Monkey See, Monkey Don't
Don't tell Nim Chimpsky, but the characters in this book have a problem with monkeys.
Monkeys represent something subhuman in I Know This Much is True. It starts with a line from a racist professor, who, when talking about comparing Native Americans to white men, says, "It was like comparing apples to oranges, monkeys to men" (13.9). That seems like a throwaway line to characterize this history teacher as a despicable racist, but monkey imagery comes up later, too.
When Domenico and his brothers move to the United States, his brother Pasquale develops an unhealthy attachment to an actual monkey named Filippo, until he finds out the monkey is pregnant, then he changes the name to Filippa. Rumors start flying faster than Tarzan swinging through the trees when Pasquale is spotted kissing Filippa on the lips.
Domenico displays his own "survival of the fittest" attitude when Pasquale dies: He takes Filippa, puts her in a sack, and dumps her into the river. It's way harsh, and makes it pretty clear that his didn't hold his brother in the highest regard—otherwise he would've shown the animal he loved so much a better time.
Later, Domenico does something similar to Prosperine, his wife's friend/cousin who lives with them. He always calls her "The Monkey" and when she betrays him (by sleeping with his wife) he has her committed to a mental institution. Again, Domenico feels no guilt over this. For him, it's evolution, survival of the fittest, a triumph to have her committed. He's proud of it.
Are you noticing the pattern here? Every time we see monkeys, they're involved in some sort of human hierarchy—and every time, the monkeys represent the bottom of the food chain in this hierarchy. And yet, as readers, we can see that the people who disrespect monkeys are actually the worst sort. Who's evolved now?