Functional morphology of the lemuriform wrist joints and the relationship between wrist morphology and positional behavior in arboreal primates

Abstract

A comparative study of carpal joint structure and function in six Malagasy lemuriforms was undertaken to test predicted morphoclines in carpal joint morphology between pronograde and orthograde arboreal primates. Patterns of movement at the wrist during locomotion were observed and described for the lemuriform species Lemur fulvus and Propithecus verreauxi. Lemur fulvus, which assumes a pronograde posture during locomotion, extends and pronates the wrist during the support phase of quadrupedal walking and running stride cycles. Furthermore, the forearm of this species exhibits some transverse movement across the proximal wrist joint during the support phase. In contrast, the indriid Propithecus maintains the hand and wrist in a flexed and partially supinated position during vertical clinging and suspensory postures. Habitual quadrupedal and vertical postures in Malagasy primates are in turn related to very different patterns of carpal joint morphology and articular mechanics. Those lemurs which are predominantly pronograde share a series of structural features related to stabilizing the antebrachiocarpal joint during extension and mediolateral deviation and the midcarpal joint during pronation: an intraarticular labrum is present on the inner portion of the radiocarpal ligament, the radiocarpal articular surface is quite flat dorsoventrally, the capitate-trapezoid embrasure is expanded dorsally, and development of the radial and ulnar styloids is more pronounced. The wrists of Propithecus, Avahi, and Lepi-lemur (vertical clingers) differ from those of quadrupedal lemuriforms in possessing a suite of morphological features related to stabilizing the wrist during antebrachiocarpal flexion and midcarpal supination: the radiocarpal articular surface is deeply curved and tilted anteriorly, the dorsal radiocarpal ligament is very broad, thick, and fibrous, the hamate's triquetral facet is directed proximodistally, and the capitate-trapezoid embrasure is dorsally constricted and expanded palmarly. These observed contrasts in carpal form and function are used to define further the morphological features related to orthograde posture in several lineages of arboreal primates.

Hands of early primates

“…They have been well studied in both "prosimian" and platyrrhine primates (Beard and Godinot, 1988;Whitehead, 1993;Hamrick, 1996aHamrick, ,b,c, 1997Hamrick and Alexander, 1996;Schwartz and Yamada, 1998) as well as in catarrhines, though the latter is not a focus of this review. The form of these bones has been linked to various functional attributes of the wrist, including its degree of mobility in flexion and extension, as well as abduction and adduction (Godinot and Jouffroy, 1984;Preuschoft et al, 1993;Hamrick, 1996bHamrick, ,c, 1997Hamrick and Alexander, 1996). The morphology of wrist bones can also reflect habitual position and direction of greatest mobility (i.e., palmarflexed, dorsiflexed, ulnar-deviated; Preuschoft et al, 1993).…”

“…The morphology of wrist bones can also reflect habitual position and direction of greatest mobility (i.e., palmarflexed, dorsiflexed, ulnar-deviated; Preuschoft et al, 1993). Finally, wrist bones can reveal stable hand positions, the directions in which forces can be effectively transmitted and sustained (Hamrick, 1996b(Hamrick, ,c, 1997Hamrick and Alexander 1996), and the relative development of the extrinsic digital flexors (Hamrick, 1997). These form-function relationships must be taken with a grain of salt, however, as some predictions of morphological variation based on biomechanical principles (Preuschoft et al, 1993) have not been upheld in comparative studies (Hamrick, 1997).…”

“…As the same facet becomes more convex and forms a more pronounced arc with the articulating capitate (the midcarpal joint), a greater degree of stable mobility becomes possible (Hamrick, 1996a,b,c). Furthermore, a greater distal extent of the triquetrohamate facet should allow greater ulnar deviation (Hamrick, 1996b).…”

The hands and feet of Archaeolemur: metrical affinities and their functional significance

“…The set of the trapezium implies a less abducted pollex than is typical of a lemur. Overall, the morphology of the carpus is more lemurid-like than indriid-like, especially with respect to the midcarpal joint (Hamrick, 1995(Hamrick, , 1996a; for example, the proximal articular surface of the hamate is more similar to those of arboreal quadrupeds than to either vertical-clingers and leapers or slow climbers (see also Hamrick et al, 2000).…”

“…To this end, we compare here the proportions of the various elements of the hands and feet of Archaeolemur to those of living Malagasy lemurs, Old World monkeys, and selected nonprimate mammals. The functional anatomy of the hands and feet is especially closely linked to posture and locomotion because these organs contact the structural environment directly and form the biomechanical link through which forces are transferred to the postcranium, an opinion we share with many others (e.g., Lessertisseur and Jouffroy, 1973;Etter, 1973;Jouffroy andLessertisseur, 1978, 1979;Gebo and Dagosto, 1988;Szalay and Dagosto, 1988;Latimer and Lovejoy, 1989;Lewis, 1989;Strasser, 1994;Hamrick, 1995Hamrick, , 1996aLemelin, 1996Lemelin, , 1999Wunderlich et al, 1996).…”