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. Specifically, primate pelage (hair/fur) pigmentation provides a substrate for selection, and is tied to functions such as crypsis, signaling, and thermoregulation. However, most of our hypotheses on the forces responsible for primate pelage pigmentation are based solely on macro-evolutionary studies. Here, we test how distinct hypotheses around climate, age, and sex might be influencing pelage color variation in a wild primate population. We characterize pelage color and pattern variation within a population of wild 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 (n = 87 individuals). 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 greater facial complexity in colder and more fragmented forests. We conclude that three explanations may explain the phenotypic pattern: isolation by distance, an environmental gradient, or local adaptation. Although it is difficult to reach a causal conclusion without genetic data, 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. We argue that primate color in wild populations may also foretell either adaptation or maladaptation (i.e., via environmental mismatch).