My friend has seven mice, four boy mice and three girl mice. The boy mice have tails which curl up into a loop, similar to that of a pig. They’re also developing growths on their shoulders and are really dumb, much stupider than previous mice. The girl mice are fine.
All seven are from the same litter. I assume that there is something sex-linked going on. Is there a mouse expert (muridologist?) who can speculate as to what’s happening?