A rare phenomenon of reversible brain shrinkage

Researchers find another brain-shrinking mammal.

European moles face an existential crisis in the middle of winter. Their maximum metabolism in mammals requires more food than is available during the coldest months. Rather than migrate or hibernate to meet the seasonal challenge, moles have come up with an unexpected energy-saving strategy: shrinking their brains.

In a recent study, a group from the Max Planck Institute for Animal Behavior led by Dina Dechmann found that European moles shrink their brains by 11% before winter and grow by 4% by summer. They are a new group of mammals known for reversibly shrinking their brains through a process known as Dehnel’s phenomenon.

However, the research does more than just add a new species to the bizarre repertoire of brain-shrinking animals; it delves into the evolutionary puzzle of what pushes them down this perilous path. When the researchers compare moles from different regions, they find that Dehnel’s phenomenon is caused by cold conditions and not just a lack of food. Reducing brain tissue helps the animals use less energy and thus withstand the cold.

Dehnel’s phenomenon was first described in the skulls of shrews, which were found to be smaller in winter and larger in summer. Dechmann and colleagues reported the first evidence that these atypical changes in shrew skulls occurred over the course of a person’s life in 2018. Dechmann and colleagues have since shown that Dehnel’s phenomenon occurs in stoats and weasels. What these mammals have in common is a lifestyle that puts them on an energetic cutting edge.

“They have an extremely high metabolism and are active year-round in cold climates,” says Dechmann. “Their tiny bodies are like turbocharged Porsche engines that go through energy storage within hours.”

It was clear to the scientists that shrinking energetically precious tissue, such as the brain, allows the animals to reduce their energy requirements. “We understood that Dehnel’s phenomenon helps these animals survive in difficult times. But we still didn’t understand what the real pressure points were, the exact environmental factors driving this process.”

Now the team has answered this by studying a new mammal at the metabolic extreme. By measuring skulls in museum collections, the researchers documented how two species of mole — the European mole and the Spanish mole — changed over the seasons. They found that the skulls of the European mole shrank by 11 percent in November and grew by four percent in the spring, but those of the Spanish mole did not change during the year.

Because the species live in vastly different climates, the researchers were able to determine that weather, not food availability, was responsible for brain change. “If it was just about food, we would see European moles shrink in the winter when food was scarce, and Spanish moles in the summer when the harsh heat made food scarce,” Dechmann says.

The research results go beyond answering questions about evolution and provide insight into how our bodies can regenerate after significant damage. “That three distantly related groups of mammals can shrink and then regrow bone and brain tissue has huge implications for research into diseases such as[{” attribute=””>Alzheimer’s and osteoporosis,” says Dechmann. “The more mammals we discover with Dehnel’s, the more relevant the biological insights become to other mammals, and perhaps even to us.”

Reference: “Winter conditions, not resource availability alone, may drive reversible seasonal skull size changes in moles” by Lucie Nováková, Javier Lázaro, Marion Muturi, Christian Dullin and Dina K. N. Dechmann, 7 September 2022, Royal Society Open Science.
DOI: 10.1098/rsos.220652