Pelage (i.e., hair/fur) is the first barrier providing mammals protection against their external environments. It is one of the defining characteristics of mammals and a likely key to their evolutionary success. Hair is so essential that basal mammals were likely unable to capitalize on terrestrial niches before the evolution of hairāwhose primary functions were possible protection from abrasion and insulation (Maderson 2003). However, pelage has multiple additional functions, including, but not limited to: individual/kin/species visual recognition (Allen and Higham 2013), camouflage (Hoekstra et al. 2005; Barrett et al. 2019), protection against pathogens (Paus and Cotsarelis 1999), and potentially providing a means for dispersing olfactory communication (Eisenberg and Kleiman 1972).
1.1.1 PRIMATE HAIR BIOLOGY
Pelage is also a central aspect of primate (including human) diversity, as it shows marked intra- and interspecific variation (Bradley and Mundy 2008). For example, in nonhuman primates, examples of pelage variation include ontogenetic hair change (e.g., dusky langurs, Trachypithecus obscurus) and facial pattern complexity within and across species (e.g., guenon monkeys, Cercopithecus spp.) (Rowe 1996; Anne-Isola Nekaris and Munds 2010; Allen et al. 2014). There is also a high degree of convergence in certain hair phenotypes across the primate clade, such as: facial hair ornamentation, dense body hair, long capes along the torso, yellow/red pigmentation (sometimes in patches), black/brown pigmentation, and black and white pigmentation (Bradley and Mundy 2008).
An earlier theory postulated that hair patterns shift through evolutionary time from agouti-banding to saturation to bleaching based on observed patterns of pigmentation in platyrrhines (i.e., metachromism) (Hershkovitz 1968). Although some early phylogenetic analyses seemed to support this hypothesis (Jacobs et al. 1995), it has since been falsified multiple times based on empirical evidence in distinct clades (Chaplin and Jablonski 1998; Santana et al. 2012). Additionally, patterns of hair pigmentation change across the primate order and generally do not adhere to phylogeny (Kamilar and Bradley 2011).
Overall, key functions for primate hair are similar to other mammals and include camouflage, communication, and thermoregulation. Primate coloration must often balance crypsis with signaling. Across all primate clades, pelage variation conforms to a classic ecogeographical rule where darker-pigmented animals are most likely found in warm and wet habitats such as rainforests or other densely forested environments (i.e., Glogerās rule) (Gloger 1833; Kamilar and Bradley 2011; Santana et al. 2012, 2013). This is because dark pigments may help conceal nonhuman primates in dense foliage from dichromatic predators, such as felids. However, selection could be acting on a distinct phenotype (e.g., immunity, cold tolerance) linked or co-evolving with color polymorphisms (Delhey 2017; Delhey et al. 2019). Yet certain aspects of primate hair color (i.e., red hues, contrast, complexity, hair tufts) may aid in conspecific communication such as kin or species recognition (Chaplin and Jablonski 1998; Sumner and Mollon 2003; Bradley and Mundy 2008; Winters et al. 2020). In fact, while both orange and black hues might be camouflaged from a dichromatic predator in a background of dense foliage, orange hues may be conspicuous to trichromatic conspecifics (Sumner and Mollon 2003). It is worth noting, though, that even in dichromatic species, such as Eulemur fulvus, females seem able to differentiate between dull and brightly colored individuals (Cooper and Hosey 2003; Jacobs et al. 2019). Thus, coloration likely aids with communication and camouflage. On the other hand, it is unclear what role primate pigmentation plays in thermoregulation. Primates of distinct colors exhibit no differences in thermoregulatory behaviors (Bicca-Marques and Calegaro-Marques 1998), but data from other organisms suggests black colors may have a thermoregulatory benefit (Fratto and Davis 2011). Nonetheless, the capacity for primate hair to act as thermal insulation is likely dependent mainly on hair density (Tregear 1965). Body hair density is known to similarly vary across populations and across body regions (Schwartz and Rosenblum 1981)āpotentially as an adaptation to climate. For example, in Neotropical primates, facial hair length increases in colder areas (Santana et al. 2012). This follows another ecogeographical rule (the āhair ruleā), which posits individuals should have longer and thicker hairs in colder regions (Rensch 1938). However, hair growth hypotheses are less well studied than those focused on color.
Like other primates, humans are covered in hair of various forms (e.g., thin, long, curly) and types (e.g., scalp, nose, axillary, pubic), spanning a swath of color profiles (e.g., blonde, brown, black) (Lasisi et al. 2016). Human hair, like that of nonhuman primates, varies across body regions and between individuals and is also correlated with ancestry (Steggerda and Seibert 1941; Seibert and Steggerda 1999). However, humans are described as āhairlessā because the hair on the torso and limbs tends to be light, thin, and vellus (Pagel and Bodmer 2003). Humans do not necessarily have less hair on their torso, but the hair they have is more sparse, thinner, and shorter than that of other primates (Schwartz and Rosenblum 1981; Sandel 2013). Thus, hair color and growth variation are defining characteristics for humans as much as non-human primates and the rest of their mammalian relatives. Unlike other primates, though, human scalp hair graying may be a uniquely human trait (Tapanes et al. 2020). Both the differences and similarities between human and nonhuman primate hair biology can provide signposts for what it means to be human. Yet the molecular and cellular mechanisms underlying human and non-human primate hair evolution remain relatively understudied.
