Mammalia na ndege wanapataje rangi yao?

The Orange-bellied parrot. Read this article and see if you can tell me where the parrot gets it’s orange color, and what about green?
Photo credit Paul Oliver.

When light hits an object, it can either be bent, reflected or absorbed. Something that is black absorbs all the light, and no light is reflected. Something that is white, reflects all the light. Something that is red absorbs all the colors of light except red, which is reflected. Something that is yellow, absorbs all the colors of light except yellow, which is reflected. Learn more about light here.

Wakati mwanga inapokutana na kitu unaweza kupindwa, kurudishwa, ama kufyonzwa. Kitu cheusi kinafyonza mwanga wote, na hakuna mwanga unayorudishwa. Kitu cheupe kinarudisha mwanga wote. Kitu chekundu kinafyonza rangi zote za mwanga kasoro mwanga yenye mawimbi nyekundu, ambayo yanarudishwa. Kitu chenye rangi ya njano vilevile kinafyonza rangi zote za mwanga kasoro ya njano, na mawimbi haya yanayohusika na rangi ya njano hurudishwa. (SCROLL DOWN FOR FULL SWAHILI POST)

Different things can affect whether an object has color. Something can actually have the color in it- or what we call a pigment. When you go to a paint shop, they take pigments and mix them into a liquid to make the paint. The other way that something can have color is because of its structure. If you hold a shiny CD, or a bubble of soap, you see a lot of different colors depending on how the light is hitting it. There’s actually no pigment there, but that thin layer of soap molecules or tiny grooves on the CD interferes with the light that hits it, and different colors are reflected back into your eye. When you look at the blue sky, what you are seeing is blue light that has been reflected by the layer of air molecules on the edge of the atmosphere. If you want to get technical, this is called scattering. At sunset, when the angle of the light hitting the atmosphere changes and depending on how much dust or moisture is in the air, determines the amazing red or orange sunsets that you can get.

One of the things that I find really interesting is that all mammals and most birds only have one type of cell that produces pigment in their skin. The cell is called a melanocyte and it can produce two pigments- eumelanin and pheomelanin. Eumelanin is black and grey/brown in color and what we normally refer to as melanin. Pheomelanin comes in shades of yellow-brown, orange-brown or red-brown in color. These cells are found in the skin and at the base of hairs and feathers and basically produce little amounts of pigment that is injected into the growing skin, hair, or feather. The pigment which is produced by the cell is influenced by genes that influence the cells to switch between the two pigments, or to stop and start producing pigment. More than 30 different genes controlling this function have been identified in mammals, but one of the more important and famous ones it is called the agouti gene. We’re going to talk more about this when we discuss zebra stripes and cheetah spots, but have you ever wondered how a porcupine produces quills that are striped black and white? As the quill grows, the agouti gene turns the melanocyte on and off, so the growing quill gets alternate doses of melanin and no melanin.

One of the questions I see often on Facebook guiding pages is about either unusually dark or black animals, or unusually light or white animals. We call animals that are darker than normal melanistic. Melanism happens when we get more melanocytes and more melanin produced by the melanocytes. It is genetic and can either be a genetic mistake in the individual or a gene it has inherited. The opposite can happen too. We can get fewer melanocytes or the melanocytes can produce less melanin. This is called leucism. Then we have a condition called albinism. Albinism can be caused by more than one genetic condition, but it is usually a mutation in a gene that produces one of the enzymes necessary for producing melanin. This means melanocytes do not produce eumelanin and as a result even the eyes have no pigment and as a result appear pink or red because you’re seeing the blood vessels in the eye.

So, if mammals and birds only produce melanins in their skin, how do we get other colors?

Another very important group of pigments are called carotenoids. Carotenoids produce the bright red, yellow and orange. I always think of bright orange carrots when I think of carotenoids. There are no cells in animals that produce them, instead carotenoids are produced by plants, bacteria and fungi and animals get them from their diet. So, when you see the bright pink on flamingos, or bright yellow weavers these colors have come from their food. But of course, it isn’t just as simple as an animal eating something with an orange pigment and turning orange. Animals still have to process the carotenoids that they eat, modify them, and then send them to the parts of the body that need them. For example, a red-fronted barbet needs to send red pigment to the forehead, and yellow to feathers in the rump. This is influenced by genes, and again is a process that can end up with mistakes- for example the black collared barbet normally has a red head, but occasionally there’s a mistake on the gene that influences the pathway that converts the carotenoid pigments that the barbet has eaten to red, and instead turns it to yellow. To understand the mechanism you would need to understand organic chemistry and how different bonds between carbon atoms affect which wavelengths of light get absorbed and reflected.

