Belyaev’s Fox Experiment – Index

After frequently finding myself encountering references to Belyaev’s fox experiment in a number of dog-related texts, I felt the need to investigate his experiment more thoroughly.  This has resulted in a lot of reading, but a lot of new found knowledge.  From this reading, I hope to have a better understanding of the connection dog-authors are trying to make between dogs and the fox experiment. I hope it also proves useful for my readers.

Part I – Introduction
A summary of the work of Belaev in his ongoing experiment with foxes.

Part II – Changes
Description of the changes observed in Belyaev’s fox experiment.

Part III – Answers
Possible reasons for the changes seen in the foxes in Belyaev’s experiment.

Part IV – Dogs?
Why does the Belyaev fox experiment matter to dogs?

I hope this series has been of interest, as I thoroughly enjoyed researching.  I did cut out some bits and pieces, so please feel free to comment if you feel I haven’t answered a burning question for you! Additionally, if you would like in text referencing, I can provide such.

References: Continue reading


Belyaev’s Fox Experiment – Dogs? – Part IV

This post is part of the series on Belyaev’s fox experiments.
(index | part I | part II | part III | part IV )

You may have read my previous three posts, which have explained details of Belyaev’s fox experiment.  And you may have wondered the relevance of studying foxes on a blog about dogs.

Obviously, foxes are not dogs. They’re not even wolves. However, they aren’t far off it.  Regions of fox chromosomes correspond with those of the dog (to be specific, fox chromosome 1 seems to indicate a fusion between chromosomes 1, 33 and 12 as we see today in the dog).  That means that we should not disregard this research because it is a different species.

Chiefly, this study can be used to examine the process of dog domestication. Because domestication and associated variability seemingly occurred relatively quickly, there have been doubts that Darwinian theories are applicable. However, this study shows that significant changes can be seen in a brief period of time and generations. Coppinger (in his book Dogs) uses this study to show how quickly a significant change can take place – in the foxes here, significant morphological and physiological changes were seen in just 8-10 generations. This all occurred with just one selection pressure – selecting for ‘tameness’.

Dogs are very different to wolves.

This is surprising on a surface level, but when considering the causes of these changes it is not so remarkable.  Indeed, these changes have been seem occur in a similar way in terms of wolf’s domestication to dogs.  For example, dogs play as adults while wolves do not, and dogs carry many other juvenile-wolf characteristics.  Furthermore, dog puppies respond to human cues like fox domesticated pups and indeed are ‘dog like’ in many behavioural ways.  It is likely that the causes of the foxes changes are also the reason the wolf is the dog we know today.

On a larger scale, this research shows that we can select for nature.  Consider that these dogs were never trained, but were selected on their genetic amicability to humans.  This is a loud message on how we should be selecting dogs to breed from.

I hope this series has been of interest, as I thoroughly enjoyed researching.  I did cut out some bits and pieces, so please feel free to comment if you feel I haven’t answered a burning question for you! Additionally, if you would like in text referencing, I can provide such.

References: Continue reading


Belyaev’s Fox Experiment – Answers – Part III

This post is part of the series on Belyaev’s fox experiments.
(index | part I | part II | part III | part IV )

There are several theories that have been put forward regarding the origins of the physical characteristics seen in Belyaev’s foxes.  I will rebut some theories, and consider the plausibility of others. There are no definite answers, just some realistic theories.

Experimenter bias

One of the most popular anecdotal suggestions is that perhaps the experimenters unconsciously selected for more dog-like physical characteristics.  Personally, I think this illustrates a lack-of-faith in the scientific method.  The nature of the tests has been clearly outlined, and we must have confidence that this method was adhered to.  If there were serious doubts, the method allows for replication.  As replication has not occurred (mostly due to expense inhibiting the experiment) we can conclude that the experiment’s results are plausible in its current form. (The empirical world loves nothing more than proving others ‘wrong’ through replication.)

Mutations and Inbreeding

Another loud argument is the notion that the initial stock was carrying mutations or unusual traits, or that these changes were as a result of mutations.  Because of the inbreeding of the experiment, these mutations were amplified. This can be rebutted in a number of ways.

Firstly, the foxes were not inbred.  This fox population was frequently outcrossed to other commercial fox farm stock, and this has meant that the domesticated fox population has an inbreeding coefficient of 0.02 to 0.07.

Secondly, many of the novel traits outlined in part II are in fact not recessive. This means that the foundation population’s mutations would have been apparent on commencement of the experiment. This was not the case – these traits became apparent over the course of the experiment, and not in the beginning stages.

