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Ferret Natural History FAQ


  1. Why are male ferrets so much larger than females?
  2. How long have ferrets been domesticated?
  3. What's the difference between a ferret and a polecat?
  4. Are ferrets "really" M. furo or M. putorius furo?
  5. What is the relationship between ferrets and other mustelids?
  6. What evidence is there that ferrets are carnivores?
  7. How likely is it that ferrets might establish feral colonies?
  8. What is a ferret's skeletal structure like?
  9. What about ferrets' teeth?
  10. How do ferrets' eyes work? How well do they see?
  11. What are the genetics and history of ferret coloration?
  12. What makes a ferret albino?

Why are male ferrets so much larger than females?

Many thanks to Bob Church, who wrote this section as well as most of the others. Bob has studied biology and archaeology and is currently pursuing a Ph.D. at the University of Missouri, Columbia.

Bob writes:

I have several male ferrets that are twice as large and three times heavier than the females. This phenomenon [different shapes and/or sizes for males and females] is called "sexual dimorphism," and occurs in most species to one degree or another. Sometimes, the females are larger (some birds, spiders, etc.) But in most sexually dimorphic mammals, the males are larger than the females, some considerably so. The reasons for this are varied, but three of them apply to mammals in general, and one in particular in the case of the polecat and ferret.

One reason is sex. The biggest guy gets the girl, so big guys produce more offspring compared to little guys. This works to a degree--until the limits of the niche are met--otherwise animals would only get bigger and bigger. Also, it doesn't explain why females tend to remain small. That is due to the second reason: that is, the bigger guys have to eat more to remain healthy (also why females tend to outlive males in survival situations, such as with the Donner Party). You have to eat a lot when you are big to maintain the body, and it's worse when you are a predator and have to raise babies. So smaller females have an advantage, and can successfully raise more offspring than the larger females. These two reasons alone help explain why so many mammalian carnivores are sexually dimorphic. But these are not the only, nor the most important reasons for our ferrets.

In some solitary hunting animals such as the polecat, sexual dimorphism seems to be more related to niche specialization than to the other two reasons. When you read in a book what a predator eats, it is rarely divided into male-preferred and female-preferred foods. Polecats are quite territorial, and for the most part, live solitary lives. Male polecats exclude other males from their territory, and females ferrets exclude females. But they will overlap and share territories with each other. How can they allow each other in their territory and still get enough to eat, much less raise offspring? One of the few hard and fast rules of biology is that you can only have one species/type/whatever per niche. If you have more, only one will remain over time. The rest will move, change niches, or become extinct. This is significantly true with predators; each one has a specific location, hunting style or time, or prey, when compared to others.

The reason polecats can do it is that the females tend to go after smaller prey than males. For example, both males and females can go down rabbit burrows, but females can also go down the burrows of smaller animals, such as rats and voles. So the males tend to hunt more rabbits and the females tend to hunt smaller rodents (although I'm sure neither would turn their noses up at either; we are speaking of overall trends, not occasional specifics). While polecats as a whole fill a specific niche, the niche is subdivided between males and females. Males compete against each other, so they are excluded from each other's territories, but because females are in a slightly different niche and don't directly compete, they are allowed within the male's area. By the same token, females exclude other females from their area, for the same reasons males exclude males.

All three reasons (plus others not mentioned) are important, but the last one is thought to be the most important to polecats in general, and mustelids as a whole. (By the way, mustelids are one of the most sexually dimorphic extant (living) group of mammals, and are only surpassed by sea lions and other pinnipeds in the degree of dimorphism exhibited.) Supporting evidence is seen in territorial animals in which ALL other members, regardless of sex, are excluded from one animal's territory. In these animals, body size is about the same, or if sexual dimorphism exists, it is relatively minor in nature. The same is true of cooperative hunters, such as some mongoose and fox species. Yes, the average male is slightly larger than the females, but not markedly so.

No one knows exactly why polecats are so sexually dimorphic, but the three reasons discussed are certainly at the top of the list. Ferrets are sexually dimorphic because they are essentially domesticated polecats, and still exhibit this powerful genetic trait. Oh yeah, it is true that SOME males are smaller than SOME females. This is because the body size of males and females each forms a continuum that overlaps the other. There will always be individuals that are smaller or larger than normal.

How long have ferrets been domesticated?

No one knows for sure. One article by Dr. Freddie Hoffman, from the FDA Veterinarian May/June 1991 Vol VI, No. III, states that "Domestication of the European ferret predates that of the cat by more than 500 years," and that "Man's use of the ferret as a hunter of rodents and rabbits can be traced back as far as the 4th century B.C."

Despite much confusion, however, the current belief is that ferrets are not mentioned in the Bible. Early translations, including the older King James versions, translate one particular word in Leviticus as "ferret," but the modern King James and most other new translations have rejected this as an error, instead using "gecko" or "lizard".

Nevertheless, it is clear from other archaeological and historical sources that ferrets have been domesticated for quite a while, at least 2500 years. According to information from Bob Church, ferrets were mentioned in 450-425 BC in plays by Aristophanes, by Aristotle around 350 BC, and by Pliny in AD 79, for example.

Bob Church writes:

The origin of the domesticated ferret, unlike that of many domesticated species, is still unknown. Historical documents from Greece mention the ferret about 450 BC, and Roman documents mention the use of ferrets to hunt rabbits at about the time of Christ, but these were already domesticated animals. To this date, no skeletal remains have been recovered which could be shown to be ancient domesticated ferrets. Most evidence supports the idea the ferret was domesticated from the European polecat, but the idea that the ferret was domesticated from the steppe polecat, although unlikely, has not been ruled out.

One popular notion is the ferret was domesticated by the Egyptians, but this idea has little evidence to support it. Considering the rich and varied faunal remains and mummies recovered in Egypt [but no ferret remains], and the lack of European or steppe polecats within that region, it is probably untrue. Another idea is the ferret was already domesticated by the time it entered the Mediterranean area. While this idea is more likely, it still suffers from a lack of evidence, but explains the historical documents and fits the modern biological evidence. A third idea is the ferret was domesticated in the Mediterranean area, perhaps in Greece or by the Phoenicans, but again, this idea is hampered by a lack of evidence.