One of the interesting things about carotenoids is that they are actually really important for an animal’s health. Carotenoids can only be produced by things that photosynthesize such as plants or blue-green algae. Carotenoids are strong antioxidants. One of their main jobs in plants it to protect the plant from harmful UV radiation from the sun. They are important for regulating different functions in the body including the immune system which means they have a direct effect on an animal’s health. We’ve all seen the bright yellow & red plumage of different species of male weavers in breeding. This isn’t just to look pretty to attract females, but because these bright colors are produced by carotenoids that are also linked to health, the bright colors are a signal of a male’s health. 

As I mentioned earlier, colors can be caused by structure rather than pigments. It is hard to believe, but the bright blue of a cordon-bleu, lilac breasted roller, or the testicles of a vervet monkey, or even someone’s blue eyes is not caused by pigment. Blue pigment is very rare in animals. When we do see blue, what is actually happening? In the case of bird’s feathers, what is happening is that the light is hitting the surface of the feathers. Feathers are made out of keratin (just like hair) and these keratin fibers can be woven in such a way that they trap a thin layer of molecules of air. When light hits these layers of air they bend and bounce and some get absorbed by the dark pigment underneath, while the blue light bounces back into our eye. The same thing is happening on a vervet monkey’s scrotum, except this time the spacing and diameter of collagen fibers in the skin cause the light to scatter. Watch this great video to learn more.

Iridescence describes the metallic shining colors that we see on starlings, beetles, and other animals. This is another example of structural color and is caused by different layers of air and keratin fibers and result in different colors shining off the surface of the feathers depending on the angle of the light. In some of the most beautiful examples of greens, golds and purples on beetles you have up to 120 layers that create the incredible color. Here is a great paper to read more.

One color that we have not talked about yet is green. We can get a green just from the structure of the surface (like blue), for example green butterflies, or we can mix blue (structural) and yellow (carotenoid pigment). This is what is happening in the beautiful green color of an African Green Pigeon. When you say something like ‘never’, or ‘the only’, there is always going to be an exception. So, let’s look at some exceptions, and these exceptions are found in the bird world, especially important if you are guiding in East Africa.

A really interesting group of pigments found in birds are called poryphorins. Poryphorins that you might have heard of are chlorophyll which gives plants the green color or heme which gives blood it’s red color. But in the bird world, poryphorins make it into the plumage and are responsible for some really interesting colors that we find in birds. Turacos for example, produce two unique pigments in this family- turacin which is red, and turacoverdin which is bright green, making it the only green pigment in the bird & mammal world. Blue eggs are blue because of another poryphorin pigment called biliverdin. The camouflage brown in bustards and nightjars is caused by a brown poryphorin.

Another pigment unique to the bird world, in particular the parrot family is called Psittacofulvin. Read the scientific paper here. It is produced in pigment producing cells and gives parrots their red, orange and yellow colors. What is even more interesting about this is that the yellows in parrots fluoresce which means it reflects ultra-violet light. We can’t see ultra-violet light, but birds can, which means when we look at the “Brown parrot”, we’re seeing something that is different to what parrots are seeing. We also see it differently because we have different types of color vision ability and that is the topic of another article.

SWAHILI VERSION

Wakati mwanga inapokutana na kitu unaweza kupindwa, kurudishwa, ama kufyonzwa. Kitu cheusi kinafyonza mwanga wote, na hakuna mwanga unayorudishwa. Kitu cheupe kinarudisha mwanga wote. Kitu chekundu kinafyonza rangi zote za mwanga kasoro mwanga yenye mawimbi nyekundu, ambayo yanarudishwa. Kitu chenye rangi ya njano vilevile kinafyonza rangi zote za mwanga kasoro ya njano, na mawimbi haya yanayohusika na rangi ya njano hurudishwa.

