This is another post that might be rather theoretic and goes pretty deep into genetics and other aspects. What I am trying to do here is to give “breeding-back” a scientific backbone. I am not a scientist of course, but my attempt is to go in-depth with the help of the knowledge that I gained in four years of studying biology at the University of Vienna and all the intense literature research I did and discussions and conversations I partook since I started being interested in “breeding-back” in 2011. And actually I think that I gathered a lot of verifiable information in these years.
Now with this article I am trying to really go deeply into a very important basic matter of “breeding-back”: the differences between aurochs and cattle on a complete, organismic level. It shows that these differences go far beyond differences in horn shape or body size but concern nearly all biological aspects of these animals. This is, in my opinion, very important as it helps to understand what the aurochs, domestic cattle and “breeding-back” results really are.
This is a subject that I have already covered partly these posts:
If you feel you are new to this subject, I suggest to have a quick read through these posts. Now it is my aim to write one big, comprehensive synthesis that clearly underlines the wide-ranging differences between aurochs and cattle.
What is very interesting, and you will hopefully see that very clearly, that all aspects of a living organism are interconnected. Developmental delays or extensions grossly affect the morphology of an animal, which is itself influenced by endocrinology. Endocrinology also affects neurology, which itself affects behaviour. It is not only that factors influence other factors directly via cascades, but some aspects are also connected because they are regulated by the same genes that influence more than one trait. For example, the same genes that affect neurology and behaviour also influence colouration. This phenomenon is called pleiotropy. A living organism is one functional entity. You cannot alter one factor without altering one or several others. This is what happened during domestication and the whole organism has been altered and modified – and not only its morphology and behaviour. And all of these factors are laid down in the genome (except for some aspects that might be influenced by epigenetics, but these are of minor importance here).
The problem now is that we do not have the wildtype, the aurochs, left alive to study and directly compare to domestic cattle. Only morphological aspects and some of its behaviour are known with certainty, as much as the genome of one individual . The fact that one genome has been resolved only means that we know the sequence of its nucleotids, not that we know the individual role of each single gene in the complex interplay that shapes an organism such as the aurochs. Therefore, to access the differences between aurochs and cattle on organismic basis, we have to take other domestic animals and their respective wildtypes as a comparison. Luckily, domestication seems to work by the same mechanisms and rules in all species that have been domesticated so far [1,2], and some of them, such as the Yak, are more or less closely related and serve as a useful analogue. Thus, we can use these species as a model for the organismic changes from wild to domestic and apply them to aurochs and cattle.
What is domestication from an evolutionary selective perspective?
Before we go into the organismic that result from domestication, I want to outline what domestication does from an evolutional. In domestication, basically a more or less small population of wild animals is snatched out from the wild and opposed it to relaxed selection on the one hand and massive directive selective pressure on the other hand. The animals are in human custody and therefore less exposed to weather and predation, seasons, are supplied with sufficient amounts of food all year round and the food includes a smaller spectrum than in the wild, they do not have to prey for themselves (in the case of predatory species) and they also have medical care (to a certain degree and not that much in prior millennia). Thus, the requirements on the physiology of the animals (acuteness of senses, endurance, strength, food, robustness, hardiness) are much lower than in the wild. To a certain degree, due to raising animals by hand and the logistics of handling domestic animals, social and parental instincts also become less important and seasonal adaptions loose meaning as well. Another very important aspect is that domestic animals experience far less intraspecific selection as breeding is controlled by humans. There is, mostly, no concurrence between the individuals for breeding success, no sexual selection via female choice or competition, and it is not necessarily the case that the most dominant or most socially competent individuals have the highest number of offspring. Intraspecific selection is a very important evolutionary factor that is lacking almost completely in domestic animals.
This is where relaxed selection slowly reduces these traits or at least allows these traits to become reduced. Directive selection is, however, actively executed by humans. They bred for certain morphologic and behavioural traits including economic value (milk, meat et cetera), tameness, trainability, and also aesthetics and whatever else requirements human farmers had and have on their animals. New alleles appear as mutations, some of which are considered advantageous by the breeders and become fixated in the population, eliminating their wildtype pendants. Furthermore, genetic drift and the founder effect play a role as well when the population is created and then spread and increased in number, causing a more or less coincidental distribution and disappearance of alleles.
Both relaxed selection and conscious active directive selection as well as genetic drift acted upon domestic animals over millennia and produced animals that differ from their wildtype in looks and behaviour but basically all aspects of the organism (genomic, developmental, neurological, morphological, behavioural, ecological to a certain degree), which is why I write of the organismicdifferences between wildtype and domestic.
