Morphological integration in mammalian limb proportions: dissociation between function and development

During mammalian evolution, fore- and hindlimbs underwent a fundamental reorganization in the transformation from the sprawled to the parasagittal condition. This caused a dissociation between serial and functional homologues. The mobilized scapula functions as the new proximal forelimb element and is functionally analogous to the femur of the hindlimb. Tarsus and metatarsus built a new functional hindlimb element that is functionally analogous to the forearm of the forelimb. Morphological covariation between serially homologous fore- and hindlimb elements can conflict with biomechanical demands when certain intralimb proportions are required for the postural stability of motion. The limb proportions of 189 mammalian species were examined to test whether intralimb proportions are governed by a general principle that corresponds to biomechanical predictions. Morphological covariation between functionally analogous and serially homologous fore- and hindlimb elements was tested by a correlation analysis. A clear relationship exists between the proportions of the first and the third elements of each limb, while the middle element is less involved in alterations of intralimb proportions. Hindlimb proportions are largely uniform across mammals and correspond to biomechanical predictions regarding postural stability. The greater variability in forelimb proportion is likely be the expression of various adaptations but might results also from constraints due to the shared developmental programs with the hindlimb.