The various "biggest cats" reported over the years have generally been morbidly obese, either as a result of overeating (as in the case of Tiddles, the Paddington Station cat) or as a result of an underactive thyroid condition which causes a sluggish metabolism which does not burn off calories. A giant cat would not be disproportionately fat though it might be "leggy". The apparently giant mutant cat "Snowball" was a hoax, but the picture gives some indication of how a true giant cat would look.

There are various forms of giantism ranging from a normally proportioned individual who is simply larger than normal, through to true giants who have an underlying disorder or inherited condition which causes abnormal growth. In dogs, a single mutant gene could explain the range of sizes, according to a study published in 2007 (Elaine Ostrander, National Human Genome Research Institute, Bethesda, US; Dr Carlos Bustamante, a member of the research team from Cornell University, Ithaca, US; Science journal). Researchers found that small dogs had the same mutation in a gene that influences size. The mutant gene is absent from most large dog breeds. The small breeds studied shared a mutant form of the insulin-like growth factor 1 (IGF-1) gene known to influence size in mice and humans. The mutant gene is present in some larger dogs; in those cases some other genetic factor must somehow override the mutant IGF-1 gene. The mutant form of IGF-1 probably arose more than 10,000 years ago.

A report of five foot long wildcats is to be found in "A General History of Quadrupeds" by Thomas Berwick (1790), but should be treated with caution. Berwick wrote: "Some Wild Cats have been taken in this kingdom of a most enormous size. We recollect one having been killed in the county of Cumberland, which measured, from its nose to the end of its tail, upwards of five feet." As well as Berwick's information being secondhand at best, animal sizes (lengths and weights) were routinely exaggerated by hunters and by farmers in order to make their kills/livestock seem more impressive.

The absence or presence of sex-hormones affect growth. Cats which are neutered before puberty tend to become leggier, but the effect is small and does not cause health problems. The end of the growth phase is delayed slightly, allowing the long bones to lengthen slightly more than in cats which are neutered at 6 months old. Though leggier, they are not normally bulkier (in terms of muscle). The effect is minor, not health threatening and (except in exhibition-quality pedigree cats where conformation is crucial) should not deter owners/rescuers from early neutering of kittens.

Although Australian feral domestic cats are sometimes described as twice the size of normal domestic cats, feral domestic cats in Australia are within the normal domestic cat size range. As with many species, a few exceptional individuals occur, but are not representative of the typical feral cat. Giant domestic cats sometimes occur due to genetic abnormalities (which often cause sterility) or pituitary abnormalities. There have been reports of Australian feral cats shot by farmers that reach 1.1 metres from nose to tail and suggestions that the cats are breeding fast enough that they are evolving into super-cats.

A supposed big cat ("black panther") in Dargo, Gippsland, Australia was shot by a Melbourne deer hunter named Kurt Engel who claimed the 1.5 metre (6 ft), 35 kilo cat had charged him. As with many photos, the cat is in the foreground and perspective makes it appear larger in proportion to the hunter. The body was photographed and disposed of, but the apparently 600 mm (26 inch) tail was sent for DNA testing at Monash University and shown to be a feral domestic cat. The tail was sent without vertebrae and could have included extra fur from the back which makes accurate size estimations difficult. A cat's tail is normally 30 cm (12 inches).

The partially decomposed carcass was examined by a Rural Lands Protections Board vet, Dr Keith Hart, in 2006. The body was 34" (just under 3 ft) and the tail 14" (total length 48"). Even considering the state of decomposition, this is substantially smaller than the size claimed by Engel. Although the measured size is within the size range of the smallest subspecies of leopard (Pantera pardus nanopardus of Somalia), the occurrence of an abnormally sized individual does not indicate that Australian feral cats are mutating into giants. For comparison, a Maine Coon has been recorded as reaching 49 in (121cm) (equalolingthe Gippsland Cat's overall length) and tails of 13" - 14" have been recorded in domestic cats.

The problem with Engel's photographs is the same as that with "Hogzilla" the giant wild pig shot in the USA. Use of perspective and the absence of a body allowed the hunter to exaggerate the pig's size. When the body was exhumed, it was found to be abnormally large, but not as large as claimed by the hunter.

