This is perhaps most obvious in animals that are naturally large to begin with. But, in 1938, German zoologist Georg Stein studied how winter affected the body weight of the smallest of mammals: shrews. He confirmed that they do, indeed, lose weight through the winter, despite already being unusually small. He was, in fact, impressed that they managed to survive at all, given how much food they normally require just to stay alive, their size requiring them to burn calories at a furious rate just to maintain their body temperature.
That aside, the fact that shrews lose weight when they don't get enough food probably doesn't come as a great surprise. Nonetheless, just over ten years later, in 1949, Polish zoologist August Dehnel took a closer look at the phenomenon... and found that that isn't what's happening.
There were two reasons why he found the obvious explanation (which, along with everyone else, was what Stein had assumed to be the case) couldn't be the whole story. Firstly, in his study of red-toothed shrews living in the Białowieża Forest, he showed that the amount by which the shrews lose body weight has nothing to do with how bad the winter is - they lose just as much in mild winters as harsh ones. Not only that, but they start bulking up again a few weeks before food started becoming plentiful again.
This suggests that they aren't losing and gaining weight because of the amount of food they are eating. In fact, it's the other way around. They become smaller in winter so that they don't need as much food, and grow back again in spring in anticipation of the bounty to come.
The key part of this, and the reason that Dehnel is now remembered, is "become smaller". The shrews are not just losing weight, in the way that other animals might. They are physically shrinking, in a process now called Dehnel's Phenomenon. Dehnel himself demonstrated this by measuring their skulls, showing that shrews' heads literally shrink as the winter approaches, with both the bone and the brain inside it becoming smaller.
So what exactly is going on here, and are there other species doing the same thing? A review earlier this year put the evidence together, and is the basis of this post.
He was looking at the common shrew (Sorex araneus) but, in 1956, another Polish scientist, Krystyna Caboń, showed that the same was true of the pygmy shrew (Sorex minutus). Since then, it has been extended to 14 other shrew species. The shrews in question are mostly red-toothed or "soricine" shrews, a widespread group distinguished by the presence of iron salts in their teeth that help to reinforce their enamel.
A significant feature of these shrews is that their typical lifespan is only about 12 months. They are born in the late spring, leaving home a month later. In July or August, even though not yet fully grown, they start to shrink, apparently preparing for the winter ahead, a process that continues through the end of the year. At some point between January and March (depending on species), they start to grow again, reaching their full adult weight in time for the breeding season. In the unlikely event they survive long enough, they shrink again in the autumn.
While best studied in red-toothed shrews in temperate Eurasia and North America, the phenomenon is not restricted to them. According to a Ukrainian study, the skull of the lesser white-toothed shrew (Crocidura suaveolens) shrinks by 10% through the winter. It is also seen in moles, which are related to shrews, but may not have shared a common ancestor since the time of the dinosaurs. More significantly, the same pattern of skull-shrinking through the winter has been reported from weasels and stoats, which are much more distantly related to shrews but which, significantly, also have a small body mass and high-speed metabolism.
One study showed that, as the winter approaches, while common shrews lose 20% of their body mass, this is unevenly distributed, and certainly is not just a loss of body fat. The spine becomes 10% shorter, the brain and kidneys shrink by 20%, and the length of the intestines by 15%. The heart, however, doesn't change at all (thus becoming larger relative to body size), while the spleen shrinks by a whopping 77%.
The phenomenon is, however, best recorded for changes in the size of the skull, which is where Dehnel first identified it. The skull becomes not just smaller, but also flatter, as specialised cells dissolve the bones of the skull roof along the edges where they join up, causing the cranium to slowly collapse. The bone then rebuilds along the same lines in the spring. The shortening of the spine, however, is slightly different, with the vertebrae not changing, but the discs between them shrinking as they lose water.
As the skull shrinks, so does the brain inside it, and how this works, and why it doesn't produce problems for the animal, is one of the more puzzling parts of the phenomenon. One thing to note, however, is that the brain typically shrinks by about the same amount as the body, so proportionally speaking, there is no real change. Nonetheless, just as some organs shrink more than others, it's also the case that some parts of the brain are more affected than others.
For example, the cerebellum, responsible for motor control and coordination, is relatively unaffected, while the neocortex, responsible for perception and other higher-order functions, shrinks the most. The hippocampus, which is primarily responsible for storing and processing memories, is somewhere in between, but that it shrinks at all is surprising, given that you'd think that would be useful in a hostile environment.
But it may be that the smaller size of these brain areas in the winter is misleading. A study in 2008 failed to find any evidence of increased cell death in shrew brains as the winter approached. Instead, what seems to be happening is that the brain loses water and shrinks for that reason, but the number of cells, and the connections between them, are largely unchanged and may even increase slightly.
The purpose of all of this is likely to reduce the need for food intake during the months when the least is available. Indeed, the metabolic rate of shrews seems to drop as the winter approaches, which would also help support what is otherwise an energy-intensive lifestyle in harsh times. However, since the process begins before the winter sets in, it can't be a response to an increasing lack of food.
The exact genetic mechanisms behind the process remain unclear, although one gene that helps cells survive during stress is more active in winter. Today, we can also use techniques such as high-resolution MRI to follow individual shrews kept in captivity to see how they respond to various conditions. From this, we have been able to discover that if you keep a shrew under conditions of constant temperature, but natural lighting, their brains fail to regrow in the spring. While this doesn't explain the start of the shrinking process, which may be due to a range of interacting factors, it does show that changing temperature, rather than day length, is a key factor for the restoration afterwards.
Whether the shrinking brain does have an effect on shrew behaviour, or whether retaining all the cells compensates for that, is another unknown. Shrews are difficult to keep in captivity, and don't respond to tests in the same way mice do, apparently becoming bored and refusing to cooperate even under normal conditions. This leaves a lot unknown, and we know even less about just how similar the process is in those non-shrew species that exhibit the phenomenon, such as moles and weasels. For that matter, it could be sitting there, undetected, in other small temperate species we haven't even looked at yet.
But the fact that it must have evolved at least three times, and probably more indicates that, for some mammals at least, a shrinking brain can be an advantage... at least, if it happens at the right time.
[Photo by Jan Sheppard, from Wikimedia Commons.]
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