The Domestication syndrome
The “domestication syndrome” describes the fact that domestication obviously affects all mammals in a similar way with the same organismic changes. These concern, for once, morphology: all domestic mammals show paedomorphy (retention of juvenile characters) in morphology, changes in size, reduction of sexual dimorphism, often hanging ears or modified appendages (changes in horn shape/size and count if present, skin flaps or enlargement of already present appendages), changes in proportion and modifications of coat and coat colour[2,3]. Furthermore, they all have the same behavioural changes which they have primarily been selected for, manifesting in reduced fear response and behavioural paedomorphy (f.e. tail wagging in dogs), agreeableness and trainability2. Domestic animals also all show similar affections of physiological traits and the same reduction of fitness of the genome called the “costs of domestication”[4, 5]. What is also very typical is a loss of seasonal adaptions. Domestic animals tend to mate and reproduce all year round, including cattle, while the reproduction circle of wild animals is adapted to the seasons of the year2.
Cattle are a perfect example of the domestication syndrome, at least for traits were we can directly compare them to their wildtype (that is mainly morphology and to some degree behaviour). For all the other aspects, other domestic animals serve as a models for other organismic differences that we can predict with high certainty because it apparently works the same in all domestic mammals. I am going to break down these differences point by point.
Morphological changes in domestic cattle
The morphological differences between aurochs and cattle are obvious and have been covered multiple times on this blog (a quick look at the Wikipedia article might teach you the most important facts). In fact, all of these differences are classic “symptoms” of the domestication syndrome: shortened legs and an elongated trunk, in most cases decreased and in some increased body size, reduced sexual dimorphism in body size and colour, reduced skull size and brain volume and paedomorphy in skull shape (shortened face, especially the snout, larger eyes and concave profile in many breeds). Also the "hump" created by the elongated processus spinosi in the shoulder area that is characteristic for the aurochs and other wild bovines, is reduced or completely absent in most breeds. According to the literature, the aurochs skeleton was more robust , indicating that the aurochs was more muscular than domestic cattle. The horns have mostly decreased, in some cases increased, in size and developed a lot of different shapes, often loosing the inwards-curve which is typical for horned domestic animals. Some breeds, mostly zebuine, have hanging and/or enlarged ears, skin flaps such as the dewlap, the scrotum and the udder is enlarged in most breeds. The fur sometimes is shorter and not as insulating as to be expected in a wild animal, some breeds, such as highland, have long hair all year round and lack the bilayered fur. Some have curly hair all over the body, some completely lost curly hair. Whether or not hair is curled is regulated by specific genes. A lot of colour mutations evolved, including those causing the typical domestic piebald colour.
Domestic cattle often have elongated appendages (skin flaps like the dewlap, elongated scrotum). I have been puzzling around what might be the cause of that and I was not able to come up with a developmental, hormonal or neurological connection so I speculate that it might caused by specific genes, perhaps such involving connective tissue formation. In wrinkled dogs, a gene involved in the production of hyaluronic acid synthase 2 causes wrinkling of the skin (also in humans) .
Affections of development in cattle
How an organism is going to take shape is determined by when and how much of specific genes is going to be produced, and when they stop. You can produce different morphologies with the same genetic programme via a developmental delay or extension. Domestication usually causes a developmental delay, what means that development stops earlier than in the wildtype. The result is paedomorphy, which is universal among domestic mammals. It is possible that a selection for reduced tameness caused paedomorphy in behaviour (reduced fear response, agreeableness) and by pleiotropic effects and developmental cascades also a paedomorphic morphology. In a sense, domestic cattle are paedomorphic aurochs. In castrated individuals, steers, the removal of the gonads extends some of the developmental pathways as the stopping signal of the gonads is off, and therefore some of the paedomorphic traits are compensated. Steers may have a somewhat more aurochs-like morphology regarding size and proportions – they are more long-legged, long-snouted and also the horns grow longer (some examples here and here). An example for the influence of developmental delay/extension that I investigated myself is horn curvature. I took the Taurus bull Latino’s skull and pushed the rather banana-shaped horn sheaths outwards to the tip of the horn bone, following the curvature – suddenly there was a perfectly aurochs-like horn curvature, the “primigenius spiral” and also the size was right (see here for a sketch). If the horns would have continued to grow significantly (note that the bull was mature) horn curvature and dimensions would have matched those of the aurochs. Thus we can speculate that the actual genes regulating these traits are not different in Latino from those in the aurochs, only the development of the horns stopped earlier in this domestic individual (this, of course, also has consequences for breeding but those will be covered in another post). In the case of drastically deviant horn shapes (such as this one), there might indeed be mutations involving horn curvature. The skull anatomy of cattle is also a classic example for paedomorphy and thus developmental delay. In breeds with a very pronounced paedomorphic skull, the snout is shortened and concave, the nose is rounded and the eyes are enlarged. Actually, in most cattle breeds both the snout and frontal area is shortened (the latter might also be related to horn size), and also the eye sockets of bulls are not as prominent as in aurochs as far as I can tell from domestic bull skulls I have seen to far. I like to call this a “calf face” and developmental delay is very likely to be the cause for these traits. Maybe in extreme examples, such as the bulldog faces of breeds like Cachena and Barrosa, special mutations might be responsible for the condition we see, just the short legs in dachshunds, but in most domestic cattle the skull shape might merely be the result of development.
