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Fig. 4. (A,B) Bone strain versus age and size in the midshaft of the goat radius and the emu tibiotarsus. Data for goats are binned by age-size groups (small, intermediate and large). Data for emu are graphed as scatterplots, with animals compared at a duty factor of 0.40 over the range of size for which data are shown. Least-squares regression was used to test for effects of size on strain. (C,D) Scaling of bone geometry. Body mass (M_b; kg); area (mm²); second moment of area (mm⁴). Cross-sectional area and second moment of area in the cranio-caudal (I_CC) and medio-lateral (I_ML) directions in goat radius and emu tibiotarsus. Light solid and broken lines depict scaling relations expected for geometric similarity (GS) and stress similarity (SS), respectively. Bold solid lines show least-squares regression slopes. P-values test for a significant difference of the regression slope from geometrically similar scaling (P<0.05). Although not shown, peak ground reaction forces (G) were scale-invariant for both species, averaging 2.0 W for the emu when running at a duty factor of 0.40, and 1.5 W for the goat forelimb at a gallop.

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