Outdoor Animal Control - Really tough squirrels ... chipmunks? Attacking lawn furniture

Below are pics of a lawn chair that I beleived to be attacked by squirrel or chipmunk. Some of these gauges are all the way through the aluminum frame of the chair, approximately 1/8" thick. All 6 chairs to this set have been attacked. Has anyone seen this before or have any idea what is causing these wild animals to attack?

http://www.csprep.com/images/chair5.jpg

http://www.csprep.com/images/chair4.jpg

http://www.csprep.com/images/chair3.jpg

TIA

Any signs of droppings or other evidence? Norway rats will gnaw metal pipes, downspouts, etc. Their incisors grow 5"/year and they need to keep them worn down by gnawing.

Norway rat info: http://icwdm.org/handbook/rodents/NorwayRats.asp

Norway rats are normally 12-18" long, including tail.

http://nematode.unl.edu/Norway_rat1.jpg
Photo Credit: nematode.unl.edu/<WBR>norwayrat.htm (http://nematode.unl.edu/norwayrat.htm) @ Google Images

one other thing porcupines will chew on anything that has sweat on it looking for salt. that is a slight possibility.
life begins when the kids leave home and the dog dies

Porcupines have four front teeth like chisels. They, too, must gnaw to keep them ground down. Although vegetarians, they will chew on anything that has a taste of salt. Do you live where there are porcupines?

This customer is in the St. Louis, Mo. area - No salt in the air. The patio set is around a swimming pool that is fenced, never seen any porcupines or rats. Any other ideas?
TIA

Identification of the animal is the first step. Droppings are a way of identifying animals. Sprinkling flour in the area to look for footprints of nocturnal animals is another way. I am not sure about St. Louis, but porcupines are native along the Missouri River.

"4 toes on the forefeet and 5 on the hindfeet, all with long, curved claws and small textured knobby pads on the bottom. The long claws make depressions far ahead of the oval tracks, which are about 3 inches long and 5 to 6 inches apart." See photos of tracks and scat here: http://wildwoodtracking.com/mammals/porcupine/porcupine01.html

http://www.doyourownpestcontrol.com/ratpag5.jpg
Photo Credit: Truman's Scientific Guide To Pest Control Operations, Courtesy of Purdue University @ doyourownpestcontrol