Most domestic animals, as a result of the developmental delay, also reach sexual maturity faster than their wild counterparts. Domestic cattle are sexually mature about one year faster than extant wild bovines.
The timing and extent of gene production plays a crucial role, or the crucial role, how an organism is going to develop and it is very likely that extensions or delays, most likely delays, of gene production as a result of selection on tameness has produced a lot of the morphological changes typical of domestic cattle compared to aurochs without necessarily involving extra mutations of the genes that actually regulate the respective body parts (see the horn example). Responsible for developmental delays and extensions are transcription factors that regulate gene activity, which are itself regulated by regulation genes and signal molecules (hormones) produced by the glands of endocrinologic system (these are, again, itself regulated by genes as well). It seems that selection on tameness alone caused a number of drastic changes in the organism in each domestic species that must be laid down in the genome , and so in cattle.
Endocrinologic changes and their consequences
Two hormone classes seem to play an important role in the domestication of mammals. One of them are corticosteroids which are involved in stress reaction. Domestic animals have a reduced fight/flight reaction, and indeed the corticosteroid level in domesticated foxes dropped by one quarter compared to the control group , indicating a causal relationship. We can assume that this similar in other mammal species as well because the types of hormones all have similar effects in mammal physiology, so probably the corticosteroid level of domestic cattle is reduced compared to the aurochs. In breeds with an inherited exaggerated fight/flight reaction, such as the Spanish fighting bull, it might be higher compared to the average of domestic breeds. Doing research on google I discovered something that fascinated me that might be of high relevance: the cushing syndrome in dogs. More precisely, it is called hyperadrenocorticism and describes an over-production of cortisol by the pituitary gland (see here). The symptoms are, among others, lethargy and a morphology that is remarkably similar to domestic cattle: enlarged belly size, reduced muscling, hanging spine (see here and here).
Another very important class of hormones involved in domestication are thyroid hormones. These hormones have a great impact on the development of many morphological, physiological and also behavioural traits . Thyroid hormones are involved in growth, stress response behaviour, hair growth, adrenal and gonad gland function and pigmentation. Hypothyroidic rats are smaller than normal ones and possess floppy ears . Domestic rats have a smaller thyroid gland than wildtype ones, which is probably linked to increased tameness . Neoteny in amphibians is caused by thyroid hormones as well, and disorders of thyroid hormones cause cretinism in humans. The symptoms of cretinism are shortened extremities, reduced body size and lowered cognitive abilities. This is remarkably similar to what we see in domestic animals. Considering that thyroid hormones are involved in hair production and pigmentation, I think Matthias Scharf’s (from the ABU) suspicion that cattle which have this colour variant that also goes hand in hand with brittle horns and hair have some metabolic or hormonal disorder is perfectly plausible.
Looking at the effects of both corticosteroids and thyroid hormones on behaviour and morphology, actually I think that we have found the key here, my friends. It seems that selection on behavioural aspects, mainly tameness, changed the hormone system in a way that the aurochs’ skeletal proportions and morphology was changed to what we see in domestic cattle: shortened limbs, reduced size, reduced brain volume and skull size, less muscling and enlarged belly, occasionally a hanging spine. This is perfectly in line with the evidence we get from other animals.
Neurology and morphology
The behaviour of animals is not only influenced by the endocrinologic system but of course also the nervous system. Thus, neurological development and neurological genes probably also were altered in selection. These, however, do not only affect behaviour but have a deep impact on the rest of the body. The neural crest is a precursor of many tissue types, influencing f.e. the adrenal gland and pigmentation . For example, melanoblasts (the precursor cells of melanocytes which are responsible for pigmentation) develop in the neural crest and migrate in the target tissue (skin, hair et cetera). Certain alleles on the KIT locus (called “Star” by Belyaev) cause a delay in this migration during which these cells die, resulting in unpigmented areas in the face (in homozygous foxes at least also body) that we see in most domestic mammals, including cattle (even in humans, where it is called “piebaldism”). Many of the “colour genes” are actually multifunctional genes that also have a metabolic or neurologic function. For example, mutations of the Dunlocus can cause neuromuscular disorders, such as rolled tails in dogs and pigs caused by myelin degeneration . The Agoutilocus also plays a role in regulation of fat metabolism in the adipocytes.
