What separates Lassie from the Big Bad Wolf or Fantastic Mr Fox? Scientists believe domestication alters the way an animal reacts to stressful situations and now they have identified the biological mechanism that could be responsible for this behavior change. A study recently published in G3: Genes | Genomes | Genetics suggests it comes down to the cells in the anterior pituitary gland, a region of the brain that controls our response to stress.
"Previous studies have found that ACTH [a stress response-driving hormone] levels in the anterior pituitary do not differ between tame and aggressive fox strains," study author Anna Kukekova explained in a statement.
"This means that differential expression of the gene encoding ACTH may not cause the differences seen in blood levels of this hormone, and some other mechanism is reducing ACTH in the bloodstream of tame foxes."
To find out what exactly, the team of researchers analyzed the brains of 12 foxes, six bred to be tame and six bred to be aggressive.
The canines came from the Institute of Cytology and Genetics in Novosibirsk, Russia – a breeding programme set up in the 1950s by a geneticist called Dmitry K. Belyaev, who wanted to replicate the domestication process using foxes. To date, these are the only foxes to have been successfully domesticated.
His selective breeding process was rigorous and by the fourth generation, the cubs were acting more like puppies than wild animals. Not only had their behavior changed (wagging tails, actively seeking out human contact, etc.), their appearance and reproductive habits were noticeably different from those of their more aggressive peers. The tamed foxes matured earlier and were able to breed out of season. Looks-wise, they were "cuter": their legs, tail, snout, and upper jaw were all shorter, their ears floppier, and their skull wider.
For the latest experiment, the researchers selected an "elite" group of individuals – essentially, the tamest and most aggressive foxes in the pack.
"Our analysis revealed that the differences between tame and aggressive foxes may lie in cells in the anterior pituitary gland, which can change their shapes to communicate with one another about when it's time to release stress hormones," said lead author Jessica Hekman.
"Their pituitary glands may produce the same amount of stress hormones but be less efficient at getting those hormones into the bloodstream."
If confirmed, this research could help explain why tame foxes appear calmer around humans than wild ones and, ultimately, how animals become domesticated in the first place.