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Our understanding of how domestication changes the neurobiology of a species may be wrong, results from a 60-year experiment to breed tame foxes suggest. The findings could also have implications for human evolution, claim researchers.

Usually, domesticated animals have smaller brains than their wild counterparts, but foxes raised in a Russian fox farm experiment in Novosibirsk haven鈥檛 followed that trend. On the contrary, fox lines purposefully bred for either a good or a bad relationship with humans had larger brains than those that weren鈥檛, says Erin Hecht at Harvard University, who is part of a team studying specimens from the experiment.

The unexpected findings 鈥渟uggest revision of existing thinking about domestication鈥�, perhaps towards an idea that brains respond to behaviour-related pressure, at least initially, by developing more grey matter, says Hecht.

Starting in 1959, project scientists began selectively breeding silver foxes (Vulpes vulpes) in a fur farm. With each generation, they chose the tamest animals to reproduce together, while also choosing the most aggressive animals to reproduce together. The researchers left a third group of foxes to mate without selection for behavioural aspects, as a control.

The idea, researchers said at the time, was to create a model of the animal domestication process. The project has led to dozens of published papers but has also seen criticism for developing captive animals that are fearful of humans.

Now, an international team of researchers has examined the brains of 30 males from recent generations of those foxes. The group took the preserved left hemispheres of the brains of 18-month-old foxes and examined them under high-resolution magnetic resonance imaging (MRI).

Surprisingly, the third group 鈥� the control foxes 鈥� actually had the smallest brains, says Hecht. Equally surprising, the 鈥渢ame鈥� fox brains were, on average, nearly identical in size and structure to the 鈥渁ggressive鈥� fox brains. In particular, the team noted that both tame and aggressive foxes had similar kinds of changes in the same parts of the brain that are apparently associated with tameness in dogs compared with wolves, including the prefrontal cortex, hippocampus and amygdala.

The study provides an unprecedented look into individual brain structures, says Ana Balcarcel at the University of Zurich in Switzerland. 鈥淭hey鈥檙e literally going deeper into the brain, and that鈥檚 such a novel thing,鈥� she says. 鈥淚t鈥檚 raising more questions than answers at this point, but that鈥檚 very exciting.鈥�

For Martine Hausberger at the University of Rennes, France, however, the conclusions are less compelling. The fox farm experiment can鈥檛 be considered a reliable model for studying domestication, in part because its 鈥渨ild鈥� animals are captive, rather than hunting and fending for themselves, she says.

Furthermore, the foxes鈥� brains may have been shaped according to how they were treated, with the tame ones certainly having more enriching interactions with handlers and the aggressive ones possibly being more mentally stimulated when handlers approached their cages. 鈥淏rain sizes evolve in each individual according to personal experience,鈥� says Hausberger. 鈥淪o are these brain differences genetic, and related to behaviour alone? I鈥檓 not convinced. It seems like a bit of a shortcut to say that they are.鈥�

If the findings do stand up, it could change the way we look at human brain evolution, says Hecht. Some anthropologists believe humans 鈥渟别濒蹿-诲辞尘别蝉迟颈肠补迟别诲鈥� by refusing to tolerate aggressive behaviour from group members. This, along with other factors, like rapid changes in the climate and the environment, might have spurred rapid changes in the brains of our own species.聽鈥淭hese are good candidates for situations that might have placed strong selection pressure on behaviour and cognition, thus driving up brain size,鈥� she says.

Balcarcel says the fossil evidence on human brain sizes is too spotty to draw these kinds of conclusions, however. 鈥淐ertainty in that area is too weak at the moment, I think,鈥� she says.

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