A Brain Shrinking Domestication Event

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Domesticated cattle (cows or steers) evolved, under domestication, into a variety of different lines (or strains), from aurochs (Bos primigenius,, now extinct) about 10,000 years ago.
Science mag news article here.
Ana Balcarcel of the University of Zurich and colleagues scanned the skulls of museum specimens, corrected for body size, and determined their relative sizes.
Domesticated strains had about 25% less brain volume on average.
She sorted the extinct and living breeds into five categories based on their primary purpose as livestock: wild, bullfighting, park (referring to cattle that live essentially as pets on rangeland), beef, and dairy. Next, she plotted the breeds’ brain sizes and looked for patterns.


She found that bullfighting breeds, which are bred for aggression and tend to have little human interaction outside fighting in the ring, have brain sizes nearly as large as those of wild aurochs. Park cattle, which have relatively little human contact, also have relatively large brains. But beef cattle have far smaller brains, and dairy cattle—which frequently interact with farmers and are bred for their milk yield and gentleness—have the smallest brains of all.
It is thought that the animals were selected for docility in order for humans to be able to work with them.
Balcarcel suspects that when breeders select for more docile animals in beef and dairy breeds, they are selecting for genes that shrink the parts of the brain that control fear, anxiety, and aggression.
The closer interaction with humans, the greater the difference for the wild aurochs.

Other vertebrate animals (pigs, dogs, sheep, cats) have similar patterns.
Some of the cattle strains have only been around for ~200 years, so the changes may have happened rapidly.
However, that kind of conclusion would depend upon what those newer strains started from, already selected to be smaller or not.
 
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