Lindos and the old town of Rhodes City are known for their cats. In each corner, at each turn you take in the small alleys, on each square that you visit, you will find cats. These cats are domestic or they turned wild. You either love them or hate them.
I imagine that these cats are highly agile and intelligent: their gaze is arrogant, knowing, mysterious, as if they are solving riddles during their naps and in the near dark. Anthropomorphic* as my imagination may be, they at least keep the island as clear as possible of mice and rats.
*Anthropomorphism: attribution of human motivation, characteristics, or behaviour 'projected' on objects, animals, or natural phenomena.
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| A young cat 'sloving riddles' in the shadows of a hot day... |
One time we found a really huge rat in Lindos, with its body spread out and a worm-like tail watching the night sky. The Cat was here. Cats are always aware and alert, thinking about tactics and strategic movements towards their prey. Though not every hunt is successful, one did catch this huge rat to let life pass away.
I imagine their view on the world is adventurous and slow at the same time. They can sleep for hours on end to spare energy and to reload their internal batteries. Their eyes will keep track of the surroundings, scanning for dangers and prey. These eyes... they are brilliantly amber coloured, almost as honey and shining bright. I wonder why they are amber in Lindos and Rhodes City; with no exception but really young kittens who start their life with blue eyes.
The European short-hair has a variety of eye colours, from grey-green to copper brown. In the Netherlands we see eye colours within this range, and cats from oriental origin with blue eyes or ‘odd-eyes’*. I didn’t encounter this array of colour schemes in Lindos and Rhodes City.
*Odd-eyed cat: an odd-eyed cat is a cat with one blue eye and one green, yellow or brown eye. It is a feline form of complete heterochromia, a condition which occurs in some other animals as well. The condition most commonly affects white coloured cats but can be found in a cat of any colour, as long is it possesses the white spotting gene.
The amber colour is seemingly due to the genes that select for a certain amount of melanin, and in relation to light refraction. My most intriguing question is: does the intensity of light has something to do with the production of melanin in the eyes of cats? The light intensity of Mediterranean countries like Greece is much higher than the light intensity of more temperate climates like in the Netherlands. Does this lead to a selection of more amber coloured eyes in Greek cats?
I didn’t find the exact answer in recent science studies, though there are implications that show that I might be on the right track.
The genetics of eye colour are complicated, and colour is determined by multiple genes. The concentration and distribution of pigmentation plays a major role, e.g. the brightly coloured eyes of many animal species are largely determined by pigments such as pteridines, purines, and carotenoids (Oliphant 1987). Pigmentation is clearly at the heart of it.
The photochemical properties of pigments make it an excellent photoprotectant: it absorbs harmful UV-radiation and transforms the energy into harmless heat to prevent indirect DNA damage. I imagine this is a more necessary feature for cats in countries with high light intensity than for cats in countries with low light intensity.
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| A creative painting on a tree in Rhodes city |
I imagine that Greek cats have developed a gene selection with lots of active melanocytes* in the cat’s eye: resulting in more melanin production and activity in the eye, colouring them yellow to copper brown. Therefore they are better equipped to cope with bright light situations and reflections of the radiation on light-shaded surfaces. Keep in mind that it's only a hypothesis.
*Melanocytes are melanin-producing cells (pigment).
What makes it more intriguing, is that melanin production in the cat's eye doesn't work exactly the same as melanin production in the skin of humans. Melanin activity in the cat's eye is fixed after becoming a sub-adult cat. In human skin the activity level of melanin production is not fixed, but fluctuating.
Recently a project is started called The Cat Genome Project. It aims primarily on understanding the genes of cats relating to diseases, and secondly on the “parentage testing, forensic analysis, and studies of evolution, including the reconstruction of domestication processes, fancy breed development, and ecological adaptation among the great roaring cats” (Genome Research 2007, Pontius et. al 2007). I didn't find the specific answer in this project, but it seems highly likely that the amber to copper coloured eyes are the result of the pressure of the surroundings: the high light intensity.
In my studies at the Open University I followed a course called Evolution. Next to the Darwin-story which is known to most people interested in Evolution Theory, it also sheds some light on how natural selection works.
The phenotype = genotype + pressure from the environment + interaction between genotype and environment.
In other words, the appearance of amber to copper coloured eyes in Greek cats (phenotype) may be the result of the genes selected (genotype), the increased light intensity (pressure from the environment) and the interaction between the genes and light intensity (interaction between genotype and environment).
I will keep up my hypothesis and track for more records on this phenomenon in the future. It just amazes me how variation works in life. In the section below, you will find literature and suggestions for further reading. I haven't had the time yet to investigate all of these articles and books, though I believe these sources might hold the key to my question. Possible progress will be reported on this blog.
Literature
Eizirik E, David VA, Buckley-Beason V, Roelke ME, Schäffer AA, Hannah SS, Narfströ K, O’Brien SJ and Menotti-Raymond M (2009) Defining and Mapping Mammalian Coat Pattern Genes: Multiple Genomic Regions Implicated in Domestic Cat Stripes and Spot, 2010 by the Genetics Society of America.
Genome Research (2007) Domestic cat genome sequenced, Cold Spring Harbor Laboratory Press, updated 2012.
Laties M (1974) Ocular Melanin and the adrenergic innervations to the eye, Trans Am Ophthalmol Soc. 1974; 72: 560–605.
Oliphant LW (1987) Pteridines and Purines as Major Pigments of the Avian Iris, Pigment Cell Research Volume 1, Issue 2, pages 129–131, September 1987.
Pontius JU, Mullikin JC, Smith DR (2007) Initial sequence and comparative analysis of the cat genome, Genome Res. 2007. 17:1675-1689.
Van Rhijn J et al (Cursusteam 2009) Levenswetenschappen 1: Evolutie, Open Universiteit Nederland, Heerlen, page 106-127.
Suggestions for further reading:
Hu DN, Simon JD, Sarna T (2008) Role of Ocular Melanin in Ophthalmic Physiology and Pathology, Photochemistry and Photobiology, Volume 84, Issue 3, pages 639–644, May/June 2008.
Różanowska, M (2011) Properties and Functions of Ocular Melanins and Melanosomes, in Melanins and Melanosomes: Biosynthesis, Biogenesis, Physiological, and Pathological Functions (eds J. Borovanský and P. A. Riley), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
Sarna (1992) New trends in photobiology: Properties and function of the ocular melanin - a photobiophysical view, Journal of Photochemistry and Photobiology, Volume 12, Issue 3, 28 February 1992, Pages 215-258.
Sarna T, Plonka P (2005) Biophysical Studies of Melanin Paramagnetic, Ion-exchange and Redox properties of Melanin Pigments and Their Photoreactivity, Biological Magnetic Resonance, 2005, Volume 23, II, Pages 125-146.
