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Feral Mink Brains Suggest That the Effects of Domestication Can Be Reversed

A black mink in the snow.
Native to North America, the American mink has become feral throughout Europe. Credit: Karol Zub.

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In a highly unexpected defiance of evolutionary biology, feral populations of the American mink have been shown to reverse key changes to their brain size that occur during domestication. The study reaffirms the amazingly plastic nature of the animal brain, even in the face of many generations of selective breeding.

The research was published in Royal Society Open Science.

Domesticated brain decreases

Domesticated animals are selectively bred for traits that can produce rapid changes to their bodies. Chickens are bulked into hulking meatballs and domesticated sheep become wrapped in excessive wool coats that require regular shearing. One trait that is seen across domesticated animals, despite not being outwardly selected for, is a reduction in brain size.

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This so-called domestication effect was thought to be an irreversible process; previous analysis of feral animals – such as pigs found on the Galapagos Islands – had shown that, even after many generations, smaller brain size was retained. The overarching theory was that “once animals lose parts of their body, such as certain brain regions, over the course of evolution, they are gone and cannot simply be regained,” says Dina Dechmann, senior author on the paper, and a group leader at the Max Planck Institute.

Regaining the brain

Now, a well-crafted study involving mink suggests that, at least in this species, lost brain mass can be restored within 50 generations of leaving captivity. “Our results show that loss of brain size is not permanent in domesticated animals,” says Ann-Kathrin Pohle, first author of the paper and a Master’s student at the Max Planck Institute of Animal Behavior. “This finding deepens our understanding of how domestication has changed the brains of animals, and how these changes might be affecting animals when they return to the wild.”

One of the most challenging aspects of the study was its design. To prove that feral populations differed from captive populations and that the cause of that difference wasn’t due to feral animals breeding with wild ones, the team had to identify a sample of animals where feral and wild animals were isolated.

Luckily, the American mink has this exact distribution. Native to North America, mink have been domesticated and are farmed in Europe for their fur. The animals that have escaped and formed feral populations there are separated from any wild mink by roughly 3,000 miles of ocean. With access to these separate groups—wild mink from North America, domesticated mink from European fur farms, and feral mink from Europe—the team was perfectly equipped to scrutinize the changes in brain size.

To measure differences in brain size without the need for living animals, the scientists adopted a clever strategy, using skull size as an indirect measure for brain size. Using skull collections from Cornell University and European fur farms, alongside a collection of feral mink skulls courtesy of Andrzej Zalewski from the Polish Mammal Research Institute, the team had all the pieces they needed to put the puzzle together. “Usually, the difficulty with skull studies is finding big enough collections to work with,” says Dechmann. “We were incredibly fortunate to work with multiple organizations to obtain the population samples we needed.”

The skull measurements suggested that the relative brain size of the domesticated mink had dramatically dropped by 25%. In contrast to what the team expected, the feral minks’ brains had regrown to almost the same size as wild mink.

Dehnel’s phenomenon

This unlikely finding, says Dechmann, could be explained by a rare aspect of mink biology. The American mink, along with shrews, moles and weasels, belong to a group of small mammals capable of seasonally altering their brain sizes—a process known as Dehnel’s phenomenon. “While other domesticated animals seem to lose brain size permanently, it’s possible that mink can regain their ancestral brain sizes because they have flexible brain size built into their system,” she says.

This cranial flexibility could offer significant advantages to rewilded mink. “If you escape from captivity back to nature, you would want a fully capable brain to navigate the challenges of living in the wild. Animals with flexible brains, like the mink, could restore their brains even if they had shrunk it during an earlier time,” says Dechmann. The team hopes to get their hands on actual brains in a future study, which would enable them to see firsthand how changes in these elastic structures affect how mink brains function.

Reference: Pohle AK, Zalewski A, Muturi M, et al. Domestication effect of reduced brain size is reverted when mink become feral. R. Soc. Open Sci. 2023;10(7):230463. doi:10.1098/rsos.230463.

This article is a rework of a press release issued by the Max-Planck Society. Material has been edited for length and content.