Face to Face With Primate Facial Diversity

Why do some primates have boldly colored faces while other species exhibit only a monotone color with little pattern? Facial color patterns likely serve several functions in primates, including intraspecific communication, species recognition, and possibly ecological or physiological roles as well (Figure 1). One hypothesis is that facial color patterns are used primarily for species recognition, with more subtle color variations used to assess individual identity.

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Figure 1. Maximum-likelihood diagram of facial color complexity in Neotropical primates. Higher facial color complexity is indicated by reds and oranges and higher numbers. Primate species illustrated include: (1) Cacajao calvus, (2) Callicebus hoffmansi, (3) Ateles belzebuth, (4) Alouatta caraya, (5) Aotus trivirgatus, (6) Cebus nigritus, (7) Saimiri boliviensis, (8) Leontopithecus rosalia, (9) Callithrix kuhli, (10) Saguinus martinsi and (11) Saguinus imperator. (Illustrations by Stephen Nash. Figure from Santana et al., 2012)

According to the behavioral drive model, social behaviors drive the evolution of increasingly complex facial colors and fur patterns. An alternative hypothesis, the metachromism hypothesis, provides a non-adaptive explanation for primate color patterns. This hypothesis posits that primate lineages exhibit predictable sequences of color changes over time beginning with the ancestral agouti condition and progressively evolving a more uniform black or red color and ending with an unpigmented bleached color.

Sharlene Santana and her colleagues at the University of California, Los Angeles, set out to test these hypotheses using New World primates. They predicted that species living in smaller groups and in sympatry with more congener species would evolve more complex facial color patterns. In addition, they tested the metachromism hypothesis using a phylogenetic approach to trace color patterns through Neotropical primate lineages. They quantified facial color patterns using photos of adult males from a wide array of Neotropical primate species (Figure 2).

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Figure 2. Primate faces (here a white-faced capuchin monkey, Cebus capucinus) were subdivided into 14 areas (b) to record hair and skin color, and hair length. These 14 areas were grouped into 5 more general regions that varied across species. (From Santana et al., 2012)

The results reveal that primate facial patterns do function in communication and species recognition. Primate species living in smaller groups and in regions with a higher number of congener species (species within the same genus) have evolved more complex patterns of facial color. There was no support for the metachromism hypothesis. In fact, ecological factors, and geographical patterns also shaped facial diversity in Neotropical primates (Figure 3). For example, primate species closer to the equator tended to have darker crowns and darker eye masks. Species living in the far western Neotropics tended to have darker noses and mouths, but lighter eye masks.

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Figure 3. Geographical trends in primate facial traits. Facial parts become darker (regions highlighted in black) or hair becomes longer (region highlighted in grey) in the directions indicated by the arrows. (From Santana et al., 2012)

Perhaps darker facial regions in more tropical habitats serve to make individuals more cryptic or protect against the powerful UV radiation in these regions. The underlying causes for these patterns is likely complex and multifaceted. Nevertheless, these results “demonstrate the interaction of behavioral and ecological factors in shaping one of the most outstanding facial diversities of any mammalian lineage.”



References

Santana, S., Lynch Alfaro, J., & Alfaro, M. (2012). Adaptive evolution of facial colour patterns in Neotropical primates Proceedings of the Royal Society B: Biological Sciences, 279 (1736), 2204-2211 DOI: 10.1098/rspb.2011.2326