reported previously 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.
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.
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," said Michael Alfaro, an
associate professor of ecology and evolutionary biology in the UCLA College of
Letters and Science and senior author of the study.
really important to how monkeys and apes can tell one another apart," 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, an adjunct
assistant professor in the UCLA Department of Anthropology and UCLA's Institute
for Society and Genetics. At the other extreme are solitary species, like the
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.
research suggests increasing group size puts more pressure on the evolution of
coloration across different sub-regions of the face," Michael Alfaro said.
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.
Photos of primate faces
federally funded by the National Science Foundation and supported through a
postdoctoral fellowship from the UCLA Institute for Society and Genetics, was
published in the journal Nature Communications.
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'
where species live presents many potential pressures that could have influenced
the evolution of facial coloration. 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.
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.
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."
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.
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
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.
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, who conducted the research as a postdoctoral fellow with the UCLA
Department of Ecology and Evolutionary Biology and UCLA's Institute for Society
and Genetics and who is now an assistant professor at the University of
Washington and curator of mammals at the Burke Museum of Natural History and
Culture. "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?
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 color 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."
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.
evolutionary biologists see these striking patterns of richness, we want to
understand the underlying causes," he said.
were not part of the analysis, although humans also belong to the Catarrhini, which includes Old World monkeys and apes