What is likely is the ferret was domesticated to hunt small animals, primarily rabbits, with the side benefit of domain mousing. It is likely that the ferret was most popular with the lower classes, which would explain the lack of early evidence and documentation (Until recently, archaeologists didn't concern themselves with the common people; royal tombs and golden artifacts were the primary goal). This emphasis has shifted in recent years, and it is likely that archaeological remains will be found that can be conclusively shown to be from the domesticated ferret. Some may already exist, stored in museums, awaiting the careful study of a future zooarchaeologist. Until then, the origin of the ferret is obscure, with a probable date of domestication prior to 2500 years ago.

What's the difference between a ferret and a polecat?

Written by Bob Church:

This is actually quite difficult to answer, and it can be approached on a variety of levels. I will do my best with such a short answer (this subject could honestly fill a book).

Taxonomically speaking -- there is no difference. Both are currently classified as Mustela putorius. When you see 'furo,' it is a subspecies designation, meaning there is some difference between the group in question, and the rest of the group; which is normally a geographical or morphological difference between populations. However, they are still "officially" the same species, and referred to as such. Usually, the term "subspecies" is used to label or designate different "races" or "breeds" within a group (not "race" as used with humans; there is no such thing unless you want to say an ethnic group is a subgroup of a subspecies. Humans use "race" to designate ethnic groups). I find the subspecies designation to be useless in domesticates because it is misused and misunderstood. For example, when the government wants to protect a threatened species, it will say that a particular subspecies is endangered (Cottontop Tamarinds, Florida (Everglades) Puma, etc.), treating the subspecies as almost a separate species. However, when it wants to keep out a species, it loses such designations, and the polecat and domestic ferret become the same thing.

Paleontologically speaking -- the relationship is unproven. Without the skull, it is virtually impossible to tell the difference between many different species of animals, such as mule deer and white-tailed deer, seals of similar size, and fisher and pine marten (this case particularly influenced by sexual dimorphism). Polecats and ferrets are so similar in their post-cranial skeleton it becomes impossible to distinguish one from the other. As for the skull, the closer you come to the point when ferrets were first being bred from polecats, the harder it is to tell the difference. You wind up calling the remains "Mustela sp." or "Mustela cf. putorius." It is very difficult to identify early domesticates (if not impossible!) Add to this the problems of sexual dimorphism (males being so much larger than females) and other closely related species (mink, fisher, marten, etc.) and its a real mess. Also, most bones are found broken and very difficult to identify. There is so much overlap in size and shape, many times bone identifications are made using distribution charts rather than morphology. The further you go back in time, the harder it is to distinguish one from the other. The result is a "lack" of evidence even though there is certainly a close connection.

Genetically speaking -- they are very similar; nearly identical. Both the ferret and the European polecat have 40 chromosomes; the steppe polecat and black-footed ferret have 38, and the mink has 30. I am not aware of any published accounts comparing the genome of the European polecat to that of the ferret, but I would expect a 99% or better compliance between the two. (The genome is the actual genetic structure. Several studies have compared the karyotype; that is, the external morphology, not the same thing.) Does that mean anything? Not much. 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, closeness in genetics does not necessarily mean the ferret is a domestic form of the European polecat. The ferret could have come from a now-extinct close relative of the European polecat, or perhaps even from the steppe polecat if some genetic event caused an increase in the number of chromosomes (it happens all the time, and explains many instances where speciation occurs). Additionally, there is some evidence that the genetic makeup of the domestic ferret shows variation in the number of chromosomes present, which muddies the waters. Without strong or compelling supporting evidence, genetic claims alone are circumstantial, forcing such save-yer-butt terms like "probably," "most likely," and "the evidence seems to indicate."

Morphologically speaking -- there are some major differences. Skull shape is different, base of skull is different, teeth are more crowded and numerically variable in the ferret, and the orbital angle is different. The internal structure of the eye is different, and there is some suggestion that there are differences in the structure of the brain. Coat colors, texture, and durability are different. Sound location is different. Balance and leaping abilities are different. In all cases, controversy exists to whether the differences are due to speciation or to domestication. The two may look alike, but they are vastly different.

Behaviorally speaking -- there are some major differences. While there exists a commonality of behavioral expression, the degree of that expression is different. Ferrets are gregarious, polecats are solitary. Ferrets will share space with other ferrets, polecats are very territorial (in a natural state). Ferrets tend to be more juvenile in behavior compared to polecats. Most differences are not in type of behavior, but of degree of expression. The same can be said for different species of polecats, so behavior does not prove speciality.

Domestically speaking --they are different species by the same rules that make dogs, cats, goats, etc, different species from their wild ancestors. A domesticate has been controlled or adapted by humans to be used for work, food, or companionship (pets), so in some way, they have had their reproduction controlled, and their morphology changed, which are two of the more important criteria in recognizing speciation. But in this case, science is inconsistent. Scientific nomenclature classifies pigs, rabbits, horses, llamas, camels, ferrets, ducks, geese, and chickens as the same species as their wild kin, but fails to do so for cats, dogs, cows, goats and sheep (short lists). All are clearly domesticated, but the same rules of nomenclature should apply to all (by the way, efforts are underway to correct this inconsistent policy). If the ability to go feral is a requirement for separate species status, then consider this. Of all the animals listed above, all have formed feral populations in island ecosystems, and all but one have formed feral populations in mainland ecosystems. Domestic ferrets alone 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, releasing thousands of animals over decades of time. Finally, it has not been reliably demonstrated that the feral populations were in fact pure-blooded domestic ferrets; they could have been hybrids, which changes things considerably.

