The speaker is fed up with chewing, and the poem turns that small, everyday act into a whole way of being in the world. Chewing here means grinding, gnawing, breaking things down. The opening images are harsh and noisy: rat's teeth on metal, a squirrel cracking a walnut. Cows, with their blank, repetitive chewing, seem dull, while meat-eaters are tied to pain and rot. Neither option appeals.
In the final stanza the speaker says what they want instead: a return to "the soup," a state before effort and violence. The images become gentle and strange. A fetus tastes the world through its belly, and a whale's mouth is like a harp, taking in krill as a long, slow song. The mood moves from irritation to a dreamy wish for a softer, more fluid way of taking in life. The poem seems to suggest that the speaker is tired of struggle and wants to absorb the world rather than tear it apart.