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

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 if
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 yield 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.