Another idea is that random mutations are the cause of these traits.  However, Belyaev determined that the rate of change in the domesticated strain was “2 or 3 orders higher than the expected frequency of spontaneous mutations”. This means that it probably not mutations that have caused the changes documented.

If we consider the mutation route as plausible, the suggestion with the most worth is Vavilov’s theory of homologous variability.  Vavilov’s theory suggested that similar gene sets can give rise to similar mutations, and so we can apply the term ‘similar gene set’ to all foxes, and mutations to their unique traits. This explains how foxes, despite being unrelated, developed similar traits just by the nature of being a fox with a fox gene set.

However, mutations probably did not have a role in the changes seem in the foxes.  What is more likely is that behaviour and anatomy may be linked in some way.

Depigmentation is a characteristic in dogs that was also seen in domesticated foxes. Photos © Ruthless Photos

Depigmentation is a characteristic in dogs that was also seen in domesticated foxes.Photos © Ruthless Photos

Selecting for many genes

The behaviour of ‘tameness’ is a varied trait, and so is controlled by a number of genes.  Because there are a number of genes involved, this means that selecting for tameness, and so also a number of genes, could have a profound affect.

Selecting for important genes

However, what is a more convincing suggestion is that perhaps this rapid change may have been as a result of selection may have been acting on relatively few genes, and genes that have an important regulatory role.  This would mean that if a ‘master’ gene was being selected for, this could have far reaching implications.  Here we reach the most convincing theory: That selecting for tameness was selecting for a major, complex, hormonal regulatory gene (or genes) which has far ranging implications on the rest of the animal.

The traits in foxes are found in many different domesticated species. Because of these similarities, Belyaev thought that early changes for amenability to domestication must be related to domesticated physiologies.  Because behaviour is regulated by neurotransmitters and hormones, modifying these elements through selecting behaviour would also have affects physiological parts of the animal.  Even though mammals are varied, their physiological processes are quite similar (their hormones, neurotransmitters, etc) – so this would be the basis for many domesticated mammal species showing similar traits.

Domestication/tamability is behaviour that is rooted in physiological changes and systems (e.g. hormones and neurochemicals).  Changing these complex systems would have far-reaching effects on the development of the animals themselves. And as all mammals are controlled by similar bigger-regulatory systems, this is seen as a reasonable explanation for the changes.

Hormone selection

‘Tameness’, ‘nervousness’ and ‘aggression’ is probably controlled by the endocrine system.  As described in the last post, serotonin, corticosteroid, cortisol, and adrenocorticotropic hormone were all found to be reduced in domesticated foxes. These hormones are responsible for behaviour that was selected for.  However, these hormones have a much bigger role in the endocrine system, so selecting for hormones would have had an extensive role and account for many of the changes observed in the domesticated animals.

Indeed, even the colour changes seen can be accounted for by hormones.  Hormones are linked to pigmentogenesis, agouti, and melanin.  The endocratic system can also explain the moulting changes in the domesticated foxes.

The endocrine system can explain many of the changes in the domesticated foxes behaviour, but this system also has a big role in development.  In selecting for genes that control behaviour, selection was also made for genes that control development.

The presence of juvenille traits (e.g play) in adult dogs was also seen in foxes. Photos © Ruthless Photos

Development mechanisms

The characteristics that the foxes adopted are those that are similar to juveniellism.  In this way, the development of the domesticated foxes can be described as ‘retarded’, as even adults have juvenile behaviours.  In this way it is thought that genes responsible for development have in some way been selected for.

These developmental changes start from embryos, with the hormones already described affecting the whole development process.  Even colouration/pigmentation has been linked to melanocyte and melanoblast activity in embryonic stage.  Neucrocrest cell migration would be delayed, which means messages to mature would not get to some body parts.  This would also have implications to the socialisation period, and be responsible for the floppy ears. Changes in the maturation timing have been seen.

Behaviour selected for seems to have been controlled by a few genes, but these genes were also responsible for a high level of regulation (i.e. hormonal level, and influencing development) and hence the foxes had a range of phenotype changes that accompanied the selected behaviours.

This is what domestication looks like

The literature review strongly stated that domesticated foxes, and their characteristics, are not terribly surprising.  Belyaev says that the “data demonstrate for foxes the kind of variability in similar characters and functions that is often observed in the domestication of other species of animals.” Because all domesticated animals have ‘done the same thing’ (in terms of phenotype traits), then this must be an implication of domestication and not an innate genetic quality of the fox population.