Big cat researcher Mike Williams claimed it was extraordinary that Australia has produced a domestic cat mutation the size of a leopard. There is currently no evidence that this is a size mutation and not the result of a medical condition in a single individual. Karyotyping would be required to rule out chromosomal abnormalities that lead to abnormally large domestic cats while giantism due to a tumour of the pituitary gland is another possibility.

Although this individual cat may be an abnormally large individual, without the whole corpse, any claims of leopard-size are hyperbole as leopard subspecies are variable in size ranging from Somalia's relatively small "nanopardus" through to the more familiar large leopards. To be meaningful, suchs claims should be accompanied with reference to the leopard subspecies.

Constitutional large size means that large animals are that way because they are simply the offspring of larger parents. In livestock and dogs, this is because the breeder has selected large individuals in order to produce large offspring. This has happened in dogs over many centuries, resulting in giant (tall) or massive (tall and bulky) breeds such as Tosas, Great Danes and Mastiffs. The Romans had huge dogs, showing how early this selective breeding began. By comparison, selective cat breeding began less than 200 years ago. In cats there is a range of sizes, though this is not as wide as the size range in dogs. There have been recent attempts to produce large cats by breeding together the current largest breeds (Maine Coon and Ragdoll) but this did not result in increased size. One such attempt to breed a large domestic cat was the Renegade.

There have been a few reports of giant, but not obese, domestic cats including a 30lb cat with a low voice (suggesting pituitary giantism) and similar sized cats in Missouri and in Bryn Mawr. Unfortunately, all of these appear to have been neutered cats so it was not possible to determine whether their condition was genetic or hormonal.

In 2005, Rick Grover sent details of 2 giant cats owned by his sister in Utah some 20 years previously. Her two random-bred cats were a light tan female and her black male sibling. Both were neutered so it could not be established whether their size could be passed to the next generation. The female was 15 pounds and the male, Loci, was over 20 pounds with no excess body fat and a very sweet temperament. Loci measured approximately 24 inches from shoulder to rump, and about 42 inches from nose to tail tip. He was well proportioned with a large head and long tail and gave the impression of an ocelot. When Rick's sister tragically died, Loci went to live with their father in the country where he learnt to earn his keep by catching mice. He lived there for several more years and reached the age of 12 - a good age for a 'working' cat at that time. Coincidentally, I received an email about a very large domestic cat and her 2 kittens being offered for sale in a neighbouring state, Nevada, in the 1990s. The female was described as being light dusty tan in colour with small ear tufts which made her somewhat resemble a caracal. It would seem that Rick is right about large domestic cats being present in that region.

To breed genetically large individuals requires several factors. Firstly the underlying genes for large size must exist. These are likely to be polygenes i.e. a whole complex of interacting genes rather than a single pair of genes. In dogs, these genes have appeared over many centuries. Because dogs have been kept as companion animals for so long, humans have taken advantage of any genetic mutation which causes large size. A larger dog is more powerful and therefore a more intimidating guard-dog or (historically) better in animal baiting sports. The different huge breeds have been interbred to compound the effects of the different genes involved.

In cats, size mutations may have occurred, but cats have not lived in close enough association with humans for these mutation to be developed. Also, cats are not as diverse in their uses as are dogs so there has been little interest in developing them into as many different forms. Interbreeding large cats has failed to increase the offspring's size because the polygenes for large size or massive build just aren't there. Until those genes occur by mutation, cat breeders just don't have anything to work with!

Nature selects for the size that works best for the species in question. When dogs are left to go feral, over many generations their shape tends to revert to something more wolf-like - medium-large size, sharp pointed muzzle, pricked ear, wolf-like body proportions. Hot climes favour a lighter build and shorter fur, cold climbs favour a more robust build and shaggier fur. Those ancestral traits proved most advantageous over millenia of evolution and, in the absence of human intervention, that is the shape that will eventually prevail - much as it has in huskies, dingoes and the various forms of pariah dog which live on the fringes of human society.

Humans have not separated the cat from this form of natural selection long enough for a wide range of shapes and sizes to occur spontaneously and to be perpetuated. Compared to dog breeders and livestock breeders, cat breeders are in the extreme early stages selective breeding. Until the appropriate genetic "palette" has occurred, cats will stubbornly remain cat-sized and cat-shaped! Until very recently (i.e. the last decade!), cat breeders have been far more concerned with fur type, colour/pattern and eye colour than with size or conformation whereas colour and pattern are secondary to size in the selective breeding of bear-baiting dogs or beef cattle.

