Chimps learn to stick grass in their ears—for no good reason

July 3, 2014 • 7:31 am

Hili’s predecessor, the wise but now-deceased tabby cat Pia, used to annoy Andrzej by sticking her whiskers in his ears when he was sleeping. We’ve now discovered that chimps show a related behavior: sticking grass stems in their ears for no good reason.

A new paper in Animal Cognition by Edwin van Leeuwen et al. documents this finding, although its significance is, to me at least, unclear. We already knew that chimps show social imitation of behaviors, like fishing termites from their mounds using a masticated grass stem, or making sponges out of chewed-up leaves to soak up water to drink. The new result here is social learning of a behavior with no clear adaptive significance.

van Leeuwen report this social learning in a captive colony of orphaned chimps in Zambia. In one colony, a chimp named Julie began sticking pieces of grass in her ears for no apparent reason, and then engaging in her normal activities with the grass hanging out of her head. From the paper:

“Grass-in-ear behaviour” (henceforth “GIEB”) was first documented in 2010 when the first author observed one female chimpanzee (Julie) repeatedly putting a stiff, straw-like blade of grass in one or both of her ears. She left the grass hanging out of her ear(s) during subsequent behaviour such as grooming, playing, and resting (Figs. 12 and Online Resource 1); the behaviour served no discernible purpose.

Here are some photos and captions from the paper:

Julie (the inventor) performing the grass-in-ear behaviour
Grass-in-ear behaviour copied by group members: a Kathy (on the left) and Julie (on the right) are grooming Jack (Julie’s son) while having grass hanging out of their ears, b Val (on the right) is grooming Julie (in the middle) while both have grass hanging out of their ears. Jack (on the left) is also visible with a straw of grass in his hands (Photo b taken by Mylène Désilets, used with permission)
Grass-in-ear behaviour copied by group members: a Kathy (on the left) and Julie (on the right) are grooming Jack (Julie’s son) while having grass hanging out of their ears, b Val (on the right) is grooming Julie (in the middle) while both have grass hanging out of their ears. Jack (on the left) is also visible with a straw of grass in his hands (Photo b taken by Mylène Désilets, used with permission)

Besides this behavior having no known function, it is clearly passed on socially, beginning with one chimp and then spreading to eight of the twelve others in the troop (two of these did it only once). The authors conclude that, because the behavior wasn’t seen in three of the other four isolated troops in the same area (well, it was one time), “ecological factors were not determiners of the prevalence of this behavior.”  Well, that’s true, though perhaps the behavior might have had some connection with ecology (parasite removal?). But I highly doubt that.

The behavior is simply an example, I think, of chimps showing social learning of stupid things. It reminds me of my own youthful behavior when I learned to stick a pair of long soda straws up my nostrils and walk around saying, “Look—I’m a walrus!” Perhaps there’s a decorative element here, and chimps feel that grass hanging out of their ears makes them feel and look attractive, but that’s a stretch. It seems to be simply a spandrel of the social learning that we already know exists in this species.

The authors’ conclusion doesn’t say much, but hey, it’s something you can publish, and is a new result:

Regardless of the precise mechanism underlying the behavioural diffusion, our observations importantly show that chimpanzees spontaneously copy arbitrary behaviour from their group members. In line with Hobaiter and Byrne (2010), we interpret our data as reflecting chimpanzees’ proclivity to actively investigate and learn from group members’ behaviours in order to obtain biologically relevant information. The fact that these behaviours can be arbitrary and outlast the originator speaks to the cultural potential of chimpanzees.

h/t: Ant

van Leeuwen, E. C., K. Cronin, et al. (2014). “A group-specific arbitrary tradition in chimpanzees (Pan troglodytes).Animal Cognition DO  – 10.1007/s10071-014-0766-8: 1-5 LA  – English.

49 thoughts on “Chimps learn to stick grass in their ears—for no good reason

    1. Maybe that’s not so far from the truth. In humans fashion can probably act to reinforce a sense of group identity. To all appearances the chimps are doing the same sort of thing – would be interesting to see the reaction of the other older chimps… maybe the same as my mum & Dad when I had long hair and wore flairs.

      1. Yes, I think so too. The first thing this reminded me of was wearing a baseball cap with the bill facing backward. It’s obviously not utilitarian, so I think it’s an attention-grab – perhaps an attempt to get promoted to a higher social rank. They copy-cats do it to associate themselves with Julie, perhaps hoping to also rise in rank on Julie’s coattails.

        1. Here in the UK… William Hague entirely destroyed his chances of leading the conservative party by wearing a baseball cap with Hague printed on it at some event… and contrary to popular remembrance of the scene (including mine before I looked it up), it wasn’t even on backwards. Just shows how sensitive we are to group differentiation cues and how we judge people when they conform to one norm or another. Hague’s mistake was that he couldn’t sustain the rebellious BB cap image whilst at the same time being conservative… and that jarring incongruity cost him his credibility.

  1. This behavior suggests to me how small children behave. The will stick things into their ear, nose, hair, navel, etc. and often seem to enjoy simply experiencing a new sensation. At least that explains its initial interest to the chimps. How long before they get bored and move on to other curious habits?

    1. I think children enjoy the reactions of others more than more than the sensation. I think that’s why the chimps do it too.

