A study of moles reveals that cold weather — not lack of food — drives the rare phenomenon of reversible brain shrinkage in mammals — ScienceDaily

A study of moles shows that cold weather — not starvation — drives the rare phenomenon of reversible brain shrinkage in mammals.

In the dead of winter, European moles face an existential problem. Their metabolism—near the upper limit of any mammal—requires more food than is available during the coldest months. Instead of solving this seasonal dilemma by migrating or hibernating, moles have adopted an unusual energy-saving tactic: shrinking their brains. In a new study, a team led by Dina Dechmann from the Max Planck Institute of Animal Behavior reports that European moles reduce their brains by eleven percent in time for winter and regrow them by four percent by summer. They represent a new group of mammals known to reversibly shrink their brains through a process known as the Dehnel phenomenon. But the study adds more than just another species to the strange canon of mammals whose brains are shrinking — it gets to the bottom of the evolutionary puzzle that drives them down this treacherous path. Comparing moles from different climates, the researchers found that the Dehnel phenomenon is caused by cold weather rather than food shortages. The breakdown of brain tissue allows the animals to reduce energy expenditure and thus survive the cold.

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Identified in the 1950s, the Dehnel phenomenon was first described in the skulls of shrews, which were smaller in winter and larger in summer. In 2018, Dechmann and colleagues provided the first evidence that these unusual changes in the shrew skull occurred over the course of an individual’s lifetime. In the meantime, Dechmann and colleagues have shown that the Dehnel phenomenon also occurs in stoats and weasels. What these mammals have in common is a lifestyle that puts them on the cutting edge.

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

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It was clear to the scientists that shrinking energetically costly tissues like the brain allows the animals to reduce their energy needs. “We understood that the Dehnel phenomenon helps these animals survive in difficult times. But we still didn’t understand what the real pressure points were, the exact environmental triggers driving this process.”

Now the team has answered that question by studying a new mammal at the metabolic extreme. By measuring skulls in museum collections, the researchers documented how two mole species – 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 back by 4 percent in the spring, while those of the Spanish mole did not change year-round.

Since the species live in very different climate zones, the researchers were able to determine that the weather and not the availability of food was responsible for the brain change. “If it were all about food, then we would have to see how European moles shrink in winter when food is scarce, and Spanish moles shrink in summer when the intense heat makes food scarce,” says Dechmann.

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The study results go beyond answering evolutionary questions and offer insights into how our bodies can regenerate after significant damage. “The fact that three distantly related groups of mammals shrink and then grow back bone and brain tissue has enormous implications for research into diseases such as Alzheimer’s and osteoporosis,” says Dechmann. “The more mammals we discover with Dehnel, the more relevant the biological findings become for other mammals and maybe even for us.”

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Materials provided by Max Planck Society. Note: Content can be edited for style and length.

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