A heat-sensitive enzyme paints the dark points on Siamese cats
Siamese kittens are born cream or white. Their dark ears, face, paws and tail appear over the first weeks because a mutated pigment enzyme only works where the skin is cooler than about 33 degrees Celsius. Owners in cold climates tend to have darker cats, and the same gene quietly rewires how their eyes connect to the brain.
The colourpoint pattern is a partial albinism caused by a change in tyrosinase, the enzyme that helps make melanin. At normal body temperature the altered enzyme stops working, but on the extremities, and on the face where air through the sinuses cools the skin, it switches on. As a result Siamese darken with age and stay paler in warm places. Early cats nearly all had seal points, almost black; blue, chocolate and lilac points were first dismissed as inferior before registries accepted them.
The ancestor is the Wichianmat, a pointed cat of Siam described in the Tamra Maew, a set of cat poems thought to date from the Ayutthaya Kingdom. A Thai legend claims that after Ayutthaya fell in 1767, the Burmese king Hsinbyushin read that such cats were as rare as gold and ordered them carried off. In 1878 President Rutherford B. Hayes received a cat named Siam, the first documented one in the United States, and in 1884 the British consul in Bangkok, Edward Gould, sent a breeding pair named Pho and Mia home to his sister Lilian, who later helped found the Siamese Cat Club in 1901.
Early imports had rounded heads and were moderately built. In the 1950s and 1960s judges came to favour ever slimmer cats, and breeding produced the modern show Siamese with its tubular body, wedge head and huge ears. By the mid-1980s the old type had nearly vanished from shows, and registries re-established it as a separate breed, the Thai. Kinked tails, once common, were bred out around the 1950s but still turn up among Thai street cats.
The albino allele also sends some optic nerve fibres to the wrong side of the brain, and many early Siamese went cross-eyed to compensate. Unlike many blue-eyed white cats, they hear normally. Crossing Siamese with Persians gave the Himalayan, and the long-haired form is known as the Balinese.
Source: Siamese cat
