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Ape facial expressions foster group harmony, study finds

April 20, 2007
Special to World Science  

Com­mu­ni­ca­tion through fa­cial ex­pres­sions is­n’t the do­main of hu­mans alone, a study has found: such ex­pres­sions may fos­ter so­cial har­mo­ny among apes and our clos­er mon­key rel­a­tives.

The re­sults seem to back up a the­o­ry that these sig­nals evolved to main­tain group co­he­sion, ac­cord­ing to the sci­ent­ist who con­ducted the in­ves­ti­ga­tion.

An­thro­po­l­o­gist Seth Dob­son of Dart­mouth Col­lege in Ha­n­o­ver, N.H. stud­ied 12 pri­mate spe­cies and found that great­er mo­bi­lity of fa­cial mus­c­les was linked to av­er­age group size. 

The size of a part of the brain that con­trols fa­cial ex­pres­sion—the fa­cial nu­cle­us—was al­so tied to group size and the length of time an­i­mals spent groom­ing each oth­er, he re­ported. For this part of the stu­dy, he an­a­lyzed pre­vi­ously pub­lished data from a some­what larg­er range of spe­cies.

Dob­son re­ported his find­ings late last month at the an­nu­al meet­ing of the Amer­i­can As­so­ci­a­tion of Phys­i­cal An­thro­po­l­o­gists in Phil­a­del­phia. He said he al­so plans to sub­mit the re­sults this year for pub­li­ca­tion to the group’s flag­ship jour­nal, the Amer­i­can Jour­nal of Phys­i­cal An­thro­po­l­ogy.

“Chim­panzees dis­play a com­plex, flex­i­ble fa­cial ex­pres­sion rep­er­toire with many phys­i­cal and func­tion­al sim­i­lar­i­ties to hu­mans,” re­search­ers wrote in the Oct. 20 on­line edi­tion of the re­search jour­nal So­cial Cog­ni­tive and Af­fec­tive Neu­ro­science. One of the sci­ent­ists, Li­sa Parr of Em­o­ry Uni­ver­si­ty in At­lan­ta, Ga., cat­e­go­rized over 250 dis­tinct ex­pres­sions in chimps, our clos­est evo­lu­tion­ary re­la­tives. (Im­age cour­tesy Anne Fis­cher, Max Plank-In­s­ti­tute for Evo­lu­tion­ary An­th­ro­po­lo­gy)

“The ev­o­lu­tion of fa­cial ex­pres­sion tracks the ev­o­lu­tion of group size and so­cial or­ga­ni­za­tion,” wrote Dob­son in an e­mail.

He added that in­creased re­li­ance on fa­cial ex­pres­sions seems to have evolved mul­ti­ple times in dif­fer­ent lin­eages. An­i­mals liv­ing in larg­er groups are un­der great­er ev­o­lu­tion­ary pres­sure “to re­solve con­flicts peace­ful­ly,” he added; fa­cial ex­pres­sions can be­come part of the ar­se­nal they have to achieve that.

This “so­cial co­he­sion hypothesis” was de­ve­loped by Da­rio Ma­es­t­ri­pi­eri of the Uni­ver­si­ty of Chi­ca­go, Dob­son noted.

Dob­son’s work 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 ev­o­lu­tion­ar­ily dis­tant or un­u­su­al groups were ex­cluded. These were the New World Mon­keys—be­lieved to have branched off the oth­er an­thro­poids about 40 mil­lion years ago—and orangutans, which are sol­i­tary but have a large fa­cial nu­cle­us. That’s prob­a­bly be­cause they made an ev­o­lu­tion­ary 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. The meth­od is an ad­ap­t­a­tion of a sys­tem used in psych­o­l­ogy for over three de­c­ades, called the Fa­cial Act­ion Cod­ing Sys­tem, he added.

The 12 spe­cies he included in the fa­cial mo­bil­i­ty study—for which he ana­lyzed videos of zoo animals—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.

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Com munication through facial expressions isn’t the domain of humans alone, a study has found: 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, according to the scientist who conducted the invest igation. Anthropologist Seth Dobson of Dartmouth College studied 12 primate species and found that greater variety of facial expressions was linked to average group size. The size of a part of the brain that controls facial expression—the facial nucleus—was also linked to 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—believed to be separated 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.