A Sense of Adventure May Distinguish Identical Twins From Each Other

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Curiosity may be key to distinguishing among identical twins — at least when it comes to rodents.

A new study of genetically identical mice throws more data into the long-running debate over how nurture affects nature — demonstrating for the first time that how inquisitive the animals are could affect the birth of new brain cells in a memory-related brain region and establish different volumes of brain neurons even in genetically identical mice. The research found that nearly one-fifth of the variation in the number of new nerve cells in the hippocampus was explained by how thoroughly the mice explored their surroundings.

The more interested the animals were in checking out new places, according to research published in Science, the more cells they had in this region as adults — even though as babies they had almost identical patterns of movement. “For the first time, we can show in animals a link between experience and activity and [structural changes in the] brain on an individual level,” says the study’s lead author Gerd Kempermann, a neuroscientist at the German Center for Neurodevelopmental Disease in Dresden.

“The most surprising and most important finding is that it shows [not only] that our brains are altered by the environment we live in, but that the life we live affects the structure of the brain,” says Jonas Frisen, a professor of molecular biology at Sweden’s Karolinska Institutet, who co-wrote a commentary on the study but was not associated with the research.

Kempermann and his colleagues studied 40 female mice that had been bred to be as genetically similar as human identical twins. They were housed together in a complicated three-level maze containing toys to challenge them. The maze was constructed with tubes connecting the levels to allow them to explore as large an area as they preferred. (Males were not included because their instinct to fight over territory would have made the research difficult to interpret.)

“It’s probably just small coincidences that make them [behave] differently to start with,” says Frisen. “What they show is that [the mice] are very similar at the onset of the study but become increasingly different over time.”

Since mice tend to be timid creatures, many of them stuck to routines and did not explore much beyond their familiar territory. But some were bolder and played more adventurously, and these mice had more cells in their hippocampus as adults. Because the environment was seeded with toys designed to engage the mice, the researchers believe those who explored over larger areas were exposed to more intellectual challenges and therefore developed more brain cells in the memory regions of the brain.

The work parallels research on London taxi drivers, who showed growth in this same area as they learned the complex routes through the city that they needed to master to be licensed. And it suggests that some of the differences in human identical twins may relate to early choices they make about how much novelty they pursue.

In his commentary, Frisen and his co-author also note that growth of cells in this region in adults — a phenomenon known as neurogenesis — is linked to the ability to make new memories of similar but subtly different experiences, such as remembering where you parked in a lot that you frequent regularly. This same part of the hippocampus can be damaged by stress and depression, leading to difficulties making such fine distinctions in these types of memories. For example, people with posttraumatic stress disorder often develop fear of places similar to those where they were initially traumatized, and depressed people tend to think that one negative event is a sign of a more general trend of rejection.

While it’s obviously a long way from genetically identical mice to identical twin humans, Frisen thinks the study suggests ways that seemingly innocuous behaviors or experiences, such as curiosity and novelty seeking, can shape personality and development.

“What they show here is that we don’t just have different memories: our brains become anatomically different depending on the life we live,” he says. “It actually affects how many new cells we generate and that’s important for certain cognitive abilities.”

In other words, the choices we make can affect how smart we eventually become by altering the structure of the brain itself.

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