Symbolism, Imagery, Allegory
Poor Ríos.
He bids on what he thinks could be a good luck charm—a tiny stuffed monkey—on Ebay. And what does he get? A huge taxidermied monkey. The guy doesn't like attention, but guess what being mailed a giant monkey gets him?
Yup. Attention.
Luckily for him, Marla's there to take the monkey off his hands. She dresses it in soldier clothes, puts it in the First Squad's Humvee, and names it Captain Yossarian, after the main character in the book Catch-22.
Catch-22 is also about war—specifically a rule of war that's designed to prevent soldiers from getting out of combat. The character Yossarian spends his time trying not to get killed and finds his commanding officers can be as dangerous as the enemy.
So it's kind of fitting when Jonesy tells Birdy the monkey's theme song:
"Survivor!" he said. He was always rocking when he drove and also hunched forward toward the wheel. "When you were peeing, Marla said that was the monkey's theme song. Destiny's Child put it out." (10.55)
Marla must have read Catch-22 (after all, she's the one who named the stuffed monkey), so she's got to know "Survivor" would have been the perfect theme song for that character. Yossarian, like Marla, Birdy, and Jonesy, is less concerned with the larger ideals of what they're fighting for than he is with staying alive.
So Yossarian the monkey riding with these modern-day soldiers is kind of a bad omen, a physical manifestation of the tough situation they're in. Like the character, the First Squad is limited by their orders. And after they get Yossarian, they get sent on some really dangerous missions...and all they can do is try not to get killed.