It is from these conclusions that the next post will start to make conclusions that relate to the domestication of dogs.  As our dogs display the domestication characteristics of the fox, this experiment is valid to our understanding of the history of dogs.

References: Continue reading


Belyaev’s Fox Experiment – Changes – Part II

ResearchBlogging.orgThis post is part of the series on Belyaev’s fox experiments.
(index | part I | part II | part III | part IV )

Animals, within the same species, can be truly varied.  Dog breeds, from Papillions to Greyhounds to Shiba Inus, are all the same species.  Belyaev commented: “Domestic animals differ from their wild ancestors, and from each other, much more than do some species and even genera.” This diversity is also seen within foxes, as there is a genetic difference between the ‘urban fox’ and the ‘rural fox’.  This is the same species, but they are significantly different because they have adapted to specific niches.

This difference was also seen between Belyaev’s selected-for-tameless line (‘the domesticated fox’) and unselected lines, as outlined below. These traits are still rare, however, with only ‘a few’ occurring out of every 1000-10000 individuals. These traits are not extraordinary, either, as these changes are typical of domestication in many other species.


Colour changes

Coat colour changes was the first change documented in the domesticated stock, in around the 8th-10th generation. Domesticated foxes are 1646% more likely to have depigmentation, 423% more likely to have brown mottling, and 400% more likely to have grey hairs, than their undomesticated cousins.  The colouration can be from patches of white on their foreheads, to spots of various colouration throughout their body, and piebaldness.

Drop ears as seen in domesticated foxes - Photos © Ruthless Photos

Drop ears as seen in domesticated foxesPhotos © Ruthless Photos


Floppy ears

Drop ears were observed around the 8th-10th generation.  Domesticated foxes are 35% more likely to have floppy ears than unselected farm foxes.


Tail changes

Tails changed position around the 8th-10th generation, with tails ‘rolled’ over the back.  Shorter tails were also observed, but at a later generation, approximately 15-20 generations into the experiment.  Domesticated foxes are 6900% more likely to have a short tail, and 1033% more likely to have a rolled tail than undomesticated farm foxes.


Reproductive Changes

Domesticated foxes reach sexual maturity earlier, give birth to larger litters, have a longer mating season than their unselected cousins.  However, there has not been any pups successfully reared to adulthood from an extaseasonal mating.  Many times the mating do not take and, when they do, the mothers are often cannibalistic.


Skulls and teeth

The domesticated foxes have smaller cranial height and width, and shorter/wider snouts than unselected farmed foxes.  Males also have a more feminine head (i.e. there is no clear difference between the sexes in terms of their head shape anymore).  Skulls in the domesticated fox are unusually broad for their length.

In regards to teeth, the domesticated foxes have smaller teeth than the unselected farm foxes.  After the 15th to 20th generation, underbites and overbites became apparent.


Hormonal changes

There are number of hormonal differences found in domesticated foxes, in comparison to the unselected foxes.  To put simply: Domesticated foxes have more serotonin (which is responsible for inhibiting aggression and generally ‘feeling good’).  This probably is responsible for the domesticated foxes having less ‘stress hormones’ in their system – because of the serotonin, they are less stressed.  As hormones have a rather complex and far-reaching role in development, it is probable that these results have implications across the animal.

The slightly more complex version of events:

Domesticated foxes have more serotonin, and more enzymes involved in the production of serotonin.  Serotonin is made by the liver and regulates mood, digestion, memory and learning, among other things.  In the foxes it is believed to inhibit aggression and development.

Corticosteroid levels, cortisol, and adrenocorticotpric hormone are all found in reduced levels in the domesticated fox.  These hormones come from the adrenal gland and the anterior pituitary gland in response to stress.  As these foxes are less stressed, they do not produce so much of these hormones.  Overall, domesticated foxes have less activity in their adrenal gland.  Additionally, the developmental spike in corticosteroid levels in domesticated foxes occurs much later than their undomesticated counterparts.

There are also changes in the level of steroid sex hormones, chiefly estradiol and progesterone in the domesticated foxes.  The role of progesterone in embryogenesis (development) is also significant.

Domesticated foxes became hormonally more relaxed - Photos © Ruthless Photos

Domesticated foxes became hormonally more relaxed – Photos © Ruthless Photos


Changes in the socialisation period

We know socialisation plays an important role for dogs, and it seems the same is the case in foxes. There is a ‘sensitive’ period of socialisation in post-natal development.

