Ape facial expressions foster group harmony

Com munication through facial expressions isn't the domain of humans alone, a study has observed: facial expressions may foster social harmony among apes and our closer monkey relatives.

The results seem to back up a theory that such expressions evolved to maintain group cohesion, as per the scientist who conducted the invest igation.

Anthropologist Seth Dobson of Dartmouth College studied 12 primate species and observed that greater variety of facial expressions was associated with average group size.

The size of a part of the brain that controls facial expression-the facial nucleus-was also associated with group size and the amount of time animals spent grooming each other, he reported. For this part of the study, he analyzed a somewhat larger range of species.

Dobson reported the findings late last month at the annual meeting of the American Association of Physical Anthro pologists in Philadelphia. He said he also plans to submit the results for publication to the group's flagship journal, the American Journal of Physical Anthro pology.

"The evolution of facial expression tracks the evolution of group size and social organ ization," wrote Dobson in an email. He added that increased reliance on facial expressions evolved multiple times in different lineages.

Animals living in larger groups are under greater evolution ary pressure "to resolve conflicts peacefully," he added; facial expressions can become part of the arsenal they have to achieve that.

Dobson focused on anthropoids, a sub-order of the primate order, the group of mammals with opposable thumbs of which humans are a member. Anthropoids are more "human-like" primates that include monkeys, apes and humans but exclude "lower" primates such as lemurs.

Even within the anthropoids, Dobson said his findings reached statistical significance only if certain more evolution arily distant or unusual groups were excluded. These were the New World Monkeys-thought to beseparated from the other anthropoids by about 40 million years of evolution-and orangutans, which are solitary but have a large facial nucleus. That's probably because they made an evolution ary switch from group to lone living just relatively recently, he said.

"This study was part of my dissertation, in which I developed a method for quantifying the mobility of the facial muscles in monkeys and apes," Dobson wrote in the email. "My measure of facial mobility allows for comparisons across species to test evolution ary hypotheses concerning facial expression."

The twelve species that Dobson studied for the facial mobility study were the chimpanzee, western lowland gorilla, white-cheeked gibbon, hamadryas baboon, lion-tailed macaque, De Brazza's monkey, black-and-white colobus, dusky leaf monkey, black-handed spider monkey, black howler monkey, white-faced saki and cotton-top tamarin. Posted by: JoslynV    Source