To develop constitutional giantism three things have to occur: the genes have arise by mutation; the mutation must occur in a fertile, breeding cat; the mutation must come to the attention of somebody able and willing to develop the mutation. This is more likely to happen in random-bred cats due to their greater genetic variability. In pure-bred cats, the gene pool is greatly reduced by selective breeding to retain consistent type and any genetic variability has been bred out.

This is illustrated by the Munchkin. In the 1930s and 1940s, short-legged cats appeared in Britain and were observed over three or four generations. No-one attempted to breed them and the mutation died. In the 1990s, a short-legged cat was found in part of the US. This time, someone was interested in breeding perpetuating the trait. Since then, other short-legged have occurred spontaneously and because the Munchkin set a precedent for this as a "breed trait", those cats form new bloodlines to expand the gene pool.

The same happened with curly-haired cats, curly-tailed cats and blue-eyed non-white cats. Those traits occur fairly regularly in random-breeding cats around the world and are often viewed as charming quirks, but of no real importance. Only recently did anyone considered selectively breeding curly tailed cats (American Ringtail, formerly the Ringtailed Sing-a-Ling) or blue eyed cats (Ojos Azules). Gene mutations for those traits kept on occurring, but until the 1990s, they had always occurred in places where the trait was either not noticed or no-one was interested in breeding the cats. Many Rex mutations were lost because they occurred in the wrong place or at the wrong time. Other "single cat" mutations vanished because the discoverer didn't understand about backcrossing (the poor old Mexican Hairless died out totally unnecessarily for this reason). There may already have been constitutional giants in the feline world, but they went unnoticed and the trait was lost, or the trait was lost through lack of interest or through mismanagement.

In wild/domestic hybrids, the hybrid offspring are back-crossed to domestic cats to get a dependable pet temperament. The end result is that the ultimate breed is closer to domestic cat size as size has to be traded with temperament.

True giantism is not the same as constitutional large size or selective breeding from large individuals to produce large (or larger) offspring. The condition is not genetic (there are no specific genes causing it), and therefore not inherited, but some families seem to carry genes that encourage the condition.

The main cause of giantism in humans is related to the pituitary gland or to a pituitary tumour (pituitary adenoma) producing an excess of growth hormone either during the main growing period or continuing to produce growth hormone after growth should have stopped. When an excess of growth hormone is present before sexual maturity, it affects all bones and organs and results in a large, but well-proportioned, individual. As well as lengthening, the bones have thickened and the internal organs have kept pace with the increase in body size.

Where growth continues into adulthood it results in a condition known as acromegaly. Only those bones which can grow lengthways can continue to grow, elsewhere in the body the bones have fused and growth is not possible. Human acromegalic giants become gangly, with large hands and feet, a jutting jaw (lantern jaw) and widely-spaced teeth. The leg bones are long, but are not proportionately thick to bear the increased weight, creating a shambling gait or crippling the individual entirely. The thickening of the bones and cartilage can restrict the function of the heart, lungs and other internal organs which have difficulty coping with the demands placed on them by an over-large frame. Acromegalic individuals are prone to hypertension (high blood pressure), breathing problems and fatigue. There is an overgrowth of soft tissues of the upper airway, causing an enlarged tongue and Adam's Apple, a small windpipe and potential breathing obstruction. The vocal cords may be affected, causing a deep voice. There is a knock-on effect on other glands which can result in common diabetes, hypothyroidism, hypoadrenalism and reduction in sex hormones. If untreated, the effects of pituitary giantism are crippling and eventually deadly.

In cats, acromegaly usually occurs in adult or elderly cats due to pituitary gland tumours that produce growth hormone. It causes widening of the facial structures which is generally easiest to appreciate by looking at the teeth, which move apart as the jaw bone grows. There may also be thickened facial folds, abdominal enlargement, increases in organ size (heart, liver, kidneys) and other growth related symptoms.

