News Release

Domesticated foxes display increased size in brain regions

Study on the classic Russian farm-fox experiment raises questions about leading theories on the brains of domesticated animals

Peer-Reviewed Publication

Harvard University

Silver Foxes

image: Fox bred for dog-like behavior towards humans. view more 

Credit: Jennifer Johnson, Darya Shepeleva, and Anna Kukekova

When Erin Hecht was earning her Ph.D. in neuroscience more than a decade ago, she watched a nature special on the Russian farm-fox experiment, one of the best-known studies on animal domestication.

The study, running since 1958, tries to replicate the natural domestication of wolves to dogs by selectively breeding two strains of silver foxes so they exhibit certain behaviors. Scientists breed one to be tame and display dog-like behaviors with people, such as licking and tail wagging. The other is bred to react with defensive aggression when faced with human contact, while a third strain acts as the control and isn't bred for any specific behaviors.

Hecht, who's now an assistant professor in the Harvard Department of Human Evolutionary Biology, was fascinated by the experiment, which has helped scientists closely analyze the effects of domestication on genetics and behavior. But, she also thought something was missing.

"In that TV show, there was nothing about the brain," Hecht said. "I thought it was kind of crazy that there's this perfect opportunity to be studying how changes in brain anatomy are related to changes in the genome and changes in behavior, but nobody was really doing it yet."

Hecht acted fast and sent an email to Lyudmila N. Trut, the scientist running the Siberian institute where the Russian foxes were being studied. Fast forward to today and that email was foundational for a new study reporting on the surprising brain changes that occur in the Russian fox-farm experiment. Published Monday in the journal JNeurosci, the paper raises questions about some of the leading theories on domesticated animals' brains.

By analyzing MRI scans of the foxes, Hecht and her colleagues showed that both the foxes bred to be tame and those bred for aggression have larger brains and more grey matter than the brain of the control group (the foxes not bred for any particular behavior). These findings run in contrast to studies on chickens, sheep, cats, dogs, horses, and other animals that have shown domesticated species have smaller brains with less grey matter, than their wild forebears.

The team of researchers from Harvard, University of Illinois at Urbana-Champaign, Emory University, Cornell University, and the Russian Institute of Cytology and Genetics, explain the increase in size and grey matter, but can't yet be sure why this happens without further study. Their leading hypothesis centers on how the tame and aggressive strain have both been bred for specific behaviors at an accelerated time frame than many other domesticated animals. Dogs, for example, have been domesticated for at least 15,000 years.

"Both the tame and aggressive strains have been subject to intense, sustained selection on behavior, while the conventional strain undergoes no such intentional selection," they wrote. "Thus, it is possible that fast evolution of behavior, at least initially, may generally proceed via increases in grey matter."

As they analyzed the MRI scans, the research team noticed another surprise: similarities in the ways that the brains of the aggressive and tame foxes were changing. Both, for instance, showed enlargement in many of the same grey matter regions, including the prefrontal cortex, the amygdala, the hippocampus, and the cerebellum. This was despite the foxes being bred for opposite behaviors.

Results suggest that selection for opposite behavioral responses, in this case tame-versus-aggressive behavior, can produce similar changes in brain anatomy. The findings also suggest that significant changes to the structure and organization of the nervous system can evolve very quickly. In fact, it can happen within the span of less than a hundred generations.

Taken together, the researchers say the study's findings suggest existing ideas of brain changes in domestication may need revising, and that the brains of other animals, including humans, may have gone through similarly abrupt morphological shifts during times a sudden selection on behavior (rapid changes in environment or climate) occurred.

Next steps in the research include observing the foxes' brains scans at a cellular level.

The researchers believe there's a lot left to be learned from the Russian farm foxes and domesticated species, in general. That's because when a species splits from its wild counterpart, its brain, body, and behavior undergo rapid changes. Studying the foxes and other domesticated animals provides a window into these complex evolutionary processes.

"It's a more simple and straightforward way to see how evolution changes brains than we can achieve with just studying naturally occurring evolved brain changes," Hecht said.


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