From BBC Earth:
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After labouring for weeks on end, pushing themselves to their physical limits, the time has now come for the beavers to reap the rewards of their hard work. Not only can signs of the beaver species hard work be seen in the successful colonisation of vast areas including that of the Taiga and Tundra in far North America, but also in their genetic make-up. Beavers teeth have evolved to endure the tough grind needed to prepare for this time of year. For instance, beaver teeth have such sharp edges they were once used as knife blades by Native Americans. In fact, beaver's front teeth do not stop growing. Which is especially useful as being particularly prolific builders means their most important wood-cutting tool never degenerates or goes blunt!
We return to the beaver family in the thick of winter. The lush vegetation, mud and stone are now frozen solid and we see exactly how the beavers hydro-engineering and science of preparation have paid off.