The public will never tire of the nature versus nurture debate but here’s a hint: the answer in biology is always both. But if you’ve ever known any twins, you know they can have quite different personalities which, you would think, are attributable to differences in nurture of one sort or another. To understand this better, some scientists did what scientists like to do which is trap some mice in a little mouse palace and watched how they behaved. These mice were isogenic so there were no genetic differences (excepting, of course, what are probably trivial mutations and some hopefully minimal epigenetic influences).
Now the mouse palace is a wonderful place but there’s not really a lot to do there beyond roaming about, exploring their environment. But not every mouse explores their environment in the same way: some mice like to explore the whole thing, some like to stay in just a few places where they are comfortable. This alone suggests that the environment has a strong impact on behavior, over and above genetics. But they also point to two other facts that they find: first, that over time the variance across the population in this exploratory difference increases. Second, more neurons are born in the hippocampus, the area related to spatial maps and learning, in the animals that roam more than in the animals that stay put.
Now although this paper is pretty cool just for Mouse Mansion (it’s Big Brother: Mice!), there’s a lot to quibble with. They never normalize the roaming variance by the roaming mean so we don’t really know that the variability is increasing. We don’t know whether neurogenesis is increasing more in the animals that increase their roaming more. And even if they did, it’s totally unsurprising that there would be more neurogenesis in the animals that explored more: because that’s just what we think neurogenesis is for! Remembering more locations! Further, from the first moment that they are recording from – 20 days (after birth ?) – the animals that explore the least continue to explore the least, and the animals that explore the most continue to explore the most, but everyone explores more as they get older. So whatever induced most of the variability happened before the behavior was recorded.
We already know a lot about how exploratory behavior arises, and my guess is if you assayed the dopamine receptor expression level, you’d find the differences that you’re looking for to explain the behavior. My naive guess as to what explains the difference is that it is mostly social – the authors don’t really demonstrate any effects of exploratory learning. We know that mice have social structure, and social structure affects serotonin and dopamine levels which in turn affect exploratory behavior. Now I don’t know if they looked at any type of social information in the Mouse Mansion, but I’d bet that the results of social play and social behavior prior to the start of the study are what creates the difference. The fact that a few weeks of social play can change your behavior for the rest of your life? Now that would be interesting.
Freund, J., Brandmaier, A., Lewejohann, L., Kirste, I., Kritzler, M., Kruger, A., Sachser, N., Lindenberger, U., & Kempermann, G. (2013). Emergence of Individuality in Genetically Identical Mice Science, 340 (6133), 756-759 DOI: 10.1126/science.1235294