Summary: Foxes bred to be either tame or aggressive had increased size in similar brain regions, a new study reports. The animals bred for specific behavioral traits had larger brains than those that were conventionally bred. The findings contradict existing theories of animal domestication.
Farmed foxes selectively bred for tameness and aggressiveness exhibit similar changes to their brain anatomy, according to research recently published in Journal of Neuroscience.
Both lineages also have larger brains than conventional farm-bred foxes, complicating leading theories on domestication.
Domesticated species provide insight into complex evolutionary processes on a condensed timeframe. When a species splits from its wild counterpart, its brain, body, and behavior undergo rapid changes.
Studies with chickens, sheep, cats, dogs, and more indicate domestication shrinks the brain. But the same pattern does not extend to foxes in the expected way.
Hecht et al. used MRI to measure the brain size and structure of foxes from the classic Russian fox-farm experiment.
The research team compared foxes selectively bred for tameness or aggression toward humans with conventional farm-bred wild foxes (the control).
Contrary to previous studies, both tame and aggressive strains had larger brains than the conventional, “wild” foxes. The tame and aggressive foxes also displayed similar changes to regions in the prefrontal cortex, despite being bred for opposite behaviors.
These results illuminate a need to rethink how brain circuits relate to behavior evolution as well as how domestication shapes the brain.
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Neuromorphological changes following selection for tameness and aggression in the Russian fox-farm experiment
The Russian fox-farm experiment is an unusually long-running and well-controlled study designed to replicate wolf-to-dog domestication. As such, it offers an unprecedented window onto the neural mechanisms governing the evolution of behavior.
Here we report evolved changes to gray matter morphology resulting from selection for tameness vs. aggressive responses toward humans in a sample of 30 male fox brains. Contrasting with standing ideas on the effects of domestication on brain size, tame foxes did not show reduced brain volume.
Rather, gray matter volume in both the tame and aggressive strains was increased relative to conventional farm foxes bred without deliberate selection on behavior. Furthermore, tame- and aggressive-enlarged regions overlapped substantially, including portions of motor, somatosensory, and prefrontal cortex, amygdala, hippocampus, and cerebellum.
We also observed differential morphological covariation across distributed gray matter networks. In one prefrontal-cerebellum network, this covariation differentiated the three populations along the tame-aggressive behavioral axis. Surprisingly, a prefrontal-hypothalamic network differentiated the tame and aggressive foxes together from the conventional strain.
These findings indicate that selection for opposite behaviors can influence brain morphology in a similar way.
Domestication represents one of the largest and most rapid evolutionary shifts of life on earth. However, its neural correlates are largely unknown.
Here we report the neuroanatomical consequences of selective breeding for tameness or aggression in the seminal Russian fox-farm experiment.
Compared to a population of conventional farm-bred control foxes, tame foxes show neuroanatomical changes in the prefrontal cortex and hypothalamus, paralleling wolf-to-dog shifts. Surprisingly, though, aggressive foxes also show similar changes. Moreover, both strains show increased gray matter volume relative to controls.
These results indicate that similar brain adaptations can result from selection for opposite behavior, that existing ideas of brain changes in domestication may need revision, and that significant neuroanatomical change can evolve very quickly – within the span of less than a hundred generations.