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January 18, 2011

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Sticks appear as “dolls” in hands of chimps

Dec. 20, 2010
Courtesy of Cell Press
and World Science staff

Young chim­panzees, and most of­ten fe­ma­les, at a na­tional park in Ugan­da some­times play with sticks in a way rem­i­nis­cent of the way chil­dren play with dolls, sci­en­tists are re­port­ing.

The prac­tice might turn out to be the first case among an­i­mals “of a tra­di­tion main­tained just among the young, like nurse­ry rhymes and some games in hu­man chil­dren,” said Har­vard Uni­vers­ity re­search­er Rich­ard Wrang­ham. “This would sug­gest that chim­pan­zee be­hav­ior­al tra­di­tions are even more like those in hu­mans than pre­vi­ously thought.”

But he added that the stick-playing is rel­a­tively rare, and un­doc­u­mented in oth­er chimp com­mun­i­ties. The find­ings, by Wrang­ham and col­leagues, are pub­lished in the Dec. 21 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy.

This is “the first ev­i­dence of an an­i­mal spe­cies in the wild in which ob­ject play dif­fers be­tween males and fe­ma­les,” said Wrang­ham. The gen­der dif­fer­ence in chimps’ ap­par­ent “doll” play al­so fits the pat­tern seen across hu­man cul­tures, he added—sug­gesting it stems from “bi­o­log­i­cal predilec­tions” rath­er than so­cial­iz­a­tion.

Al­though both young male and female chimps play with sticks, fe­males do so more of­ten, and they oc­ca­sion­ally treat them like moth­er chimps tend­ing their in­fants, the re­search­ers said. Ear­li­er stud­ies of cap­tive mon­keys had al­so sug­gested a bi­o­log­i­cal in­flu­ence on toy choice, ac­cord­ing to Wrang­ham and col­leagues: when young mon­keys are of­fered sex-stereo­typed hu­man toys, fe­males grav­i­tate to­ward dolls, where­as males tend to go for “boys’ toys” such as trucks.

The new find­ings are based on 14 years of stud­ies of the Kan­yawara chim­pan­zee com­mun­ity at Ugan­da’s Kibale Na­tional Park. Wrang­ham and co­au­thor Sonya Kahlen­berg of Bates Col­lege in Maine found that chim­panzees use sticks in four main ways: as probes to in­ves­t­i­gate holes po­ten­tially con­tain­ing wa­ter or hon­ey, as props or weapons in ag­gres­sive en­coun­ters, dur­ing sol­i­tary or so­cial play, and in a be­hav­ior the re­search­ers call stick-car­rying.

Wrang­ham said they had seen stick-car­rying from time to time over the years and sus­pected fe­males were do­ing it the most. De­tailed in­ves­ti­ga­t­ion has con­firmed that, they added. “We thought that if the sticks are be­ing treated like dolls, fe­males would car­ry sticks more than males do and should stop car­rying sticks when they have their own ba­bies,” Wrang­ham said. “Both of these points are cor­rec­t.”

Young fe­males some­times took their sticks in­to day-nests where they rested and some­times played with them cas­u­ally in a man­ner that evoked ma­ter­nal play, the re­search­ers re­ported.

It’s not yet clear wheth­er this form of play is com­mon in chim­panzees, the re­search­ers say. In fact, no one has pre­vi­ously re­ported stick-car­rying as a form of play, de­spite con­si­der­able in­ter­est among chim­pan­zee re­search­ers in de­scrib­ing ob­ject use. “This makes us sus­pect that stick-car­rying is a so­cial tra­di­tion that has sprung up in our com­mun­ity and not oth­ers,” Wrang­ham said.

Be­cause stick-car­rying is un­com­mon even in the Kan­yawara chimps that Wrang­ham and Kahlen­berg stud­ied, they said, they won’t be sure un­til re­search­ers stu­dy­ing oth­er com­mun­i­ties re­port its ab­sence. They note that chimp play is gen­er­ally poorly doc­u­mented be­cause chimp com­mun­i­ties are usu­ally small with few young­sters at any one time.

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Young chimpanzees, and most often females, at a national park in Uganda sometimes play with sticks in a way reminiscent of the way children play with dolls, scientists are reporting. The practice might turn out to be the first case among animals “of a tradition maintained just among the young, like nursery rhymes and some games in human children,” said Harvard University researcher Richard Wrangham. “This would suggest that chimpanzee behavioral traditions are even more like those in humans than previously thought.” But he added that the stick-playing is relatively rare, and hasn’t been documented in other chimp communities at all. The findings, by Wrangham and colleagues, are published in the Dec. 21 issue of Current Biology. This is “the first evidence of an animal species in the wild in which object play differs between males and females,” said Wrangham. The gender difference in chimps’ apparent “doll” play also fits the pattern seen among human kids, he added—suggesting that it comes from “biological predilections” rather than socialization. Although both young male and female chimpanzees play with sticks, females do so more often, and they occasionally treat them like mother chimpanzees caring for their infants, the researchers noted. Earlier studies of captive monkeys had also suggested a biological influence on toy choice, according to Wrangham and colleagues: when young monkeys are offered sex-stereotyped human toys, females gravitate toward dolls, whereas males tend to go for “boys’ toys” such as trucks. The findings are based on 14 years of studies of the Kanyawara chimpanzee community at Uganda’s Kibale National Park. Wrangham and coauthor Sonya Kahlenberg of Bates College in Maine found that chimpanzees use sticks in four main ways: as probes to investigate holes potentially containing water or honey, as props or weapons in aggressive encounters, during solitary or social play, and in a behavior the researchers call stick-carrying. Wrangham said they had seen stick-carrying from time to time over the years and suspected females were doing it more than males. Detailed investigation has confirmed that suspicion, they added. “We thought that if the sticks are being treated like dolls, females would carry sticks more than males do and should stop carrying sticks when they have their own babies,” Wrangham said. “Both of these points are correct.” Young females sometimes took their sticks into day-nests where they rested and sometimes played with them casually in a manner that evoked maternal play, the researchers reported. It’s not yet clear whether this form of play is common in chimpanzees, the researchers say. In fact, no one has previously reported stick-carrying as a form of play, despite considerable interest among chimpanzee researchers in describing object use. “This makes us suspect that stick-carrying is a social tradition that has sprung up in our community and not others,” Wrangham said. Because stick-carrying is uncommon even in the Kanyawara chimps that Wrangham and Kahlenberg studied, they said, they won’t be sure until researchers studying other communities report its absence. They note that chimp play is generally poorly documented because chimp communities are usually small with few youngsters at any one time.