What is this?

I took this photo at the zoo. I am always looking down checking the ground out. Floors, pavement, etc. It used to be a self-esteem thing, due to puberty, acne, and shyness, but now I do it because of the weird things I find. I have found over $300.00 in my life just looking down. I have found cassette tapes, CD's, G.I. Joes, Star Wars Figs, and more. I have discovered almost every manhole is unique and different. Smoker's are the trashiest people on the planet followed closely by beer drinkers and Fast food junkies. You never find organic milk cartons on the side of the road or in your yard. Wal-mart receipts however seem to be able to travel for hundreds of miles. I have even found a winning scratch off lottery ticket in my yard. This photo though is one of those things that just begs the question, What is this? I mean, it's a nut with a rusty washer around it with a white circle painted around that, but what for? Is it a super special nut? Does it release the lions? Who knows? There it was though, in the pavement on the path at the zoo. Kinda looks like a booby.