The fifth digits (D5) of 22 species of Australian bats were compared. Unloaded, all species had positive wing camber values between 6.5 and 11.4%. During the measurements on live individuals two species were observed to entirely straighten D5, including the inherently cambered metacarpal-5 (M5) (1.5 to 5.6%). To explain this capability, preserved specimens of all species were assessed in terms of the metacarpal's unloaded camber, tendon alignments, cross-sectional aspect ratio, various length ratios, bending stiffness, bone density and tendon insertions onto the metacarpo-phalangeal joint. The four species that migrate or commute long distances at high speeds have anatomical mechanisms that could eliminate the camber of their fifth digit, including its metacarpal, thereby optimising their high flight speed performance (Mormopterus beccarii, M. loriae, M. planiceps and Tadarida australis). They were distinctive among the 22 species assessed because: (1) their extensor tendon was aligned directly above the M5's elastic axis over the full length of the bone, (2) this tendon had additional insertions in the metacarpo-phalangeal joint that were correctly positioned to straighten the positive camber inherent in the M5, (3) the metacarpo-phalangeal joint capsule was appropriately reinforced to support the additional insertions, (4) their M5 showed a distinctive pattern of bending stiffness values in the sagittal plane appropriately graded for straightening the bone and (5) when exposed and tensioned manually, the extensor tendon did straighten the shaft of M5.
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