This article is from the WSSF 2016 AFRMA Rat & Mouse Tales news-magazine.

By Karen Robbins

Tiny Tails Hammocks and Animal Accessories, Facebook
Q I was curious what caused this! My Rattie Nonnie has tags on her right ear. I always thought maybe it was an injury she got as a baby. Even though she’d never be a show rat, she sure is the perfect little friend!!!

A Normally the notches your rat shows are from fighting. One other way for a rat or mouse to have notches in their ears, is for it to be purposely done to be used as a form of ID (labs will use this method sometimes and notch out different areas of the ears which corresponds to a number; also done with livestock). We have a couple articles telling about different forms of ID:

ID For Rats
Telling Rats Apart

Other ways for ear problems to happen are possibly from a birth defect, mom cleaning too hard when they are little, or some other type of injury.