Is clear is that the domestication process in foxes has resulted in a different, extended socialisation period.  Firstly, they respond to sound two days earlier (and open their ears quicker) and open their eyes one day earlier (and also open their eyes quicker) than unselected foxes.  The socialisation period of the domedicated fox closes 3 weeks later than non-domesticated lines (i.e. the unselected foxes have a socialisation period of 40-45 days, while unselected foxes have a socialisation period of 60-65 days). Domesticated foxes have accelerated maturation intitially, and then this is retarded.  This means that foxes have an extended window, which allows them to learn more about human ways and how to respond to them.

Belyaev experimented with fox cubs from domesticated lines and lines selected from aggression.  From day 30 onwards, differences in the pups were clear.  Domesticated fox pups are more likely to move around in a new environment than aggressive lined fox pups.  But it is from day 40 that the most difference is seen.  Domesticated fox lines are less fearful, and more willing to overcome the fear in a new environment.  Aggressive lined pups were inclined to hide in a corner during the testing period – hunched, scared, and sometimes snarling.  This kind of fearful behaviour exacerbates difference between the lines, as fearful pups interact less with the environment and so have more foreign items to be afraid of in the future.


Other Difference

Domesticated foxes sound different (they sound ‘doglike’), they experience excitement urination, they have thinner bones, and moult in different ways to unselected stock.  Around the 15th to 20th generation, shorter legs were seen. Fox puppies from the domesticated line also respond to human cues (such as pointing) “as skilfully” as dog pups.



Overall, these traits all fit into the descriptor of ‘pedomorphosis’.  This is the term to describe the retention of juvenile traits in adults.  This includes physical changes (in this example, the skull shape) to behavioural changes (such as whining and barking). As these foxes reach reproductive maturity despite the pedomorphosis, they are experiencing a form of pedomorphosis called neoteny.

The reasons for this neoteny, and related traits, will be examined in depth in Part III of my posts regarding Belyaev’s fox experiment.


An Illustration

My friend, Stefan Psarkos, made a physical model of the changes seen in Belyaev’s foxes. A photo of his model is shown below.  This model illustrates the depigmentation, floppy ears, and erect tail set that selection for tameness has been associated with.

On the left is the ‘typical’ fox, with erect ears, normal colouration and a low set tail. The fox on the right is what Belyaev got after selecting for tameness – a fox with piebaldism, an erect and curly tail, and drop ears.



References: Continue reading


Belyaev’s Foxes – Introduction – Part I

ResearchBlogging.org This post is part of the series on Belyaev’s fox experiments.
(index | part I | part II | part III | part IV )

After frequently finding myself encountering references to Belyaev’s fox experiment in a number of dog-related texts, I felt the need to investigate his experiment more thoroughly.  This has resulted in a lot of reading, but a lot of new found knowledge.  From this reading, I hope to have a better understanding of the connection dog-authors are trying to make between dogs and the fox experiment. I hope it also proves useful for my readers.

The version of events in much of the dog literature is that selecting foxes for tame behaviour results in foxes developing more ‘dog like’ physical characteristics.  As such, conclusions regarding dogs have been made from this experiment.

However, a little more sophisticated version is that foxes have simply developed traits that many domesticated animals have – that is, domesticated foxes and dogs are not much different to how horses, cows, sheep, and cats have also responded to domestication.

This is the first of a series of three (or four) posts that will examine what the literature often refers to as Belyaev’s fox experiment.  This post will give a general understanding on the method of the experiment, while following posts will look at the changes found in the domesticated fox line, and their potential causes.  Finally, I will make a post to attempt to explain why this is relevant to dog science.

Belyaev’s fox experiment

This experiment has spanned 40 years, has involved 45 000 foxes, and resulted in 100 very domesticated foxes which actively seek human interaction. The fox experiment started in the 1950s.  Darwinian ideas were unpopular at the time, and fox farming was a suitable guise for Belyaev’s experimental purposes.

Belyaev described the main task of the experiment was “by means of selection for tame behavior, to obtain animals similar in their behavior to the domestic dog.” I think it’s important to note that Belyaev himself describes the main task of his experiment was to develop dog-like behaviour in the fox. Belyaev also theorised that selecting for domestic-type behaviour influences the reproductive patterns of animals.

