The passage that starts on 564 concerning the lemming named Ursula is very interesting. It bring up several of the motifs we've explored in previous posts- suicide, being "passed over", etc.

"One lemming, kid?"
"I've had her for two years," he sobs, "she's been fine, she's never tried to-- I dont know. Somthing just came over her."
"Quit fooling. Lemmings never do anything alone. They need a crowd. Its gets contagious. You see, Ludwig, they overbreed, it goes in cycles, when ther are too many of them they panic and run off looking for food."

Slothrop finds it very hard to believe that Ludwig's lemmming would be the single lemming to survive the famously cliched mass-suicide of the lemmings.