Born in spring, the common shrew attains about 70% of its adult body mass by summer but loses some of it over the following winter. Even its brain becomes smaller.
The common shrew (Sorex araneus). Credit: Karol Zub
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
How do some of the world’s smallest mammals survive temperate winters? Keeping the internal thermostat on high, drains energy at a time when food isn’t easy to come by. Birds cope by flying off to warmer climes. Large mammals stuff themselves with food from spring to autumn, putting on enough fat to insulate themselves through hibernation. Small mammals like rodents and shrews can neither migrate nor hibernate.
Their high metabolism needs plenty of food. But winter is not a period of plenty, and shrews would be hard put to find enough soil invertebrates. Keeping warm is seemingly unsustainable.
Mammals in cooler, higher latitudes are larger than their warm-weather counterparts, a principle called the Bergmann’s rule. Shrews stand this rule on its head – becoming smaller in some of the coldest parts of Europe and Asia. In fact, two species brave life in the Arctic Circle.
Compacting their size allows less body heat to escape, but the ratio of surface area to body mass makes smaller bodies feel the cold much more keenly. Indeed, shrews grow a thicker fur coat that reduces the loss of body heat, but that’s not enough. They continue to burn fat reserves to keep warm. So they opted for an amazing workaround: reduce mass for more energy savings.
In 1949, Polish zoologist August Dehnel discovered the skulls of shrews in winter were smaller than ones caught in summer. Since then this adaptation takes his name, the Dehnel phenomenon. Not only do shrews and small rodents reduce their overall body size, they shrivel up in winter, losing body and brain mass and shortening their skeletons.
Typically, mammals that don’t hibernate and weigh less than one kilogram seem susceptible to this principle. Snowshoe hares, for instance, weigh more than a kilogram and do not lose body mass in winter.
More than rodents like voles and hamsters, the common shrew (Sorex araneus), among the smallest Eurasian mammals, shrinks to a dramatic degree. It doesn’t usually live to see more than one winter. Born in spring, it attains about 70% of its adult body mass by summer but loses some of it over the following winter. Even its brain becomes smaller, reaching a minimum in March. The following summer, when the soil crawls with insect delicacies, not only does the shrew regain what it lost but it piles on even more. Then it reproduces and dies before the second winter.
By conducting postmortem studies on many shrews, scientists discovered that the insectivores lost between 9% and 28% in body mass depending on which part of Europe they lived in. The spinal column shortens, the vertebral discs flatten, the cartilage in them is absorbed and, most dramatic of all, the brain and skull shrivel.
Once they start growing, few animals can downsize and expand their bones and brain cases. How does the shrew skeleton grow and shrink? When humans lose weight, for instance, they lose tissue mass and bone density. But bones don’t shorten nor do skulls shrink. As they age, the elderly become shorter as their vertebrae compress and arch, and they become susceptible to a higher rate of bone fractures. Once bones compact, it’s impossible for them to regrow.
However, the bones of astronauts who spend many days in space shrink in length and density. A year after their return to Earth, they regained their original bone structure, but they didn’t recover their strength in that time. So they work out to prevent this loss from occurring at all.
X-ray images of one individual as a juvenile in July (left), size-decreased subadult in February (centre) and re-grown adult in May (right).
But in shrews, shrinking bones appear to have no consequences. Perhaps, they don’t reduce in size at all and there’s another explanation for this phenomenon.
In those earlier postmortem studies, “each individual could be examined only once,” Jan Taylor of the University of Białystok, Poland, told The Wire. “Such cross-sectional studies have a serious disadvantage.” They cannot rule out that hefty individuals die in autumn and researchers catch the runts that survived into winter.
To get to the bottom of this strange effect of winter on shrews, researchers would have to track the same animals in different seasons. This is what researchers from the Max Planck Institute of Ornithology and University of Konstanz, Germany, and Columbia University, US, set out to do by weighing and measuring 12 wild shrews every month.
