There's actually a lot of evidence in primates and other animals that they return favors.

After coitus every animal is sad.

The codfish lays ten thousands eggs, The homely hen lays one. The codfish never cackles To tell you what she's done. And so we scorn the codfish, While the humble hen we prize, Which only goes to show you That it pays to advertise.

I like pigs. Dogs look up to us. Cats look down on us. Pigs treat us as equals.

The Discovery Channel had a fascinating show on the mating habits of hyenas. They said that the male hyena often will get angry at the female hyena while they are having sex. It doesn't help that the female hyena is laughing at you all the time.