Beaver Skull (IMAGE) Northwestern University Caption Northwestern University researchers found that the pigmented enamel of beavers, which contains iron, is both harder and more resistant to acid than regular enamel, including that treated with fluoride. Credit Credit: Michael Graydon, Toronto Usage Restrictions Credit must accompany image License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.