Hair phenotype diversity across Indriidae lemurs

Hair (i.e., pelage/fur) is a salient feature of primate (including human) diversity and evolution-serving functions tied to thermoregulation, protection, camouflage, and signaling-but wild primate pelage biology and evolution remain relatively understudied. Specifically, assessing multiple hypotheses across distinct phylogenetic scales is essential but is rarely conducted. We examine whole body hair color and density variation across the Indriidae lemurs (Avahi, Indri, Propithecus)-a lineage that, like humans, exhibits vertical posture (i.e., their whole bodies are vertical to the sun). Our analyses consider multiple phylogenetic scales (family-level, genus-level) and hypotheses (e.g., Gloger's rule, the body cooling hypotheses). Our results show that across the Indriidae family, darker hair is typical in wetter regions (per Gloger's rule). However, within Propithecus, dark black hair is common in colder forest regions, which may implicate thermoregulation and is the first empirical evidence of Bogert's rule in mammals. Results also show pelage redness increases in populations exhibiting enhanced color vision and may thus aid conspecific communication in forested environments. Lastly, across Indriidae, we find follicle density on the crown and limbs increases in dry and open environments-rare empirical evidence supporting an early hypothesis on hominin hair evolution. We find an effect of body size on hue (red hair) and hair density but not on brightness (black hair). This study highlights how different selective pressures across distinct phylogenetic scales have likely acted on primate hair evolution. Lastly, since hair does not fossilize, the results of follicle and hair density variation across this clade offer us some potential insight into contextualizing human hair evolution.