Squirrels are one of the few animals that can descend a tree head first. Hypermobility is a condition that describes where joints stretch farther than normal. Colloquially, when this condition is present, it is also referred to as being double-jointed.
One of the few mammals with this condition, Squirrels are double-jointed in their rear legs and can turn them around in order to hang down from trees upside down. The ability to anchor their back feet allows squirrels to stretch out their forearms and grab on to adjoining branches or bird feeders.
This condition also gives squirrels their agility in racing up and down trees.