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.
However, despite much confusion, 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.
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 Tamarians, 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 [1]). 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 [7]. 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?....
Colin P. Groves, "On the nomenclature of domestic animals," 1995 Bulletin of Zoological Nomenclature 52(2):137-141.
This is a short and easy to read paper that clearly illustrates some of the problems I've been discussing for the last six months--at least from the viewpoint of taxonomic nomenclature. If you are well practiced "speaking biologica" you will have no problems; if not, a dictionary will help in a few places.
The paper points out a great inconsistency in taxonomic nomenclature: the rules are applied differently to different species. For example: the dog, cat, and ferret (and at least 15 other species) were named prior to the naming of the wild species (if it was known or clear). If they are the same species, then the rules of priority should follow, and the wild animal should take its name from the prior-named domesticate. As far as I can tell, in all but one case, the two names remain, leaving the impression of two separate species (i.e., Canis lupus and Canis familiaris [dogs], Felis sylvestris and Felis catus [cats], Equus caballus and Equus ferus [horses], etc). The ferret is an exception; not only did the "two species" become one, but they took the younger name (Mustela putorius) rather than the older one (remember the rules of priority? The namers didn't...).
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.
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...
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.
The significance of this passage becomes apparent when you realise 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.
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 he "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 [3]. (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.