Why do shrews lose brain size in winter but grow bigger noses? Scientists uncover a bizarre seasonal adaptation in tiny mammals

A tiny Japanese mammal is showing scientists that surviving winter can require some unusual biological trade-offs. Researchers have found that long-clawed shrews grow wider and taller snouts during the coldest months, even as their brains and the surrounding skull become smaller.

The unexpected seasonal change was discovered in 136 skull specimens of long-clawed shrews collected on Japan's Hokkaido island. The finding adds a new twist to Dehnel's phenomenon, a rare adaptation in which shrews and some other small mammals reduce the size of their brains, skulls and other tissues during winter.

The study was published in Proceedings of the Royal Society B.

Shrews shrink their brains but enlarge their noses

For decades, scientists have known that some shrews undergo dramatic physical changes as winter approaches.

Their bodies and organs can become smaller when food is scarce, reducing the amount of energy required to survive. The brain is particularly expensive to maintain, making seasonal brain reduction a potentially important adaptation.

But the new study shows that the process is not simply a matter of everything getting smaller.

Researchers from Toyo University in Tokyo examined museum specimens of long-clawed shrews (Sorex unguiculatus) collected between 1948 and 1988. Using photographs and specialised software, they measured numerous points on each skull before comparing the dimensions with the season in which the animals had been collected.

The expected reduction in the braincase appeared clearly in the winter specimens.

Then researchers noticed something unusual.

The shrew's snout changes in the opposite direction

While the braincase became smaller, the animals' snouts became noticeably larger.

During winter, the shrews' snouts were approximately 7% taller and 6% wider than those seen in other seasons. Measurements reached about 2.43 millimetres in height and 2.19 millimetres in width.

When spring arrived, the snouts returned to their smaller dimensions.

Researchers describe the unusual pattern as a possible “reverse Dehnel's phenomenon”, because the nasal region expands at the same time that other parts of the skull contract.

The discovery surprised the scientists because previous research had largely framed Dehnel's phenomenon as a seasonal reduction in body size and organ mass.

Why would a shrew need a bigger nose in winter?

The researchers have proposed two possible explanations for the seasonal nose growth.

One possibility involves the way the animals breathe in extremely cold weather.

A larger nasal cavity could provide additional space for blood vessels. As freezing air enters the nose, the expanded vascular network may help warm the air before it reaches the respiratory system and, potentially, the brain.

That could be particularly useful for an animal whose brain has already undergone seasonal shrinkage.

Another possibility is linked to the shrew's remarkable sense of smell.

An expanded nasal structure could increase the surface area of tissue involved in detecting odours. For a small predator searching for food beneath snow, having a more effective sense of smell could be especially valuable when winter makes food harder to locate.

The researchers believe both mechanisms could potentially contribute to the seasonal change.

An unusual example of animal adaptation

Dehnel's phenomenon has been documented in several small mammals, including certain shrews, weasels, voles and moles. The reduction in body and organ size is generally viewed as a way of lowering energy demands during periods when food is difficult to find.

The Hokkaido shrews suggest that the phenomenon is considerably more flexible than previously thought.

Instead of following a simple pattern in which the animal becomes smaller across the board, different parts of its anatomy may respond differently to the demands of winter.

The researchers say the finding offers a broader perspective on skeletal plasticity, showing that seasonal anatomical changes can involve both shrinkage and growth within the same animal.

Scientists want to know what triggers the nose growth

The next question is whether the unusual snout enlargement occurs only in severe winter environments or is common across the long-clawed shrew's wider range.

Jan R. E. Taylor, an evolutionary ecologist at the University of Białystok who was not involved in the research, suggested that studying populations living in milder climates could help determine which environmental conditions drive the change.

Researchers also want to examine what happens inside the nasal cavity as the seasons change.

Future work could involve studying the animals' behaviour and physiology, as well as changes in blood vessels and other nasal tissues. Such research could reveal whether the enlarged snout primarily helps shrews warm freezing air, detect food more effectively, or perform both functions.

For now, the tiny mammals of Hokkaido have offered scientists an unexpected lesson: winter adaptation does not always mean becoming smaller.

Sometimes, even while the brain shrinks, the nose knows better.