THE CORRELATION BETWEEN THE MECHANICAL ADVANTAGE OF THREE FORELIMB JOINTS AND HABIT SPECIALIZATION IN EXTANT MAMMALS

All living mammals use the forelimb in some capacity to maintain a lifestyle specialization or habit. The bones of mammalian forelimbs are shaped to facilitate specific behaviors associated with a habit, and yet phylogenetic variations obscure such behavioral signals. By focusing on mechanical characters, it may be possible to remove such obscurity and find the connection between shape and habit. In order to quantitatively test this hypothesis, we focused on the mechanical advantage of forelimb joint motions. Several multivariant statistical analyses were performed on a database containing species name, habit type and mechanical characters for 205 extant mammal specimens representing 189 species, 150 genera and 16 orders. The habit types were coded as fossorial (scratch-digging, humeral rotation digging, hook-and-pull digging and lateral digging), arboreal, ricochetal (adapted for jumping), semi-aquatic, aquatic or terrestrial. The mechanical characters involved the three forelimb joints with 6 shoulder (flexion, extension, adduction, abduction, internal and external rotation), 2 elbow (flexion and extension) and 2 wrist motions (flexion and extension). The shoulder, elbow and wrist muscle moment arms in relation to the out-force lever arm (the total length of the humerus, ulna, and hand respectfully) were used to calculate mechanical advantage. Preliminary analyses suggest separation between the fossorial and the non-fossorial taxa, with varying degrees of separation within each. Separation among habit types, further supports the separation within fossoriality as proposed by previous authors. Among all habit types, the greatest separation occurs within the mechanical advantages of shoulder flexion and elbow extension with the greatest difference between shoulder adduction and wrist flexion. Future work includes incorporating additional taxa and applying ranges to the mechanical advantages found in extinct mammal taxa forelimbs to reveal the most efficient motion and therefore preferred habit.