It seems that there is an issue with captured animal's maintenance. On the face of it, they do not count toward unit maintenance, as sacrificing them do not change the number of supported units. On the other hand, some of them do- I sacrificed a large number of animals, and some of them did lower the number of supported units. I think bears were one of them. Another similar issue I encountered is when constructing buildings with animals, such as feather worker (from a pigeon), sometimes my income would decrease, instead of rise. While there might be clear reasoning for what is going on, at the best it's unclear, and at worst a bug.