A Rare Phenomenon of Reversible Brain Shrinkage

Researchers find another brain-shrinking mammal.

European moles face an existential crisis in the depths of winter. Their high-limit mammal metabolisms need more food than is available during the coldest months. Instead of migrating or hibernating to deal with the seasonal challenge, moles have devised an unexpected energy-saving strategy: shrinking their brains.

In a recent study, a group from the Max Planck Institute of Animal Behavior headed by Dina Dechmann found that European moles shrink their brains by 11% before the winter and grow them back 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.

European moles are the latest species of mammal known to reversibly shrink their brains before winter. Credit: Javier Lázaro

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The research, however, does more than just add another 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 various regions, they discover that Dehnel’s phenomenon is caused by cold conditions rather than a lack of food alone. Reducing brain tissue helps the animals to 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 the winter and larger in the summer. Dechmann and colleagues reported the first evidence that these atypical changes in shrew skulls happened throughout the course of an individual’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 knife edge.

Skulls of European moles shrink before winter and regrow in spring in a process known as Dehnel’s phenomenon. Credit: Lara Keicher/ Max Planck Institute of Animal Behavior

“They have extremely high metabolisms and year-round activity in cold climates,” says Dechmann. “Their tiny bodies are like turbocharged Porsche engines that burn through energy stores in a matter of hours.”

To the scientists, it was clear that shrinking energetically costly tissue, such as the brain, allows the animals to reduce their energy needs. “We understood that Dehnel’s phenomenon helps these animals survive when times are tough. But we still didn’t understand what were the real pressure points, the exact environmental triggers, driving this process.”

Now, the team has answered this by studying a new mammal on the metabolic extreme. Measuring skulls in museum collections, the researchers documented how two species of mole – the European mole and the Spanish mole – changed across seasons. They found that the skulls of the European mole shrank by eleven percent in November and regrew by four percent in spring, but those of the Spanish mole didn’t change throughout the year.

Because the species live in vastly different climates, the researchers could pinpoint that weather, not food availability, was responsible for brain change. “If it was just a question of food, then we should see European moles shrinking in winter when food was scarce and Spanish moles shrinking in summer when harsh heat made food scarce,” says Dechmann.

The study findings go beyond answering questions of evolution, offering insights into how our bodies can regenerate after sustaining 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 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