Cheap thermometer calibration technique?

I don't know about rabbits (maybe something to do with digesting cellulose in grass, which is notoriously difficult to do). Cows do something similar, but it's done internally (i.e. chewing the cud).

Many animals have to eat their parents' excrement -- that's how they get the right bacteria into their gut to breakdown the type of food found in that locality. Elephants are probably the best known example of this, and adults will also repeat this if they are moved to a different location where the diet changes.