Holistic Animal Behavior: The Effects of Domestication on Animal Physiology and Behavior

Part 1, The Effects of Domestication on Animal Physiology and Behavior

For as much time as veterinarians devote to domestic animals, veterinary education pays little attention to the effects domestication has on animal physiology and behavior. However, not only does this process significantly alter an animal’s physiology and behavior, it represents a dynamic phenomenon capable of altering an individual animal’ physiology and behavior on a daily basis and that of an entire species over time.

The most comprehensive study of the effects of domestication come from the Institute of Cytology and Genetics of the Russian Academy of Science where geneticist Dmitry Belyaev began a study in 1959 that still continues today. Unlike other scientists, Belyaev believed that the animal’s behavior was the key selective factor for domestication. To prove this, he began systematically breeding wild foxes, Vulpes vulpes, for only one quality which he called tameability (which we might call “friendliness”).

This elegant experiment has yielded two results of interest to holistic veterinary practitioners:

  1. The existence of a bodymind connection
  2. The role of neoteny or pedomorphosis in domestication

By breeding for this single behavioral quality, the researchers created some dramatic changes in physiology and morphology which mirror those seen in other domestic animals.

By the tenth generation, 18% of the foxes were eager to establish human contact, vocalized to attract attention, and sniffed and licked experimenters like pet dogs. By 1999, 70-80% of the foxes displayed this behavior. The new foxes look markedly different from their wild counterparts, too. The exhibited floppy ears, monomorphism (males and females look alike), and the solid or piebald coat colors common to domestic animals. An increase in under- and overbites suggests that the shorter muzzles, more prominent eyes, and smaller craniums and brains found in other domestic animals and birds loom in the offing. In short, the adult domestic foxes retained the physical characteristics of the young, a condition known as neoteny or pedomorphosis. Consequently, we can say that domestication freezes the animal in a behaviorally and morphologically immature state.

Physiologically, the tame foxes reach sexual maturity earlier, give birth to litters an average one pup larger, experience longer breeding seasons, and some have mated twice a year. Although such mating behavior might appear more “adult,” among other reasons it could also result from the delayed timing of the fear response and its associated corticosteriods.

What happens during the first few weeks of an animal’s life will determine how that animal responds to members of other species. In wild canids, this window of opportunity opens when the pup begins to sense and explore its surroundings, and it closes when it begins to fear unknown stimuli. Wild fox pups begin to respond to sound at 16 days and their eyes are completely open by day 18-19; domestic study pups respond to sound an average of two days earlier and open their eyes one day sooner. Wild fox pups display the fear response at 6 weeks, domestic ones at 9 weeks or later. The longer the window remains open, the more time an individual has to accept humans or others as one of its own. Put another way, domestication reduces the animal’s sense of its unique species self/identity for a longer period of time.

At the same time, the later an animal displays the fear response, the later the surge of corticosteriods linked to it. In wild foxes the level of corticosteriods rises sharply between 2 and 4 months and reaches adult levels by 8 months of age. In the domestic foxes, the later the fear response, the later the surge of cortisteroids. However, by adulthood the domestic foxes’ blood cortisol levels are about half that of their wild counterparts. Given the far-reaching effects of corticosteriods, it’s conceivable that this surge and subsequent lower levels contributed to the changes seen in the reproductive cycles of the domestic animals.

What does this mean to those of us who endeavor to treat the animal as a complete entity rather than a collection of isolated parts? For one thing, the fact that domestic animals evolve as a bodymind unit strongly suggests that what we see behavioral and morphologically may have physiological corollaries. For example, it doesn’t seem beyond reason that the domestic animal’s immune response might experience a similarly enlarged window of opportunity. The result of this fuzzier barrier between self and other on the cellular and subcellular level might leave the animal more susceptible to allergies, auto-immune and immune deficiency diseases. The higher levels of brain serotonin found in the domestic foxes gives rise to images of higher levels of serotonin throughout the body and all the far-reaching effects these might have, including on the immune response.

Additionally, the domestication studies raise the specter that certain physical conditions that we see as causing certain behaviors might, in fact, be the result of them. For example, archeozoologist Susan Crockford of the University of Victoria (British Columbia) who specializes in ancient dogs notes that all of the changes in the developmental rates that produce the physical characteristics of neotony that we associate with domestication appear to fall under the direct or indirect control of one biochemical: thyroxine. Taking that one step further, we can postulate that the more domesticated the animal, the more stress placed on the thyroid. The more stress on the thyroid, the greater the potential for hypothyroidism.

In fact, we may find some support for this theory if we compare our most and least domesticated companion animals, the dog and the cat. Whereas hypothyroidism occurs fairly often in dogs, hyperthyroidism most commonly assaults the most recently domesticated (and some might argue still not domesticated) cat. In the few cases where hyperthyroidism occurs in dogs, some veterinary endocrinologists speculate that this may represent a period of hypertrophy preceding the crash of the organ in that particular individual. Perhaps this is what the feline species as a whole is experiencing as it struggles to adapt its naturally solitary bodymind to our social human one.

As if all this weren’t complex enough, we also need to bear in mind that domestication does not represent a point-in-time phenomenon, but rather an ongoing phenomenon. Early bond studies linked a stronger emotional human attachment to those dogs which displayed the infantile characteristics of short muzzle and big eyes coupled with a more dependent personality. While we can easily relate this to Yorkshire terriers and King Charles spaniels, we can also see a similar trend in golden and laborador retrievers and rottweilers as they made the transition from working animals to pets. Most Americans who see an European or Russian working dog of these breeds will remark on its “pointy nose” and “beady eyes.” Similarly, the shorter muzzles and prominent eyes of many purebred cats indicates that the human preference for neoteny continues in that species, too.

One final piece in the exceedingly complex mosaic of domestication comes from DNA studies of dogs performed at UCLA by Robert Wayne and Carles Vila. By comparing the mitochondrial DNA of 67 breeds of dogs with that from wolves, coyotes, and jackals they discovered that canine domestication originated from at least four different lines much longer ago than the 14-20,000 years most commonly quoted. Based on the rate at which DNA sequences changes, they estimate that the original split with wolves, i.e., domestication, occurred about 135,000 years ago, about the time those first humanoids started babbling to each other.

In the next installment, we’ll see how this long-term evolution of bodymind neotony in both human and canine species could set the stage for than most complex of all bodymind diseases: aggression.


Davis, Kate: Mind of a Dog, New Scientist, March, 2000.

Trut, Lyudmila. “Early canid domestication: the farm-fox experiment.” American Scientist, March-April, 1999.

Crockford, Susan. “Dog evolution: a role for thyroid hormone in domestication changes.” In: S. Crockford (ed.), The domestic dog: Issues and methods in the analysis of archaeological remains (Proceedings of the 1st ICAZ Symposium on the History of the Domestic Dog). Archetype Books, London.