A form of giantism is found in certain big cat hybrids due to the interaction of mismatched genes. Lion/tigress hybrids (ligers) often exceed the size of either parent, but are not disproportionately leggy. Male ligers are almost always sterile (the apparent exceptions are anecdotal) and are reported to be relatively docile (probably due to a deficiency of testosterone). One suggestion is that giantism in ligers is due to endocrine (hormonal) abnormalities resulting in a longer growing period during which they attain a larger size before becoming mature. Another suggestion is that gene mismatches cause them to produce abnormal amounts of growth hormone compared to the parent species. A third theory is that they inherit different genes for large size from each parent and these genes (which were never intended to meet up in the natural course of things) have a compound effect on the hybrid.

By far the most plausible explanation for giantism in ligers(and a corresponding tendency to small size in tigons [tiger/lioness]) is due to "genomic imprinting" - the unequel expression of genes depending on parent of origin i.e. whether certain growth genes are inherited from the male or the female. A number of genes are contributed unequally by the male and female parents affect the size, general health and longevity of the offspring. This has been demonstrated in the laboratory using rodent species. The result is called growth dysplasia.

In simple terms, this is linked to the species' lifestyle and breeding strategy - whether the female mates with only one male while in heat (non-competitive) or whether she mates with many males (competitive). Genes contributed by the father promote size of the offspring to ensure that his offspring survive and out-compete any other offspring in the womb at the same time. Genes from the female inhibit growth to ensure that as many offspring as possible survive and that they all have an equal chance. This works fine if both parents are the same species as the two effects are held in equilibrium.

Lions live in prides led by several adult males. Each of those males mates with a female on heat. Each male wants his offspring to be the ones to survive, so his genes will promote larger offspring. To compensate, the female's genes inhibit the growth of the offspring. The effects of the male and the female cancel each other out. In contrast, tigers are largely solitary and the female normally only mates with one male when on heat. There is no competition for space in the womb so the male tiger's genes do not need to promote larger offspring - it would best serve his interests if all the offspring stood an equal chance of survival. There is therefore no need for the female to compensate by restricting the growth of the offspring, so growth goes uninhibited.

When a male lion mates with a tigress, his genes promote large offspring because lions are adapted to a competitive breeding strategy. The tigress does not inhibit the growth because she is adapted to a non-competitive strategy. Therefore the offspring (liger) grows larger and stronger than either parent because the effects do not cancel each other out. In contrast, when a male tiger mates with a lioness, his genes are not promoting large growth of the offspring because he is not adapted to a competitive breeding strategy. However, the lioness is adapted to a competitive strategy and her genes still inhibit the growth of the developing cubs. This uneven match means that the offspring (tigons) are often smaller and less robust than either parent.

This mismatch results in "growth dysplasia" which has other effects: the size of the placenta may be affected (causing miscarriage), the embryo may be aborted at an early stage due to abnormal growth, the cub may be stillborn or may only survive a few days. In some rodents, mating Species A males with Species B females produces offspring half normal size, but mating Species B males with Species A females cause the offspring to be aborted as they try to grow to several times the normal size.

In domestic cats, the reproductive strategy is competitive therefore the genes contributed by the female domestic cat act to inhibit the offspring's size. If a male domestic cat were to be mated with a cat species with non-competitive strategy, this might result in larger sized offspring. However this size increase might not be passed on to the next generation (assuming the offspring are fertile).

Another syndrome associated with large size is klinefelter's Syndrome, an abnormality of the sex chromosomes. In males, it is known as klinefelter's and results in XXY individuals. In females, the equivalent condition is called Triple-X syndrome and results in XXX individuals. More rarely, there can be XXXY or XXXX individuals. The XXY condition can occur in cats and is one of the causes of male tortoiseshell cats. In fact, it is usually only noticed in tortoiseshell male cats, because the tortoiseshell colouration is not normally found in males! In fact it may be more common than we think, going generally unnoticed except perhaps where a pedigree cat's infertility is investigated.

klinefelter's/Tiple-X individuals are often born undersized, but grow faster than usual, especially in the limbs. They may be taller than same-age individuals. They tend to become long-legged adults, but are not noticeably giants in adulthood. klinefelter's males have a lanky physique, may lack certain secondary sexual characteristics (e.g. beardless human males) and are frequently infertile. A few have feminised features. XXY tortoiseshell cats are also infertile, may be feminised in appearance or behaviour (one individual solicited other males to mount him) and although they have testes, they may lack the thickened jowls of typical unneutered male cats.

