  We constructed a modular, biomimetic red panda paw with which to
experimentally investigate the evolutionary reason for the existence of the
false thumbs of red pandas. These thumbs were once believed to have shared a
common origin with the similar false thumbs of giant pandas; however, the
discovery of a carnivorous fossil ancestor of the red panda that had false
thumbs implies that the red panda did not evolve its thumbs to assist in eating
bamboo, as the giant panda did, but rather evolved its thumbs for some other
purpose. The leading proposal for this purpose is that the thumbs developed to
aid arboreal locomotion. To test this hypothesis, we conducted grasp tests on
rods 5-15 mm in diameter using a biomimetic paw with 0-16 mm interchangeable
thumb lengths. The results of these tests demonstrated an optimal thumb length
of 7 mm, which is just above that of the red panda's true thumb length of 5.5
mm. Given trends in the data that suggest that smaller thumbs are better suited
to grasping larger diameter rods, we conclude that the red panda's thumb being
sized below the optimum length suggests an adaptation toward grasping branches
as opposed to relatively thinner food items, supporting the new proposal that
the red panda's thumbs are an adaptation primary to climbing rather than food
manipulation.
