Domestic Ferret vs European Polecat
by Bob Church
The European polecat (Mustela putorius), is a member of the subgenus Vison, which includes several extant species, the domestic ferret, American mink, steppe polecat, and black-footed ferret, and the extinct sea mink. Other members of the genus Mustela include weasels, ermines, stoats, and European mink. While the exact relationships are still being investigated, it seems certain that members of the subgenus Vison shared a common ancestor in the recent past, most likely between 1,000,000 and 200,000 years ago.
Species within the genus Mustela includes the smallest members of the Mustelidae. Typically, the body is long muscular, tube-shaped, and possessing a short tail. Short legs and small rounded ears are also shared-characteristics, although more variable. Members of the subgenus Vison tend to be fossorial to some degree, making their homes in self-constructed burrows, or those taken over from other species. Extant members are adapted to an aquatic or semi-aquatic lifeway, displaying partially-webbed digits and thick fur, waterproofed by oily secretions. All have well-developed anal sacs, common to all carnivores, containing an oily malodorous secretion used for territorial marking and defence. The sense of smell and hearing is acute, although in domestic members hearing can be somewhat diminished. Vision is adapted for fossorial or aquatic lifestyles - that is, maximised for short distances. Except for the domestic ferret, all are generally solitary predators with strong territorial instincts. All possess hierarchical dominance behaviours at some point in their life, most noted in the gregarious domestic ferret.
The diet of each species within the subgenus can vary with the exact niche filled, but is usually composed of small mammals, birds, amphibians, fish, and various invertebrates, including shellfish, insects, spiders, and snails. Mustelids are opportunistic feeders, searching for whatever is available, rather than primary hunters like felids and larger canids. They have a strong caching instinct, and will often hoard or store surplus food.
It is clear that a strong and close relationship exists between the different members of the subgenus Vison, which makes the investigation of the ancestry of the domestic ferret more difficult. There is little doubt that the domestic ferret is more closely related to the European polecat (Mustela putorius) and the steppe polecat. (.eversmanni) than to the mink and black-footed ferret. Convention identifies the domestic ferret as Mustela putorius furo, a subspecies of the European polecat, but is that an accurate appraisal?
This is very difficult to answer in a short non-scientific paper because of the complexity of the questions involved. I will attempt to answer the basic question on a variety of levels.
There is no difference in taxonomy between the domestic ferret and the polecat. Both are the same species, Mustela putorius. When you see "furo," you are seeing a subspecies designation, which means there is some minor difference between the two groups in question. Normally, this difference is of a geographical or morphological difference between populations. However, they are still the same species. Subspecies labels usually designate different "races" or "strains" with a group. I find the subspecies designation to be useless for two reasons. First, all species have some degree of variation and defining a subspecies can be arbitrary. Second, it is useless because it is politically misused. For example, when government agencies want to protect a particular subspecies, such as cottontop tamarins, the Florida puma, or Washington white tailed deer, they will classify the subspecies as endangered, treating the subspecies as almost a separate species. However, species which are considered pests by these same government agencies lose such designations, and the European polecat and domestic ferret become one.
The exact evolutionary relationship between the domestic ferret and the European polecat remains unproven. Without the skull, it is virtually impossible to tell the difference between the post-cranial bones of many closely-related species, such as mule and white-tailed deer, seals of similar size, and fisher and pine marten. Even the skulls are difficult. European polecats, steppe polecats and ferrets are so similar in their post-cranial skeleton that it is virtually impossible to distinguish one from the other. As for the skull, the closer you come to the point where ferrets were first being bred from polecats, the harder it is to tell the difference. Add to this the problems caused by sexual dimorphism (males being so big compared to females of the same species), and other closely related species in the same area (mink, fisher, marten, etc.), and it becomes a real mess. Additionally, most bones are broken or missing from ancient skeletons. Even with modern populations and comparative collections, there is so much overlap in size and shape that bone identifications are often made using species distribution charts rather than by morphology. As a result, skulls and post-cranial remains are often classified as Mustela sp. or Mustela cf. putorius. So despite quantities of remains, there is very little evidence of the exact relationships, especially between the domestic and wild forms.
The genomes of the domestic ferret and polecats must be very similar or nearly identical. I am not aware of any published accounts comparing the genome of the European and steppe polecats to that of the ferret, but I would expect a 99% or better compliance between the two. Does this prove anything? Consider chimps and humans are 96-97% genetically similar, and dogs are at least 99% identical to wolves, so a very small difference can be quite important. Also the studies that have been done are flawed in the sense that they only tested living species. This is like killing the siblings then testing the cousins. Of course the cousins will be the closest relatives. The domestic ferret could have been developed from a now-extinct close relative of the European polecat. Additionally, there is some evidence that the genetic makeup of the domestic ferret shows variation in the numbers of chromosomes present, much muddies the waters. Without strong or compelling supporting evidence, genetic claims alone are circumstantial, forcing such save-your-butt terms like "probably," "most likely," and the ever-popular "evidence seems to indicate."