Reproductively speaking -- they are very similar. While it is true that European polecats and ferrets can interbreed forming viable offspring, that alone does not prove they are the same species. Wolves can interbreed with coyotes and domestic dogs (essentially any member of the genus Canis), and form fertile offspring, yet they are classified as separate species. Different species of felines can likewise interbreed forming fertile offspring, as can cattle and bison (different genera), mule and white-tailed deer, the steppe polecat and the black-footed ferret, and many, many others. Ernst Meyer says such interbreedings are mistakes, otherwise the two species would merge into one superspecies. They stay separate, so they are separate species.

Specifically speaking -- they are very similar. However, even when two different species share the same genetic make-up, can successfully interbreed forming fertile offspring, but fill different niches or live in different geographic areas, which form a barrier to reproduction, they can be classified as separate species. The designation is supposed to show reproductive isolation has taken place, and that speciation is occurring. While the domestic ferret and the European polecat are clearly of the same genus, it has yet to be demonstrated that they share the same species designation.

Mo' Bob speaking -- it is my opinion that domestic binomials are all screwed up, with one set of rules naming one group while another set names the other group. (By the way, this is the key to why some states can classify the ferret as domesticated and others classify it as wild.) I personally would like to see all domesticates take the binomial of the species of origin, with a subspecies designation to indicate domestication. Thus dogs would become Canis lupus domestica (or familiaris), cats would be Felis sylvestris domestica/catus, horses would be Equus caballus domestica, and ferrets would be Mustela putorius domestica/furo (I prefer domestica over any other designation for the clarity it provides, but am somewhat in the minority.) In the meantime, I suggest using Mustela furo, which is common in Europe, when applying a binomial to the ferret.

You can see the issue is not very clear, mostly because no one has studied the relationships between polecats and domestic ferrets in a depth sufficient to fully answer the questions. Someday, these questions will be answered, but as for now?....

Are ferrets "really" M. furo or M. putorius furo?

Also from Bob Church:

Zoological nomenclature is determined by an international committee. Basically, once a species has been described, it remains with the first name given to it, unless someone else comes along and demonstrates the species is something else, or more closely related to some different group. At that point a petition is make to the international committee on zoological nomenclature, and the committee accepts or rejects it. Membership can vote on the issue. Sometimes a revision or description is made through some other publication, and no formal application is made to the committee, but it accepted as legitimate none-the-less. Under any system, rules of priority preserve the first name given to the animal.

The ferret and the European polecat were first described by Linnaeus in 1758. On page 46 of "Mammalia Ferae," the European polecat is assigned the name Mustela putorius (entry 6), and the ferret is given the name Mustela furo (entry 7). This was first published in "Systema Naturae: Regnum Animale" in 1758. When the chromosomal evidence came in during the late 1970s and early 1980s, it was decided the two were the same species, and because the polecat was named first, the ferret would take that name due to the rules of priority. Because it was a domesticated version of the polecat, it would be given the subspecies name of furo; hence the designation Mustela putorius furo.

In and of itself,this would be the end of it. Except for several problems. First, the practice is not administered equally because no guidelines exist to deal with domesticated animals. During the same time when the ancestor of the ferret was being determined, the ancestors of several species, such as the dog and cat, were discovered, yet they continued to maintain the "incorrect" name. For example, the dog is Canis familiaris, but is in fact a domesticated wolf, Canis lupus. If the rule was applied to dogs the same as ferrets, they would be Canis lupus familiaris.

Also see the following article:
Colin P. Groves, "On the nomenclature of domestic animals," 1995 Bulletin of Zoological Nomenclature 52(2):137-141.

Secondly, while the chromosomal studies appear to be convincing, they are in fact not studies of the genetic structure, but rather on the number and external morphology of the chromosomes. This is only circumstantial evidence, which is somewhat refuted by studies of the cranium and teeth, which show the ferret to be more closely related to the steppe polecat rather than the European polecat. That is not to say genetic studies are fruitless; its just that the study of the karyotypes gives different results than the study of the genome. For example, there are more than 6 billion people with the same karyotype (excluding those with genetic defects which alter the number, shape, and type of chromosomes.) However, each person in that populations has a unique genome (excluding identical twins, although some of them are unique as well).

What happened with the ferret is comparative studies were made on the possible ancestors of the ferret, and the only one with a comparable karyotype was the European polecat. The problem is, there could be a closely related animal that went extinct as the ferret was domesticated, as in the horse and the camel. Or, it could have been domesticated from the steppe polecat, but during the domestication process, the karyotype was altered to superficially match the European polecat.

However, this ignores the basic problem of what to do with domesticated animals. Many different ideas have been offered, from lumping them with the original species, to giving them their own position. Whatever the ultimate decision, it should be applied to all domesticates equally, regardless of current standing. My opinion is domestication alters the gene frequencies of an animal, much like natural selection, so domestication is a speciation event and domesticates are new species.

Currently, there is a movement within the committee to return the original Linnean names to all domesticated species. Part is to reduce confusion, and part because domesticated species are seen to be different from the wild counterpart. It is very likely that the committee, having been forced to address this issue by crazed anthropologists determined to have a separate species name to reflect human-mediated evolutionary processes, will agree, and the binomial will officially become Mustela furo again.

Remember, not all scientists currently agree with the binomial as it now stands, and regularly use Mustela furo in their publications. These tend to be Old World scientists mostly, and seems to be a habit of mustelid experts. Plenty of New World scientists agree; just run the subject "mustela furo" on Medline, Current Contents, or any number of other scientific bibliographic services, and you will find numerous current entries.

The bottom line is that technically, the accepted binomial is M. p. furo. But if you use M. furo, as I suggest, you have placed yourself within the company of many brilliant scientists, including Caroline King and Juliet Clutton-Brock.

While taxonomists and those who deal with species concepts generally understand what is going on, it is not clear to most, and the flaws become exploitable and damaging by the ethically- or educationally- challenged. The main problem is, it has yet to be proven that the ferret actually descended from the European polecat, so it is premature to take its binomial. So, go ahead and call the ferret "Mustela furo." What are they gonna do? Take away your birthday? (They can take away mine anytime they like...) Tell them you'll get your act together as soon as they do.

