The Milk Mystery and Imitation


A Blue Tit (Cyanistes caeruleus). Via Wikipedia.

In 1921, people who had milk delivered to their door in Swaythling, England had a mystery on their hands; someone was drinking all the cream out of their milk bottles. Local youths and other potential culprits were named, but the phenomenon began to spread across the country, no one being quite sure who was behind the theft of the high-butterfat layer. Eventually, however, the freeloaders were identified; Blue Tits (Cyanistes caeruleus) were opening the tops of the bottles and skimming off the best part of the un-homogenized milk. Various bottle caps were tried over a number of years but most proved to be ineffective, groups of birds actually waiting for milkmen when they would arrive to make their deliveries each morning, and the behavior spread throughout the European continent. Surely these birds must have been imitating birds that they saw open the bottles, but the real answer is simpler while being less immediately apparent.

Imitation, “monkey see, monkey do” to borrow a colloquialism, is actually a complex cognitive ability. It might come easy to us and to great apes in a number of tasks, but it is either more simplistic or absent in many primates and other animals. Some tamarins, for example, can be taught to open containers using either their mouth or their hands based upon they way their observe a container being opened by a modeler, but it is more difficult for them to imitate more complex tasks. Indeed, we must tread carefully as even we can’t always get things right the first time around, and my primatology professor Ryne Palombit found an elegant way to demonstrate this point. Picking a member of the class who did not know how to tie a Windsor knot, he quickly tied a Windsor knot in front of the class and then asked the student to do the same. She could not do it, so does this show that she cannot imitate behaviors? Of course not, tying this particular knot being far too complex to simply see it once and then do it, showing that negative results might mean that there’s something wrong with the particular test we’re giving. Still, there is a large disparity between our ability to imitate and the ability of tamarins to imitate behavior, and what might look like imitation doesn’t always turn out to be so.

In the case of the Blue Tit, birds that knew how to open bottle caps were observed by birds that did not know how to open bottle caps, and then the observers were presented with a capped milk bottle to see if they could replicate what they saw. They could not. What the birds did do, though, is focus their attention on the bottle cap and fiddle around with it even though they did not immediately open it they were in which they were shown. Their attention was drawn to the bottle cap by the behavior of the modeler, even if the subsequent birds had to learn how to actually open the cap themselves (which they were quite capable of doing). Thus there was something of a cultural transmission, the birds that opened these bottles to get at the cream each having to learn the behavior on their own but often after being giving a “clue” by watching another bird do it first. Some could have very well figured out how to open the bottles without observing the other birds (Lefebvre, 1995 suggesting a second site for this behavioral innovation and the cultural transmission of it), but the speed and spread of the behavior shows that there was quick transmission of this behavior based upon watching an ever-growing body of modelers.

A similar problem arises in the case of Imo, the famous Japanese Macaque (Macaca fuscata) that washed her potato before eating it, supposedly founding a culture of potato washing that spread through the population. While it is true that other members started to wash their tubers, the rate of recruitment was very slow, a new “washer” only showing up on the average of every two years. Furthermore, the keepers of these monkeys provisioned them with the food, giving preferential treatment to monkeys that washed their food over others because people liked seeing the macaques wash their food. Given these considerations (plus the fact that the data is sparse given that the researchers were not studying cultural transmission), it’s difficult to say whether the other monkeys were really imitating Imo, whether each one was learning on its own based upon what Imo was doing (like the Blue Tits), or whether they were simply washing to get a reward in a more simple Skinnerian model. The overall trend suggests that it was not imitation (imitation usually occurring very fast as more and more modelers are present, thus more individuals observe the behavior), although the information seems insufficient to say precisely what was going on.

The rigorous study of imitation in nonhuman primates and other animals is relatively new and there’s still much work to be done, especially since imitation may not be as common as it may first seem. It’s possible to culturally transmit behaviors that need to be learned by each individual, but these patterns are typically much slower than actual imitation. Even outside understanding this difference, the reason why members of one species vary in certain behaviors across various populations (even ones that are located near each other) is yet another mystery, especially in apes where different populations of chimpanzees, for example, have different tool cultures. Indeed, imitation and cultural transmission can be a difficult thing to study, but it is utterly fascinating and hopefully more attention will be turned to this topic in the coming years.

