How Long Does Domestication Take?

How long does it take to domesticate a species? If the species naturally produces some less skittish animals, the answer can be as few as 4 generations. This has been tested in silver foxes, American mink and otters and scientists are beginning to unravel the genetic basis for tameness - which could explain why some species have never been domesticated. This a short summary of some of the research into the domestication process.

Dmitry Belyaev geneticist at the Institute of Cytology and Genetics, Novosibirsk began selectively breeding tame silver foxes in 1959. He obtained 130 relatively friendly silver foxes from a fur farm. In each generation, he allowed only the tamest animals to breed. Within four generations, some of the foxes had started to wag their tails. After eight generations, his tame foxes showed pied colouration. In successive generations, they showed floppy ears, shorter tails, wider skulls and would breed at any time of year. Within 20 years, Belyaev had created dog-like tame foxes that made good pets, as devoted as dogs but as independent as cats.. Physiologically, they had reduced activity of the hypothalamic-pituitary-adrenal axis, the nervous and hormonal signals involved in controlling an animal's response to stress. The tame foxes have higher levels of serotonin in their brains, inhibiting aggressive behaviour.

In 1972, Belyaev began breeding rats - beginning with wild rats and selecting one group for tameness and the other for aggression. After only 30 years of selection, the researchers had a tame group that enjoyed human contact and a viciously aggressive group. Belyaev then acquired 200 relatively tame American mink. After 4 generations of selective breeding he produced mink with clear evidence that domestication was occurring: temperament, new coat colours and other anatomical changes. In 1980, the experiment was repeated with wild river otters. After 13 years and 3 generations later, the percentage of tame otters had risen from 10% to over 30% and the tame otters had lighter markings, changed reproductive cycles and changes in brain chemistry. Belyaev died in 1985, but his work is continued by Lyudmila Trut. When the Societ Union broke up in the early 1990s, the experiments ran out of funding and the foxes and mink were reduced in number while the otter breeding ceased altogether (some tame otters and their descendents are now at Moscow Zoo).

At Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, Scante Pääbo and his team are now looking at the genetic changes for tameness. This might help us understand why the horse-like zebra and African Buffalo are intractable and might even explain why some humans behave more aggressively or anti-socially than others. Pääbo acquired 15 tame and 15 aggressive rats from Novosibirsk to establish colonies in Germany. To check the behaviour was genetic, not nurture, pups from aggressive rats were fostered on the tame mothers and vice versa. However, the pups behaved according to their genetic parents, rather than learning tame or aggressive behaviour from their foster parents. Ruling out whether maternal hormones affected the developing foetus had been done previous by transplanting embryos in tame and domestic silver foxes; the offspring still behaved according to their genetic parents.

The researchers then cross-bred the rats; the second generation of hybrids showed a spectrum of behaviour from very tame to very aggressive. These were assessed for tameness according to whether they could be handled or whether they violently attacked the handler. The results were compared with 200 genetic markers and several key regions of the genome seemed to have a strong effect on tameness. At least 5 genes interacted to produce the overall effect of tameness or aggression. Further study is needed to identify the mutations and exact effects. It might then be possible to replicate these in the genomes of other species that man wants to domesticate, but which have proven intractable so far.

Most domesticated mammals differ from their wild ancestors in terms of body shape, markings and breeding patterns. Darwin believed these changes came slowly and cumulatively. Belyaev suggested humans selected one useful trait - tameness - and took only a few generations to develop a tame population. Human hunter-gatherers might have captured some of the less skittish prey animals and opted to breed them as a reliable food supply kept close to home. Other traits associated with domesticated animals might be byproducts of breeding for tameness as Belyaev found in his research.