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Becoming an individual twin isn’t about genetics or environment, but how you experience them

The views expressed are those of the author and are not necessarily those of Scientific American.

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Have you ever known a pair of identical twins? Not just the ones that look alike, but identical twins that really were part of, at some point, the same egg and sperm combination, that then split early in development to create two “identical” people, with the same genetics.

If genetics really were the be all and end all of our behavior, you might expect these identical twins to look the same, act the same, speak the same, move the same. They have the same DNA, they should essentially, be the same people.

But they’re not.

I have known several pairs of identical twins. Yeah, the resemblance is more than a bit uncanny, but without a doubt, they are not the same people. They have different interests, different behaviors. One is more outgoing, while one is more reserved. One may be into art while the other is into engineering.

You might then think that the environment had something to do with it. After all, often twins are exposed to different things. There are famous twin studies of identical twins exposed to different situations (often reared separately), which help to understand the interactions between genetics and environment.

But then, most identical twins are reared together, not separately. They go to the same school, know most of the same people, often share the same room. Their environments do not differ all that much. In theory, in terms of environmental influence, many identical twins should have the same exposure.

And they do. But it’s not just the environment that matters, or just the genetics that matter. It’s how you use them.

Freund et al. “Emergence of individuality in genetically identical mice” Science, 2013.

Same face. Different people.


The differences between identical twins aren’t limited to humans. As a good example, many scientists who work in animal behavior work with genetically identical mice. And they are VERY identical, inbred over many generations to make one mouse pretty much the same as another. This helps to decrease variability of behaviors and biology, so that when we are looking for differences (say, a response to a drug or treatment), we can better separate them from the noise.

But while these mice are genetically identical…they aren’t behaviorally. Yes, the variability is reduced, but one mouse will usually be more anxious than another, or more active than another, or smarter than another, or fatter than another, even within the same strain. We can get identical genetics, but the behavior and physiological result is never perfect, the mice remain individuals.

And the question becomes: what produces this individuality?

To examine this, Freund et al took a large group of genetically identical female mice (C57 Black mice, the mouse of choice for many studies, in this case the kind from the Charles River supplier, which is something that really can make a difference). Half were used as controls, in control cages with cage mates, and normal food and water available. The other half received a higher enriched environment. While sometimes “enriched” can mean a larger cage with some tubes and chew toys, in this case, the enriched environment was a mouse paradise, with multiple levels, loads of tubes, different water spouts, nesting boxes, the whole  nine yards.


(Figure 1 from Freund et al, 2013)

The enriched mice were also equipped with tags so that their activities could be tracked.

Then, the controls were left, and the enriched mice were left, for 12 weeks. That’s a significant portion of a mousey lifetime.

At the end, the mice were examined for changes in hippocampal neurogenesis. Hippocampal neurogenesis, the birth of new neurons in the hippocampus, occurs throughout adult life, but can be affected by environmental stimuli or drug treatment. Long term antidepressant treatment, for example, can increase hippocampal neurogenesis, while stress can decrease it. Hippocampal neurogenesis can also be increased in response to things like new environments that animals have to learn about. And it is increased in response to things like an enrichment.

So it was no surprise that, in the mice exposed long term to the awesome mouse paradise, hippocampal neurogenesis was increased. But it wasn’t increased to the same degree in all the mice. There were individual differences. And the authors of this study took the opportunity to try and examine WHY those differences in neurogenesis might be occurring.

They used the RFID tags on the enriched mice to track where they had gone during the course of the experiment (3 months of data!). And they found that, while some mice stayed close to “home” in limited territories, others ranged all over the place.

(Figure 2B, 2C, Freund et al, 2013)

You can see on the left the more limited range of a more “cautious” mouse. This mouse had low roaming entropy, it liked its small range and didn’t need to venture out of it. On the right, however, you can see a high roaming entropy mouse. This one ranged all over. And on the bottom graph, you can see that these actions diverged over time, a subset of the enriched mice stuck to their limited range, while the other subset roamed widely.

And this roaming CORRELATED with the amount of hippocampal neurogenesis. Mice that had limited roaming had lower hippocampal neurogenesis than mice that had higher roaming. This shows a solid set of individual differences, in genetically identical mice, exposed to exactly the same environment. It wasn’t the genetics, it wasn’t the environment, instead, it was how the animals experienced the environment they were in.

