Associations of physical strength with facial shape in an African pastoralist society, the Maasai of Northern Tanzania

Abstract

Objectives: Previous research has documented associations of physical strength and facial morphology predominantly in men of Western societies. Faces of strong men tend to be more robust, are rounder and have a prominent jawline compared with faces of weak men. Here, we investigate whether the morphometric patterns of strength-face relationships reported for members of industrialized societies can also be found in members of an African pastoralist society, the Maasai of Northern Tanzania.

Materials and methods: Handgrip strength (HGS) measures and facial photographs were collected from a sample of 185 men and 120 women of the Maasai in the Ngorongoro Conservation Area. In young-adults (20-29 years; n = 95) and mid-adults (30-50 years; n = 114), we digitized 71 somatometric landmarks and semilandmarks to capture variation in facial morphology and performed shape regressions of landmark coordinates upon HGS. Results were visualized in the form of thin-plate plate spline deformation grids and geometric morphometric morphs.

Results: Individuals with higher HGS tended to have wider faces with a lower and broader forehead, a wider distance between the medial canthi of the eyes, a wider nose, fuller lips, and a larger, squarer lower facial outline compared with weaker individuals of the same age-sex group. In mid-adult men, these associations were weaker than in the other age-sex groups.

Discussion: We conclude that the patterns of HGS relationships with face shape in the Maasai are similar to those reported from related investigations in samples of industrialized societies. We discuss differences between the present and related studies with regard to knowledge about the causes for age- and sex-related facial shape variation and physical strength associations.

Fig 1. Scatterplot of handgrip strength as a function of age, separately for Maasai men (left) and women (right).

Cut-off points for the two age groups included in the shape analysis are indicated with vertical, dashed lines. The curved line is the quadratic regression of physical strength on chronological age (men: R² = 0.10, n = 183; women: R² = 0.14; n = 118).

Fig 2. Visualization of the association of handgrip strength (HGS) with the corresponding facial shape scores within age-sex groups.

The test statistics related to the depicted regression lines are given in Table 2. The association is significant in all sex-age groups, but stronger for women and young men than for mid-adult men.

Fig 3. Facial shape correlates of handgrip strength (HGS) in the Maasai.

Thin-plate spline deformation grids (on the left) depict the facial shape changes with HGS as deformations from the average facial configuration (middle column) to −5 SD of HGS (left column) and to +5 SD (right column), separately for each sex-age group. This magnification factor was applied to facilitate interpretation. The same facial configurations were also visualized through image unwarping and averaging (on the right). Thin-plate spline deformation grids and facial morphs were aligned at the height of the pupils and scaled for interpupillary distance.