Sababu tofauti zinaweza kutoa rangi ya kitu. Mara nyingine, kitu kinaweza kuwa na hiyo rangi ndani yake – kitu kinachoitwa ‘pigment’ kwa kiingereza. Ukienda kwenye duka la rangi, wanachukua rangi (pigment) tofauti na kuyachanganya kwenye mafuta au maji ili kupata rangi ya kupaka. Kwa namna nyingine, kitu kinaweza kuwa na rangi kwa sababu ya muundo wake. Ukishika CD, au povu ya sabuni, unaona rangi tofauti kulingana na namna mwanga inavyopenya ndani. Hakuna ‘pigment’, lakini kulingana na tabaka nyembamba ya molekuli ya sabuni au michirizi kwenye nyuso ya CD kuingiliana na mawimbi ya mwanga yanayorudisha nuru, na mawimbi ya rangi tofauti hurudishwa kwenye macho yako. Ukiangalia angani unaona rangi ya buluu, lakini hakuna ‘pigment’ ya blue angani. Unachoona ni mwanga wa buluu unayorudishwa na tabaka za molekuli za hewa kwenye angahewa. Ukitaka kujua neno la kifundi, kutawanya mwanga hivyo inaitwa ‘scattering’. Wakati jua lina zama, pembe za mwanga hugonga angahewa kwa badiliko kutegemeana na kiasi cha vumbi au unyevu angani, na kusababisha rangi za nyekundu na machungwa tunayoyaona kiajabu.

Kitu kimmoja kinachonivutia kuhusu mammali na baadhi ya ndege, ni kwamba kuna aina mmoja tu ya chembe yenye uwezo wa kutengeneza rangi (pigment) kwenye ngozi yao. Hizi chembe zinaitwa ‘melanocyte’ na zinaweza kutengeneza aina mbili za pigment- zikiwa ‘eumelanin’ na ‘pheomelanin’. Eumelanin ni nyeusi na rangi ya kijivu, na huwa kawaida tunaiita melanin. Pheomelanin inarangi tofauti ya njano-kahawia, nyekundu kahawia, na machungwa-kahawia. Hizi chembe zinapatikana kwenye ngozi na kwenye shina la nywele na manyoya, na hua zinatengeneza kiasi cha pigment inayoingizwa kwenye ngozi, nywele, na manyoya yanavyoota. Aina ya pigment inayotengenezwa inasababishwa na jeni amabayo inaweza kusababisha kubadilika kati ya pigment mbili, kutengeneza au kutokutengeneza pigment. Kuna aina ya jeni 30 tofauti yenye uwezo wa kudhibiti kazi ya hizi chembe kwenye mammalia, na yenye umaarufu zaidi  zinaitwa jeni ya ‘agouti’. Nitaiongelea zaidi ninapongea kuhusu mistari ya pundamilia na madoa ya duma, lakini kama umewahi kufikiria namna nungunungu anavyotengeneza miiba yenye mistari nyeusi na njano, ni jeni la agouti linalosababisha chembe za melanocyte kuwaka na kuzimika. Hili linasababisha mwiba ukiota ipate dosi tofauti ya melanin na kunyimwa melanin.

Kuna swali naona mara kwa mara kwenye mitandao ya Facebook kuhusu wanyama wenye rangi nyeusi au nyeupe kuliko kawaida. Wanyama weusi kuliko kawaida wanaitwa ‘melanistic’. Melanism inatokea wakati wanyama wanpata chembe za melanocyte zaidi na melanin zaidi kutengenezwa na hizo melanocyte. Hii hali inatokana na jeni na inawezakuwa kosa kwenye jeni au kutokana na kurithi jeni kwa wazazi. Kinyume chake kinaweza kutokea pia. Tunaweza kupata upungufu wa chembe za melanocyte au hizo melanocyte kutengeneza upungufu wa melanin. Hali hii inaitwa leucism. Halafu kuna hali inaitwa albino au zeruzeru. Albinism inaweza kusababishwa na hali ya jeni tofauti, lakini kawaida ni ‘mutation’ kwenye jeni zinazotengeneza kimeng’enya kinachohitajika kutengeneza melanin. Hii inasababisha melanocytes kushindwa kutengeneza melanin na kwa hivyo hata macho yanakosa rangi na huwa yanakuwa nyekundu kwa sababu unaona rangi ya damu kwenye macho.

