Can you imagine an animal shrinking its brain and regrowing it over time? Some shrews shrink their own brain during winter, reducing the organ by as much as a fourth, to survive the colder months. These shrews then regrow much of their brain in the spring.
This process of shrinking and expanding the brain and other organs with seasons is known as Dehnel phenomenon, and allows animals to reduce calorie-consuming tissue when temperatures decline. According to an article by The Washington Post, common shrews, also known as the Eurasian shrews, shrink their brain in winter and regrow the organ in spring. The common shrew, with the scientific name Sorex araneus, has velvety dark brown fur and a pale underside.
Dehnel phenomenon
Other small animals which undergo seasonal shrinkage in their skulls are high-metabolism mammals such as weasels and moles. The Dehnel phenomenon not only affects the brain, but also other major organs such as the liver and kidneys. The phenomenon is named after Polish zoologist August Dehnel.
Etruscan shrew also shrinks its brain in winter
The Etruscan shrew, with the scientific name Suncus etruscus, also attempts to save energy during winter by shrinking its brain, according to an article published by The Scientist in 2020. Also known as the Etruscan pygmy shrew or white-toothed pygmy shrew, it is the smallest terrestrial mammal.
How much weight does the Etruscan shrew lose in winter?
The Etruscan shrew needs to eat eight or more times its body weight every day. As a result, it cannot hibernate. The Etruscan shrew loses 28 per cent of the volume from its somatosensory cortex, according to an article published in Proceedings of the National Academy of Sciences (PNAS) in 2020. This technique helps the shrews conserve energy. The somatosensory cortex is the region of the brain which receives and processes sensory information from the entire body.
Red-toothed shrews also shrink their brains in colder months of the year
According to the article published by The Scientist, red-toothed shrews, which belong to a group separate from the Etruscan shrew, are born and grow to their full body size in a single summer, and start to shrink all over in autumn. Their skull, bones, spine length, liver and body weight reach their smallest size in the winter.
Shrews start regrowing their brains in February
Then, around February, their brains start to regrow again and reach a second peak size. During spring, shrews sexually mature. The shrews reproduce just once, shortly after which they die.
Shrews can decrease brain size in summer if food is scarce
A team of researchers repeatedly conducted MRI scans of the brains of 10 white-toothed shrews each season for a year, starting in summer. The researchers found that brain volume decreased in the winter, despite keeping the shrews under a constant 12-hour light-dark cycle, at a consistent temperature, with unlimited access to food. When different shrews were offered limited food in the summer, a decrease in brain thickness was observed, indicating that Dehnel phenomenon in these animals was associated with both internal cues related to their age or to the passing of time, and external influences, such as the availability of food.
What happens to shrews’ somatosensory cortex in winter?
The team used another group of animals to highlight the shrinkage in the brain to the somatosensory cortex, according to the article published by The Scientist. The somatosensory context is the region that receives sensory input from the shrews' whiskers, which the animals use in hunting.
The article said that the width of one layer of the somatosensory cortex decreased by 28 per cent in winter and increased by 29 per cent the following summer. In the somatosensory cortex, neuron numbers increased by 42 per cent from winter to summer.
What happens to neurons in somatosensory cortex in winter?
The researchers found that the physical changes in the brain translated to changes in the organ's function. The team classified three groups of neurons in the somatosensory cortex, namely activated, suppressed, and unaffected by whisker touch. According to the study, neurons that were suppressed in the somatosensory cortex had a more than two-fold higher activity in spring and summer than in fall and winter. Therefore, a change in activity could help the animals conserve energy, the authors hypothesised. No changes were found in other regions of the brain.
Shrews have equally active metabolism in summer and winter
According to a study conducted by researchers at Max-Planck-Gesellschaft, animals such as shrews which do not hibernate in winter, have equally active metabolism in summer as well as winter. The study was published in April, 2020, in the journal Royal Society Open Science. Common shrews do not need to increase their metabolism even at sub-zero temperatures, according to the study.
Common shrews are highly metabolic mammals
Common shrews have one of the highest metabolic rates among mammals, and hence, must consume a considerable amount of energy for their relatively low body weight. Since shrews' fat reserves are quickly used up, they often starve to death after only a few hours without food. Despite these constraints, forest shrews are highly evolutionarily successful.
Common shrews do not hibernate
Instead of storing food or hibernating, common shrews grow rapidly to a maximum size after their birth in summer. They begin to shrink and lose approximately 10 to 20 per cent of their body weight in autumn. Their fat and muscle mass and brain size decrease. In February, the shrews begin to grow again until they reach their maximum size in spring. Some tissues such as the brain only partially grow again, according to the study.
Since shrews have an unfavourable body surface to body mass ratio, one would expect them to cool down more easily in winter temperatures. In other words, they are expected to lose more heat to the cold ambient temperature. Dina Dechmann from Max-Planck-Gesellschaft and her team measured laboratory shrews' metabolism at the respective outdoor temperatures of the different seasons. In a statement released by Max-Planck-Gesellschaft, Dechmann said the common shrew somehow manages to "cheat evolution".
Shrews produce large amounts of heat due to high metabolic activity
Even though temperatures in winter fluctuate by more than 30 degrees, and despite shrews' reduced size, the animals do not consume more energy per gram of body weight.
According to the study, shrews constantly produce excessive amounts of heat because of their high metabolic activity, as a result of which they do not need to increase their metabolic rate in winter. Also, they shrink their brains because a smaller body size implies they can consume less energy overall, which is advantageous in winter due to the scarcity of food.
Why is it important to understand shrews’ mechanism of brain shrinkage and regrowth?
By understanding how shrews restore their brain power, scientists can help find ways to treat Alzheimer's, multiple sclerosis (a disease which affects the central nervous system) and other neurodegenerative diseases in humans, according to the article by The Post.
Dechmann's team observed that the unorthodox strategy of shrews to reduce their brain power may help them save energy during the winter, but it comes at cost. The team had conducted experiments on shrews placed in a sandbox. It was observed that larger-brained shrews in the summer outperformed the smaller-brained counterparts in the winter. However, after regrowing their brains in spring, their ability to solve lab puzzles appeared to return, according to the article.
The researchers found that the shrew's brain does not grow uniformly. While the hippocampus expands back to normal, the neocortex does not. Both the hippocampus and neocortex are parts of the brain associated with memory functions.
The article stated that the lipid-rich white matter strewn throughout the shrew brain appears to be disappearing. This suggests that the shrew's body may be consuming portions of its own brain to make it through the winter, the article said.
White matter of the brain helps relay information in the organ, and its deterioration is a symptom of multiple sclerosis and other neurodegenerative diseases. Therefore, discovering the proteins and other triggers responsible for the shrinkage and regrowth of shrew brains could provide pathways to treat brain diseases.