Do You Recognize Me?M G BellNovember 23, 2013
Research at UCLA on the coloration and complexity of primate faces (monkeys and apes) has shown that there is a correlation between those attributes and the size of social groups, although the effect was less marked among apes as opposed to monkeys, which is thought to be because apes have a more highly developed set of facial expressions which they can use to aid recognition. Past a certain point, muscular expressivity might be a more flexible way of demonstrating individuality than coloration and physiognomical variation.
"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," 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.
"Faces are really important to how monkeys and apes can tell one another apart," he said. "We think the color 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."
"Our research suggests increasing group size puts more pressure on the evolution of coloration across different sub-regions of the face," Michael Alfaro said.
Other variables affected coloration such as distance from the equator and the degree of forest cover (true also of humans), but the degree of facial complexity was linked only to group size.
"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."
Evidently, one must distinguish between group behaviour linked to mutual protection (such as among shoals of fish or herds of bison) and intra-group 'social' behaviour in which characteristics are attributed to individuals, requiring a mechanism to distinguish one individual from another. But it would be unjustified to make a hard and fast distinction between such types of groups. No doubt there is a continuum of intermediate situations between, at the most basic, a group whose members are simply recognizable as conspecifics because of their size, shape, color, or smell, and a complex social group in which individuals' behaviour towards each other depends on memory of previous interactions with a particular individual, emotional drives and external situations (to pick just a few out of many factors determining inter-personal behaviour).
It is an interesting question, as to how far back up the evolutionary tree animals first became able to distinguish indivuals from conspecifics in general. Do dogs recognize each other? – obviously yes; do lizards recognize each other? – hmmm; do sharks recognize each other? There is a lot we don't know!