I've been trying to identify this animal for a couple of years. I even submitted it to the zoology department at the Field Museum and they were unable to identify it.

I photographed it near the Shedd Aquarium in Chicago. It was in one of the gardens. It was a bit larger than a gerbil but smaller than a rat. Here is a link to a photo:

http://www.flickr.com/photos/garysibio/5374693264/in/set-72157622943930233

Any help would be greatly appreciated.

I was not familiar with this species, so I asked a mammalogist with the Illinois Natural History Survey to take a look at the photo. He confirmed that it is not native to North America. He thinks it may be in the family Muridae. There are over 700 species, several of which are bred by the pet trade for their color and docility. He thinks this is an escaped or released pet. If you find out what species it is, please let me know.