Introduction 1972 1972 1965 1967 1974 1979 1983 1985 1983 1982 1994 2004 Materials and methods Samples 2 Appendix 1 Table 1 Parameter values for apes   Gorilla Gibbon Orangutan Skin thickness (mm) ~1.5 ~0.6 ~2.3 Hair Length (cm) ~6 ~4 ~10 ∅ (μm) 66 ± 2 (21%) 52 ± 2 (19%) 120 ± 4 (24%) α 17 ± 2 (25%) 6.0 ± 0.6 (33%) 32 ± 1 (13%) F E 0.38 ± 0.03 (30%) 0.18 ± 0.02 (36%) 0.49 ± 0.02 (12%) F r 0.67 ± 0.09 (39%) 0.29 ± 0.04 (47%) 0.79 ± 0.04 (17%) S r 0.26 ± 0.05 (56%) 0.28 ± 0.05 (52%) 0.07 ± 0.01 (51%) E (GPa) 5.0 ± 0.6 (42%) 2.8 ± 0.3 (43%) 2.8 ± 0.2 (36%) n α F E F r S r E α 1931 Methods L o L L L o L o 1994 1993 2003 2 μ Results Tensile measurements 1997 1994 1 1 1 Fig. 1 a b  1931 1933 1997 2 1994 1993 2003 1957 1997 Fig. 2 a b c d a E P PY B b c d a α F E  1997 F E α E 9 2 α F E 2 Appendix n 2 1994 2004 α F E 2 F r S r 1 α F E F r S r E F E F r 1 F 2,155 p E 2 t Table 2 Statistics for tensile parameters of ape hairs   F E α F r S r Gibbon × Orangutan t 10 11 18 8.4 4.4 p <0.0001 <0.0001 <0.0001 0.0003 Gorilla × Gibbon t 10 5.3 6.2 4.1 0.32 p <0.0001 <0.0001 0.0008 0.75 Gorilla × Orangutan t 10 3.1 7.6 1.3 4.0 p 0.007 <0.0001 0.2 0.0009 ANOVA F 2,27 44 119 20 9.3 p <0.0001 <0.0001 <0.0001 0.0009 n t t 10 p p E 1994 1993 Appendix Appendix F E F E N F E Appendix 1994 F E robust Infant carrying (1931 1968 2 2 1931 1981 rhd rhd rhd 1981 rhd 2 1965 2 1931 1968 1931 1968 1972 1965 1985 1997 1984 1972 1965 1985 1967 1967 Weight limit for clinging in African apes F E 2 1968 2 F r 1 2 1979 1997 1997 1997 1 2 1987 2004 2 1982 1979 1967 1997 Dorsal clinging position 3 θ θ Fig. 3 θ 1  W t W c μ 1956 1 \documentclass[12pt]{minimal}
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\begin{document}$$\mu W_{\text{t}} \cos \theta  + W_{\text{c}}  > W_{\text{t}} \sin \theta $$\end{document} W c θ c θ c μ 1 1 μ Friction measurements 1994 μ W 3 μ 4 μ W μ W A μ W 1952 1952 2 \documentclass[12pt]{minimal}
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\begin{document}$$\mu  = a + b/W$$\end{document} Fig. 4 a μ W 2 line 2 b μ W circles 2 squares 2 triangles 2 lines  a 1952 b 1959 2 4 A μ A μ a b a b 4 a b A a A b A b/W 1 W A 2 1981 1975 μ μ 1994 1956 1959 μ μ μ 1994 1994 Equilibrium condition in African apes 1 W c W t 1979 1997 1 μ θ 2 1965 1979 θ 2 1968 1967 It may be concluded that slipping imposes clear limits on the maximum body angle attained by heavy ape species carrying infants, representing a hindrance against evolution towards bipedality. This might explain the persistence of knuckle-walking among the great African apes. Effect of humidity on friction 1994 μ 1959 1994 μ 1994 μ μ 1997 1994 1 5 W t 1 θ μ W c Fig. 5 W t θ 1 μ top W c dashed line line with cross W c μ θ  Discussion θ W c μ θ 5 if they could avoid it 1954 Homo 1985 something Homo 2001 It seems that the not trivial mechanical constraint of safe carrying of heavy infants has never been considered before. 2001 1994 1998 1 in the dorsal clinging position θ Concluding remarks and future perspectives An important conclusion from this work refers to interest in systematic study of tensile properties of hairs from an evolutionary point of view. The other clear conclusion is that models on locomotion and bipedalism evolution must focus on females carrying infants, who are the target of the strongest selective pressures, and this conclusion is independent of the form of locomotion of hominid ancestors (knuckle-walkers, terrestrial or arboreal quadrupeds, or full-time arboreal climbers). To disentangle the divergence between apes and hominins, several directions of research become clear from this work. Studies of living apes focused on the mechanical problem of infant carrying, as well as determination of the elastic limits of the ape skin and the limits for grasping capacity of ape infants, which are all essential to identify the critical factor responsible for emergence of bipedalism. It should be stressed that in a scenario with bipedalism emerging for safe infant carrying, the selective pressure would act particularly on females. This is not a problem, as only genes in the nonrecombinant part of the Y chromosome are not transmitted to both sexes and therefore cannot be associated with species locomotion. The process correlates with increase in the mother–infant bond, characteristic of the human lineage. 2001 plausible necessary complementary consequence necessary 1989 1871 1970 rhd 1981 1982 1985 1992 1996 1980 1992 1996 1996 2004 2003 Homo