Why are you who you are? At a simple level, biology says you’re a product of your genes and your environment—or, nature and nurture. But the explanation is trickier when you consider identical twins raised in the same home: they have the same genes, and grew up in the same environment, but, if you’ve ever met a pair you’d probably agree, they’re different people.
A study on mice has tried to get to the bottom of this puzzle—and it finds that the ways identical mice interact with their environment differs, changing both their brain structure and their subsequent behavior. Mice that begin indistinguishable thus grow increasingly different, by virtue of how they interact with their surroundings.
Researcher Julia Freund and colleagues at various German universities began with 40 genetically-identical young female mice. These mice were housed in a custom-built mouse paradise: a cage with five levels, tubes to climb through, boxes to hide in and toys throughout.
To monitor how the mice interacted with their playground environment, scientists implanted RFID tags (like the ones that are plastered on CDs to prevent shoplifiting) under the mice’s skin and tracked their movements with small antennas throughout the cage. They gave the mice unlimited food and water and sat back and waited.
After three months, all the mice had grown more active and adventurous. But though the mice at the start all demonstrated a similar level of wanderlust, by the end of the experiment their travel patterns were decidedly different. While some mice hung around a home area and occasionally ventured outside their comfort zones, others spent equal parts of time in all the cage’s corners. What’s more, the mice’s wanderlust at the end of the experiment was correlated with how many new neurons they’d added in their hippocampus, one of two known areas where new neurons are born in adult mammals’ brains.
It’s been known that physical activity promotes adult neuron-growth (called neurogenesis), but in this case the researchers found it didn’t fully explain the differences. Mice that were very active but in a limited range showed less neuron-creation than mice that wandered over a greater area. The researchers thus concluded that the mice’s divergent experiences of their environment were driving their brain changes. In the end, about one-fifth of the differences in neurogenesis between the mice was attributable to how far the mice wandered, the researchers report in Science.
Thus identical twins, though they start with the same genes, likely develop different personalities in the same environment partially based on how they interact with their environment. This lived experience, in turn, probably changes their genes: Previous research has found that human identical twins accumulate epigenetic changes as they age, making them more dissimilar over time. In this way small initial personality differences could snowball—changing behavior, which changes brain—and result in our colorful, unique selves.
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