AboutZoos, Since 2008


Biol­o­gists find an evo­lu­tion­ary Face­book for mon­keys and apes

pub­lished 19 Novem­ber 2013 | mod­i­fied 03 Novem­ber 2014

Why do the faces of some pri­mates con­tain so many dif­fer­ent colours — black, blue, red, orange and white — that are mixed in all kinds of com­bi­na­tions and often strik­ing pat­terns while other pri­mate faces are quite plain?

Monkey faces apes lineagesBiol­o­gists of the Uni­ver­sity of Cal­i­for­nia Los Ange­les (UCLA) reported Jan­u­ary last year on the evo­lu­tion of 129 pri­mate faces in species from Cen­tral and South Amer­ica. This research team now reports on the faces of 139 Old World African and Asian pri­mate species that have been diver­si­fy­ing over some 25 mil­lion years.

With these Old World mon­keys and apes, the species that are more social have more com­plex facial pat­terns, the biol­o­gists found. Species that have smaller group sizes tend to have sim­pler faces with fewer colours, per­haps because the pres­ence of more colour patches in the face results in greater poten­tial for facial vari­a­tion across indi­vid­u­als within species. This vari­a­tion could aid in iden­ti­fi­ca­tion, which may be a more dif­fi­cult task in larger groups.

Species that live in the same habi­tat with other closely related species tend to have more com­plex facial pat­terns, sug­gest­ing that com­plex faces may also aid in species recog­ni­tion, the life sci­en­tists found.

Faces are really impor­tant to how mon­keys and apes can tell one another apart
Michael Alfaro, senior author, asso­ciate pro­fes­sor of ecol­ogy and evo­lu­tion­ary biol­ogy, UCLA Col­lege of Let­ters and Science »

“Humans are crazy for Face­book, but our research sug­gests that pri­mates have been rely­ing on the face to tell friends from com­peti­tors for the last 50 mil­lion years and that social pres­sures have guided the evo­lu­tion of the enor­mous diver­sity of faces we see across the group today,” he said. “We think the colour pat­terns have to do both with the impor­tance of telling indi­vid­u­als of your own species apart from closely related species and for social com­mu­ni­ca­tion among mem­bers of the same species.”

Most Old World mon­keys and apes are social, and some species, like the man­drills, can live in groups with up to 800 mem­bers, said co-​author Jes­sica Lynch Alfaro. At the other extreme are soli­tary species, like the orang­utans. In most orang­utan pop­u­la­tions, adult males travel and sleep alone, and females are accom­pa­nied only by their young, she said. Some pri­mates, like chim­panzees, have “fission-​fusion soci­eties”, where they break up into small sub-​groups and come together occa­sion­ally in very large com­mu­ni­ties. Oth­ers, like the hamadryas baboons, have tiered soci­eties with harems, clans, bands and troops, she said.

“Our research sug­gests increas­ing group size puts more pres­sure on the evo­lu­tion of coloura­tion across dif­fer­ent sub-​regions of the face,” Michael Alfaro said. This allows mem­bers of a species to have “more com­mu­ni­ca­tion avenues, a greater reper­toire of facial vocab­u­lary, which is advan­ta­geous if you’re inter­act­ing with many mem­bers of your species,” he said.

The research, fed­er­ally funded by the National Sci­ence Foun­da­tion and sup­ported through a post­doc­toral fel­low­ship from the UCLA Insti­tute for Soci­ety and Genet­ics, was pub­lished 11 Novem­ber in the jour­nal Nature Com­mu­ni­ca­tions.

Monkey faces Vervet monkeyLead study author Shar­lene San­tana used pho­tographs of pri­mate faces for her analy­sis and devised a new method to quan­tify the com­plex pat­terns of pri­mate faces. She divided each face into sev­eral regions; clas­si­fied the colour of each part of the face, includ­ing the hair and skin; and assigned a score based on the total num­ber of dif­fer­ent colours across the facial regions. This numer­i­cal score is called the “facial com­plex­ity” score. The life sci­en­tists then stud­ied how the com­plex­ity scores of pri­mate faces were related to pri­mates’ social systems.