What is the relationship between ferrets and other mustelids?

Written by Bob Church:

The family Mustelidae has been around for a very long time; it is probably the oldest extant (living) family in the Carnivora, which means there are lots of different subgroups within the major group. Weasels include weasels, mink, ferrets and polecats; martens include martens and fishers; skunks include all types of skunks; badgers include badgers and wolverine; and otters include sea and river otters. Sometimes martens are grouped within the badgers, usually with the weasels, and lately they are being kept separate.

[Native] mustelids are found worldwide (except Australia, Antarctica, and most oceanic islands), and are perhaps the most successful of all the carnivores. They include 25 genera, and about 70 species. In terms of ferrets, their closest relatives would be the European polecat and the steppe polecat, with the most probable ancestor being the European polecat because it has the same number of chromosomes. The steppe polecat goes by several common names, one of which is the Chinese polecat, which are being used in the black-footed ferret (BFF) [breeding] program. This beastie can successfully breed with the BFF, producing viable, reproducing offspring. Skulls of this polecat have been recovered in Alaska, dating between 12 and 10 thousand years ago. Some have proposed that the BFF is a subspecies of the steppe polecat, not unreasonable. Others say they are different species and that the speciation event coincides with the beginning of the Holocene and the flooding of the land bridge between Asia and North America.

As for the question of any other mustelid being able to breed with the ferret, scant evidence exists. Outside of the lab, where even human and mouse genes have been combined, the only mustelid shown capable of breeding with the domesticated ferret has been the European polecat. There are many rumors of breeding with other mustelids, some of which might be true. None are published nor proven. Could it happen? Sure, and I could win the lottery tomorrow. But I wouldn't bet on it...

What evidence is there that ferrets are carnivores?

By Bob Church:

First, the digestive system of the ferret is very short. There is no caecum (a pouch or tube at the junction where the large intestine meets the small intestine), nor appendix, and the junction between small and large bowel is not visually apparent. This is not uncommon in highly carnivorous mammals, including sea mammals and many specialized carnivores. In contrast, the caecum of herbivores is often very large, and can form pouches quite long relative to the length of the large intestine. The determining factor in the length of the caecum appears to be the amount of ingested cellulose in the typical (averaged) diet. The more cellulose ingested by the species, the longer the caecum tends to be. The ferret's problem in digesting plants are threefold; first they lack a caecum to hold the bacteria which breaks down the cellulose, second, they lack several of the enzymes found in the rumin of most plant-eaters, and third, the passage time from oral- to anal-aperture is too fast to digest the nutrients locked up in the plant fibers.

Second, there are very few "true" carnivores or herbivores. The vast majority of mammals are omnivores in practice; the designation is generally applied according to the major dietary preference. Compounding the misunderstanding is the name applied to a group of mammals that are generally meat-eaters, the Carnivora. Many Carnivora are in fact herbivores (pandas) or omnivores (bears, raccoons). I like to use the term "strict carnivore" in describing the ferret because although it will eat fruits and nuts, it evolved a body and digestive system designed to be primarily a meat eater.

Third, any animal can be fed an "unnatural diet" and survive. It happens all the time. The animal may have health problems, a shortened life, depressed reproductive ability, etc., etc., but it can survive. I have read authoritative accounts indicating ferrets should be fed "sops" (milk and bread) or cooked cereals, and they survived. But it is not an optimum diet, nor does it promise the healthiest and best possible life for the animal. Additionally, starving animals will consume just about anything to survive. I was once asked to perform a necropsy on a dog found dead at a kennel. The kennel people claimed the dog was ill, but I found bits of rubber, stones, sticks, grass, and nails in the dogs stomach and intestine. The dog was ingesting anything it could find to sate its hunger, including eating portions of a garden hose. Lacking meat, most carnivores will eat vegetation to get by. Heck, they will even eat plastic.

Fourth, ferrets imprint (via smell) on foods at a very young age, which means they can learn to eat foods that would not normally be part of wild animal's diet. So just because a domesticated ferret can learn to eat an artichoke (Bear loves them) doesn't mean it would sustain them in a wild state, nor would it even be seen to occur except perhaps by starving animals. Trust me, whatever [vegetable] Bear eats does not change much on its passage through his digestive system; he could not get enough from it to survive.

Fifth, if you were in my lab, I could show you Cuvier's trick, a generally accurate way of determining the diet and carnivory/herbivory ratio for any animal. You just look at the teeth. The ferret has four molariform teeth, one in each quadrant. The lower molariform teeth are nothing more than tiny pegs, each slightly smaller than the head of a sewing pin. The upper molariform teeth are about 1/3 the size of the largest tooth in the mouth, which is a cutting tooth called a carnasial (or sectorial) tooth [a cheek tooth]. This indicates that while the ferret primarily cuts meat (carnivores do not technically chew food--they cut it with the carnasial and swallow the pieces), it does eat things that require crushing. This includes insects, snails, spiders, fruits, berries, and nuts. The cellulose in the vegetation passes through, and the ferret claims some of the carbohydrates, vitamins, and proteins. Like most mammals, ferrets crave the high energy and low work-cost of many fruits, and depend on their sugars to build the fat reserves to make it through the winter. (Yes, I know ferrets are domesticated, and so are we, but we both suffer from a physiology that still thinks we live hand-to-mouth in the wild.) As a professor once told me about temperate carnivores, "...meat allows them to maintain their bodies and reproduce, but sugar (carbohydrates) allows them to survive the winter."