In September 2003, I heard from Laura K Baker of Dublin, Ohio who owns a Klinefelter calico male called Brody who shows a degree of giantism. Brody was saved from being euthanized at the vet where someone dropped him off as a scrawny kitten. He has feminine cat features and misshapen feet, but the primary distinguishing feature is that he is a Klinefelter giant and weighs over 25 pounds. Predominantly white, Laura describes him as shaped like a small sheep! In temperament he is intelligent and sweet-natured and very huggable. Brody needed some operations and special treatment as a kitten due to a broken hip and hates vets. His misshapen feet are apparently a birth defect. Vets are generally puzzled by him. Initially, the vet explained that Brody had Klinefelter's Syndrome and probably wouldn't live very long, or that he would be perpetually sickly if he survived long-term. Luckily both prognoses were wrong and possibly Brody is a mosaic of XXY and either XX or XY cells which would explain his ongoing good health (many surviving Klinefelter individuals turn out to be mosaics). Although small and scrawny as a kitten, he ballooned up around age 2 and seems to get bigger every year.

BRODY BAKER - male tortoiseshell Klinefelter giant, Dublin, Ohio, USA (by Laura K Baker)

He started gaining serious weight around age 2. Initially we tried a lot of different techniques to help him lose the weight. I even started taking him outside for "walks." Despite all that, he continued to gain weight at a rapid pace. Various vets said he seemed to be healthy despite the extra pounds. Now, at age 7, I realize and accept this is just his unique body shape and as long as he is healthy and happy, I'm OK with that. At last weigh-in he was 25 pounds. I don't know if this is a trait of all XXY cats, but I have heard that it can be -- and I wanted to share this story and pictures. He's a light in our lives and we love him unconditionally -- not because he's rare or special -- but because he's Brody - a kind-hearted soul who just happens to have an extra chromosome.

In humans, Marfan Syndrome is a common cause of excessive tallness. It increases height, but does not increase bulk. It results in excessively long limbs and arachnodactly ("spider fingers"); eye problems and problems with the heart and blood vessels which have weakened walls and become dilated. The major blood vessel (aorta) is prone to tearing and this has to be surgically prevented. It is due to a single dominant trait, is not linked to gender and many cases arise as fresh mutations. It has not, to the best of my knowledge, been reported in cats.

Known as Sotos Syndrome, this is noticeable in early childhood. The child becomes tall with large hands and feet, poor circulation and delayed development. The head is also enlarged. The bones basically hit their growing spurt early on. Sexual maturity also occurs early and the adult size is not noticeably giant. Some cats do hit their growth stage early and reach sexual maturity early (some males attaining full size, being fertile and having secondary sexual characteristics as early as 4-5 months of age), but this appears to be natural variation rather than cerebral giantism.

The breeding of large-size cats is feasible in the long term, provided the correct gene mutations/combinations occur and are capitalised on be cat breeders. This would be a new direction for cat breeding to take. Currently, genetic diversity in cats is far less than it is in dogs for the simple fact that dogs have been selectively bred for many centuries, giving a far longer time period in which the mutations could occur or be noticed and be perpetuated. Some of those mutant genes would have existed in the ancestral stock (i.e. prior to domestication) but remained hidden because nature did not select for those traits. Following domestication, humans noticed and bred the traits they found attractive. By contrast, cats have only been selectively bred to create distinct breeds since the mid 1800s.

Of the various conditions which are already known to cause large size, not all can be inherited. There remains the possibility of increasing the size of the domestic cat by crossing it to larger cat species although this would also affect temperament and perhaps create a variety unsuited to being a pet; in improving the temperament by back-crossing to domestic cats, there be a trade-off in terms of decreased size.

Ultimately cats can be scaled in the same way as dogs, but in dogs this took centuries to accomplish. Although a giant cat may be feasible, is it desirable? Would it be controllable? And since cats are obligate carnivores (they have no option but to eat meat), a giant cat would also be extremely expensive to feed and as long as predatory instincts remain, it could be dangerous to its owner's family in the same way that certain dog breeds remain dangerous to humans.