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Chimp culture reaches new heights with “grass-in-the-ear” trend

July 3, 2014
Courtesy of Springer Journals
and World Science staff

Chim­panzees are cop­y­cats and, in the pro­cess, they form new tra­di­tions that are of­ten spe­cif­ic to just one group.

Such are the find­ings of an in­terna­t­ional group of sci­en­tists, who wad­ed through over 700 hours of vi­deo foot­age to un­der­stand how it came about that one chim­pan­zee stuck a piece of grass in her ear and started a new trend.

Unfortunately, Julie, the in­vent­or of the trend, died. It continued without her.

Im­age cour­tesy of Spring­er Sci­ence + Busi­ness Me­dia


The find­ings of the stu­dy, led by Ed­win van Leeu­wen of the Max Planck In­sti­tute for Psy­cho­lin­guist­ics in The Neth­er­lands, are pub­lished in the re­search jour­nal An­i­mal Cog­ni­tion.

It was in 2010 that van Leeu­wen first no­ticed how Ju­lie re­peat­edly put a stiff, straw­like blade of grass for no ap­par­ent rea­son in one or both of her ears. She left it there even when she was groom­ing, play­ing or rest­ing in Zam­bi­a’s Chim­fun­shi Wild­life Or­phan­age Trust sanc­tu­ary. On sub­se­quent vis­its, van Leeu­wen saw that oth­er chim­pan­zees in her group had started to do the same.

This aroused his in­ter­est to find out if they cop­ied what Ju­lie did by watch­ing and learn­ing from her through so-called so­cial learn­ing. The re­search team, in­clud­ing Zam­bians who mon­i­tor the chim­pan­zees dai­ly, col­lect­ed and an­a­lyzed 740 hours of foot­age that had been shot dur­ing the course of a year of 94 chim­pan­zees liv­ing in four dif­fer­ent so­cial groups in the sanc­tu­ary. Only two of these groups could see one anoth­er.

The re­search team found that only one of the four groups reg­u­larly per­formed this so-called “grass-in-the-ear” be­hav­ior. In one oth­er group one chim­pan­zee once did the same. Eight of the twelve chim­pan­zees in Ju­lie’s group re­peat­edly did so. The first to copy her was her son, Jack, fol­lowed by Kathy, Mir­a­cle and Val with whom she reg­u­larly in­ter­acted. Gen­er­ally at least two of the chimps put grass in their ear at the same time. Interest­ingly, the chim­pan­zees Kathy and Val kept up the cus­tom even af­ter Ju­lie, the orig­i­nal in­ven­tor, died.

The ob­serva­t­ions show that there’s noth­ing ran­dom about in­di­vid­ual chimps stick­ing grass in­to their ears, the re­search­ers ar­gue. The an­i­mals spon­ta­ne­ously cop­ied the ar­bi­trary be­hav­ior from a group mem­ber. Chim­panzees have a ten­den­cy to learn from one anoth­er – clearly a case of “mon­key see, mon­key do” in fact. Van Leeu­wen sug­gests that those an­i­mals that find a spe­cif­ic be­hav­ior some­how re­ward­ing will con­tin­ue to do so on their own, even if the chim­pan­zee they have learn­ed it from is no long­er around.

“This re­flects chim­pan­zees’ pro­cli­vity to ac­tively in­ves­t­i­gate and learn from group mem­bers’ be­hav­iors in or­der to ob­tain bi­o­log­ic­ally rel­e­vant in­forma­t­ion,” says van Leeu­wen. “The fact that these be­hav­iors can be ar­bi­trary and out­last the orig­i­na­tor speaks to the cul­tur­al po­ten­tial of chim­pan­zees.”


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Chimpanzees are copycats and, in the process, they form new traditions that are often specific to just one group. Such are the findings of an international group of scientists, who waded through over 700 hours of video footage to understand how it came about that one chimpanzee stuck a piece of grass in her ear and started a new trend. The findings of the study, led by Edwin van Leeuwen of the Max Planck Institute for Psycholinguistics in The Netherlands, are published in the research journal Animal Cognition. In 2010, van Leeuwen first noticed how a female chimp named Julie repeatedly put a stiff, strawlike blade of grass for no apparent reason in one or both of her ears. She left it there even when she was grooming, playing or resting in Zambia’s Chimfunshi Wildlife Orphanage Trust sanctuary. On subsequent visits, van Leeuwen saw that other chimpanzees in her group had started to do the same. This aroused his interest to find out if they copied what Julie did by watching and learning from her through so-called social learning. The research team, including Zambians who monitor the chimpanzees daily, collected and analyzed 740 hours of footage that had been shot during the course of a year of 94 chimpanzees living in four different social groups in the sanctuary. Only two of these groups could see one another. The research team found that only one of the four groups regularly performed this so-called “grass-in-the-ear” behavior. In one other group one chimpanzee once did the same. Eight of the twelve chimpanzees in Julie’s group repeatedly did so. The first to copy her was her son, Jack, followed by Kathy, Miracle and Val with whom she regularly interacted. Generally at least two of the chimps put grass in their ear at the same time. Interestingly, the chimpanzees Kathy and Val kept up the custom even after Julie, the original inventor, died. The observations show that there’s nothing random about individual chimps sticking grass into their ears, the researchers argue. The animals spontaneously copied the arbitrary behavior from a group member. Chimpanzees have a tendency to learn from one another – clearly a case of “monkey see, monkey do” in fact. Van Leeuwen suggests that those animals that find a specific behavior somehow rewarding will continue to do so on their own, even if the chimpanzee they have learned it from is no longer around. “This reflects chimpanzees’ proclivity to actively investigate and learn from group members’ behaviors in order to obtain biologically relevant information,” says van Leeuwen. “The fact that these behaviors can be arbitrary and outlast the originator speaks to the cultural potential of chimpanzees.”