According to Guns, Germs, and Steel, what are the four ways domesticated animals help feed larger populations?

The answer to this question can be found on p. 88 of Guns, Germs, and Steel. There, Diamond explains why domesticated animals (in particular, certain species of large domestic animals) can help feed large populations.  These four reasons are 1) that they provide meat, 2) that they provide milk and other sources of protein, 3) that they provide manure, and 4) that they can pull plows.

What this means is that there are two general ways in which large domesticated animals can help people.  First, they can directly provide people with food.  If people have abundant sources of fat and protein, they are more likely to thrive. Large domesticated animals provided this, thereby making it easier to support large populations. Second, they can indirectly provide people with food by helping them grow plants.  Large animals can pull plows.  When people can plow land, they can get better yields from that land. Domesticated animals can also provide manure that people can use to fertilize their fields.  This, too, allows them to get better yields. For these four reasons, domesticated animals can help support larger human populations.