Domestication syndrome: The effect of a human-animal relationship

By Jemima Frame

Domestication is described as the process of selectively breeding a wild animal until, overtime, they become genetically determined to be tolerant of humans, however, there is no clear distinction as to when exactly a wild animal evolves into a domesticated one. At present humans have domesticated 14 large mammals, including sheep, pigs and cows, and have engineered hundreds of domestic dog species (Daly, 2019). Artificial selection is the mechanism that drives domestication, and overtime, as humans have become an ever more dominant presence in the worlds ecosystems, they threaten to bring a new direction to evolution, straying from natural selection to artificial selection. 

The ‘domestication syndrome’ was first theorised by Charles Darwin when he noticed shared features between domesticated mammals. These included increased tameness, coat colour changes, changes in ear and tail formation, e.g. floppy ears, and a reduction in brain size (Wilkins et al., 2014). These changes between the domesticated animal and their wild counterparts are shown very well in dogs and wolves. The formidable predator that avoids human company transformed into a smaller, floppy eared animal who actively seeks out human company. Domestic animals often have different proportioned body parts. For example an English Bulldog has a short skull with a prominent underbite, whereas a wolf has a very long skull with the muzzle sticking out further than its eyes, and a pig has additional bones in their spines and lengthier bodies than their wild boar ancestors (Pilcher,  2020). Darwin recognised this array of shared featured in domestic animals, however, he had no knowledge as to why this ‘domestication syndrome’ occurred. He questioned if humans had purposely selected for these features, e.g. selectively bred floppy eared animals, or whether these features were just a by product of domestication. 

In 1959 a Russian scientist, Dmitri Belyaev, began an experiment that would provide an answer to Darwin’s question. He bred silver foxes at a fast rate, to provide an accelerated process of evolution. The first generation consisted of wild foxes which were in no means domesticated, and from this batch he selected 10% of the population which were slightly less aggressive and chose these animals to breed. When these offspring grew to breeding age, the process would be repeated (Pilcher, 2020). His theory behind this method was that this is how dogs must have been domesticated; by humans selecting the tamest wolves, and if tameness had a genetic element, this tameness would become more pronounced with every generation. By the fourth generation the foxes had begun wagging their tails, and by the sixth they were licking the scientists faces. Physical changes happened as well as behavioural changes, with some foxes in the tenth generation developing floppy ears and curly tails. By generation 45 they were almost all behaving like friendly dogs. Remarkably, studies showed that they were also thinking more like dogs, with the foxes now having the ability to decipher human gestures, which dogs are well known to do. This experiment showed that our distant ancestors did not select for certain physical features, such as floppy ears and curly tails, but rather these characteristics evolved as a by-product of selecting animals for their tameness. We therefore did not deliberately engineer the domestication syndrome; it was a natural process. 

In 2004 scientists put forward a theory which proposed an explanation for the domestication syndrome. They proposed that the domestication syndrome was due to small neural crest cell deficits developed during the embryonic stage (Wilkins et al., 2014). During a vertebrate’s embryonic development, cells from the neural crest will migrate to different areas of the body and form numerous different cells and tissue types. They contribute to forming ears, teeth and pigment-producing cells, as well as helping to form the adrenal gland, which is responsible for the ‘fight or flight’ instinct. In dogs the neural crest cells will travel all the way down to the tail where they help to give its structure and size (Pilcher, 2020). Scientists now think that as tameness is selected for over generations and animals begin the process of domestication, something changes in the neural crest. The neural crest cells start migrating but most of them do not reach their final destination meaning they are not able to assist in the formation of certain body parts. Therefore, the ears flop, the snouts elongation becomes stunted, and the tail no longer stands straight, and is rather curly. Most importantly, the adrenal gland is never fully able to mature, so the fear response is inhibited. The outcome is an animal that is never able to mature properly, physically or mentally. Domestication essentially disrupts normal development and traps animals in a state of everlasting youthfulness. 

Domestication is not a simple thing to define, the boundary between wild and tame is fuzzy and unclear. The domestic cat is an excellent example of this, as they have remained largely similar to their wild counterparts. They still hunt wild food and can interbreed with wild cats, and their physical features remain almost the same as a wildcat. Therefore, it is hard to distinguish at which point a wildcat becomes a domestic cat. It is not surprising that domestication is not easy to define, as there is no specific moment when a wild animal reaches domestication, it is, rather, a never-ending journey. Every generation is subtly different from the last, and domesticated animals, such as dogs, cows and chickens are still transitioning. Afterall, domestication is just a human-guided form of evolution.

References:

Daly, N. (2019) Domesticated animals, explained. National Geographic. Available from: https://www.nationalgeographic.com/animals/reference/domesticated-animals/ [Accessed 29th September 2020]

Driscoll, C., Macdonald, D., O’Brien, S. (2009) From wild animals to domestic pets, an evolutionary view of domestication. PNAS. Available from: https://doi.org/10.1073/pnas.0901586106 [Accessed 29th September 2020] 

Pilcher, H. (2020) Life Changing; How Humans are Altering Life on Earth. London, Bloomsbury Sigma. 

Wilkins, A., Wrangham, R., Tecumseh Fitch, W. (2014) The “Domestication Syndrome” in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics. Genetics. 197 (3), 795-808. Available from https://doi.org/10.1534/genetics.114.165423 [Accessed 29th September 2020]

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