The Milk Mystery and Imitation

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]

References;

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