Evolution of the digital tendon locking mechanism in bats and dermopterans: A phylogenetic perspective

Abstract

A tendon locking mechanism (TLM) in the digits of the feet has been described previously only in bats and birds. In bats, this mechanism typically consists of a patch of tuberculated fibrocartilage cells on the plantar surface of the proximal flexor tendons, and a corresponding plicated portion of the adjacent flexor tendon sheath. The two components mesh together like parts of a ratchet, locking the digit in a flexed position until the mechanism is disengaged. This system apparently allows bats to hang for long periods of time with reduced muscular activity. In this study, we document for the first time the presence of a similar tendon lock in dermopterans, an occurrence that provides additional support for the hypothesis that dermopterans and bats are sister taxa. The present work also includes observations on the morphology of the digital tendon system in chiropteran species not previously examined, including members of the Craseonycteridae, Mystacinidae and Kerivoulinae. Unlike other bats that have a TLM, Craseonycteris and Kerivoula have a plicated proximal tendon sheath but lack distinct tubercles on the flexor tendon. This condition may be related to small body size or may represent an evolutionary intermediate between the presence of a well-developed TLM and the complete absence of this structure. Phyllostomids apparently lack the ratchet-like TLM typical of other bats, instead exhibiting modifications of the tendon sheath that may contribute to its function as a friction lock. Consideration of the distribution of TLM structures in the context of previous phylogenetic hypotheses suggests that a ratchet-type tendon lock was lost and reexpressed at least once and perhaps several times within Microchiroptera. The friction lock is an autapomorphy of Phyllostomidae.

Friction between human finger flexor tendons and pulleys at high loads

“…Schutt (1993) investigated the mechanism more precisely in bats and Quinn and Baumel (1993) finally called it TLM and compared it in different bat species (chiropterans). A similar mechanism was described in climbing mammals by Schaffer (1905) and Haffner (1996), in birds by Quinn and Baumel (1990) and in dermoptera by Simmons and Quinn (1994). The digits of most bats consist of three phalanges and a conjoint tendon instead of a FDP and a FDS tendon.…”

Microbat paraphyly and the convergent evolution of a key innovation in Old World rhinolophoid microbats

“…Given the absence of tarsal modifications that unite bats with other archontans, the primary rationale for including bats in Archonta is the suite of novel features that bats share with flying lemurs (37,39,41). Bats and flying lemurs share a patagium with two unique features not seen in other gliding mammals: a ME logdet, minimum evolution with logdet distances; ME-ML, minimum evolution with ML-corrected distances; GTR, general time reversible model; ⌫, gamma distribution of rates; I, allowance for invariant sites; HKY, Hasegawa-Kishino-Yano model of sequence evolution.…”

“…humeropatagialis muscle and interdigital patagia on the manus (41). Bats and flying lemurs also have a tendon locking mechanism (TLM) on their feet that allows them to hang without continuous contraction of the flexor muscles (41).…”

“…Bats and flying lemurs also have a tendon locking mechanism (TLM) on their feet that allows them to hang without continuous contraction of the flexor muscles (41). Also, megabats (not microbats) and flying lemurs share features such as peculiar primate-like retinal projections to the superior colliculus (2).…”

“…Thiele et al (46) contested the claim that megabats, primates, and flying lemurs share the same unique retinal projections. Chiroptera and Dermoptera share the unique ratchet TLM that allows members of these orders to hang with minimal energy expenditure (41). However, the dermopteran TLM differs from that of bats in having cartilage nodules and a more complex pattern of tendon division and insertion (41).…”