went back to straight angus bulls.