Ontogeny and Adaptation:
A Cross-Sectional Study of Primate Limb Elements

Abstract

How primates achieve their adult skeletal form can be ascribed to two broad biological
mechanisms: genetic inheritance, where morphological characters are regulated by an individual's
phenotype over development; and plastic adaptation, where morphology responds to extrinsic
factors engendered by the physical environment. While skeletal morphology should reflect an
individual’s ecological demands throughout its life, only a limited amount of published research
has considered how ontogeny and locomotor behaviour influence limb element form together. This
thesis presents an investigation of long bone cross-sectional shape, size and strength, to inform
how five catarrhine taxa adapt their limbs over development, and further, evaluate which limb
regions more readily emit signals of plasticity or constraint along them. The sample includes Pan,
Gorilla, Pongo, Hylobatidae and Macaca, subdivided into three developmental stages: infancy,
juvenility and adulthood. Three-dimensional models of four upper (humerus and ulna) and lower
(femur and tibia) limb elements were generated using a laser scanner and sectioned at proximal,
midshaft and distal locations along each diaphysis. Three methods were used to compare geometry
across the sample: 1) principal and anatomical axis ratios served as indices of section circularity,
2) polar section moduli evaluated relative strength between limb sections and 3) a geometric
morphometric approach was developed to define section form. The results demonstrated that
irrespective of taxonomic affinity, forelimb elements serve as strong indicators of posture and
locomotor ontogenetic transitions, while hindlimb form is more reflective of body size and
developmental shifts in body mass. Moreover, geometric variation at specific regions like the midhumerus
was indistinguishable across all infant taxa in the sample, only exhibiting posture-specific
signals among mature groups, while sections like the distal ulna exhibited little or no intraspecific
variation over development. Identifying patterns of plasticity and constraint across taxonomic and
developmental groups informs how limb cross-sections either allometrically or isometrically scale
their form as they grow. These findings have direct implications to extant and extinct primate
research pertaining to body mass estimation, functional morphology and behavioural ecology.