Every so often I am asked about "super-cat size" ferals by people who have heard stories that high-protein diets are causing feral cats to grow to huge dimensions. For a full explanation of why cats don't grow to giant sizes (and for the hoax giant cat "Snowball" picture), see Giant Cats. Gigantic ferals, of Labrador dog” dimensions, are entirely mythical. A visit to a cat show demonstrates that cats vary in size, but stories of a race of monster feral cats are often fuelled by anti-cat individuals or organisations in attempts to further vilify the already much maligned feral cat.

THE GENES FOR LARGE SIZE

Unlike dogs, cats lack the genetic plasticity to breed to huge sizes. 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 second-hand 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.

In August 2012, Stephen Harris, Professor of Environmental Sciences at Bristol University, suggested that feral cats in Britain are growing to large sizes to fill the "large predator" niche once occupied by wolves. He claimed domestic cats can be as long as 4ft from nose to tail-tip in Britain and that he had seen domestic cats exceeding 5ft in Australia. In Britain, their size is assessed by analysing indistinct photos rather than by shooting and measuring the animals themselves. There is no scientifically verifiable evidence supporting the Australian claims as the carcases of purported "giant ferals" shot in Australia have tended to disappear. Forced perspective and photographic angle gives the impression that animals are larger than they really are.

The genes for huge size simply don't exist in cats, and regardless of improved diet, cats have stayed pretty much the same size since they were first domesticated. The largest breeds of domestic cat are the Maine Coon and the Ragdoll, though these are generally under 20 lbs - hardly gigantic - and are bred for mellow temperaments. Even crossing domestic cats with a larger wild species such as the serval or jungle cat has only slightly increased the size. A number of F1 and F2 hybrids reach impressive sizes, but the need to back-cross to domestic cats to preserve the domestic temperament also proves a limiting factor on size.

There have been a few reports of giant, but not obese, domestic cats including a 30lb cat with a low voice (suggesting pituitary giantism). Rare abnormalities such as Klinefelter syndrome or acromegaly (pituitary giantism) can also result in over-sized individuals. Pituitary giantism brings with it a host of health problems including deformity, joint problems and shortened life expectancy; it does not create huge ravening felines. Far from being more effective predators, affected individuals would have a hard time surviving in the wild due to the harmful effects of these conditions.

HIGH PROTEIN DIETS

Another claim is that high protein diets are causing feral cats to grow larger. Firstly, where are the feral cats getting these high protein diets? Most are hunting the same prey and getting the same levels of protein as their predecessors. Some are being fed by cat lovers, but they are eating the same balanced cat foods that domestic cats eat with the same level of protein that we feed to our normal-size pet cats. Many of the ferals living on handouts have also been neutered as part of the deal, so they won't be breeding kittens of any size. Others are living on scraps and restaurant leftovers; these scraps are probably high in fats and carbohydrates, rather than high in protein.

The idea that high-protein diets have caused successive generations of feral cats to grow huge is also erroneous. Cats naturally evolved to metabolise fats and proteins rather than carbohydrates. Their diet must be high in protein in order for them to reach a normal size. Lower protein diets or poor quality foods stunts normal growth. If feral cats have access to high protein diets we will see them reach normal cat-size, sometimes the upper range of normal cat size, rather than being stunted; but we won’t see an emerging race of super-ferals from good diet alone. To produce a larger-than-average race of feral cats requires mutation followed by natural selection, as well as good diet (bigger cats need more food).

If cats eat a diet of high levels of protein, but which is deficient in other essentials, we get unhealthy cats with vitamin and mineral deficiencies or overdoses. Too much or too little of certain vitamins can be lethal. Too little calcium from eating nothing but muscle meat causes bone problems. Too much liver causes Hypervitaminosis A and skeletal problems. Too little vitamin D causes rickets. To grow large and healthy, cats need balanced diets, not just protein. Cats generally regulate their food intake (pet cats overeat through boredom) and feral cats expend much of their energy intake in hunting and foraging activities. A feral cat that gets a high protein diet will probably regulate its intake by hunting or foraging less often. It may be fitter, less scrawny and not stunted, but diet alone won’t make it – or its offspring - super-cat sized. Abundance of food means they may breed more successfully and more kittens survive, but the increase will be an increase of numbers, not an increase of individual dimensions. Eventually the number of cats means more competition for a finite food source and this could go 2 ways. Either the largest, strongest cats survive and breed, or the smaller cats which need less food will survive.