Loss of physiological fitness
Not much is known about the physiology of the aurochs, as there are no living aurochs to study, unfortunately. But there is a comparably close relative of the aurochs that has been domesticated and is still extant in both its wild and domestic form: the yak. This makes it a useful model for the physiological changes that domestic cattle might have undergone during domestication. The differences between wild yak and domestic yak in physiology are considerable. For example, the proportions of g-globin to total globin is higher in the wild yak than in domestic yaks, probably linked to higher stress resistance. Wild yaks also have a higher endurance as the activity of lactate dehydrogenase which prevents muscle fatique is higher in the wild yak. The respiratory metabolism of the wild yak is probably also more efficient as liver, lung, kidney and heart consume less oxygen and the quantity, size and bulk of red blood cells is higher. The wild yak might also be more efficient in digestion as the content of free amino acid in the serum is four times higher than in the domestic yak, and wild yaks keep on increasing in weight during winter, whereas domestic yaks decrease (domestic cattle do so as well) . The death rate in winter is 4% lower in the Datong yak (which is a hybrid breed of domestic and wild yak) , so it might be even lower in wild yaks. Wild yaks also have a higher quantity and quality of ejaculate. I think that the physiological differences between aurochs and cattle must be of a very similar manner. The yak was apparently domesticated 4500 years ago , and therefore its domestic history is only half as long as in domestic cattle, and basically all of them are landraces and thus not as protected from abiotic and biotic factors as more derived cattle breeds. Therefore I actually expect the physiological differences between aurochs and cattle to be more intense than between wild and domestic yak, at least in the more derived cattle. In pigs domestication also influenced leukocyte count, growth traits, meat character and disease resistance , so these factors might also be affected in cattle. In horses, domestication apparently also affected skeletal muscle performance, joints and skeletal articulation, balance, locomotion and the cardiac system . Surely each of these changes reflects the purposes the species was domesticated for, and horses were primarily selected on work purposes. But at least some of these aspects might be affected in domestic cattle as well, especially because the domestic life does not have the same physical requirements as living in the wild as an aurochs. Last but not least, and this is the only case where we have direct evidence from the aurochs genome we have, genes for milk production and quality have been altered by selection .
Loss of genetic fitness
It is not known whether aurochs and cattle differed in karyotype, or if cattle experienced genomic mutations such as duplications or deletions of entire genes. But what we can say with certainty that the genome must have “suffered” from domestication in a similar manner as in all domestic animals. This includes a loss of genetic diversity and therefore an increase in homozygosity due to the founder effect, genetic drift and numerous bottlenecks . The absence of purifying selection for evolutionary fit traits plus new mutations probably led to an increase of deleterious alleles in the cattle genome (mutation accumulation), because this is what we find in other domesticated organisms [1,4]. This phenomenon is described as the genetic costs of domestication and the deleterious alleles can affect all possible aspects of the organism, including morphology, growth, disease resistance, metabolism, reproduction, neurology and many others [1,4,5]. Conscious inbreeding, especially in the latest two centuries, further intensified the loss of genetic fitness of domestic cattle. This process is still on-going, especially since the more derived breeds are becoming an increasingly important part of the domestic cattle gene pool and absorbing the less derived landraces all over the world.
Summary.It occurs that there is a long way from an aurochs to modern cattle on organismic basis, despite both being assigned to the same biological species. Apparently selection on tameness and other characters drastically affected the timing and amount of development of the organism. Developmental delays and alterations of endocrinology such as the corticosteroid and thyroid hormones as a result of selection on behaviour changed the morphology of the aurochs dramatically and produced a phenotype with (mostly) smaller size, shorter legs, paedomorphic skull with reduced brain volume, reduced sexual dimorphism, enlarged belly, reduced muscling and altered horn shape and size. Selection on behaviour also influenced the neural crest development, resulting in affected pigmentation showing in unpigmented areas (piebaldism) in many breeds. Additionally to that, new colour mutations evolved, which might also affect metabolism and neurology. For a number of extreme phenotypes, such as stubby legs and faces, additional new morphological mutations might be responsible. Modifications in behaviour made domestic cattle much more tame, agreeable, trainable and lethargic compared to the aurochs – these changes correlate with the morphological traits because of genetic and hormonal interconnections. As a result of relaxed selection, not only the seasonal adaptions but also the physiological fitness of domestic cattle must have reduced significantly, probably concerning metabolism, digestion, respiration, endurance and stress resistance, which would very likely impact the survival rate under natural conditions as in the case of the yak. From a population genetic/evolutionary perspective, domestication probably reduced the genetic diversity and fitness of the population as a whole compared to the wildtype, which is typical for domestic animals.
