the-awkward-turt:

LOVE the point about herbivorous animals occasionally supplementing their diet with protein sources. Just because an animal supplements its diet with something does not mean it can survive or thrive on a diet of just that thing.

Butterflies supplement their diets with mud, that doesn’t mean they can live off of mud. Chickens supplement their diet with insects and small vertebrates, that doesn’t mean they would thrive or be healthy on an all-meat diet.

Dogs adapted to better supplement their diet with carb-rich scraps from humans, that does not mean they can live healthy lives without meat composing a significant portion of their diet.

Human-ancestors, on the other hand, evolved from largely herbivorous apes and only became true omnivores relatively recently in our evolutionary history. We started supplementing our diet with more meat and eventually evolved to digest it better, but we still have most of the biological equipment to get all the nutrients we need from plant-based sources like our far-off ancestors used to. That’s why humans can thrive on a vegan diet.

Dogs and cats did not have ancestors that derived most of their nutrition from non-meat sources so they don’t have the biological equipment to thrive on that diet. Even if dogs WERE omnivores they would still have evolved from mostly meat-eating ancestors and so would not have the biological tools to live off a diet without any meat. Evolutionary history matters.