Comparative limb bone loading in the humerus and femur of the tiger salamander Ambystoma tigrinum: testing the ‘mixed-chain’ hypothesis for skeletal safety factors

Abstract

Locomotion imposes some of the highest loads upon the skeleton, and diverse bone designs have evolved to withstand these demands. Excessive loads can fatally injure organisms; however, bones have a margin of extra protection, called a 'safety factor' (SF), to accommodate loads that are higher than normal. The extent to which SFs might vary amongst an animal's limb bones is unclear. If the limbs are likened to a chain composed of bones as 'links', then similar SFs might be expected for all limb bones because failure of the system would be determined by the weakest link, and extra protection in other links could waste energetic resources. However, Alexander proposed that a 'mixed-chain' of SFs might be found amongst bones if: (1) their energetic costs differ, (2) some elements face variable demands, or (3) SFs are generally high. To test whether such conditions contribute to diversity in limb bone SFs, we compared the biomechanical properties and locomotor loading of the humerus and femur in the tiger salamander (Ambystoma tigrinum). Despite high SFs in salamanders and similar sizes of the humerus and femur that would suggest similar energetic costs, the humerus had lower bone stresses, higher mechanical hardness and larger SFs. SFs were greatest in the anatomical regions where yield stresses were highest in the humerus and lowest in the femur. Such intraspecific variation between and within bones may relate to their different biomechanical functions, providing insight into the emergence of novel locomotor capabilities during the invasion of land by tetrapods.

One foot out the door: limb function during swimming in terrestrial versus aquatic turtles

“…Differences in functional change between the forelimbs and hindlimbs have been noted for other taxa spanning evolutionary transitions in habitat [11]. The reduction of propulsive force from the hindlimbs that appears likely with the loss of ankle flexion in tortoises may contribute to their inability to swim into flowing water during our trials.…”

SegmentGeometry: A Tool for Measuring Second Moment of Area in 3D Slicer

“…Therefore, the shape of the salamander forelimb may reflect its loading regime and be stiffer in these directions. Bone loading analyses and second moment of area calculations on the humeri of the terrestrial A. tigrinum confirmed that resistance to bending was greatest in the dorsocaudal direction, reflecting the directionality of the ground reaction forces ( Kawano et al. 2016 ).…”

Biomechanical factors influencing successful self-righting in the pleurodire turtle,Emydura subglobosa

“…To elicit self-righting attempts, turtles were inverted and placed on their carapace, such that the dorsal surface of the head contacted a custom-built force platform (K&N Scientific, Guilford, VT, USA). Specifications of the force platform and signal processing are reported by Butcher and Blob (2008) and Kawano et al (2016). Three-dimensional forces were recorded at 5000 Hz using a customwritten LabVIEW (v.6.1, National Instruments, Austin, TX, USA) routine, while being filmed with digitally synchronized high-speed video in dorsal and frontal views at 100 Hz (Phantom v 5.1, Vision Research Inc., Wayne, NJ, USA).…”