The foundation population

Belyaev defines domestication as animals who are able to breed in captivity, and be non-fearful of man, and indeed perhaps ‘obey’ man.  He started with foxes that, even though they had been raised in a captive situation for 80 years, still had wild, undomesticated habits: their reproductive rhythm, moulting, and behaviour was ‘wild like’.  30 male foxes, and 100 vixens were the foundation stock from a fox farm.  The most tame animals from this population, and their offspring, was bred from (which was only 4-5% of the male offspring, and 20% of the female offspring).

(Click here to view a video of a farm-fox being evaluated.)

This initial stock responded to man in varying degrees.  30% were very aggressive, 20% were fearful, 40% were fearful and aggressive, and 10% were quiet and more exploratory (but were still by no means friendly, and in fact Belyaev describes them as “dangerous”).  The initial population did have some overlaps, with animals not clearly fitting into any of these groups.  However, over the course of the experiment, soon these overlaps dissipated.

Evaluating ‘tameness’

The ‘tameness’ of the foxes was evaluated by tests. Belyaev (in 1979) describes a different process of evaluation than Trut (in 1999).  My best guess is that methodology for evaluation has changed over time, and this accounts for different evaluation methods between the two authors.

Belyaev says that the foxes were tested twice at 2-2.5 months and then again at 4.5-5months, with very little variation seen between the two tests (i.e. an animal that was defensive at 2.5 months is likely to have a similar score at an older age).  The fox’s reaction to the experimenter’s presence, attempts to touch, and attempts to give it food were recorded.

According to Trut, the foxes were evaluated once a month, starting from 1 month of age and continuing until the fox was 6-7 months old.  In these tests, the experimenter offers food to the fox and tries to stroke the animal.

Regardless of the evaluative test, at the conclusion of these tests, the foxes were then given a tameness ‘score’.

  1. Class III foxes would flee from man, or bite.
  2. Class II foxes would let themselves be patted or handled, but were not considered friendly.
  3. Class I foxes are friendly, wag their tails, and whine.
  4. After six generations, they added Class IE (for “elite”).  These foxes are super eager to have human contact – they seek human attention and basically act like dogs.

By the tenth generation, 18% of fox pups were class IE, and by the 20th generation, 35% were class IE. From 1985 to the present, as the experiment continues, about 80-90% of foxes are Class IE.

Basically, each fox was tested and the foxes that were the most tame in their generation was bred to other tame animals.  This was a strict process, and only 15-20% of the population was ever allowed to breed on. They continued this process over generations, and it was just 10 years before foxes were bred who were attracted to humans.  These animals were not ‘trained’ to like people – the tameness was an innate behaviour pattern.  All contact with humans was time dosed to ensure no training occurred.

Controls & Inbreeding

This experiment also accounted for controls. Alongside the domesticated strain, selected for tameness, there was also an unselected strain and a strain selected for aggression.  These have been useful for evaluating the differences between the domesticated and undomesticated strains of foxes.

There was always concern about inbreeding in this fox population, too, and the animals were frequently outcrossed with farmed-foxes to ensure genetic diversity.

Results of Experiment

This is how Belyaev describes ‘his’ foxes: “…[The] foxes are quite tame, not as a result of training or taming, but due to prolonged selection for a tame genotype. Moreover, some quite new ethological characters have appeared, unusual even in the tamest animals bred on ordinary farms. Like dogs, these foxes seek contact with familiar persons, tend to get close to them, and lick their hands and faces. In moments of emotional excitement, they even sound like dogs. … at the sight of even a strange person, they try actively to attract attention with their whining, wagging of tails, and specific movements.”  This population is different to other farmed foxes: They are not scared or aggressive to people, and actually seek them out.

(Click here to view a video of a tameness bred fox being evaluated.)

These results have been somewhat replicated.  Kenttamies and colleagues conducted an experiment that was similar, but concerned with confidence in domesticated foxes.  Confidence was measured by how quick a fox would eat in an experimenter’s prescence.  The experiment spanned over 4 years, but, by selecting for more confident individuals, the confidence improved in just 3 years.  At the conclusion of the experiment, intensely fearful pups made up less than 10% of the selected population.  The research concluded that confidence was low to moderately inherited.

The experiment proves that tameness (and defensiveness) is a genetic trait.  This is a significant implication of this research.  However, it does not seem very extraordinary that selecting for tame foxes has resulted in a line of tame foxes in this current era.  What has been interesting is that alongside the tameness, appeared traits that were not selected for.  This includes floppy ears, changing coat colours, smaller teeth, smaller bones, curly tails, and overall puppy like characteristics – or, in other words, very dog like characteristics.  These changes and their potential causes will be examined in my next post.

References: Continue reading