horses and cattle, imported during the First Empire, run wild on the plains where they are hunted by predators unless protected by men;
some maris (barbarians) both ride and eat the moose-like native ayuks which live on the seven-inch-long, egg-laying hive-rats;
wood- and plant-eating hive-rats are not carnivorous but are vicious when trapped and hundreds will help one that is wounded;
most, though not all, native animal life is humanly edible, although lacking in essential vitamins, but native plants are metal-storing, therefore poisonous to human beings - the animals separate out the metal;
the maris, dependent on their ayuks, must move around the plains as the ayuks eat all the hive-rats in each locality.
"'It must be true, as the priests say, we came here from another star long ago, else why would God have put us where we cannot eat?'"
-Jerry Pournelle, King David's Spaceship (London, 1984), p. 108.
This reasoning, theistically expressed, is essentially correct. Human beings could not have evolved in an environment where they were unable to eat the plants.