Basi, kama mammali na ndege wanatengeneza aina za melanin peke yake kwenye ngozi, tunapataje rangu tofauti.

Kuna kundi la pigment muhimu linayoitwa ‘carotenoid’. ‘Carotenoid’ zinatengeneza rangi za njano, nyekundu na machungwa. Huwa ninakumbuka neno hili nikifikiria rangi ya karoti. Hakuna chembe kwenye wanyama yenye uwezo wa kutengeneza ‘carotenoid’, badala yake ‘carotenoid’ zinatengenezwa na mimea, bakteria, na ukungu, na wanyama huwapata hizo rangi kwenye chakula chao. Inamaana ukiona rangi ya pinki kwenye heroe (flamingo), au njano kwenye manyoya ya weavers inatoka kwenye chakula chao. Lakini, sio rahisi tu kama vile mnyama anakula kitu chekundu na kubadilika kuwa mwekundu. Wanyama baado wana mchakato, na wanahitaji kubadilisha zile carotenoid, na kuzitumia kwenye sehemu zinazohitajika. Kwa mfano, red-fronted barbet anahitaji kutumia nyekundu kwenye kipaji cha uso, na njano kwenye mkia. Hii nayo inasababishwa na jeni, na vilevile ni mchakato unaoweza kupata makosa- kwa mfano, kawaida Black-collared barbet ana kichwa chekundu, lakini inaweza kutokea kuwepo na kosa kwenye jeni zake zinazochochea mfumo wa carotenoid kwenye chakula chake kubadilishwa kuwa nyekundu, na badala yake kuwa njano. Kuelewa njia hii tungehitaji kuelewa kemia ya kikaboni na kuelewa namna mivutiano tofauti kati ya atomi za kabon husababisha mawimbi tofauti ya mwanga kufyonzwa au kurushwa. 

Kitu kimoja cha kuvutia kuhusu carotenoid, ni kwamba ni kemekali au virutubisho muhimu kwa afya ya wanyama. Carotenoid huwa zinatengenezwa na viumbe venye uwezo wa usanidimwanga kama mimea na algea ya buluu. Carotenoids zina nguvu sana kama antioxidant. Kazi yao kubwa ni kulinda mimea kuzuia mawimbi makali ya mwanga wa UV. Ni kemekali muhimu kwa mifumo tofauti ya mwilini na zinaweza kuchochea afya ya mnyama. Wote tumeona mfumo mzuri wa manyoya ya dume ya weavers kipindi cha kuzaliana. Hii sio tu kuvutia majike, lakini kwa vile hizi rangi za kuvutia zinatokana na carotenoid ambazo zinahusika na afya, zinaonyesha ishara ya nguvu za yule dume. 

Kama nilivyotamka kabla, rangi pia zinaweza kutokana na muundo badala ya ‘pigment’. Ni ngumu kuamini, lakin rangi ya buluu kwenye manyoya ya cordon-bleu, lilac-breasted roller, au kwenye korodani ya dume ya ngedere, au hata macho ya blue kwenye binadamu, haisababishwa na pigment. Pigment ya buluu ni adimu sana kwenye wanyama. Huwa tukiona buluu, ni nini kinachotokea? Kwenye manyoya ya ndege, kinachotokea ni kwamba mwanga inapiga kwenye uso ya manyoya. Manyoya yameumbwa kwa kitu kinachoitwa keratin (kama nywele) na hizi uzi za keratin zinaweza kuungwa kwa namna ya kudaka tabaka ndogo ya hewa. Mwanga ikigonga hii tabaka ya hewa inapinda na kudunda na mawimbi mengine yanafyonzwa na pigment chini yake, lakini mawimbi ya buluu yanarudi kwenye macho yetu. Hii nayo inasababisha korodani ya ngedere kuwa buluu, lakini kwenye ngedere ni mfumo wa uzi za collagen kwenye ngozi zinazosababisha mwanga kusambazwa.

Iridescence ni neno linaloelezea mfumo wa rangi zinazong’aa kama kwenye starling, mbawakavu, na wanyama wengine. Hii nayo ni mfano wa rangi ya muundo badala ya pigment, na inasababishwa na tabaka tofauti ya hewa na uzi za keratin na kusababisha rangi tofauti tofatui kurudishwa kwenye uso la manyoya kutegemeana na pembe ya mwanga. Kwenye mifano ya rangi za kijani, dhahabu, na zambarau kwenye mabawakavu, unaweza kuwa na tabaka hadi 120 za kuumba rangi hizo.

