B. (IMAGE) Aarhus University Caption Cut-through narwhal tusk displaying the individual year rings. Analyses of the individual layers of narwhal tusks have provided information about their food choice and their exposure to mercury throughout their life. Credit Photo: Rune Dietz. Usage Restrictions None 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.