Phylogenetic structural scaling of the appendicular skeleton: relationship with loading regime and locomotor behaviour

Scaling studies examine how structures change in response to increasing animal size. Most previous scaling studies have characterized bone shape with length and midshaft diameter and quantified loading on the bone with the mass of the animal. In this interdisciplinary study we will integrate engineering analyses, including image analysis, detailed mechanics, kinematics measurements and statistical shape modelling to determine how bones change across a range of species and sizes. We will investigate 5 species from each of the following clades: bipedal birds, mammalian carnivores (cats, dogs, wolves, lions), Macropodoidea (kangaroos, wallabies), bovine artiodactyls (hoofed mammals), and catarrhine primates (terrestrial monkeys). We will use computer tomography images, similar to 3D x-rays, to determine the structural characteristics of the bone that affect the bone's mechanical properties and its ability to withstand load. We will measure the locomotor behaviour of each animal using a multi-camera motion analysis system and force plates. This will allow us to determine joint angles, ground contact forces, limb positions, and stride characteristics that are integral components of locomotor behaviour and hence bone loading regimes. Using a range of statistical techniques we will determine the relationship between bone structure and the locomotor behaviour. The shapes of some irregularly-shaped bones, such as the scapula and pelvis, are difficult to quantify. Unlike long bones where structural measures, such as cross-sectional area, are simple to determine, irregular bones have no obvious dimensions to measure. Statistical shape modelling overcomes this difficulty by creating a virtual model of the bone and comparing its shape to an average bone. The technique considers all variability and highlights were the most variability in shape occurs. These statistical shape models can be used to predict the shape of the corresponding bone in a joint. This technique would prove extremely useful for palaeontologists who often have incomplete skeletons and must predict the shapes of missing bones. It will also show how closely the shapes of articulating bones are linked via their interrelated loading regimes. Bone is made up of tiny spicules called trabeculae. The size and orientation of the trabeculae are influenced by the loading on the bone. Few studies have examined how the trabecular structure varies across species. We will obtain trabecular bone samples from each species and image them using microCT, which has a resolution up to 5 micrometers. We will obtain standard 3D measures of the trabecular structure, such as trabecular thickness and orientation. Trabecular structural measures will be correlated with locomotor parameters in order to determine how the loading, size, and locomotor behaviour influences bone micro-structure. Our study will also take research on musculoskeletal scaling into a new, extremely integrative direction. We have designed a study that (1) examines all bones in the appendicular skeleton at multiple structural levels, (2) uses a wide range of taxa and sizes, (3) applies a mechanistic approach to understanding bone structure and locomotor behaviour, and (4) employs rigorous statistical techniques to determine relationships between structure and function.

Scaling studies examine how structures change size and shape across species with a range of masses. By adding detailed mechanical analysis of bone structure and gait, we hypothesize that a better understanding of the relationship between bone shape and function will be obtained. We will examine the structural and locomotor characteristics of 5 species in each of 5 clades: bipedal birds, Carnivora, Macropodoidea, bovine artiodactyls, and catarrhine primates. We will measure the macro and micro-structure of the bones using CT and micro-CT imaging, respectively. From the images we will construct 3D models of the bones in order to determine macro-structural properties (such as second moment of area) and micro-structural properties (such as trabecular thickness and orientation). We will characterize locomotor behaviour using a multi-camera motion analysis system and force plates to obtain measures of joint angles during gait, ground reaction forces, speed, and stride length. From these measures we will calculate the joint forces and moments, a first approximation of the loading on the bone during gait. To obtain locomotor data for each study animal, we will use previous studies that have extensively studied gait as well as collect our own data at local zoos and wild animal parks. We will additionally measure variability in complex bones such as the pelvis and scapula using statistical shape modelling, which indicate the major modes in shape variability. We will use these models in canonical correlation analysis to determine if bone shape can be predicted across a joint. We will employ advanced statistical techniques to determine how bone structural parameters are related to locomotor behaviour. Our study will take research on musculoskeletal scaling into a new, extremely integrative interdisciplinary direction, by employing detailed rigorous mechanistic analysis to understand locomotor behaviour influences bone shape across species.