Department of Anthropology, University of Michigan, Ann Arbor
Saturday 8:00-8:15, Parlors
Humans are unique among primates in our apparent hairlessness. Schwartz and Rosenblum (1981, AJPA 55: 9-12) investigated the role of body size on anthropoid hair density. They found that as body surface area increased, “relative hair density” (RHD: hairs/cm2/body surface area) decreased. They concluded that larger anthropoids have systematically less dense hair due to thermoregulatory adaptations. This hair pattern may result simply from differences in growth. In humans, the absolute number of hair follicles appears to be determined in utero: increases in surface area postnatally decrease the density of hair follicles. If a similar pattern of follicle formation occurs in all primates, I hypothesize that larger primates exhibit lower hair follicle densities than smaller primates due to increasing body surface area. Therefore, I plotted body mass (g) against hair density (hairs/cm2) in 22 anthropoids using data from the literature. The RHD variable obscures the physiological significance of hair density and makes a scaling relationship less intuitive, as RHD is a count/cm2/cm2 regressed against surface area (cm2). Consistent with Schwartz and Rosenblum, I found negative relationships between body mass and scalp, chest and back hair density. Body mass, however, explains only 50-60% of variance. RHD regressed against body surface area appears to have created a specious relationship between hair density and body size. Further study of the morphology and ontogeny of hair will elucidate the size-density relationship, and provide insights into the human hair phenotype and the function of hair in primate evolution.