Abstract
The greying of human head hair is arguably the most salient marker of human aging. In wild mammal populations, greying can change with life history or environmental factors (e.g., sexual maturity in silverback gorillas). Yet, whether humans are unique in our pattern of age-related hair depigmentation is unclear. We examined the relationship between pigmentation loss in facial hair (greying) to age, population, and sex in wild and captive chimpanzees (Pan troglodytes). Digital facial photographs representing three chimpanzee populations (N = 145; ages 1–60 years) were scored for hair greying on a scale of one [~100% pigmented] to six [~0% pigmented]. Our data suggest that chimpanzee head and facial hair generally greys with age prior to mid-life (~30 years old), but afterwards, greying ceases to increase incrementally. Our results highlight that chimpanzee pigmentation likely exhibits substantial variation between populations, and that both 'grey' and pigmented phenotypes exist across various age classes. Thus, chimpanzee facial hair greying is unlikely a progressive indicator of age beyond mid-life, and thus facial greying in chimpanzees seems different from the pattern observed in humans. Whether this reflects neutral differences in senescence, or potential differences in selection pressures (e.g. related to conspecific communication), is unclear and worthy of more detailed examination across populations and taxa.
Highlights
Head hair greying is one of the most important phenotypic markers for human aging, especially after mid-life [1]
We examined residual plots to be sure that the assumptions of the models were met
The greying pattern in chimpanzees is unlike the pattern seen in humans (Fig 1B) where the frequency in hair greying reaches up to 91% in old age, while exhibiting a markedly linear and progressive pattern [3]
Summary
Head hair greying is one of the most important phenotypic markers for human aging, especially after mid-life [1]. The age of hair greying onset and its progression throughout life are known to vary between human populations and geographical origin [3]. Age-related or not, is interesting to examine in chimpanzees (Pan troglodytes ssp.) because, like humans, they are long-lived and use facial cues to discriminate faces of known individuals [16].
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