http://www.boingboing.net/2010/09/29/why-squirrels-mastur.html
This image has not been digitally altered.
It's a Cape ground squirrel, writes sci-blogger Ed Yong, and they all (or, anyway, all the dude squirrels) look like that. And if you think this is impressive, you should see their penis*.
Perhaps somewhat unsurprisingly, Cape ground squirrels are known for masturbating.