Mechanical analysis of infant carrying in hominoids

Abstract

In all higher nonhuman primates, species survival depends upon safe carrying of infants clinging to body hair of adults. In this work, measurements of mechanical properties of ape hair (gibbon, orangutan, and gorilla) are presented, focusing on constraints for safe infant carrying. Results of hair tensile properties are shown to be species-dependent. Analysis of the mechanics of the mounting position, typical of heavier infant carrying among African apes, shows that both clinging and friction are necessary to carry heavy infants. As a consequence, a required relationship between infant weight, hair-hair friction coefficient, and body angle exists. The hair-hair friction coefficient is measured using natural ape skin samples, and dependence on load and humidity is analyzed. Numerical evaluation of the equilibrium constraint is in agreement with the knuckle-walking quadruped position of African apes. Bipedality is clearly incompatible with the usual clinging and mounting pattern of infant carrying, requiring a revision of models of hominization in relation to the divergence between apes and hominins. These results suggest that safe carrying of heavy infants justify the emergence of biped form of locomotion. Ways to test this possibility are foreseen here.

Fig. 1

Micrographs showing orangutan hair, whose light color allows better visualization; a external appearance with typical cuticle scale structure, b transversal cross section with external cuticle, thicker cortex, and inner medulla

Fig. 2

Measured force × strain (relative elongation) curves for hominoid a human, b gorilla, c gibbon, and d orangutan hairs. In a are stressed the initial Hookean linear elastic region (E), the plastic plateau (P), the post-yield region (PY), and the breaking threshold (B). The different curves in b, c, and d correspond to ten hairs of each individual ape. The inset in a is a magnification of the elastic linear region, showing the slope α and the force at the elastic limit F_E

Fig. 3

Sketch of ape carrying infant. For simplicity, in the sketch, the angle θ of the inclined plane where the infant stands (to which Eq. 1 refers) coincides with the angle defined by the knuckle-walking position of African great apes, which is not necessarily so, as discussed in the text

Fig. 4

Results for friction coefficient at 80% RH. a friction coefficient μ as a function of W for a gorilla skin with smaller area (16 cm²). The line shows a fit to the data using Eq. 2. bμ × W for (circles) gorilla with larger area (76 cm²); (squares) gorilla with smaller area (16 cm²), and (triangles) gibbon with intermediate area (42 cm²). Linear fits to the data are shown in lines

Fig. 5

Curves of total weight W_t supported in dorsal position as function of the inclination angle θ of the infant base on the mother’s trunk, obtained from Eq. 1, for several values of the friction coefficient μ (0–0.4, as indicated on top), with W_c = 5 kgf (limit value shown with dashed line). The line with cross is for W_c = 10 kgf and μ = 0.4. Vertical position corresponds to θ = 90°