Taking a shift away from my review article last week, I am going to focus on an experimental paper this week. The paper is titled "Cultural Transmission of Feeding Behavior in the Black Rat (Rattus rattus)" by Joseph Terkel of Tel Aviv University. This one talks about black rats, found in Israel, that were discovered to be pulling pine cones from tree branches, stripping all the scales and getting to the seeds located inside. The experimenters were initially shocked at the behavior that they witnessed, since there were no squirrels in Israel, but they laid out a series of traps and discovered that it was these black rats doing the dirty work.
The researchers then conducted a series of experiments to determine how these black rats had learned this behavior. They wanted to know if the pine cone opening behavior was transmitted genetically or acquired by learning, and whether they learned it through trial and error or through imitation.
The first experiments were done to examine trial and error learning. Naive (those who had never stripped pine cones) rats before were not given food for 48 hours to make sure that they were hungry, and then put in a cage with pine cones. The rats certainly did try to open the pine cones, but not a single rat (out of the 32 tested) were able to successfully strip the pine cones. The primary response of the rats was to gnaw at the cones, which is the way rats typical approach eating their food. In addition, a second experiment was done where naive, adult rats were paired with experienced pine cone strippers of the same sex. This, too, proved ineffective as the naive rats were unable to learn the technique necessary to the strip the pine cones. A third experiment was done where naive pups, were fed milk from their naive mothers who were fed pine seeds. This was done to see if there was some flavor cue that would enhance the young rats' abilities to strip a pine cone. This too proved ineffective.
The most effective type of learning occurred when pups who were born to stripper mothers (mothers who knew how to strip pine cones) or born to naive parents but were reared by a stripper mother were able to socially learn the stripping behavior. This is distinctly different from the previous experiments conducted because the rats are all young (pups) and have the plasticity to learn new behaviors, unlike adult rats who are more fixed in their ways. Also, the rats got paired up with a female stripper mother, as opposed to a rat of the same sex. The influence a mother has over pups is much greater than one an experienced adult has over other adults. Pups of any species must look to the mother to learn how to feed and forage for food, otherwise they will die. It is this necessity and situation that causes the pups to socially learn the behavior from mothers, whereas adult rats do not learn the behavior from other rats, instead opting to gnaw at the pine cones - a strategy that has provided weak results.
I am impressed with how extensive the research conducted was. The researchers also wanted to test how naive rats come to learn the stripping behavior, so they decided to assist the rats and give them clues to opening the pine cones. They gave rats a series of cones with different grades of the scales stripped. For example, they gave rats cones with 4 scales stripped off and found that they were able to finish the job of stripping the pine cone and get to the seeds inside. When they were given a fully intact pine cone afterwards, they were unable to strip the pine cone, however. It was found that ninety percent of the rats were able to open pine cones, when a steadily decreasing number of scales was presented to them. For example, one week they were given a pine cone with 4 scales removed, then 3, then 2, then 1, and the rats were able to open them all. It would have been interesting to see if the rats were able to open the pine cones with a gradually increasing order of the scales removed to see if they were able to open a pine cone with 1 scale removed, as opposed to starting off with the easiest task of opening up a pine cone with 4 scales removed.
Finally, the research went into another style of opening the pine cones - a method called shaving. In this method, the rats would strip one side of the pine cones and extract the seeds from that side, as opposed to stripping the entire pine cone. When "assisting" the rats, this time by pulling 4 scales off the cone, but only going halfway around the circumference of the base, they were able to complete the task by stripping one side of the pine cones and extra the seeds from that side only. This goes to show you that rats are able to learn to strip cones based on what they see in nature. If their earliest experiences with pine cones came from seeing scales stripped from one side, then they would shave the cones. If their experiences came from seeing the scales stripped all the way around, then they would also strip the cones all the way around. Animals are very social creatures, so observation and plasticity at a young age plays a very important role in development. The article I read made a great point about how "In nature, the likelihood that a pup will encounter a partially opened cone in the vicinity of its mother, is much greater than the likelihood of an adult finding a partially opened cone. This experiment, therefore, stimulates a condition under which the feeding technique could be acquired and passed on to the next generation."
There are two ways of testing that can be done to see how rats acquire the ability to strip pine cones - observation and experimental testing. Both have their advantages and disadvantages which I wanted to discuss briefly. The observational approach would provide more accurate results as the exact method by which rats acquire their skills, but it is often difficult to carry out because the rats feed at night, and in areas that are not always accessible. On the other hand, experimental approaches are effective because any variable can be manipulated and tested, but the external validity of these tests is not very high, since rats are not in a cage or in a laboratory in the real world. As the experimenters alluded to quite a few times, the rats were shy or scared to strip cones while people were watching them, and sometimes it seemed like they became desperate to do something different because they were starving and people were watching. Maybe the Clever Hans phenomenon was at play in these experiments. This term comes from the the situation whereby a horse seemed to know how to count and add numbers, but was actually just responding to the unintentional cues given by the experimenter whenever it had reached a correct answer. Maybe the experimenters in this study let out a sigh of relief when the rats began stripping away the scales of the cones, and that was how they learned the behavior. Although it may be a legitimate study, you can never know for sure exactly what is going on in nature simply through experimental procedures.
In conclusion, rat pups were seen to be most capable of social learning (through observation and trial) when reared by a mother with pine cone stripping experience. Many rats have been seen to take pine cones from their mothers after the mother has partially stripped the scales off. This further supports the experiments conducted in this study where rats learned how to strip a pine cone after they were partially stripped already. And, of course, it was concluded that the stripping behavior was not passed on genetically as was originally postulated. The first rats to ever learn this behavior must have seen some pine cones in nature that had lost part of their scales and experimented with ripping the scales off completely. I think these experimenters did an excellent job breaking down the process at every level and continually investigating the causes and factors at play for social learning. This was one of the best experiments I had ever read about as no corners were cut and a multi-layer investigation was conducted to finally learn how it is that rats do what they do.
Bibliography:
Terkel, Joseph. Cultural Transmission of Feeding Behavior in the Black Rat (Rattus rattus). Tel Aviv,
Tel Aviv University, 1996.
Nice coverage of one of my favorite behavioral analyses of all time. A few comments.
ReplyDeleteYou write: "Pups of any species must look to the mother to learn how to feed and forage for food, otherwise they will die. It is this necessity and situation that causes the pups to socially learn the behavior from mothers,..."
I don't think the authors conclude that the rat pups learned the proper stripping technique by watching the mothers strip the pine cone. Rather, they managed to occasionally steal the partially stripped pine cones from their mother, providing them with the necessary conditions for trial-and-error learning (i.e., instrumental learning) to allow them to learn the proper technique for stripping the pine cones efficiently.
Also, I doubt any type of "Clever Hans" effects introduced by the experimenters could have biased the results toward the pattern they report having observed in their experiments. If so, we should have seen a lot more positive results as the "sigh" exhibited by the experimenters would presumably have been present in all of the experiments. Nevertheless, only a few of the experiments were successful (e.g., the cross-foster experiment), and then only for the group of rat pups raised with the experienced stripper mother, not with the naieve stripper mother. Thus, even in this experiment, rat pups in both groups should have been exposed to the "sighs" of the experimenter and thus have learned to strip the cones. Finally, rats have not been shown to be so highly influenced by subtle human-derived cues the way dogs and horses have been.