Body size and scaling of the hands and feet of prosimian primates

Abstract

The hands and feet of primates fulfill a variety of biological roles linked with food acquisition and positional behavior. Current explanations of shape differences in cheiridial morphology among prosimians are closely tied to body size differences. Although numerous studies have examined the relationships between body mass and limb morphology in prosimians, no scaling analysis has specifically considered hand and foot dimensions and intrinsic proportions. In this study, we present such an analysis for a sample of 270 skeletal specimens distributed over eight prosimian families. The degree of association between size and shape was assessed using nonparametric correlational techniques, while the relationship between each ray element length and body mass (from published data and a body mass surrogate) was tested for allometric scaling. Since tarsiers and strepsirrhines encompass many taxa of varying degrees of phylogenetic relatedness, effective degrees of freedom were calculated, and comparisons between families were performed to partially address the problem of statistical nonindependence and "phylogenetic inertia." Correlational analyses indicate negative allometry between relative phalangeal length (as reflected by phalangeal indices) and body mass, except for the pollex and hallux. Thus, as size increases, there is a significant decrease in the relative length of the digits when considering all prosimian taxa sampled. Regression analyses show that while the digital portion of the rays scales isometrically with body mass, the palmar/plantar portion of the rays often scales with positive allometry. Some but not all of these broadly interspecific allometric patterns remain statistically significant when effective degrees of freedom are taken into account. As is often the case in interspecific scaling, comparisons within families show different scaling trends in the cheiridia than those seen across families (i.e., lorisids, indriids, and lemurids exhibit rather different allometries). The interspecific pattern of positive allometry that appears to best characterize the metapodials of prosimians, especially those of the foot, parallels differences found in the morphology of the volar skin. Indeed, relatively longer metapodials appear to covary with flatter and more coalesced volar pads, which in turn slightly improve frictional force for animals that are at a comparative disadvantage while climbing because of their larger mass. Despite the essentially isometric relationship found between digit length and body mass across prosimians, examination of the residual variation reveals that tarsiers and Daubentonia possess, relative to their body sizes, remarkably long fingers. Such marked departures between body size and finger length observed in these particular primates are closely linked with specialized modes of prey acquisition and manipulation involving the hands.