Rangi mmoja ambayo sijaliongelea bado ni kijani. Rangi ya kijani yanaweza kutokana na muundo wa uso peke yake (kama buluu), kwa mfano kwenye vipepeo ya kijani, au tunaweza kuchanganya buluuu (rangi ya muundo) na njano (carotenoid). Hii ndio inachotokea kwenye kijani cha kuvuti ya njiwa wa kijani. Ukisema huwa kitu hakitokei au ni ya pekee, lazima hitilafu itakuwepo. Basi, tuangalia hitilafu hizi, na baadhi zinapatikana kwenye ulimwengu wa ndege, na ni muhimu kama unaongoza wageni Africa Mashariki.

Kuna kundi la pigment ya kupendeza zinazopatikana kwenye ndege zinazoitwa poryphorin. Poryphorin ambazo utakua umeshayasikia ni kwa mfano ‘chlorophyll’ inachoipa mimea rangi ya kijani, au ‘heme’ inachoipa damu rangi nyekundu. Lakini, kwenye ndege, poryphorin zinafikia kwenye manyoya na ni sababu mmojawapo ya rangi za ajabu kwenye ndege. Kwa mfano, Turaco wanatengeneza pigment mbili kwenye familia hii ya rangi- turacin, ni rangi nyekundu, na turacoverdin ni rangi ya kijani. Hii ni rangi peke ya kijani inayopatikana kwa mammalia na ndege. Mayai ya buluu ni buluu kwa sababu ya poryphorin nyingine inachoitwa biliverdin. Rangi ya kahawia kwenye bustard na nightjar inasababishwa na aina nyingine ya poryphorin.

Pigment nyingine ya pekee kwa ndege inapatikana kwenye familia ya kasuku na inaitwa Psittacofulvin. Soma nakala ya kisayansi hapa: here. Hii rangi nyekundu, ya machungwa, na ya njano haitokani na chakula chao, na inatengenezwa kwenye chembe pekee. Kitu cha kupendeza zaidi kuhusu hii rangi ni kwamba rangi ya njano ina “flouresce”, kwa maana inang’aa kwenye mwanga wa UV. Sisi hatuwezi kuona mwanga wa UV, lakini ndege wanaweza, na inamaanisha tukimwona “brown parrot”, tunaona rangi tofauti kabisa na wanachokiona kasuku. Vilevile, sisi tunaona rangi tofauti kwa sababu utafuati wa macho yetu, na hii ndiyo mada ya nakala nyingine. 

Sources

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Caro, T. Mallarino, R. Coloration in Mammals. https://doi.org/10.1016/j.tree.2019.12.008 Trends in Ecology & Evolution, Vol 35, April 2020.

Bagnara, J. Fernandez, P. On the Blue Coloration of Vertebrates. Wiley Online. Jan 2007. https://doi.org/10.1111/j.1600-0749.2006.00360.x

Siracusa, L. The agouti gene: turned on to yellow. Trends in Genetics, Volume 10, Issue 12, 1994 https://doi.org/10.1016/0168-9525(94)90112-0

Dyck, J. Reflectance Spectra of Plumage Areas Colored by Green Feather Pigments. The Auk , Apr., 1992, Vol. 109, No. 2 https://www.jstor.org/stable/4088197

David P.L. Toews, Natalie R. Hofmeister, Scott A. Taylor, The Evolution and Genetics of Carotenoid Processing in Animals, Trends in Genetics,Volume 33, Issue 3, 2017, https://doi.org/10.1016/j.tig.2017.01.002.

McGraw KJ, Nogare MC. Distribution of unique red feather pigments in parrots. Biol Lett. 2005. https://pubmed.ncbi.nlm.nih.gov/17148123/

Jiyu Sun, Bharat Bhushan, Jin Tong, 2013 Structural Coloration in Nature. RSC Advances, 20013. 3. DOI: 10.1039/c3ra41096j (https://www.researchgate.net/profile/Jiyu_Sun/publication/255772388_Structural_coloration_in_nature/links/53ec9bd50cf250c8947cd2f2.pdf)