As this is an imprecise science I don’t believe that you can provide a confident answer to a question about the life expectancy of a Siamese cat. However, there are clear indications that the Siamese cat’s life expectancy is shorter than average. It is similar to that of the contemporary Persian cat. Both these cats have inherited genetic diseases and of the two the Siamese has the most. The reason why the Persian cat’s average lifespan is relatively short is because they inherit PKD in large numbers.
Based on my research some time ago, the Siamese cat has the longest list of inherited genetic diseases which you can read about by clicking on this link if you want to. These combine, in my view to reduce the lifespan. Perhaps their immune system is compromised. Perhaps overdoing selective breeding to create the extreme elongated and slender appearance of the modern Siamese cat is a contributing factor? For example they have eye problems.
Siamese cat lifespan 4 years less than average
It is said that the median lifespan of the cats in the Siamese group i.e. the family of cat breeds which are associated with the Siamese cat, is somewhere between 10 and 12.5 years. The top end of that range is 12.5 years. The average domestic cat is going to be a random bred cat and if that cat is well cared for you could expect him or her to live to about 15-17 years of age in my opinion. Therefore, the Siamese cat’s lifespan is about four years shorter than average which is identical to that of the Persian.
A study looking at over 4,000 domestic cats found that random bred cats had an average life expectancy of 14 while purebred cat on average live for 12.5 years. That implies that the average for all purebred cats is similar to that of the Siamese and Persian cats. I disagree with that. I think that a good percentage of purebred cats live to a similar age, on average, as random bred cats.
Difference between random bred and purebred lifespans
What I have said hints at a difference in lifespan between purebred cats and random bred cats. The purebred cats are the cat breeds. They are often pedigree cats and sometimes, the best examples, are show cats. There is certainly a difference.
Why should the lifespan of a purebred cat be shorter than that of a random bred cat? It can only be put down to selective breeding. Purebred cats are cosseted and often kept indoors and well cared for. This can’t be a question of poor cat caretaking. It must be due to selective breeding which allows deleterious recessive genes to manifest themselves resulting in shortened lifespan.
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Inbreeding depression and hybrid vgor
There are also issues regarding “inbreeding depression” and “hybrid vigour”. Inbreeding can bring about a decline in vigour or general weakness as Robertson’s genetics describes it. It is called inbreeding depression. It is linked to homozygosity of “an increasing number of genes with harmful effects”. The cumulative effect of deleterious polygenes can “mount up and eventually become noticeable”. In random bred cats most of these genes are present as heterozygotes “so that their existence may be unsuspected until exposed by the inbreeding”.
In short, inbreeding can reduce the lifespan of purebred cat. Selective breeding is essentially inbreeding and all purebred cats are selectively bred. That is what cat breeding is all about.
Siamese cat group
The Siamese cat group inludes breeds such as the Thai, Balinese (long-haired Siamese), Oriental Shorthair and Javanese. And there are variations in Siamese cats: Applehead, classic and old style plus the modern variant I have referred to.