Many times in a rehab situation with wildlife we have to winter over animals or have them in rehab a bit longer than normal. For those animals that love to run, this wheel is ideal. This is without a doubt the best wheel I have ever seen. And for those Nocturnal animals, you'll finally be able to sleep at night! This wheel is almost totally quiet. We have a flying squirrel in our clinic for the winter and he was keeping the gray squirrels awake at night with his old wheel, they don't even know he's there anymore! This wheel is available in two sizes: Medium and Large. We recommend the medium for use on Red Squirrels, Flying Squirrels, and smaller animals such as mice, gerbels. The larger wheel would be recommended for Opossums, Guinea Pigs, and other animals of a larger size. This wheel is designed to hang in the cage or be free standing. Mini Wheel 4.5" $9.95 Appropriate for mice and other mammals of mouse size Medium Wheel 6.5" $10.95 Appropriate for Flying Squirrels, Red Squirrels, Hamsters. Please note: Gray Squirrels and Fox Squirrel generally do not desire the use of a wheel. Large Wheel 12" $24.95 Appropriate for larger mammals such as opossums. Please make your selection in the drop down box. We've included a video of Champ on his wheel, it looks a little scary but he really does love this thing. The Wheel shown in this Video is the Medium 6.5"