There's been a noticeable uptick in squirrel graffiti this fall, as pumpkins all over town have been literally defaced, or perhaps refaced. My daughter carved a fine ghost at a halloween party, only to have the ghost consumed the next day by a resident rodent.
One interpretation is that squirrels have turned their substantial intelligence and expressive range to the art of pumpkin carving. Since they aren't afraid of ghosts, they choose to refashion pumpkins with what they consider to be far scarier images--car wheels and cats, for instance. Thus, the circular nature of their carvings, with something ear-like sticking out on top.