Yesterday’s design was made by a beaver as it removed bark from a tree. The light-colored, curved little “bumps” that run horizontally across the middle of the tree were made by the two incisors in the beaver’s upper jaw. When eating the sought-after cambium layer of a tree, beavers grip the tree with their two upper incisors as they scrape towards their upper jaw with their two bottom incisors, sometimes creating this pattern. (Individual marks where the upper incisors gripped the bark and the four incisors didn’t quite meet can be seen in the insert.)
The ever-growing incisors of rodents are harder on the front surface (outer layer is hard enamel, colored orange from iron in a beaver’s diet) than the back (softer dentine), so the back of each incisor wears away faster than the front, creating a sharp, chisel-like edge to these four specialized teeth. So functional are beavers incisors as cutting instruments, Native Americans used to insert a beaver incisor in a wooden handle and use it to cut bones and to shape their horn-tipped spears.
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