For humans, the face is a very integral part of living in society. It is usually the first thing people notice about each other. Now a new monkey face study reveals that primates also rely on facial features to know each other as their social circles grow in complexity.
The research, conducted by UCLA researchers, sought to find out why some primate species have complicated features with more colorful faces compared to other species with plain features and less face coloration. They found that as their species grew in number, monkeys and apes would find it more difficult to tell each other apart. Hence, facial colorfulness and features became increasingly more important in helping them recognize each other and spot their enemies despite their increasing population.
The monkey face study looked at Old World monkeys and apes only. These primates are indigenous to Africa, Asia and the Middle East. Various species differ in their social complexity. For example, Mandrills can be found living in groups of 800 members. Orangutan males, on the other hand, sleep alone while females stay with their young only. Chimpanzees live in small groups and only occasionally join a larger group. Peaking in social complexity are the hamadryas baboons. Their social hierarchy consists of clans, troops, bands and even harems.
The research involved studying the facial features of 139 Old World primate species. Sharlene Santana, a postdoctoral fellow and part of the UCLA research team, rated the primates’ facial features for complexity based on their photographs. Dividing the primate face into parts, she classified the colors found on each face part including hair and skin. She also scored each face for the number of different colors observed.
Apart from their growing population and social complexity, the scientists also considered the effects of habitat on primate facial complexity. They looked at geographic location, tree canopy thickness, rainfall and temperature to assess their impact on facial complexity in primates.
Results revealed, however, that facial complexity corresponded to the size of a primate’s group and the number of similar species living in the same habitat. Meanwhile, geographic factors were found to influence monkey face pigmentation in how dark or light his face complexion was found to be. Results were published online Nov. 11 in Nature Communications.
They also found that darker-faced Old World primates in Africa lived closest to the equator while lighter-faced apes and monkeys inhabited areas farther away. Primates in tropical, dense forests were also found to have darker complexions.
A previous study on New World primates of South and Central America displayed a contrasting pattern from the Old World monkeys. New World primates living in large groups actually have simpler facial features.
“Our research suggests that faces have evolved along with the diversity of social behaviors in primates, and that is the big cause of facial diversity,” said Michael Alfaro, senior author of the study.
Of course, not all animals use facial features to identify each other. For example, penguins recognize each other from the vocal sounds they utter.
In primates, however, this study of the monkey face offers insight into understanding how facial features, population density, and social complexity are related to each other.
By Fatema Biviji