[Hat-tip to my professor Ryne Palombit for inspiring this post, which is based upon a recent lecture given by him on the topic of culture]


Galef, B.G. (1976) “Social Transmission of Acquired Behavior: A Discussion of Tradition and Social Learning in Vertebrates.” Advances in the Study of Behavior, Vol. 3, 77-100

Lefebvre, L. (1995) “The opening of milk bottles by birds: Evidence for accelerating learning rates, but against the wave-of-advance model of cultural transmission.” Behavioural Processes Vol. 34, 43-54


  1. #1 Sven DiMilo
    December 4, 2007

    Great post. There was an article in Science a few years ago claiming evidence for visual imitative learning in octopuses. I wonder what data exist for cognitively advanced birds like crows and parrots. Most comparative psychologists are shamefully primate-centric.

    I can’t encounter the milk-cap story without smiling because of an incident that occured when I was in grad school, TAing an introductory course in ecology and evolution. The somewhat oblivious and absent-minded professor was lecturing to an auditorium of 300 students, and mentioned these birds called Blue Tits. Naturally this provoked some Beavis & Butthead-style giggling and jostling among the freshmen (tee-hee! he said “tits”), which the prof evidently interpreted as confusion. To clarify, he went to the whiteboard and wrote in letters about 3 ft. high the word “TITS.” Of course, this just made everybody laugh more, throughout the rest of the lecture. From the back of the room I wondered what if the Dean or somebody’s mom came in and saw this guy lecturing against the backdrop of TITS?

  2. #2 Peter Midford
    December 4, 2007

    There have been a couple of field studies of social learning in free-living Corvids, in both cases New World jays:

    Langen T.A. 1996. Social learning of a novel foraging skill by white-throated magpie-jays (Calocitta formosa, Corvidae): a field experiment. Ethology 102: 157-166.

    and my own, more culture-focussed study:

    Midford, P. E., Hailman, J. P. & Woolfenden, G. E. 2000. Social Learning of a novel foraging patch in families of free-living Florida scrub-jays. Animal Behavior 59:1199-1207.

    There are also studies on Ravens:

    Fritz, J. & Kotrschal, K. 1999. Social learning in common ravens, Corvus corax. Animal Behaviour 57 , 785-793.

    and I’m sure a lot more.


  3. #3 DDeden
    December 4, 2007

    I haven’t seen the actual milk caps. Many small birds spend lots of time attempting to get food under things and in crevices, so I wonder if the actual manuever was so difficult for them, once the first blue tit came across an open filled bottle and slurped, then simply repeated the same thing at another closed bottle with a bit of prying or pecking.

    I’m just thinking that there may have been very little learning involved, once the “open bottle = tasty cream” experience occurred in the first blue tit.

  4. #4 Dave Godfrey
    December 4, 2007

    In the UK milk bottle tops are thin pieces of foil sealed over the neck of the bottle. (The lids are cheap and disposable because the milkman collects the empties to refill back at the dairy). If you have long enough fingernails you can easily break the top yourself. With a bit of fiddling a blue tit would be able to get into the bottle- but it would need to know that it was worth getting into, and that the lid was the target.

  5. #5 Sven DiMilo
    December 4, 2007

    Thanks Peter!
    (by the way, I was really sorry to hear recently of Glen Woolfenden’s death–he was a field biologists’s field biologist)

  6. #6 Christopher Taylor
    December 4, 2007

    I can’t resist pointing out that “Imo” is the Japanese word for potato.

  7. #7 Peter Midford
    December 4, 2007

    I was stunned and disappointed when I learned of Glen’s death. It was apparently
    unexpected. I learned quite a bit from him and I’m only sorry I didn’t take more
    advantage of the time I was around him at Archbold.


  8. #8 thomas
    October 7, 2010

    Isn’t all social learning fundamentally based in imitation? If you look at the blue tit example, and break down the steps required to actually get at the milk. Step 1: imitate landing on milk bottle. Now what? Step 2: experiment with opening bottle top (non imitative) or imitate what other tits are doing. In any case the cultural transmission of this learning throughout tit populations is dependent on imitation of step 1. What happens after that is variable. If it were the case that the bottle tops could only be opened one way (by means of a ring pull for example), then all tits would open the tops the same way and you would likely say the whole process was a single imitation from landing on bottle top opening. But it would still progress in at least two distinct imitation steps, and after tits learned to land on the milk bottle they would need to imitate the opening of the top.

  9. #9 film izle
    October 10, 2010

    An exceptionally preserved fossil primate is pretty exciting, but that’s not why the publicist for tomorrow’s AMNH event wrote one of the most overblown press releases I have ever seen. No, the paper, which will be released in PLoS One tomorrow, claims that this particular primate is of vital importance to the origin of anthropoid primates (or monkeys and apes, with our species being included in the latter category). As might be expected during this significant year.

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