In short, a high-protein diet won’t lead to gradual increase in stature to create super-cat size animals. The effect of too much good food and the feline inclination to conserve energy can be seen in the overweight housecats of Britain and America. It still suits anti-cat organisations to exaggerate tales of feral monster moggies able to dispatch brown hares, new-born lambs and even domestic pets. Much of the damage attributed to monster feral cats (and to escaped “panthers”) is caused by stray and feral dogs, but many people prefer to ignore the fact that "man's best friend" is still a wolf at heart.

THE GIPPSLAND GIANT CAT

Tales of feral cats attaining giant sizes have been spread in Australia, where there is a vociferous anti-cat lobby, in the USA (where an ill-advised book portrayed feral cats as huge and dangerous) and in Britain. A few years ago, a faked photo of a man holding a supposedly giant cat ("Snowball") did the rounds accompanied by a story that the cat was due to the effects of mutagens on feral cats. The photo is known to be a hoax and story of the photo and its originator is reported on numerous websites.

Australian feral domestic cats are sometimes described as twice the size of normal domestic cats. Most are within the normal domestic cat size range, though a few exceptional individuals occur. Giant domestic cats can 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" (48" total length). 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 leopard-sized predators. A pet Maine Coon wass recorded as reaching 49 in (121cm) total length (equalling the Gippsland specimen's size) 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. Genetic testing would be required to rule out genetic abnormalities that could lead to abnormally large size. Giantism due to a pituitary gland tumour 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. Claiming something to be “leopard-sized” is meaningless if the leopard subspecies isn’t specified! Scientists claim this abnormally large cat is proof that Australia feral cats grow to enormous proportions and present a significant threat to small wallabies.

EVOLVING INTO A NEW LANDRACE

Just as the dingo became a pariah dog (a de-domesticated form of domestic dog), the unowned descendants of ship’s cats and settlers’ cats in the 1700s and 1800s easily went feral, adapting to their new environment and becoming subject to natural selection. Some Australian researchers believe the genetics of feral cats furthest from human settlements should be studied to see if they are becoming a new landrace. Since their introduction, some cats are claimed to have become a quite different animal that fills the apex predator niche where dingoes and foxes are scarce.

Commentators tend to compare them to the Maine Coon (USA), Sokoke (Kenya) and Cimarron Uruguayan dog (Uruguay) in the way they have adapted to the environments into which they were introduced. The Cimarron Uruguayan is descended from mastiff-type dogs introduced by Spanish colonists and allowed to go feral. The hardy Maine Coon is descended from farm cats and domestic mousers. The Sokoke, also descended from European house cats, has a more recent origin, is not a large breed and has a more oriental (Siamese) shape.

The removal of the thylacine – Australia’s only marsupial apex predator at the time Europeans arrived – to protect introduced livestock, left a gap in nature. The ancestors of the domestic cats evolved to thrive in semi-arid regions and hunt the small rodents also introduced into Australia by settlers. Without competition and with plentiful prey – native and introduced – cats had only each other to compete with. In some regions, natural selection seems to favour those with genes for large size, perhaps to exploit larger prey.

Efforts to eradicate feral cats through shooting and poisoning are largely ineffective as they quickly repopulate cleared areas. In 2023 a “goo gun” was developed that shoots sticky poison onto cats so that they ingest the poison when they groom the goo off themselves (and colony members that groom each other also get poisoned). Cat pelts have long been exported from Australia, but a more recent suggestion is to exploit some people’s urge to own large apex predators – not as large as lions or tigers etc, but a feline predator larger than the typical domestic moggy. In essence, to export oversized ferals out of Australia instead of killing them.