The habi­tat where species live presents many poten­tial pres­sures that could have influ­enced the evo­lu­tion of facial coloura­tion. To assess how facial colours are related to phys­i­cal envi­ron­ments, the researchers analysed envi­ron­men­tal vari­ables such as geo­graphic loca­tion, canopy den­sity, rain­fall and tem­per­a­ture. They also used sta­tis­ti­cal meth­ods that took into account the evo­lu­tion­ary his­tory and rela­tion­ships among the pri­mate groups to bet­ter under­stand the evo­lu­tion of facial diver­sity and complexity.

While facial com­plex­ity was related to social vari­ables, such as group size and the num­ber of closely related species in the same habi­tat, facial pig­men­ta­tion was best explained by eco­log­i­cal and spa­tial fac­tors. Where a species lives is a good pre­dic­tor of its degree of facial pig­men­ta­tion — how light or dark the face is.

“Our map shows clearly the geo­graphic trend in Africa of pri­mate faces get­ting darker nearer to the equa­tor and lighter as we move far­ther away from the equa­tor,” Lynch Alfaro said. “This is the same trend we see on an intra-​species level for human skin pig­men­ta­tion around the globe.”


Species liv­ing in more trop­i­cal and more densely forested habi­tats also tend to have darker, more pig­mented faces. But the com­plex­ity of facial colour pat­terns is not related to habi­tat type.

“We found that for African pri­mates, faces tend to be light or dark depend­ing on how open or closed the habi­tat is and on how much light the habi­tat receives,” Alfaro said. “We also found that no mat­ter where you live, if your species has a large social group, then your face tends to be more com­plex. It will tend to be darker and more com­plex if you’re in a closed habi­tat in a large social group, and it will tend to be lighter and more com­plex if you’re in an open habi­tat with a large social group. Dark­ness or light­ness is explained by geog­ra­phy and habi­tat type. Facial com­plex­ity is bet­ter explained by the size of your social group.”

In their research on pri­mates from Cen­tral and South Amer­ica pub­lished last year, the sci­en­tists were sur­prised to find a dif­fer­ent pat­tern. For these pri­mates, species that lived in larger groups had more plain facial patterns.

“We expected to find sim­i­lar trends across all pri­mate radi­a­tions — that is, that the faces of highly social species would have more com­plex pat­tern­ing,” said San­tana. “We were sur­prised by the results in our orig­i­nal study on neotrop­i­cal (Cen­tral and South Amer­i­can) pri­mates.” In the new study, they did find the pre­dicted trends, but they also found dif­fer­ences across pri­mate groups — dif­fer­ences they said they found intrigu­ing. Are pri­mate groups using their faces differently?

“In the present study, great apes had sig­nif­i­cantly lower facial com­plex­ity com­pared to mon­keys,” Lynch Alfaro said. “This may be because apes are using their faces for highly com­plex facial expres­sions and these expres­sions would be obscured by more com­plex facial colour pat­terns. There may be com­pet­ing pres­sures for and against facial pat­tern com­plex­ity in large groups, and dif­fer­ent lin­eages may solve this prob­lem in dif­fer­ent ways.”

“Our research shows that being more or less social is a key expla­na­tion for the facial diver­sity that we see,” Alfaro said. “Ecol­ogy is also impor­tant, such as cam­ou­flage and ther­mal reg­u­la­tion, but our research sug­gests that faces have evolved along with the diver­sity of social behav­iours in pri­mates, and that is the big cause of facial diversity.”

Alfaro and his col­leagues serve as “evo­lu­tion­ary detec­tives,” ask­ing what fac­tors pro­duced the pat­terns of species rich­ness and diver­sity of traits. “When evo­lu­tion­ary biol­o­gists see these strik­ing pat­terns of rich­ness, we want to under­stand the under­ly­ing causes,” he said.

Human faces were not part of the analy­sis, although humans also belong to the “clade Catarrhini”, which includes Old World mon­keys and apes.

(Source: UCLA News­room news release, 18.11.2013)

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