Research History of Manx Cats

Most well-established breeders of Manx Cats have been in "the business" for ten to fifteen years, as I have been, but some prefixes have been around for much longer, having been established prior to 1970. What follows is a summary of four research projects that were under way at that time, which gives us an indication of the ‘evolution’ of our understanding of the Manx Cat.

Neil B Todd, 1961

Todd explored the manner in which the Manx factor was inherited in his paper, The Inheritance of Taillessness in Manx Cats, published in the Journal of Heredity in 1961. This research was conducted as his Undergraduate Honours Programme at the University of Massachusetts (for the uninitiated, ‘Honours’ is a specialist, optional year of study/research following a first degree).

He set about establishing a limited breeding programme and supplemented his findings through questionnaires and interviews supplied by breeders. All the breeding stock came, either from the Isle of Man or were known descendants of such cats. Matings were made between Manx Females x Tailed Males, Manx x Manx and a few Manx Males x Tailed Females.

Among other things, he noted that virtually all of his females had passed on a normal-tail allele (evidenced in tailed offspring), and that a reduction of litter sizes occurred in matings involving Manx females. His conclusion was, basically, that lethality or partial lethality could not be ruled out as an explanation of the absence of homozygous, or "true-breeding", Manx cats – even after 50 years of selective breeding for this by Manx fanciers.

Joyce M Howell & P B Siegel, 1963

Howell & Siegel conducted their programme at Virginia Agricultural Experiment Station at the Virginia Polytechnic Institute. Their findings on the Phenotypic Variability of Taillessness in Manx Cats were published in the Journal of Heredity in 1963.

Their report was based on a breeding program, which was commenced in 1958. The breeding stock was composed of known descendants of Isle of Man imports to the States. 78 matings were conducted involving various combinations of rumpy, riser, stumpy, Manx-normal and domestic-normal tailed cats. By this time, the Manx factor was accepted as a semi-lethal, and research continued on this assumption.

Their results indicated a clear reduction in litter sizes of matings involving Manx rumpies, risers and stumpies. This supported previous research and the assumption of semi-lethality. Further, they suggested that the variation of tail lengths of Manx offspring were due to "incomplete penetrance" of the Manx factor over the normal, tailed condition. That is to say, the Manx factor was showing only partial dominance to a greater (rumpy) or lesser (stumpy) degree.

Neil B Todd, 1964

Todd, now working at the Animal Research Centre, Harvard Medical School, Boston, published The Manx Factor in Domestic Cats, in the Journal of Heredity in 1964. He introduces his paper by confirming the semi-lethal nature of the gene, calling it an "autosomal lethal". He describes the lethal homozygous condition (MM) as "a tailless cat, without the cat"! He also quotes a personal communication from Siegel. This cites two matings of tailed cats of Manx parentage (indistinguishable from normal cats), that had produced stumpy progeny and that none of these stumpy offspring had survived to reproductive age. Further matings of this type had failed to produce any Manx offspring. This seemed to confirm a less than 100% dominance of Manx factor over normal tail.

Todd proposed the possibility of ‘modifiers’ involved in determining the degree of taillessness and proceeded on this theory. This paper was written against data gathered on matings of a single Manx female acquired in 1953 from the Isle of Man and does not take tailed offspring into account. He classed Manx offspring as either rumpy (including risers) or stumpy only. The female was mated to tailed cats and of the Manx produced, 50% were rumpy. She was then mated to 3 of her rumpy sons and of the Manx produced, 78% were rumpy. One rumpy male had sired no stumpy offspring.

The tentative conclusion drawn was that the female may bear one modifier for ‘rumpy’ and one for ‘stumpy’ and that the male cited may have only ‘rumpy’ modifiers. In summary, he suggests that there may be a simple genetic basis for inheritance of the rumpy or stumpy phenotype, supporting an earlier note that breeders had met with some success in selecting for rumpy vs. stumpy phenotype.

Todd also noted that the female had a chronic anal inflammation, associated with microperforate anus. This condition was passed to most of her Manx offspring, usually along with rectal prolapse in the females. These conditions were noted not to occur in other strains of Manx, but were associated with the Manx allele of the primary female in the study. None of her tailed offspring developed these problems. It was also noted that inbreeding intensified the abnormality. The female was mated to an unrelated, inbred Siamese and this produced the greatest number and severity of cases.

Joyce M Howell & P B Siegel, 1966

Howell & Siegel now became more interested in other effects of the gene on the spinal column. In their paper, Morphological Effects of the Manx Factor in Cats, published in the Journal of Heredity in 1966, they confirm the opinion that the Manx factor is semi-lethal and the effect of the primary genes is influenced by modifiers. They established a breeding programme and studied 52 adult Manx and 10 domestics. They also autopsied 20 kittens from that programme, which were stillborn or died prior to 12 months of age.

Generally, they found that all Manx cats had shorter vertebrae than normal cats - even more so in female Manx than in males. Skeletal malformations were observed in 9 rumpy and riser individuals, including reduction in the number of lumbar vertebrae; fusion of vertebrae; absence of sacral vertebrae (and in 5 cats, no sacral vertebrae at all); spinal column malformations. Autopsied kittens showed very high incidence of gut and skeletal deformity, with only 3 of the 20 having any sacral vertebrae at all. Two autopsied kittens had only 3 lumbar vertebrae (normal is 7 lumbar and 3 fused sacral vertebrae).

Their conclusions were that, even though the whole spine was affected to some degree, the reduction of vertebrae was concentrated mainly in the posterior region. That the greater the deletion of caudal, sacral and lumbar vertebrae, the greater the associated malformations and the higher the juvenile mortality rate. And that a simple genetic explanation could not account for all the variations of length and number of vertebrae in Manx cats.

So, What’s in it For Breeders Today?

Indeed. What can we glean from all that hard work?

For starters, we can now say what the Manx factor really is. Todd states in his 1964 paper that it is best described as "a lethal allele manifesting lack of dominance towards its wild-type (normal tailed) alternative" (italics mine). Or, you could say it is a semi-lethal with a complex of associated modifiers. Not an easy thing to work with!

Also, now we know why to select breeding stock with care. Sacral vertebrae are very important. The sacrum distributes all those nerves associated with the rear workings of the cat, i.e., the sacral plexus, which serves the legs and anal/genital region. Eat into that and we are begging for trouble. Risers are much more likely to have the important sacrum intact than ‘dimple’ rumpies would be. This is why it is preferable to keep risers and short stumpies for breeding, rather than ‘dimple’ rumpies.

The really short-backed Manx may be also hiding a reduced number of lumbar vertebrae and other malformations not immediately obvious except by radiograph – traits some may not wish to perpetuate in future generations. After all, Manx have measurably shorter vertebrae anyway, without selecting for it.

The "use stumpies, get stumpies" ethic may have a ring of truth to it, it seems. But it’s not that simple. As far as I am concerned, if using rumpies all the time runs the real risk of producing kittens with reduced or missing lumbar or sacral vertebrae, and the problems associated with that, I don’t mind using stumpies sometimes. For the sake of the health of the kittens.

After all, we aren’t in it just for the money - are we?