“Although the shrews are everywhere, you rarely ever see them, unless your cat catches one,” said Moritz Hertel, a behavioural neurologist at the Max Planck Institute of Ornithology and one of the researchers.
Using wooden traps, Javier Lázaro, a doctoral student, caught juvenile shrews in the first summer of their lives. These animals cannot survive longer than two hours without food. So Lázaro had to check on the traps all the time.
The researchers knew from the shrews’ looks whether they were juveniles or adults. “Animals in their first summer are easily distinguished from second-year animals since they have shiny fur and straight tails,” Hertel told The Wire. “These animals are very territorial – males as well as females fight a lot. Older animals in their second summers are easy to recognise by their scars and general body condition.”
Lázaro anaesthetised the animals to take precise X-ray measurements of their skeleton and brain case. Before releasing the animals, he fitted them with microchips for identification. Their high metabolism made even the brief period of anaesthesia challenging, says Hertel. “Last but not least, every animal was released back into the wild and we had to recapture them in competition with all the owls, foxes and cats out there to get the next data point.”
Taylor commends the novel techniques used in the study. “I would expect that X-rays might be harmful to these small high-metabolic animals. Apparently, the authors applied the correct dose of radiation. All my trials to anaesthetise shrews failed, presumably because I used the wrong drugs or dosage.” Taylor wasn’t involved in this study by Lázaro and Hertel.
Hertel and his colleagues found that in autumn, the average body mass dropped 17% and shot up 83% in spring, while the average loss in brain case size was 15% that regrew by 9%.
“Reducing head size – and thus brain size – might save energy disproportionally as the brain is energetically so expensive,” Lázaro said in a press release.
Indeed, a study of Djungarian hamsters (Phodopus sungorus) estimated that by reducing its mass in winter and growing thicker fur, it saved energy by 37%.
A common shrew in a trap. Credit: Javier Lázaro
One study catalogued seasonal changes that the brain undergoes – the volume of neocortex decreases accompanied by an increase in the olfactory bulbs and cerebellum during winter and an increase in the hippocampus during the summer breeding season. Another study published earlier this year showed the shape of the skull also undergoes a dramatic change. Juvenile shrews with newfound independence may need large brains to set up territories and find shelter. Come winter, they neither move much nor confront rivals and court mates, and this is the time when their brains are the smallest.
The researchers don’t understand how the skull shrinks, but evidence suggests that as the brain tissue gets reabsorbed, the bone encasing it also reduces.
“We thus document the greatest postnatal reversible individual change in mammalian skull size along with mass known to date,” write the researchers.
“That something as rigid as the bone of mammals is able to undergo such profound changes is amazing,” says Hertel.
Why do shrews grow 70% in their first summers only to lose about 20% mass in winters and then regrow 80% in the following summers? Why don’t they grow to their winter size as juveniles and then reach full adult size, following an incremental growth plan like average mammals?
“Bear in mind that evolution does not follow logic,” says Hertel. A life form undergoes random changes to survive in its environment. There’s some speculation that bones could function as a kind of calcium reservoir to tide over a poor calcium winter diet. But shrews shrink and their bone density reduces when they still have plenty to eat.
“We know little about the costs of shrinking the bones, brains and other organs and later regrow them,” says Hertel. “The animals cope very well with their situation in winter, but maybe reproduction, the search for a mating partner and the struggle for a suitable territory to raise a litter, need very different body configuration.”
Does the reduction in brain size imply cognitive decline?
“Nothing is known yet about the effect of the brain size on the cognitive functions in shrews,” says Taylor. “What is the mechanism of the brain shrinkage? Proliferation and death of neurons in the shrew brain cannot explain the phenomenon.”
The researchers suggest the answers to the shrew mystery could one day help humans suffering from degenerative bone diseases.
The study was published in the journal Current Biology on October 23, 2017.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.