        1. That could be part of the fun, but I suspect that they first started doing it to enjoy the reactions of other chimps.

  2. It’s a function of being so well-off (since they’re in a colony, I assume food is brought to them & they have no need to forage) that they can indulge in useless behavior.

  3. The reason is obvious. Their ear is itching; just like we humans dangerously stick cotton swabs into ours :).

  4. Funny thing, you remember Pia’s nasty habit, but she used to explain, that it wasn’t intentionally.

    1. Never ever ever stick such things in ears if you value your hearing. They are dangerous & impact cerumen!

      1. To quote a medic of my past acquaintance : “the smallest thing a person should stick in their own ear is their own elbow.”

      2. nonsense. I have done so for 48 years and have yet to have any problems at all. And yes, I have ear exams regularly thanks to having many ear infections when a child and having one ear drums burst from the inside and the other lanced. One simply has to use such tools with intelligence, like any tool.

  5. It’s only a matter of time until “bean-in-nose behavior” (henceforth “BINB”) is observed.

  6. I find this quite attractive. It is classy, not so damaging or permanent like a piercing or a tattoo (also quite attractive but chimps are hard to train in these practices) and shows a bit of humour too. Next time I’m out and about in Torquay I should try this out and take some extra grass for a friend or two. Maybe it will catch on with those humans.

    If you are thinking about trying to attract a chimp you could always try magic. This cutie fell in love with the magician very quickly.

  7. The q-tip/cotton swab thing mentioned several times above makes me think, do the chimps also get itchy/cough when they put those straws in their ears?

    I also had a look at Frans de Waal’s facebook page to see if he had any comments; he put up something about the paper, but no further comments by him, unfortunately.

  8. The idea that this is a spandrel from social learning seems reasonable. I also wonder if Julie is a high ranking chimp, so many of the chimps would be more likely to follow her lead.

    1. There is an ENT journal that had a n xray of a man who tried suicide by placing biros into his nose then ramming down on a desk. He survived. When I was a child I stuck plasticine in my nose… why???!

  9. I wonder if she had some foreign body in her ear to start with, then just carried on with the behaviour. Why do children stick things into ears/nose etc?

    1. Why do children stick things into ears/nose etc?

      To see what happens when you stick things into your ears/nose etc. ( what exactly do you mean by etc. btw? )

            1. You know it happens.
              But if you insist … this link includes mention of a concrete enema, which I remember reading a formal paper on. ISTR that it was in something like the “American Journal of Forensic Medicine and Pathology”, which was always good for opening a random number at a random page and just reading. It relieved the boredom of researching chronic H2S poisoning – and that’s my excuse!
              Someone has written (a | another) book recently. Which has been getting reviews.

              1. But if you insist … this link includes…

                LOL. Thanks for that. 🙂

  10. We are still doing it. Some people love to display their foot-in-mouth behavior, which else serves no discernible purpose. (E.g. Egnorism.)

  11. These two phrases’ definitions — within seemingly many, many instances — equal the very same endeavors, not ?

    “showing social learning of stupid things” =s “hey, it’s something you can publish” … …

    Not having yet read even a wee bit of this post and within the title on my computer — by the time I had gotten read past its first line ( the words of this title after “for” ), I just burst out guffawing over … … “for no good reason.”

    Appears to me that, now if not at nearly all of the ages before, within academia ? That is soooo what I witness somewhere in some activity of it … … every freakin’ day !

    Funny, funny posting.

  12. Seriously though, why can’t this behavior be to remove earwax or scratch an itchy ear canal? I’m not saying it could be for no reason at all but “no discernible purpose” sounds debatable.

    1. When you want to remove ear wax or scratch an itch, do you stick something small in your ear and leave it there? No. You stick something in your ear (probably your finger) and twist/turn/wriggle it around in order to remove the irritant. You keep wriggling it aronud until the irritant is gone, and then you remove your finger.

      This (the chimp’s) behavior does not at all resemble what a primate does to treat an itch. Apes (and monkeys), like humans, scratch when they have an itch.

  13. Jerry Coyne, knowing that you lost your belief in God suddenly in a flash while listening to the Beatle’s “Sergeant Pepper’s” album, I was briefly wondering if you got the walrus imitation idea from “Magical Mystery Tour”. However, a check of the Wikipedia article shows you would have been 19 when it came out. So I guess not. 🙂

  14. What seems obvious to me is that it is a distinguishing “decoration”. Much like the earrings that many human females (and now males) wear. No one in my family does this and we generally consider it rather silly but mostly harmless (excepting ear or other body part piercing of any small children).

  15. One wonders if Julie has had the opportunity to observe humans going about their normal business with bits of jewelry dangling from their ears for no apparent reason.

  16. The new result here is social learning of a behavior with no clear adaptive significance

    Social learning *is* adaptive though

  17. As behavior variability goes, and its relevance to the success of evolutionary adaptations, I find the behavior quite fascinating.

    The behavior may in fact provide a) sensory stimulation, b) social reinforcement or c)escape. Only further analysis can determine that. If the behavior has no function it is unlikely that it would be supported. Understanding behavioral variability is key to explaining the success of physical adaptations and reproductive success. An adaptation may function in isolation, but coupled with behavioral change it can maximize the efficiency, or a behavioral adaptation alone may provide an advantage of some kind.

    Disparaging remarks indicate to me a lack of appreciation for the potential significance of the behavior, funny as it is.

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