Specks. Stripes. Red fur. Black fur. Eye masks. Bald spots. Beards. Moustaches. New World monkeys are nature's motley crew. Their faces display an extraordinary range of colours and patterns. Some are simple and straightforward, others intricate and complex. Take the bald uakari. Its hypervascularized, red skin is striking, but uniform. The uakari's nose is just as red as its forehead. Other species have more complex facial patterns, such as the marmosets. Large tufts of hair extend from around their ears. Specks of white and brown are islands in a sea of grey.
Evolutionary biologist Sharlene Santana was struck by this diversity. She wanted to know what evolutionary forces shape the colours and patterns on a primate's face. Why is the marmoset's face so much more complex than the uakari's?
For us primates, faces are an important source of information. We are visual and social animals, reading faces is what we do. A face can tell to which group or species a primate belongs (species recognition), or reveal the identity of a fellow group member (individual recognition). Santana came up with two hypotheses to explain how these two modes of recognition affect the evolution of the colours and patterns on primate faces.
Santana's first hypothesis is that a complex face aids in the recognition individuals. A face that consists of multiple distinct components also has more potential variations and combinations. If Mico has longer hair tufts and a darker eye mask than Sue, it's easier to tell them apart. According to this scenario, primates living in large, social groups should have the most complex faces. Seeing who's who at a glance is more important for them than it is for solitary animals.
In Santana's second hypothesis, species recognition is the main driver of facial complexity. The roles are reversed in this explanation: solitary primates or primates living in small groups should now have the most complex faces. They only meet others of their kind sporadically, so they have to be able to rapidly identify that potential mate or territorial aggressor when they see one. This is easier when faces are distinctive and intricate.
To test these hypotheses, Santana collected photo's of 129 different primate species, together with colleagues from the University of California. She scored all these faces on their facial complexity. She first divided the faces in 14 different regions and then tallied how many different colours occurred across these regions. A simple approach, but it is the first time that the facial colours and patterns of so many different species have been measured and compared. "Past work has been mostly focused on particular species or one feature of the face", Santana says.
Sure enough, Santana found a correlation between facial complexity and sociality: primates living in smaller groups have more complex facial patterning. This is in line with Santana's second scenario, where species that rarely interact with others of their kind have to identify and classify other primates as quick as possible. When species live in the same habitat as a high number of closely related species, they also tended to have more complex faces. A distinctive and recognizable face is even more important when things get crowded.
The New World monkeys that live in large groups must have other means to recognize their fellow group members. Perhaps they can distinguish between subtle differences in shape and structure of noses, lips and eyes, or on differences in colour intensity, rather than the shape of facial patterns. Social primates also display a wider range of expressions on their faces. Perhaps there's a trade-off between the evolution of complex facial musculature and of complex facial patterning. No one can see your grin if it is covered by a massive moustache, after all. These are interesting ideas, but they cannot be resolved until more primatologists have gathered more data on the expressions and facial musculature of New World monkeys.
When Santana compared the facial complexity of primates with their geographical distribution, she identified several other drivers of facial evolution, aside from sociality. Eye masks become darker towards the Equator and the east, to shield eyes from glare in open and sunny surroundings. In more temperate regions, beards, moustaches and hairs grow longer. In the forested west, monkeys have darker noses, to aid in camouflage. None of these ecological rules are absolute: a primate's face is shaped by the combination of behavioural, ecological and social pressures.
It's interesting that Santana never set foot in the Amazon rainforest or the Brazilian Caatinga for her research. She collected the primate pictures came from databases like All The World's Primates and Arkive, she mined the information on average group size came from scientific publications and literature, and the data on their geographical ranges came from the database InfoNatura. All the data was there, but it Santana and her colleagues connected all the dots. "I think we are coming to an interesting point at which there is a 'critical mass' of data and resources for many species, all of which is allowing us to conduct these broad comparative and integrative studies. These would have been virtually impossible in the past", Santana says.
Ever since Humboldt travelled Latin America, and Darwin described the emotions of man and animals, our collective scientific knowledge has increased. There are ever more dots to draw lines between and unforeseen patterns to uncover. Data-driven biology is the next chapter in study of life.
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 DOI: 10.1098/rspb.2011.2326