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Chimpanzees Socially Transmit Grass-Insertion Fads in Captivity

A July study finds ear-to-rear grass habits among sanctuary chimpanzees spread through peer observation after caretakers modelled the action under relaxed care routines.

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Chimps Are Sticking Grass In Their Ears And Rears As They Embrace “Pointless” Fad

Overview

  • Network-based diffusion analysis published in Behaviour confirms that grass-in-ear and grass-in-rectum behaviors spread through social learning rather than independent invention.
  • The trends emerged independently in two groups at Chimfunshi Wildlife Orphanage Trust, with only those under caretakers who cleaned their own ears with grass or matchsticks adopting the fad.
  • Researchers found no evidence that the grass served any practical purpose such as itch relief, classifying the actions as non-instrumental traditions.
  • These arbitrary behaviors have not been observed in wild chimpanzees and appear only in captivity, where consistent provisioning frees time for experimentation.
  • Study authors suggest the fads may strengthen social bonds within groups and offer comparative insights into the evolutionary origins of human cultural practices.