Under these circumstances, many people would claim the ferret was an omnivore, but that would be incorrect. Ferrets are strict carnivores; that is, they are biologically adapted to eat [only] meat. Besides, fruits, berries, and nuts are only available for a short time; the major portion of the year is spent eating, you guessed it, small rodents, leporids, amphibians, fish, and insects. The ferret is as true a carnivore as they come, second perhaps to the vampire bat or sea mammals (interestingly enough, mink lack a caecum, while cats have one; so if you classify a ferret as an omnivore, you will have to include mink and cats). In reality, food preferences form a continuum with pure herbivory at one end and true carnivory at the other. Ferrets are at the top of the carnivore end of the food-preference spectrum.

How likely is it that ferrets might establish feral colonies?

This section was contributed by Bob Church. Although this book is almost entirely dedicated to rabbits, there are bits and pieces of ferret wisdom contained on its pages. In particular, the following statement appears on p. 131 (regarding the introduction of rabbits into Australia): "Ferrets (Mustela furo) have been used many years to catch rabbits but despite thousands of accidental releases they have not established feral populations even in those areas where rabbits were very abundant, probably due to lack of alternative prey when rabbits stop breeding."

The significance of this passage becomes apparent when you realize 1) the ferret cannot establish itself in a damaged ecosystem despite a lack of competition and predators and lots of introduced natural prey, 2) these ferrets are whole, that is, able to breed, yet they could not establish a population, and 3) ferrets are presumably domesticated polecats, and still need the environment originally adapted to by the polecat.

The journal "Ecology" is common at most colleges. This is a fine paper that discusses the ability of introduced organisms to invade and become established, and the few exceptions to the "rule of ten." (The rule of ten states that only one of ten importations lead to an introduction, only one of ten introductions lead to an establishment, and only one of ten establishments become a pest. So the chances of any given importation becoming a pest are about 1/10 x 1/10 x 1/10, or about 1 in a thousand. No one knows why it works, but it does.)

This could be a nice reference for Californians wanting to argue against the Dept. of Fish and Game's claims that the ferret could become established as a pest--even though it has been in the New World since white faces and hasn't established feral populations anywhere. The paper explains how the exceptions to the "rule of ten" have special unique characteristics (not found in California), and details what is required for such invasions to be successful.

This paper also supplies evidence to my earlier contention that no one is really sure if the animals in New Zealand are feral domesticated ferrets, European polecats, or hybrids. My contention is they must be hybrids of one degree or another, because it is improbable that they would not interbreed with European polecats released at the same time . (It was--and is--common practice to breed ferrets to polecats to increase their hunting instincts, which must have occurred during the release program.) According to Groves (and references contained within the paper), without human intervention, the two groups would readily merge into one. Since both domesticated ferrets and European polecats were released on New Zealand, any survivors would have interbred, meaning their offspring (virtually all feral fitch on the island) would be domesticated-wild hybrids; technically NOT domesticated ferrets.

What is a ferret's skeletal structure like?

Written by Bob Church:

The skeleton of all mammals is basically the same, so if you study one, you study them all. I'm in love with the skeleton of mustelids because of their strength and functionality. The skeleton is superbly adapted to the evolutionary habits of the little beasties, and shows little variation throughout most of the Mustelids; in other words, the skeletons of all mustelids are pretty much the same.

Humans have about 204 bones in their skeleton, however, it actually runs between 196-214 depending on what you include. Younger mammals can have 2-3 times more bones than adults, but they ultimately fuse together. If you look closely for differences in the ferret skeleton, you will find some of the vertebrae are a little bit longer (proportionately) than in most mammals. This is especially true of the neck vertebrae. The limb bones are a little bit shorter than in most mammals. Finally, the skull of ferrets is very long compared to most mammalian skulls. You know what a human skull looks like. Imagine someone flattening the top of your skull until it was level with your eyebrows, then pulling your nose and jaw out, say 7 or 8 inches so you can no longer see anything under your nose. Now, pull the back of your head and stretch the skull almost a foot. You now have the human equivalent of a ferret skull. Cool. Now, find a San Diego Padres ballcap that fits.

In the weasel group in particular (which includes ferrets, mink, and polecats) the mandible attaches about halfway down the skull, and the ridges along the top of the skull (the sagittal crest) and the back of the skull (the occipital and nuchal crests) are extremely developed. This shows the bite of the weasel group to be powerful, perhaps the most powerful, pound per pound, than any other member of the Carnivora. The skulls look pretty much the same, down to the teeth, and differences in size can almost be used to differentiate species, but sexual dimorphism can confuse the issue. The skull is quite similar to other mustelids, although the farther away the evolutionary relationship, and the larger the species the shorter and fatter the skull, and the closer to the back of the skull the mandible attaches. So in badgers, the skull is relatively broad, and the mandible attaches closer to the hole that allows the spinal cord to exit (the foramen magnum).

The skull is long and flat for several reasons. First, it makes for a powerful biting force, proportionately one of the strongest in mammals, which is very useful in killing animals near your size having teeth that can bite through a pencil (hey, imagine a giant beaver snapping at your nose, with incisors as thick as your middle finger). It also turns the body into a streamlined tube, which is really neat when running down tunnels and you don't want to bump your head or you need to turn around really fast. It also puts your teeth right out in the front.

The end result of each vertebra being a little longer than usual (plus the long skull) is a body that is proportionally quite long. Shorten each leg bone a little, and you magically have the ferret, adapted to run down tiny tunnels, and still be able to carry their catch without tripping over it. Imagine a dachshund trying to carry another dachshund in its mouth and get somewhere fast. Those legs would trip over something with every step and the wienie dog would get nowhere fast. But add a long neck, and now the loser can be carried enough forward to keep out of the way of the victor's feet. Trim those ears a bit, and tie something stinky to its butt, and you have a really funny looking dog disguised as a ferret.

