Postcranial adaptations for leaping in primates

Abstract

Leaping primates are specialized for hindlimb-propelled locomotion within arboreal habitats. As a group, they include members of Galagonidae, Lemuriformes and Tarsiidae. Postcranial characters analysed here include humeral and femoral diaphyseal rigidity, articular surface areas and lengths. Data for leaper taxa are compared with corresponding data for less specialized small primates. The more generalized comparative primates include both closely related prosimians and distantly related platyrrhines (New World monkeys). In addition, the leapers are subdivided for further analysis according to body size and taxonomic association. Questions addressed concern the identi®cation of functionally and/ or phylogenetically linked traits in leaper postcrania. Results indicate that leapers as a group have relatively higher femoral diaphyseal rigidity and longer femora than do more generalized primates. These traits are also present in Pithecia pithecia, a platyrrhine leaper included for comparison. These enhanced properties probably function in resisting large hindlimb forces incurred during leaping, and in producing longer, more ef®cient leaps. Most of the large-bodied lemuriform leapers are further distinguished in having relatively bigger femoral heads and reduced humeral rigidity. The small-bodied leapers, galagonids and tarsiids, do not differ in either femoral head surface area or in any of the humeral properties from more generalized primates. Pithecia has a large femoral head like lemuriforms, but it is not reduced in humeral rigidity. Aspects of hip joint structure and mobility may be related to femoral head size in lemuriforms and Pithecia. Explanations regarding reduced lemuriform humeral rigidity are also explored. Differences between lemuriform primates are also present, most notably in aspects of the distal humerus. Indrids are characterized by relatively reduced trochleae, while many lemurids have relatively small capitula. These features are interpreted with regard to frequencies of suspensory behaviour and quadrupedalism.

Additional remains ofWadilemur elegans, a primitive stem galagid from the late Eocene of Egypt

“…2G, various geometric properties can be calculated, including the cortical bone area and the second moments of area and section moduli about the anteroposterior and mediolateral axes (see Table 6, which is published as supporting information on the PNAS web site). Although the midshaft is typically the point along the shaft at which these parameters are measured and compared across taxa (33)(34)(35), the absence of a distal end makes determination of the midshaft impossible. These data can nevertheless be useful to illustrate the cross-sectional geometry of the specimen and to infer possible loading conditions at various levels along the shaft.…”

“…These data can nevertheless be useful to illustrate the cross-sectional geometry of the specimen and to infer possible loading conditions at various levels along the shaft. The section moduli, Z a-p and Z m-l , reflect the bending strength in the anteroposterior and mediolateral planes, respectively, and extant leaping strepsirrhines have been shown to have greater femoral rigidity, in general, and greater anteroposterior bending strength, in particular, than primates with more generalized locomotor behavior (33)(34)(35). The index of Z a-p to Z m-l in DPC 13999 indicates that the cortical bone is distributed in such a way as to best resist bending loads in the parasagittal plane rather than in the coronal plane.…”

“…Some of the characters shared with modern galagids include a cylindrical femoral head, a large fovea capitis, a short neck oriented generally perpendicular to the shaft, a large, anteriorly developed greater trochanter that overhangs the anterior aspect of the shaft, and a straight, and presumably long, shaft with an anteroposteriorly elliptical cross-section (31)(32)(33)(35)(36)(37)(38)(39)(40). The primary observation driving the alleged correlation of these femoral characters with vertical clinging and leaping locomotion is their shared presence in both Galagidae and Tarsiidae.…”

Mechanical significance of femoral head trabecular bone structure inLorisandGalagoevaluated using micromechanical finite element models

“…As compared to lorises, who have relatively isotropic trabecular bone throughout the femoral head, galagos have a less dense and more anisotropic trabecular structure. Distinct differences in locomotor behaviors and external locomotor anatomies have been noted between the specialized, saltatory galagos and the slow quadrupedal and climbing lorises (Anemone, 1990(Anemone, , 1993Connour et al, 2000;Demes and Jungers, 1993;Gebo, 1987;Runestad, 1997). Although measurements of hip joint forces are not available, the magnitude and orientation of joint loading in these two taxa is expected to be quite different, especially considering the large differences in hindlimb ground reaction forces generated during locomotion (15 times body weight in Galago vs. 0.5-1.0 times body weight in Loris; Demes et al, 1990;Gunther, 1985;Ishida et al, 1990) and the differences in locomotor kinematics (Burr et al, 1982).…”

“…It is generally thought that, just as with external bone anatomy and diaphyseal cross-sectional geometry (Anemone, 1990;Connour et al, 2000;Demes and Jungers, 1989), this variation reflects differences in joint loading and limb use during various activities. It is not known, however, whether these structural differences can be fully explained as the result of epigenetic or evolutionary functional adaptation, or whether other nonstructural causes such as genetic, nutritional, hormonal, or gender factors may play a role as well.…”