The descendants of black-bear-sized giants who could haul massive rocks with their mouths, modern Castor canadensis once numbered as many as 400 million in North America. Felling mighty trees with their orange, iron-infused, nearly invincible incisors, they created their terraforming, hydro-engineering, half-submerged eco-homes based on blueprints stored in their DNA. Now, thanks to merciless fur-hunting by humans, their population has plummeted by 97.5%.