Overall, the vertebral column of mustelids is long and supple, allowing the animal a great deal of flexibility and power. In the sprinting mode, the muscles along the back release a great deal of energy, and even though most ferrets don't show it, the ability to reach warp speeds (I once had a fisher run over my foot to get to a tree. All I saw was a brown streak, and I could only identify the animal by waiting a couple of hours until it poked its head out of a hole.) The supple back allows a great deal of flexibilty in changing directions while running, for carrying heavy loads (2-3 times their body mass in typical), and for turning around in burrows and on tree limbs. It also helps in jumping and landing, and acts as a shock absorber in falls. Most mustelids can leap 2-4 times their body LENGTH and in some cases more, which in humans would be 10-25 feet depending if you were me or some exomorphic basketball player. Much of the power for those leaps comes from the muscles of the back, and their relationship to the vertebral column.

Arm bones: The ferret clavicle is a tiny little ossification in the muscle where the clavicle should be, and is not always found, but the scapula, humerus, radis, and ulna are about the same as ours. Their elbow has an extension on the end of it to make their arms very strong for digging. The wrist bones (carpals), the hand bones (metacarpals), and the finger bones (phalanges) are almost the same, except for the 3rd phalanx which has a claw-like process on the end to support the nail. As far as I know, all members of the mustelid family have all five digits front and back. Ferrets have more carpal-metacarpal sesamoids than we usually do, which are tiny bones within the tendons where they pass over joints. Most of us have 1 or 2 of them at our thumbs, but ferrets generally have 2 for every digit. Oh, the thumb has 2 phalanges and the fingers have 3 for both humans and ferrets. The thumb in the ferret is like a finger. Just imagine what a ferret could do with a human thumb....

Leg bones: The femur, tibia and fibula are very similar. The ankle bones are sightly different to improve jumping ability. The foot bones are like the hand bones. In almost every mustelid (not as noticeable in the fisher group), the limbs are short and powerful. This is most noted in the badgers, who can dig faster than Bubba Bill can run. Short limbs allow access to burrows, but are also quite powerful for digging, running and climbing. As a result, they are some of the strongest animals on the planet, pound per pound. Due in part to the angle and position of the muscle attachments, even though they are tiny little creatures, they are quite strong for their size. This makes for a great little digger and climber, not to mention leaper. The short legs also make for a good set of shock absorbers, which, along with the powerful vertebral column, allows leaps exceeding 5-10 times their body length without serious injury. Imagine being about to jump down from the 3rd or 4th floor of a building without injury and you get an idea of the relative distances these little guys can jump. (A great deal of this also has to do with body mass as well).

The pelvis (composed of 2 os coxae and 1 sacrum) is similar to other carnivores; human pelves are twisted because of our upright posture, and widened to allow our infant's hypertrophied cranium to pass through (Ouch is right). Oh yes, the male ferret has a bacula (os penis) and the female an os clitoris. Humans don't. Figure it out.

Back and Chest bones are very similar to all other carnivores, except the length of the vertebrae. Human vertebral columns are modified for upright posture, and the neck bones are much shorter. Both ferrets and humans have 7 cervical vertebrae (as well as giraffes), so the length of the neck is related to the length of each vertebrae. Ferrets have 15 thoracic vertebrae, so they have 30 ribs (sometimes only 14T with 28 ribs); humans have 12 T-vertebrae. Their sternum is made up of 8 bones, humans usually 1 or 2. Ferrets have 5 or 6 lumbar vertebrae, humans 5. Ferrets have 3 sacral vertebrae, humans 5. Oh, humans don't have a long tail you big ape; we have 3-4 caudal vertebrae, ferrets about 18.

Head bones: In humans, there is a single hyoid bone and in ferrets have 9. The ferret's jaws do not fuse in the center like a human, and their upper incisors are rooted in a premaxillary bone (2 total) not found in humans. The rest of the skull is about the same except for a couple of bones that fuse in humans. The place where the jaw articulates with the skull is different, and the ferret has a bony covering over the middle ear called the auditory bulla which protects the ear and improves hearing.

Now there are quite literally thousands of ways the ferret skeleton can be told from the human (besides size), but for the most part, they are more alike than different. This is true of most mammals (excluding those that fly or swim) and sometimes you can get fooled. The bear paws look close enough like human hands that even police have been fooled. When I was in California, the police asked me to help determine the sex and ethnic background of a fresh human femur. It was from a black bear.

What about ferrets' teeth?

Written by Bob Church:

The ferret has two sets of dentitions; the deciduous (baby) and the permanent. The deciduous dentition is composed of 28-30 teeth; 12-14 incisors, 4 canines, and 12 premolars. The teeth begin erupting at about 3-4 weeks following this basic sequence: canines, 3rd and 4th premolar, then 2nd premolar. The incisors are highly variable in their sequence of eruption, but come in soon after the canines start to erupt. The dental formula for the ferret's deciduous dentition is 2(i3-4/3 c1/1 p3/3) = 28-30, or more commonly:

This follows the practice of only describing half the mouth, top and bottom. The lower case letters stand for the type of teeth (i = incisor, c = canine, and p = premolar. In the permanent dentition, the letters are capitalized, and M = molar). The first number refers to the number of teeth in the upper jaw, and the second number refers to the number of teeth in the lower jaw. So 3-4/3 indicates the ferret has 3 or 4 upper teeth and 3 lower teeth in either the right or left side of the jaws.

The permanent dentition is 2(I3/3 C1/1 P3/3 M1/2) = 34, however, the number of incisors is still variable. Missing or extra incisors are common. The permanent teeth erupt from 50 to 74 days of age in the following sequence:

About 50 days: Upper Canines, Lower Canines, 1st Molar
About 53 days: Upper 1st Molar
About 60 days: Upper 2nd/3rd/4th Premolar, Lower 2nd Premolar
About 67 days: Lower 3rd Premolar
About 74 days: Lower 4th Premolar, 2nd Molar

This sequence is variable depending on the health of the ferret, genetic variability, and nutrition. Also, the exact replacement of the premolars are variable, however, they almost always follow the eruption of the 1st molars. Generally, the permanent teeth form under the deciduous tooth (in the case of molars) or beside it (in incisors and canines.) The deciduous tooth is not lost until the permanent tooth has nearly erupted, so the replacement in rarely seen except for those who notice 'double fangs' when the canines are being replaced. This type of replacement allows the kit to be weaned on solid food at a very early age, and still be able to eat even though losing the baby teeth.

