Pathways to primate hip function

Abstract

Understanding how diverse locomotor repertoires evolved in anthropoid primates is key to reconstructing the clade's evolution. Locomotor behaviour is often inferred from proximal femur morphology, yet the relationship of femoral variation to locomotor diversity is poorly understood. Extant acrobatic primates have greater ranges of hip joint mobility-particularly abduction-than those using more stereotyped locomotion, but how bony morphologies of the femur and pelvis interact to produce different locomotor abilities is unknown. We conducted hypothesis-driven path analyses via regularized structural equation modelling (SEM) to determine which morphological traits are the strongest predictors of hip abduction in anthropoid primates. Seven femoral morphological traits and two hip abduction measures were obtained from 25 primate species, split into broad locomotor and taxonomic groups. Through variable selection and fit testing techniques, insignificant predictors were removed to create the most parsimonious final models. Some morphological predictors, such as femur shaft length and neck-shaft angle, were important across models. Different trait combinations best predicted hip abduction by locomotor or taxonomic group, demonstrating group-specific linkages among morphology, mobility and behaviour. Our study illustrates the strength of SEM for identifying biologically important relationships between morphology and performance, which will have future applications for palaeobiological and biomechanical studies.

Keywords: functional morphology; locomotion; morphological system; path analysis; primate evolution; structural equation modelling.

Figure 1.

(a) The grasping envelope of the hindlimb (red outline) should differ between stereotyped and acrobatic primates. (b) Diagram of baseline recursive path model depicting relationships between morphological trait and hip abduction variables. Femoral head anteversion (ANT) and declination (DEC), fovea-trochanteric angle (FTA), neck-shaft angle (NSA) and abduction angle were all measured in degrees. Femoral neck length (NL), shaft length (SL) and abducted knee position were measured in millimetres. Femoral head relative surface area (SA) is a dimensionless ratio of posterosuperior subchondral bone area relative to the total surface subchondral bone of the femoral head.

Figure 2.

Diagrams of final path models depicting relationships between femur morphological traits and hip abduction measurements in the (a) Anthropoid group, (b) Hominoid group and (c) Monkey group. All path coefficients shown are standardized estimates.

Figure 3.

Diagrams of final path models depicting relationships between femur morphological traits and hip abduction measurements in the (a) Acrobatic group and (b) Stereotyped group. All path coefficients shown are standardized estimates.