By Dr. Karen Shaw Becker
Animals have many intriguing ways of surviving harsh winters. Hibernation is one survival trick, used by animals such as bears, who fatten themselves up during the summer months then fall into a state of near suspended animation over the winter. Not all animals have this luxury, however, including shrews, which are too small to store up enough fat to hibernate. Plus, they have high metabolisms that require near-constant eating in order to survive.
Some animals that don't hibernate get around winter by migrating to warmer, more hospitable locales. Here, too, shrews miss out, as they are unable to migrate. Forced to hunker down in place even as temperatures plummet, they've got a different trick up their sleeves that makes winter survival possible: shrinking skulls.
Shrews' Shrinking Skulls May Help Them Survive Winter
In the 1940s, zoologist August Dehnel noticed that shrews seemed to have different-sized heads at different times of the year.1 The "Dehnel phenomenon," as it came to be named was not scientifically proven, however, until now.
Researchers from the Max Planck Institute for Ornithology in Germany and colleagues captured shrews from the summer of 2014 to the fall of 2015.2 Their body mass was measured and an x-ray taken of their head before they were microchipped and released. Thirty-seven of the animals were later recaptured and measured once again.
The findings revealed that along with a seasonal change in body mass index (rising by more than 83 percent in the spring and falling by nearly 18 in the fall), the shrews' braincases also changed in size seasonally, decreasing about 15 percent during the autumn and increasing by more than 9 percent in the spring. Study co-author Javier Lázaro told The Guardian:3
"Now for sure we can say this is happening [within] individuals — we can really talk about the shrinkage and regrowth … These animals cannot hibernate [and] they cannot migrate and they live in a very seasonal environment — so they need some alternative strategy to deal with winter … If you shrink an organ like the brain which is disproportionally more 'expensive' than other kinds of tissue you might save energy."
It's unknown why the shrews' braincases only partially regrow come spring or how the tissue and bone is reabsorbed by the body. It appears, however, that shrews may reabsorb and regenerate bone at different seasons, which could have implications on research into bone diseases like osteoporosis.4
Interestingly, weasels also appear to display Dehnel's phenomenon, with the braincase shrinking and regrowing seasonally, but only in males. Writing in the journal Scientific Reports, researchers explained this could be due to certain male-oriented demands:5
"[W]hile the shrinking likely facilitates survival during seasonal low resource availability in these high-metabolic mammals with year-round activity, the regrowth may be most strongly influenced by high investment into reproduction and territories, which is male-biased in the weasels."
Pigmy Shrews Can Starve in as Little as Two Hours During Winter
Although pigmy shrews are small, weighing about 5 grams each, they must eat constantly to keep up with their high energy demands, particularly during winter. They feast on insects and spiders (earthworms are typically too large) and must eat up to 125 percent of their body weight daily. The animals are so small, and their metabolic demands so high, that they can starve to death in as little as two hours without food.6
Even larger common shrews, which weigh anywhere from 5 to 14 grams, must eat every two to three hours in order to survive. In addition to their shrinking skull and brain, which helps them conserve energy in winter, shrews become less active during the winter months, and their body mass and liver shrinks as well, which means they need less food to survive. Even so, if they survive one winter it may very well be their last, as shrews have a short lifespan of about one year.
Shrews typically live alone, except while mating or raising young, and are very territorial (though their individual territories are small). Despite their small size, they can be aggressive and may fight with one another, letting out high-pitched squeaks, especially during the summer.7
Although shrews are not endangered, they are protected in the U.K. under the Wildlife and Countryside Act of 1981. This means they may only be trapped with a license and traps must be checked frequently because the animals cannot go long without food. In addition, widespread agricultural activities, including the use of pesticides and loss of natural habitat, are a potential threat to shrews.
The Wildlife Trusts, which care for 2,300 nature reserves in the U.K., is working with farmers and landowners to develop wildlife-friendly practices to protect shews and other animals, and encourages leaving rough areas of grass and shrubbery in which these creatures can hunt and hide.8