The Upper 3rd Premolar and lower 1st Molar are the carnassial (or sectorial) teeth. These teeth are specially modified to cut through tissues and bone, but can also be used to crunch kibble. The lower jaw of the ferret locks into the skull (in some cases, the lower jaw stays attached to the skull even in skeletonized specimens.) This type of attachment keeps the jaw from dislocating when biting large animals or from the strength of the bite. But it effectively prevents the type of chewing you see in herbivores and even omnivores. In fact, the ferret doesn't actually chew; it cuts the food into pieces with the carnassials, and then swallows the pieces. When you see a ferret "chewing" kibble, it is actually cutting the kibble into pieces small enough to swallow. When a herbivore eats plants, it chews the plant into a fine mush; chewing the cud takes this practice to the extreme. This allows the maximum amount of nutrients to be released from the food. When a ferret eats plants, it cuts them into swallowing-sized bits and down they go. The minimizes the amount of nutrients that can be absorbed from the food.

The non-carnassial molars are small and primarily used to crunch invertebrates. Generally, the incisors have little to do, they are said to help hold the prey in the mouth, but with the size of the canines, it is unlikely they actually do much. The ferret has no ill effects if they are lost. The canines are used to puncture, rip, tear and grip the prey.

Like humans, ferret teeth are subject to the build-up of plaque (tartar), caries, abnormal wear, and fracturing. Sometimes the tip of the canine will break off, usually in the ferret because of falls or biting part of the cage. They are sometimes cut off to prevent biting, once common in ferreting and fur production. The teeth will sometimes turn dark, which could be due to the death of the tooth, a cavity, or mineral discoloration. They can also turn transparent with age, due to the root canal becoming filled in with dentine.

Ferrets generally have neutral or slightly fishy breath; bad odors are generally associated with gum disease, decaying teeth, or gastrointestinal problems. Caries [cavities] and gum disease can be at least partially controlled by brushing the ferret's teeth manually, using a non-fluoridated toothpaste. This is especially effective after sticky or sweet treats, which tend to promote the decay.

How do ferrets' eyes work? How well do they see?

Written by Bob Church:

The eyes do look like little black beads, don't they?

They are like all mammalian eyes in that they have a retina, lens, iris, etc. Their pupil is round (like ours) when fully opened, and slits (like cats) when closed down in bright light; but unlike cats, the slits are horizontal, not vertical. As far as I know, all mustelid eyes are fully muscled and they can turn their eyes independently of their head much like we do, albeit with less range. (Besides, most people will turn their heads to look at something instead of cranking their eyes around. That is, unless you're scoping out the fox/hunk next to you.) There are some significant differences, however, in the overall shape of the eye, the ability of the eye to focus, and of course, in the structure of the retina. Because ferrets are predators, the eyes mostly point forward (right down the nose), but because they are also prey for larger predators, the eyes bulge somewhat (for a wider view, like a wide-angle lens) and are somewhat centered to the side and top of the head when compared to large predators or humans (again, for a wider view). So the ferret basically looks forward to see stuff, but they have a wider view of their surroundings than we do. Cool, eh?

The vast majority of ferret's eyes look either red (in albinos) or black. There are some ferrets with various other colors, but most owners just hear about them, or only notice them when the light is right. The red eyes are red because the eye completely lacks pigments, and the color comes from the blood circulating in the vessels of the retina. Most other ferrets have eyes so dark you can't see the pupil unless you look from the side while shining a penlight into their eye. Then you can see the pupil without a problem. Ferret eyes also have the white part (sclera), but it is behind the eyelids and is not normally seen. If you lift the eyelid (be very careful) you can see some of it in the corners of the eye.

The retina is different from human retinas in that it has very few cones (cones see colors) and a high density of rods (rods see tones of grey). Also, they have a reflecting layer behind the retina that bounces light forward, which is why ferret eyes seem to glow in the dark.

Finally, the lens is structured so that the ferret can see very well up close, but like the stereotypical bookworm, can't see clearly across the room. That is not to say it can't see anything; ferrets are actually better than people in noticing movement or shadow, they just can't read the fine print until they get closer to see if the box says raisins or prunes, that's all. But they don't need to, because they are specially adapted to hunt animals that live in burrows, which are dark places without much visual distance. So their eyes are perfectly suited for their job; that is, hunting rodents in dark burrows.

And that is why the eyes are so dark. Basic physics says light colored things reflect light and dark colored things absorb light, which I learned at about three when I would walk barefoot across parking lots. I would race across the asphalt yelling bloody murder, then stand on the white painted lines until the fire went out. I crossed parking lots from line to painted line. The reason the asphalt was so hot compared to the white stripe is because dark things absorb light. Same with eyes. You want to see well in the dark, you need dark eyes. Now, cats get away with light eyes because their eyes are so big and their pupil is constructed in such a way to allow it to open quite large compared to round pupils, but ferrets run around underground, and thus have tiny eyes (helps keep the dirt out, helps keep facial injuries to a minimum from fighting rats, and don't have much use for them in the dark anyway). So the eyes have to be dark to do their job. They only see in tones of gray because rods are far more sensitive to light than cones (they can actually see a little red). The reflecting part of the retina bounces light forward so the same light photon can stimulate the same rod twice, giving it a double stimuli and increasing its ability to see in the dark tremendously. And they see best up close because they don't have to see the rat from across the street--just down the tunnel a few feet.

Now, while the ferret's eye is basically the same as ours, they are not nearly so dependent on vision as we are. Instead, they use their nose to find their way from place to place. Foster, my 12-year-old, has recently lost 90% or more of his sight. I'm not sure of what he can see anymore, but it doesn't matter much because he gets along just fine using his nose. I have noticed he never walks in a straight line anymore, but sort of zigzags, using his nose as a homing device. He startles if you grab him, but I just call his name and let him sniff my hand first and he is fine.

