Of Mice and Men

Cleaning behind a door today I found two mice huddling together. They were obviously juveniles, and seemed very sleepy. So I found a piece of cardboard, slid it under them, and deposited them gently in a cardboard box. They continued to huddle.

Curious about this behavior I took a closer look. One mouse was looking very dead, but the other continued to snuggle up to it. As they were both about the same size, and both were smaller than average, I wondered if the surviving sibling was...