Should a dog be vegan? There are compelling reasons to say yes. The environment, for one: American dogs and cats eat roughly a quarter of the animal-derived calories consumed in the US each year, according to UCLA geographer Gregory Okin. By his calculations, our pets’ current diets are responsible for as much as 64 million tons of greenhouse gases each year. For context, that’s about what you’d get driving 13 million cars for a year.