A tale of a shrinking brain

by Hinal Kharva: Somewhere in Europe, winter is coming and a chipmunk is busy collecting nuts and berries to survive the harsh weather.

Chipmunk (to Shrew friend): Hey buddy, all set for winter?

Shrew: Yes, am I not looking slimmer to you?

Chipmunk: Oh my god, yes you do. But why? Instead of putting on some extra fat for winter, you are losing weight?

Shrew: Ha Ha, unfortunately, I do not get to sleep (hibernate) like you my friend. I will be out all winter as I am in summer and spring. I have become smaller.

Chipmunk: Are you nuts?

Shrew: I have to tell you something about my family history. Don’t worry, it’s not a big secret. Way back around the 1940s polish zoologist August Dehnel found out that our family and the weasel family shrunk their bones, skull, and some major organs during scarcity of food. He also observed that it happens just before the winter season arrives. It’s called Dehnel’s phenomena. My parents told me we are the smallest mammal on the planet. And they said, unlike you guys, we take meals around 8-10 times a day. You know I can’t go to sleep without food.

My great grandparents and their friends volunteered (2013-2014) for a scientific study to understand why do we lose weight. They were taken to the Max Plank Institute for Animal Behavior in Radolfzell. They received free X-ray photos of themselves and a microchip. If I remembered correctly, they visited the institute even in summer and spring.

Chipmunk: Do your parents still have those microchips? I would like to see how it looks like.

Shrew: No, they took it back. But that’s not important. At the end of volunteering, they found out that not only their body lost some weight, but their brains also got rid of about 20-30% of the original size.

Chipmunk: Wait, what are you saying – their brains shrunk?!!  

Shrew: Apparently, like other organs, our brains also shrink during winter and during scarcity of food. I thought it was all fake until I experienced it myself. Everyone in the family lost some weight. It has nothing to do with age. My cousin, the Etruscan shrews (Suncus etruscus), were also part of a recent study in 2019 to understand which part of the brain actually shrunk. I heard they did an MRI of the brain and found a region called the somatosensory cortex, which loses around 28% of its thickness. This area receives sensory information from our whiskers while we are hunting.

Chipmunk: Are you saying you lost 28% of the brain to save energy? You must be so low on energy.

Shrew: Maybe I did, but I don’t really feel anything.

Chipmunk: Ah, you people are so small-minded.

Shew: Not for so long. Once winter is over, I will regain my brain and body size just about the time of my birthday during summer. My cousins were telling me that there is a huge discussion going on among scientists about what regulates our body weight and skull size. The scientist wonder if these are regulated by our internal body clock or changes in the external environment we live in.

One of them said it has to do with saving body energy during a harsh season. They are also interested in knowing how we are able to regrow the neurons. It seems adult neurogenesis (generating new neurons) is very rare. So, as a mammal, going through this regrowing process is extremely special and fascinating.

Chipmunk: Wow! That is interesting. I learned something new about my little friends today. Now, while I take a nap, you can go and look for another meal. 

Shew: Yes, hope I can visit you after the snow. Till then, Happy holidays 🙂


  1. Saikat Ray S, Li M, Koch S.P, Mueller S, Boehm-sturm P, Wang H, Brecht M, Naumann R.K (2020) Seasonal plasticity in the adult somatosensory cortex. PNAS 117(50) pp.32136-32144 doi:10.1073/pnas.1922888117
  2. Lazaro J, Dechmann D.K.N,Lapoint S, Wikelski M, Hertel M (2017) Profound reversible seasonal changes of individual skull size in mammal. Current Biology 27(22) pp.3576 doi: 10.1016/j.cub.2017.08.055.

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