The American mink grows its own brains in a rare reversal of the domestication process

Farm animals look different from their wild counterparts in many ways, and there is one consistent difference: Their brains are smaller than those of their ancestors. From sheep to pigs to cows, domestic animals have relatively smaller brain sizes compared to their wild counterparts—a phenomenon known as the domestication effect.

Now, a study by the Max Planck Institute for Animal Behavior (MPI-AB) has discovered a rare reversal of the domestication effect. Over the course of breeding in captivity, the American mink experienced a decrease in relative brain size, but the populations that escaped from captivity were able to almost completely recover their ancestral brain size within 50 generations. The study was published today in Royal Society for Open Science.

“Our results show that brain volume loss is not permanent in pets,” says Ann-Kathryn Buhl, MPI-AB master’s student and first author on the research paper. “This discovery deepens our understanding of how domestication changes the brains of animals, and how these changes may affect the animals when they return to the wild.”

Understanding the lateral brain

When animals lose their brain size through domestication, it is often considered a one-way street. Animals never seem to recover the brain sizes relative to their ancestral forms, even in feral populations that have been living in the wild for generations. “Once animals lose parts of their bodies, such as certain regions of the brain, over the course of development, they are gone and simply cannot be regained,” says Dina Deichmann, senior author on the paper and group lead at MPI-AB.

Examining whether wild animals can recover the relative brain sizes of their wild counterparts is also challenging methodologically. To do it right, Dishman says, “You’d need to find an animal with separate wild and feral populations to reduce the chance of the populations mixing. And you’d need to find an animal that could be studied with adequate brain and skull measurements.” In other words, you will need an animal such as the American mink.

Native to North America, the American mink has been domesticated for the fur trade for over a century. After being bred in Europe to grow fur, captive animals escaped to form feral populations that spread across Europe. And so this natural history presented the separate populations that Dishman and her team needed: wild mink from North America, domesticated mink from European fur farms, and feral mink from Europe.

To explore changes in brain size, the team turned to a proxy: skulls. “Braincase size is a good proxy for brain size in mink, and this allows us to take measurements from existing skull ensembles without the need for live animals,” says Buhl. A museum collection from Cornell University was used to study the skulls of wild American mink while European fur farms provided skulls of domestic animals.

For the feral audience, Dechmann and Pohl collaborated with Andrzej Zalewski at the Polish Mammal Research Center who had a collection of skulls obtained from a feral mink eradication program. “Usually the difficulty with skull studies is finding groups that are large enough to work with,” Diekmann says. “We were incredibly fortunate to work with multiple organizations to get the population samples we needed.”

The team took measurements from the skulls to calculate the animals’ relative brain size. They found that, according to the well-documented process of domestication, the brains of bred mink shrank by 25% compared to their wild ancestors. But, contrary to expectations, the brains of feral minks have grown almost to wild size in 50 generations.

Flexible minds

Dishman suspects she knows why this particular animal has achieved what was thought unlikely. The American mink belongs to a family of small mammals with a remarkable ability to change the size of its brain seasonally in a process known as the Dehnell phenomenon. Dishman, an expert on the process, has documented Dehnell in shrews, moles, and weasels.

“While other pets seem to lose their brain volume permanently, it is possible for mink to regain the brain volumes of their ancestors because they have a flexible brain volume built into their system,” she says.

This flexibility would have provided advantages for re-wild minks. “If you escape from captivity into nature, you will need a brain that is fully capable of meeting the challenges of living in the wild. Animals with flexible brains, such as mink, can restore their brains even if they had shrunk earlier.”

The results do not reveal whether the brains of feral minks function in the same way as wild minks. To find out, the team will have to examine the animals’ brains, a step for a future study.