CHI*, K.-J.; ROTH, V.L.; Duke U.; Duke U.: Scaling of foot contact area and its mechanical implications for mammals of different foot postures

Foot contact areas (FCA) from ~180 spp. of mammals of a wide range of body masses (BM) and different foot postures (P: plantigrady; D: digitigrady; U: unguligrady) were calculated from images of feet or footprints. Log-transformed raw data suggest that at a given BM, P>D>U for FCA; however, U>D>P for the exponent b in the FCA-BM relationship. Differences in b among groups are less significant for phylogenetically corrected data. Because b<1 in all groups, plantar pressure and hence stress within the footpads increases with BM. At greater BM and plantar pressure, to maintain functional integrity, footpads must reduce internal stress by adding tensile material and consequently increasing compressive stiffness. What parameter, if any, do footpads appear to optimize? H1: Constant stress may be maintained within the pad tissue, in which case our hydrostatic model relating size, morphology, and mechanics of pad tissue predicts that pad stiffness increases disproportionately slower than BM. Alternatively, H2: perhaps pad stiffness scales in proportion to BM; our model then suggests that internal stress within pads of bigger mammals is reduced. To test these hypotheses, we used digitigrades as a model system and measured the compressive stiffness of fore and hind paw pads from 17 canid, felid, and hyaenid individuals at biologically relevant strains. Analyses reveal that stiffness scales differently in fore and hind pads: proportionally to BM in fore pads (H2), but between values predicted by H1 and H2 in hind pads. In both cases, stiffness is greater than that predicted by H1, implying a reduced internal stress within the pads of bigger mammals. The scaling of pad stiffness with BM in fore pads implies that maintaining a constant foot-ground impact regime across a range of body sizes is a principle of footpad design.