Science

A rare phenomenon of reversible brain shrinkage

A rare phenomenon of reversible brain shrinkage

Concept of brain energy signals

European moles shrink their brains by 11% before winter and grow them back by 4% by summer.

Researchers have found another mammal that shrinks its brain.

European moles face an existential crisis in the depths of winter. Their high mammalian metabolic rate requires more food than is available during the coldest months. Instead of migrating or hibernating to cope with the seasonal challenge, moles devised an unexpected energy-saving strategy: shrinking their brains.

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

European Mole

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

The research, however, does more than just add another species to the bizarre repertoire of brain-shrinking animals; delves into the evolutionary puzzle of what drives them down this dangerous path. When researchers compare young from different regions, they find that Dehnel’s phenomenon is caused by cold conditions, not just a lack of food. Shrinking brain tissue helps animals expend less energy and thus withstand the cold.

The Dehnel phenomenon was first described in shrew skulls, 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 during an individual’s lifetime in 2018. Dechmann and colleagues have since shown that the Dehnel phenomenon occurs in shrews and weasels. What these mammals have in common is a lifestyle that puts them on the edge of an energetic knife.

Comparison of mole skulls

The skulls of European moles shrink before winter and grow back in spring in a process known as the Dehnel phenomenon. Credit: Lara Keicher/Max Planck Institute for Animal Behavior

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

It was clear to scientists that reducing energy-dense tissue, such as the brain, allowed animals to reduce their energy needs. “We realized that the Dehnel phenomenon helps these animals survive when times are tough. But we still didn’t understand what the actual pressure points were, the exact environmental triggers, that were driving this process.”

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

Because these species live in vastly different climates, the researchers were able to pinpoint that weather, not food availability, was responsible for the changes in the brain. “If it’s just food, then we should see European moles shrinking in winter when food was scarce, and Spanish moles shrinking in summer when high heat made food scarce,” says Dechmann.

The study’s findings go beyond answering evolutionary questions, offering insight into how our bodies can regenerate after suffering significant damage. “That three distant 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





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