Model-based assessment of muscle forces and strain distribution in the femur of Cabot’s Tragopans(Tragopan caboti)

The hindlimbs play a crucial role in bird locomotion,making the biomechanical properties of the musculoskeletal system in these limbs a focal point for researchers studying avian behaviour.However,a comprehensive analysis of the mechanical performance within the long bones of hindlimbs during locomotion remains lacking.In the present study,the strain and deformation of the femur of Cabot’s Tragopans(Tragopan caboti)were estimated.We employed inverse simulation to calculate the force and moment of femoral muscles during mid-stance terrestrial locomotion and conducted finite element analysis to calculate femoral strain.Results showed that during mid-stance,the femur experiences combined deformation primarily characterized by torsion,bending,and compression.It emphasises the importance of considering the influence of varying loads on bone adaptation when investigating bone form-function relationships.Muscles were found to play a significant role in offsetting joint loads on the femur,subsequently reducing the deformation and overall strain on the bone.This reduction enhances femoral safety during locomotion,allowing birds to meet mechanical demands while maintaining a lightweight bone structure.Notably,the M.iliotrochantericus caudalis significantly reduces torsional deformation of the proximal femur,protecting the vulnerable femoral neck from high fracture risk induced by rotation load.Given that the femur torsion during terrestrial locomotion in birds is associated with changes in hindlimb posture due to their adaptation to flight,the characteristics of M.iliotrochantericus caudalis may provide insight into the locomotor evolution of theropods and the origin of avian flight.

Femoral mechanical performance of precocial and altricial birds:a simulation study

Background:As the major load-bearing structures,bones exhibit various properties related to mechanical perfor-mance to adapt to different locomotor intensities.The habits and ontogenetic changes of locomotion in animals can,thus,be explored by assessing skeletal mechanical performance.Methods:In this study,we investigated the growing femoral mechanical performance in an ontogenetic series of Cabot’s Tragopans(Tragopan caboti)and Pigeons(Columba livia domestica).Micro-computed tomography-based finite element analysis was conducted to evaluate the stress,strain,and strain energy density(SED)of femora under axial and radial loading.Results:Femora deflected medio-laterally and dorso-ventrally under axial and radial loading,respectively.Femora deformed and tensed more severely under radial loading than axial loading.In adult individuals,Cabot’s Tragopans had lower strain and SED than pigeons.During ontogeny,the strain and SED of pigeons decreased sharply,while Cabot’s Tragopans showed moderately change.The structural properties of hatchling pigeons are more robust than those of hatchling Cabot’s Tragopans.Conclusions:Limb postures have dominant effect on skeletal deformation.The erect posture is preferred by large mammals and birds to achieve a high safety factor of bones during locomotion.Adult Cabot’s Tragopans have stronger femora than pigeons,reflecting a better bone adaption to the terrestrial locomotion of the studied pheas-ant species.Changes in strain and SED during growth reflect the marked difference in locomotor ability between precocial and altricial hatchlings.The femora of hatchling Cabot’s Tragopans were built with better energy efficiency than deformation resistance,enabling optimized mechanical performance.In contrast,although weak in mechani-cal function at the time of hatching,pigeon femora were suggested to be established with a more mature structural design as a prerequisite for rapid growth.These results will be helpful for studies regarding developmental patterns of fossil avian species.