From the afterwords in Out of the Silent Planet (p156):
For example, can I even make you understand how I know, beyond all question, why it it is that the Malacandrians dont keep pets, and in general, dont feel about their lower animals as we do about ours? Naturally, it is the sort of thing they themselves could never have told me. One just sees why when one sees the three species together. Each of them is to the others both what a man is to us, and what an animal is to us. They can talk to each other, they can cooperate, they have the same ethics; to that extent a sorn and a hross meet as two men. But each of them finds the other different, funny, attractive as an animal is attractive. Some instinct staved in us, which we try to sooth by treating irrational creatures almost as if they were rational, is really satisfied in Malacandra. They dont need pets.
Part of me wants to put this as an answer to DestrierDragon's earlier topic because he also had some very good Lewis citations that are relevant, however, I'd like to give this excerpt special focus. Lewis has several like it elsewhere, in MN, also in That Hideous Strength, and so on. I'm being selective here so as not to cast too wide a net.
(Sorry if this is even more inscrutable than the above.)
They [the Sorns] were astonished at what he [Ransom] had to tell them about human history- war, slavery, and prostitution... things that particularly stuck in their minds about our world... the fact that we had only one kind of hnau: they thought this must have far reaching effects in the narrowing of sympathies and even of thought.
"Your thought must be at the mercy of your blood," said the old sorn, "for you cannot compare it to thought that floats on different blood."
7/22 very interesting comments, I see the cuetlaxmeh have weighed in. I'll link my earlier comments in a few days after I pick my cousin's brain on a point or two.
7/24 - BTW, the Sorns kept livestock (for cheese)... that's really a side issue...