I wish to share an early experiment with my fellow evolutionary biologists. The key result of the experiment suggests that the length of an event staying in memory may be directly associated with the effect of the event on fitness.
The experiment was done many years ago on white-footed mouse (Peromyscus leucopus) when I was a graduate student at University of Western Ontario. It has never been published partly because I developed strong allergy to mice before the study is incomplete.
The story started when I was trapping white-footed mice with Longworth mouse traps and accidently caught a least weasel (Mustela nivalis), the most deadly enemy of wild mice. Out of curiosity I brought it back to our animal facility housing my mouse colonies on which I carried out genotyping and breeding experiment. The presence of a least weasel somewhere was almost immediately sensed by all the mice – they stopped regular eating and drinking.
For wild mice, there is no news worse than the arrival of a least weasel in their vicinity.
The news, while depressing for the mice, was found exciting by Dr. Martin Kavaliers who was at that time studying signal-transduction pathways of analgesics. Martin then did an experiment on my white-footed mice with a hot plate. The control mice, when being put on the hot plate, would lift and lick their feet constantly when the hot plate was turned on. In contrast, the mice exposed to the smell of the least weasel became so focused on the smell as to be oblivious of the burning heat under their feet – they did not lift or lick their feet as they should have when there is no weasel smell. Martin incidentally reported the least weasel as short-tailed weasel (Mustela ermine) which is bigger than the least weasel and would have a difficult time squeezing its way into a Longworth trap.
While Martin’s experimental mice were field-caught, the same phenomenon was also observed in lab-born white-footed mice that never had any real encounter with the least weasel, so that “memory” of the bad thing (i.e., the least weasel) may have been somehow encoded in DNA and re-enacted through proteins (which would be truly intriguing). In this way, the memory of the weasel becomes permanent and will not be eroded by the passing of time. (Of course there is also the possibility of new-born mice learning from others.)
I then did an experiment in my semi-natural enclosure which has been described in some of my early publications. The experiment also involved a hot plate, enclosed within a card box, and placed inside the semi-natural enclosure. In one part of the experiment (Memory-Of-Good-Thing-Expt), sunflower seeds and Purina rat chow were scattered on the hot plate, but the hot plate was never turned on.
White-footed mice are crepuscular and active mostly during twilight hours. Similar to any other field mice, they are always cautious to new things added to their environment, and would investigate the box carefully before entering. After some initial exposure, however, the mice would recognize the box as a good source of food and would enter the box quickly to eat, and females would bring the rat chow back to their nest-boxes. If you take the box away for one day and put it back at the same location, the mice would still recognize the box and enter it readily without further investigation – their memory told them that it was a godsend.
By the way, the white-footed mice have reasonably good memories and memorize the routes along which they dart from one hiding place to another. If you put a card box on such a route, they will typically dart head-on against the box, producing a loud sound breaking the silence of night, a consequence that the card box is not in their memory of the routes. Sometimes I wonder if they would run right into a predator if the predator knew their routes and just sit there with a wide-open mouth (they don’t, thanks to their smelling capabilities). Of course, there is also benefit of darting from one hiding place to another by using the map in the memory instead of by vision. These mice don’t see very well during twilight hours (Mice are not cats or owls). Relying on their vision might delay their escape from predators.
Leaving aside the many interesting biological aspects of the mice, let’s come to the other part of the experiment (Memory-Of-Bad-Thing-Expt), in which the setup is the same as Memory-Of-Good-Thing-Expt except that the hot plate did get turned on when mice were inside (Make sure that you do not have a thick layer of food on the plate preventing the heat reaching the mice). These mice, experiencing the bad feeling of burning, avoided entering the box in spite of the inviting food inside.
The boxes were then taken away for three weeks, after which the mice were again exposed to their respective boxes. Those mice in the Memory-Of-Good-Thing-Expt seemed to have forgotten altogether the good food associated with the box. They resumed the cautious approach and restarted the investigation. In contrast, those mice in the Memory-Of-Bad-Thing-Expt appeared to instantly recognize the box as evil and avoided entering the box.
In short, memory of bad things lasts longer than memory of good things, at least in this particular case. From an evolutionary point of view, this is not difficult to understand. While the good thing may not enhance their fitness very much, the bad thing (the burning and the weasel) could do serious damage. I very much wish the conclusion generalized because it is important.
Mistakes are typically associated with bad feelings, but we learn from mistakes, with the consequence of a lot of bad feelings staying in our memory and lasting for a long time.
One corollary from the conclusion is that, when one gets old, if there is no constant reinforcement of good things and good feelings, then one could potentially be left with memory of bad things and bad feelings. This corollary was made particularly pertinent when a former colleague, a well-established Peromyscus researcher, Dr. Jerry O. Wolff, committed suicide, exactly seven years ago, on May 10, 2008. I met Jerry several times, mainly through conferences and through his visit to my former supervisor’s (Jack Millar) lab. We regarded Jerry as an excellent colleague and did not, and still do not understand why he would have chosen suicide to end his life. Had he lost good things and good feelings in his memory? Did bad things and bad feelings took an upper hand? While our knowledge of science and technology has progressed so much, our knowledge of hope and love has lagged so far behind.
We often see old people indulged in the so-called “good old days”, the kind of nostalgia that historians, psychologists and politicians all unanimously dismiss as mass delusion, i.e., a Garden of Eden that has never existed. Indeed, if we measure wealth, health, life span, degree of democracy and rationality in human behaviour, etc., we will find the present day beating all “good old days” by a wide margin. But do we have more human interactions to reinforce good things and good feelings in our memory if we get old today? Will our brain be gradually poisoned by the bad things and bad feelings?
The mouse experiment described above might also be relevant to parenting as well. Parents perhaps should be particularly cautious when dispensing punishment to their children, especially when parents sometimes do wish their children to remember the bad feeling of punishment for a long time. A child is likely to become troublesome and lost without constant reinforcement of good things and good feelings in his/her memory. A home should not be a transient Biblical Garden of Eden with a cruel one-strike law.
May we keep alive our fond memories of love and friendship until the end of our days!
(Written on May 10, 2015)