Rats: Reproduction.

Commentator: "One pair of rats can produce 2,000 offspring in one year alone. They're the most prolific of all mammals. A single pair of rats left to reproduce unhampered will grow to 360,000,000 in just 3 years."

Oh, holy cow. Did you know that one pair of humans can produce 44 children within their breeding lifetimes? Yep. It's a potential problem. Fortunately "unhampered" is a key factor here: If a rat were to reproduce all willy-nilly, there'd be a shortage of food, now wouldn't there? And mom would be feeling pretty tired and would probably be DEAD by the time that she was 2 months old. Oh, and don't forget that the average lifespan of a single rat in the wild is 8 months. Unfortunately it does get cold out there, there are oodles of threats that we humans don't experience on a regular basis, and there are no hospitals around that are willing to help them out if there are complications..."