Ferrets seem to be quite nearsighted, no doubt a response to their adaptation to a burrowing lifeway and reliance on smell to find prey. As far as can be determined, their close-up vision is as sharp as ours, and perhaps a wee bit sharper. One of my e-mail pals is doing some work on astigmatism in animals as an adaptation to hunting and predation pressures, and preliminarily suggests the ferret has an eye that is astigmatic in the horizontal mode, which she suggests is an adaption to flying animal predation [i.e., having to avoid being eaten by a bird of prey].

As for color, wild polecats see the long and short ends of the spectrum; that is, the blues and reds. However, some studies suggest domesticated ferrets have lost the ability to see the blues, so all the color they see is the reds. This is also probably an adaptation to a burrowing lifeway, but is also common in predators that hunt in the dark.

Can they see their food? Sure, except for the time it is directly under their nose, and a couple of studies have shown the ferret suffers a blind spot in that location. So they can see the food as they walk up to it, but when they get there, it's up to the nose to find it. If you watch the little boogers closely, they start a shallow and rapid breathing at that time, presumably to home in on the food with their noses.

Can they see their reflections? Well, not if they are of the vampire variety of ferret, somewhat common on the FML I've been told. Sure they can, as long as they are close enough. They can also see the images on TV, bright light spots from flashlights (some of mine love to chase them), floating dust motes, and dangling earrings. My son found my old Pong game (am I that old?) and hooked it up to the TV in his room, and several of the ferrets (Bear, Chrys, and Nosette) spent considerable time watching the dot bounce on the screen. Andrew thought the game was lame, but had a good time with the ferrets. Sam-Luc, Sandy, and Ballistic regularly go after stuff on TV programs, especially if the object is small and moving horizontally about the speed of a mouse.

What are the genetics and history of ferret coloration?

By Bob Church:

I am not sure of the exact genetics of ferret coloration, but I would expect fur coloration to be carried in 3 or more places. It would require a minimum of three different locations to explain the existing main colorations; sable, panda, and cinnamon. Some breeders will surely object to this simplification, but from what I've seen and read, all other "breeds" are in fact either variations or combinations of these three, excluding the albinos of course. Personally, I think fur coloration may turn out to be carried on even more than three locations, perhaps as many as five, but the extra locations are either close to one or more of the other locations so become linked to nearby traits. Which is why some colorations exhibit a high incidence of deafness or lowered mental abilities. Occasionally, because of cross-overs or whatever, they throw off expectations and you get a different look than expected. (OK, I admit I did some nasty math to get to the figure of three locations. I would tell you how I did it, but then I would have to kill you...)

Historically, I think the tendency was for albinos to be called ferrets, and sables, easily just as domesticated, to be called either fitch, fitch-polecat or polecat. Linnaeus described an albino in setting up the ferret binomial, but I think he understood ferrets existed in many coloration schemes, and used the albino as a type species because he could be sure the breed was pure. They may not have had a clue about genetics in the 18th century, but they fully understood husbandry, and knew to keep albinos white. Linnaeus knew ferrets were sometimes back-crossed with wild polecats to improve the hunting instincts, and wanted his type specimen to be a true representation of the ferret. Not long after Linnaeus described the ferret, it was described in a British paper as having two coloration schemes, the red-eyed white, and the fitch, which was lighter and less masked than the polecat, a coloration scheme that still generally holds.

Without a doubt, the ancestral coloration of the ferret was sable. No doubt, no argument. In fact, sable, or a very close variation, is ancestral to ALL members of the weasel subgroup, including mink, weasels, polecats, and ferrets, as well as most of the Mustelidae. During the domestication process and undoubtedly due to inbreeding, albinism developed, and was obviously selected for, perpetuating the trait. I cannot answer the question, "Why did they want albinos?" because I wasn't there, there is a curious lack of archaeological evidence, and because the people who did it are long dead and they didn't write down their reasons. They just seemed to like albino ferrets, and you can speculate as to why, but any speculation is 100% pure storytelling.

My apologies if in simplification or condensation the genetics or breeding portions of this post are unclear. I will be happy to write much much more and send it privately.

What makes a ferret albino?

By Bob Church:

White pants? Ooops, I mean white jeans. Eh, make that albino genes. As far as I know, no one has worked out the genetics of albinism in ferrets (except in a basic manner), but I would expect it to follow the basic rules and conditions of albinism in other mammals, BTW, all vertebrate groups can and do exhibit albinism from time to time. It is a common mutation.

Essentially albinism is caused by a mutation that prevents the formation of pigmentation. The external appearance (phenotype) can be the result of several different mutations within the chromosomes (genotype). Albinism can be caused by a mutation that prevents the body from manufacturing the pigments, a mutation that prevents the body from recognizing the proteins that key for the pigments, or even the body's inability to make the proteins that tell the body to make pigments. Three different reasons (from a possibly infinite number), but all look the same. However, in most cases of mammalian albinism, the reason is a mutated gene that prevents the manufacture of pigments.

Other factors beside albinism can result in light-colored coats. Usually this is the result of a protein that blocks the formation of pigments, such as the seasonal "albinism" of northern mammals, like in the weasel, hare and fox. This is not true albinism, because the pigments still exist in the skin and eyes, and some of the fur, and they return with the next molt. Other times, animals can have a white or light colored coat, such as in the Black-eyed whites, and this trait can blend with other coat colors to form intermediates of some type. Again, these are not albinos, and the lack of pigment in the fur reflects changes in the genetics at a different location than for albinism. In fact, albinos can have the same genes as your typical sable, and would look like one if they could make the pigments. BTW, the eyes are red because you can see the red blood vessels on the retina (very vascular). The eyes of dead albinos turn white or light bluish.

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Copyright © 1994-1998 by Pamela Greene <pamg@rice.edu>
I am not a ferret expert and cannot guarantee the accuracy of this information.
Last modified: 27 Feb 1998.