In this respect, bears are like trees. The most accurate way to tell the age of a bear is to count the rings in a cross-section of a tooth root using a microscope. The outer part of the root, called cementum, adds a new layer each year. The new layer has two parts: a narrow dark line that forms during hibernation and a light area that forms during the growing season. Black bears are born in January, so the dark lines mark birthdays. The spacing between the dark lines tells more. For a typical female bear there will be wide spacing between the lines in her first eight years as she grows to full size. Narrow spaces between some of the lines would show years when she raised cubs: much of her calcium would go into milk instead of into cementum.

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