Hair phenotype diversity across Indriidae lemurs

Objectives: 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 evolution remains 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 Indriidae (Avahi, Indri, Propithecus)—a lineage that,
like humans, exhibits vertical posture (i.e., their whole bodies are
vertical to the sun). Materials and Methods: Our analyses considers
multiple phylogenetic scales (family-level, genus-level) and hypotheses
(e.g., Gloger’s rule, the body cooling hypotheses). We obtain hair color
and density from museum and/or wild animals, opsin genotypes from wild
animals, and climate data from WorldClim. To analyze our data, we use
phylogenetic generalized linear mixed models (PGLMM) using Markov chain
Monte Carlo algorithms. Results: Our results show that across the
Indriidae family, darker hair is typical in wetter regions. However,
within Propithecus, dark black hair is common in colder forest regions.
Results also show pelage redness increases in populations exhibiting
enhanced color vision. Lastly, we find follicle density on the crown and
limbs increases in dry and open environments. Discussion: This study
highlights how different selective pressures across distinct phylogenetic
scales have likely acted on primate hair evolution. Specifically, our data
across Propithecus may implicate thermoregulation and is the first
empirical evidence of Bogert’s rule in mammals. Our study also provides
rare empirical evidence supporting an early hypothesis on hominin hair
evolution.