What’s causing this different environmental experience? The authors put forth several hypotheses. One of them is epigenetics, the modifications made to the “outside” of the genome that determine how it is transcribed, and which can produce differences in gene expression. There are also options like intrauterine position (as mice have litters of pups, often up to 10-14 at a time, the position in the uterus, whether you’re between two boys or a boy and a girl or two girls, for example, could expose you to different hormone levels during development which could then affect behavior (probably through altering epigenetics). And of course there is the distinct possibility of random genetic mutation.

All of these are possible. But it will be interesting to see where the group goes with this work, and how they decide to look at these new, individual mice. Because the enriched environment showed that genetically identical mice, in an identical environment, can become individuals. Like twins, it’s not just the environment, or the genetics, but the combination, and how you experience it.

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

Scicurious About the Author: Scicurious is a PhD in Physiology, and is currently a postdoc in biomedical research. She loves the brain. And so should you. Follow on Twitter @Scicurious.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. N a g n o s t i c 11:35 am 05/20/2013

    Define identical. One person’s perfect is another’s sloppy.

    Machines produced with exacting tolerances and operating under essentially identical conditions will wear and break down at different rates.
    Subatomic particles behave in individually capricious ways, so there’s nothing suprising about complex organisms deviating from an expected path.

    If a human was replicated via use of a matter duplicator, I suspect the time length during which the original and copy were identical would be so small as to defy measurement. These individuals would assume noticeably unique states rather quickly in my view.
    As the author noted, twins in utero start deviating from identicality, due to womb placement and variations in hormone exposure.

    In the end, all this can be philosophically fascinating stuff, yet I fail to see the scientific usefulness of this research. The whole “nature versus nurture” thing is tiresome, as we’re talking about the same thing – nature alone.

    What fundamental questions are to be answered by splitting hairs over mice and twins?
    The article didn’t say.

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  2. 2. scicurious 11:41 am 05/20/2013

    Nagnostic: I’m particularly interested in the results of this research because of the application for mouse studies. We use mice frequently as models for various diseases, I think it’s very important to see how the variability in these mice occurs, and what it could then mean for the outcomes of those studies. Understanding how and why mice vary could help us understand some of the differences we see, for example, in drug responses, and could also help us relate these differences to human conditions. While mice are by no means a perfect model, they are an important preliminary model for many treatments and for helping us understand human systems, so understanding the mouse system itself is of particular importance.

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  3. 3. N a g n o s t i c 12:05 pm 05/20/2013

    I love how the author capitalized “correlated”.
    Good for you Scicurious, here’s a gold star. Ooops, never mind, I’m making a value judgement. The whole class gets a star!

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  4. 4. tania_v 3:47 pm 05/20/2013

    “Scientific usefulness”? Science’s goal is to gather as much knowledge as possible about how the Universe works.

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  5. 5. shadyw 5:06 pm 05/20/2013

    At a parent of identical twin girls, I can tell you that intrauterine position can play a role with human twins too, and may be positional as much as hormonal.

    One of my daughters settled into a head-down position, ready to be born, months before full-term. That left the other twin draped over her feet, constantly being kicked and moving position many times each day. In the ultrasounds, the head-down daugther is curled up, happily sucking her thumb in every image. The less comfortable twin, by contrast, would get hiccoughs, turn cartwheels and generally wriggle.

    As soon as they were born (by c-section) it was immediately apparent which twin was which. The comfortable twin was a calm, quiet baby. The uncomfortable twin was more nervous, noisy and restless.

    This clear difference at birth has formed the basis of their personalities (now aged 14) although not in any obvious, causal way. We simply observe that their personality traits started to be formed at the same time that their brains were forming in utero, and have continued to diverge into hugely different “identical” twins over time.

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  6. 6. Ianthegreen 9:35 am 05/21/2013

    Meaney and Szyf have shown the quality of maternal care rats received early in life modified the expression of genes involved in stress response. Not a great leap (at all) that an enriched/deprived environment could have similar consequences. so if the theory is as follows: Differing input->differing epigenetic state-> differing gene expression/phnotype observed, then why not quantify that? Looking for global epigenetic changes (in DNA methylation or chromatin mark enrichment) seems the logical next step, it is quite disappointing they didn’t hold off and publish with a fuller picture. Cool study though!

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  7. 7. faucet 2:26 pm 05/21/2013

    I wonder if regularly pushing mice out of the “home” area, forcing them to spend significant time in the more varied environments, would increase hippocampal neurogenesis or induce neurogenesis inhibiting stress responses. I would imagine if the mice were kicked out of the house regularity they would become comfortable in less explored areas and the perceived stress of dealing with the unique would eventually vanish, leading to more hippocampal neurogenesis. Cool stuff!

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