There are major morphological differences between the domestic ferret and the European polecat. Skull shape, especially the base of the skull is different. The domestic ferret's teeth are often numerically variable, and are more crowded on a slightly shorter jaw. The orbital angle is different, decreasing stereoscopic vision in the domestic ferret. The internal structure of the eye is different, and there is some indication that the internal structure of the brain has been affected through domestication. Colour, texture, and durability of the pelt are different. Sound location is different in domesticated ferrets.
There are major behavioural differences between the domestic ferret and the European polecat. While there exists a commonality of behavioural expression, the degree of that expression different. While both possess a hierarchical dominance structure, polecats are usually solitary while domestic ferrets are gregarious. Ferrets will normally share living space, displaying territorial behaviours to newcomers; polecats are very territorial to all other polecats. Ferrets tend to be more juvenile in behaviour compared to polecats. Polecats grow out of play behaviour while domestic ferrets engage in such behaviour all their lives. Most differences are not in type of behaviour, but in degree of expression. Because there is little difference between the behaviours of the steppe polecat when compared to the European polecat, behaviour alone cannot be used to prove species ancestry.
The domestic ferret is a different species by the same rules that make dogs, cats, goats, different species than their ancestors. A domesticate has been controlled or adapted by humans to be used for work, companionship, or food. Scientific nomenclature classifies such domestic animals as pigs, rabbits, horses, llamas, camels, ducks, geese, and chickens the same species as their wild kin, but fails to follow the same rules for cats, dogs, cows, goats, and sheep. All are clearly domesticated, but with different rules of nomenclature being used. The ancestry of dogs is well known to be from the wolf; modern arguments are concerned about geographical and subspecies origination. All have the ability to form fertile hybrids with ancestor species. If the lack of ability to go feral is a requirement for separate domestic species status, consider this. Of all the animals listed above, all have formed feral populations in island ecosystems, and all but one has established feral populations in mainland ecosystems. Domestic ferrets have not established scientifically verifiable feral populations in any mainland ecosystem that I can document. Furthermore, the feral fitch populations that do exist in island ecosystems were expressly and artificially established by humans. Finally, it has not been demonstrated that any feral populations were in fact pure-blooded domestic ferrets; they could have been hybrids, which changes things considerably.
Intraspecies vs. Interspecies Fertility:
The domestic ferret can reproduce with the European polecat, forming fertile hybrids. This does not prove the two are the same species. Wolves can interbreed with domestic dogs, as well as with coyotes, forming fertile offspring, yet they are classified as a separate species. Different species of felines can likewise interbreed and produce fertile offspring as can mule and white-tailed deer, the steppe polecat and the black-footed ferret, and others. Breeding between different genera can result in fertile offspring, such as between the cow and the bison (although somewhat diminished), and many different species and genera can interbreed, producing infertile offspring. While genetic closeness increases reproductive success and offspring fertility, it does not necessarily prove the two are the same species. Ernst Meyer says such interbreedings are mistakes otherwise there would not be two distinct species.
The domestic ferret is very close to the European polecat. However, there are many cases where virtually identical species, sharing identical genetic make-up successfully interbreed producing fertile offspring, and are morphologically similar, can be classified as different species because they fill different niches or live in different geographical regions. The different species designations reflect the existence of barriers to reproduction and that speciation is occurring. In the case of the domestic ferret, such conditions exist - there is very little interbreeding except for the occasional ferret breeder - however, there is no recognition of this genetic isolation.
In nearly every case, the specimen used to describe a species has been saved as the type specimen, to be used for comparisons with other animals. The domestic ferret was first described by Linnaeus, who named it Putorius furo, and considered it distinct from wild polecats. Later, the genus was changed to Mustela, more accurately showing relationships within the Mustelidae. The type specimen used by Linnaeus was lost; and there is some question as to what he was describing; a polecat, domestic ferret, or hybrid. The bottom line is, exactly what he described is unknown.
It is clear that the assignment of taxonomic binomial nomenclature to domestic species is done in an arbitrary manner, and may accurately reflect anthropomorphism and political agendas rather than true species relationships. It would be more accurate to utilise one set of rules for all domesticates, such would reduce confusion and misunderstanding in the private sector. Ferrets are as domesticated as the dog or cat, yet the taxonomic nomenclature does not reflect this shift in genetic structure.
The exact relationship between the domestic ferret and the European polecat is unproven. Of the living species, the ancestor of the domestic ferret is most probably the European polecat, but not necessarily so; it could be an extinct relative. Until those relationships are better understood, and until a better system of designation binomials for domestic species are established, I feel assigning the binomial Mustela putorius is premature. For this reason, I suggest the use of Mustela furo to describe the domestic ferret.