I've started acquiring animals to breed and use for food / hunting, but they keep starving. How do I feed captured/caged animals?
If you're running a version at or newer than 31.19, herbivorous animals now require a pen/pasture with grasses for them to eat (i, n, then Shift + n to assign animals). Grasses grow on above-ground and cavern soil spaces, and grew on any soil space in versions before 31.21.
If the pasture isn't big enough to regrow grass before they eat enough of it, your animals will starve, so you will either need to rotate your animals between different pastures to give them time to regrow or make the pasture big enough to begin with. Some animals, like Yak, are very heavy grazers, and will require sizable pastures to keep from starving. As far as I know it is not possible to feed a caged animal, corroborated by the fact that there is no way to gather grasses in the first place.
Also, once food starts becoming scarce or they are simply kept in an area too small, animals will begin fighting with each other, which will generate combat reports.