All these changes in sum comprehensively concern the whole organism of the animal. Despite being assigned to the same species and being able to reproduce readily with each other, aurochs and cattle are two completely different types of organisms – wild and domestic. Surely the extent of these differences will vary between breeds, especially between less-derived landraces such as Maronesa or Turano-Mongolian breeds and highly-derived breeds such as the Holstein-Frisian and others, but the differences outlined in this article are very basic and must be universal among domestic cattle as they simply describe the state of being domesticated. I expect variation among breeds only to concern the extentof these changes, not whether the total of these changes is there or not. Furthermore, while it is true that some cattle might be closer to the aurochs than others, I consider it more than likely that all modern domestic cattle are closer to each other than to the aurochs, and that the distance to wild aurochs is quite considerable even in a less-derived breed like Maronesa.
Consequences for breeding-back
I think that diagnosing those wide-ranging differences between a wild animal and its domestic counterpart, especially between the aurochs and domestic cattle, has highly relevant implications for the understanding of what “breeding-back” results are. Actually, having researched these dramatic differences that thoroughly makes “the aurochs was larger, had a specific colour and different horns” rapidly loose meaning and almost sound like Kindergarden stuff to me. And judging on a total organismic level it actually it is, in a sense. When I started being interested in the aurochs and “breeding-back” in spring 2011, I looked at wonderfully bred individuals of the Wörth Heck cattle line such as the bull Aretto or the cow Erni and I did not quite understand why those animals should not be considered aurochs. My thought was: the phenotype matches, and the phenotype is a product of the genotype, so why not? Nowadays I am not only aware of the phenotypic differences (size, morphology, behaviour and so on) but I also understand that the differences go far beyond, affect nearly all organismic aspects of the animal and that the genotype definitely would not match in any case. I hope this becomes obvious from this post. Back in the time of the Heck brother’s experiments, for example, these differences where not even known to nearly the same extent. Apart from that, the Heck brothers overlooked many of the morphological differences between aurochs and cattle and failed to properly judge what they bred (for more details, see here). Why should one of their bulls be a resurrected aurochs? Because of similarities in colour and horn shape? From a modern perspective I would say: you cannot be serious, this is not even 2% of the story. So even if breeding manages to fix body size, proportions, horn shape, snout length and other aspects, “breeding-back” results will still remain domestic animals and therefore different animals. The interconnections between morphology, behaviour, development and all the other aspects are so complex and involve so many loci that I do not think that traditional breeding can truly reverse the changes caused by domestication. This is why I mostly write “breeding-back” under quotation marks and strongly reject the term “rebred aurochs”.
However, some modern projects claim to execute “breeding-back” on a genetic level in order overcome these problems to achieve a more authentic result that might deserve the title “aurochs 2.0”. In the next post, I am going to present my personal take-on to the facts behind this claim.
Most of the research for this article I did on my own, but I also have to say that some of the references, especially those on the yak physiology, I would have never discovered without the precise and thorough research work of Roberta on the Carnivora Forum Aurochs thread. It helped me great time to complete my body of theories on this subject. Many thanks for that!
1 D. Wright: The genetic architecture of domestication in animals. 2015.
2 L. Trut: Early canid domestication: The farm fox experiment. American Scientist. 1999.
3 K. Dobney & G. Larson: Genetics and animal domestication: new windows on an elusive process.2006
4 Moyers et al.: Genetic costs of domestication and improvement. 2017.
5 Schubert et al.: Prehistoric genomes reveal the genetic foundation and costs of horse domestication. 2014.
6 W. Frisch: Der Auerochs: Das Europäische Rind. 2010.
7 Hans-Peter Uerpmann: Der Rückzüchtungs-Auerochse und sein ausgestorbenes Vorbild.
8 Kemper, Visscher & Goddard: Genetic architecture of body size in mammals. 2012.
11 Zhonglin: Development of a new yak breed through utilization of wild yak genetic resource – serial technologies of the development of the Datong yak breed.2004.
12 Lanzhou Institute of husbandry and pharmaceutical sciences: The 5thinternational conference on yak. 2015.
13 Lensch, Schley, Zhang: Der Yak (Bos grunniensis) in Zentralasien. 1996.
15 Orlando, L: First aurochs genome reveals the breeding history of British and European cattle. 2015.