Melanism in a wild sifaka population: Darker where cold and fragmented

Abstract

Pigmentation is one of the most striking examples of diversity in the natural world. Mainly, pelage (hair/fur) pigmentation provides a substrate for selection (i.e., crypsis, signaling, thermoregulation) and is capable of rapid change. Thus, this trait may be the one potential early signal of adaptation (or maladaptation) in wild primate populations. However, most of our hypotheses on the forces responsible for primate pelage pigmentation are based solely on macro-evolutionary studies. Here, we characterize pelage color and pattern variation within a population of wild primates, diademed sifakas (Propithecus diadema), exhibiting striking diversity in coloration (melanic to tri-colored). Our approach jointly assesses climate and pelage variation across the region. We score pelage using a semi-quantitative methodology. We then test if pelage variation is associated with climatic or demographic factors (i.e., sex-class, age-class) across the Tsinjoarivo forest, Madagascar. We find darker bodies and less complex faces occur in colder and more fragmented forests. We explore three hypotheses that may explain this phenotypic pattern: isolation by distance, an environmental gradient, or unique local adaptation. Importantly, each scenario signals the need for enhanced conservation of diademed sifakas in the Tsinjoarivo forest. More studies on primate pigmentation in wild populations will be needed to contextualize if this pattern is exceptional or typical. It is likely that in other primate populations pigmentation may also foretell of adaptation or environmental mismatch.