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A representation of an aurochs’s brain set inside a CT scan of its skull

Ana Balcarcel

The average brain size of cattle has shrunk as we have domesticated them, and their brains seem to get smaller the more time they spend around humans.

In general, domesticated animals tend to have smaller brains than their wild counterparts. This phenomenon had not been tested in cattle, as their wild ancestor has been extinct for several hundred years.

To estimate the brain size of wild cattle, Ana Balcarcel and her colleagues at the University of Zurich in Switzerland measured 13 skulls from the extinct aurochs (Bos primigenius), the ancestor of modern cattle. They then compared these to known measurements of 317 skulls from 71 domestic cattle breeds (Bos taurus).

Based on the size differences in the skulls, they team estimates that the brains of domestic cattle are about 26 per cent smaller on average than those of wild cattle.

“Really, the surprising result was that when we compared not just the wild versus domestic but all the different breeds within the domestic populations, we found differences there,” says Balcarcel. “Those differences correlate strongly with the amount of time these animals spend with humans. The intensity of the human contact really has an effect on how much the brain reduces.”

The team found that cattle that are around humans more often, such as dairy cows, have the smallest brains, while bullfighting cattle that only have limited contact with humans had larger brains, closest to the size of their wild ancestors.

Breeding cattle to be more docile seems to be driving these changes in brain size, says Balcarcel.

Dominic Wright at Linköping University in Sweden says this may be an overly simple explanation. “There does seem to be a shift in relative brain size from the wild to the domestic,” he says.

“The problem is, we don’t know if modern cattle definitely came from these aurochs, which [this team] happens to be using in their study, but it seems like there’s a good possibility that they are representative of their wild progenitors,” says Wright.

Journal reference: Proceedings of the Royal Society B: Biological Sciences, DOI: 10.1098/rspb.2021.0813

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