‘Ghost Birds’ and albinism

7 Feb

Naturalist, birder and blogger Marcia Bonta recently posted an excellent examination of the complexity of bird coloration (‘Ghost Bird’). As I have alluded to in many of my animal posts on this blog, coloration in birds and mammals can confuse attempts at accurate identification. Also,  guide books often lack information on the many color morphs of animals during their life cycles, seasons, and between sexes.

Adding to the confusion, the occurrence of white aberrations due to albinism and leucism*, which people often conflate into one term, ‘albino’, usually exacerbates identification. However, the two conditions are phenotypic (observable) traits arising from a variety of genotypic (of or related to genes and gene expression) origins. Nevertheless, gene mutations can result in both leucism and albinism.

Then there is the antithetic trait of a darker coloration than normal, called ‘melanism’, exemplified by the rare totally black cougar, leopard or jaguar. All of these are usually called ‘Black Panthers’. Many people assume that all black variants in these big cats are a single species, but they are not. Again, these observable traits have similar origins with changes in pigmentation, especially complete (or partial) lack of coloration. The commonality originates in changes of melanin production in the body, such as in albinism. Leucism, on the other hand, is more complex.

Albinism is associated with altered production of the pigment melanin, either complete absence or reduced amount. White animals can be the result of an complete absence of melanin production and are considered albinos. ‘Melanistic’ animals are the opposite, with an over-production of melanin. However, depending on the genetic cause, some animals can have a only reduced production of melanin, which may impart a much lighter overall color than the typical dark animal coloration; for example, light brown or beige instead of black.

Albinism is inherited alterations in the genes which result in albinism preventing the body from making the usual amounts of melanin. Thus, altered coloration depends on if an animal inherits either a single trait or set of traits that alter melanin production. A non-albino can carry a recessive mutation in a gene that affects melanin production. If that animal mates with another that also carries the same mutation, a portion of their offspring may be albinos. Very rare cases of albinism result in dominant heredity.

Melanin is the primary pigment that determines the color of a mammal`s (including human’s) skin, fur, and eyes. But albinism also occurs in fish, birds, reptiles, and amphibians. Often confused as a protein, melanin is a group of pigments produced in specialized cells, called melanocytes, in the body. Although the pigments are derivatives of the amino acid tyrosine (an important amino acid used to build proteins), they are actually fine granular polymers. Melanin is strongly associated with coloration of skin and fur in mammals, ranging from black to sandy and reddish-brown.

Animals manufacture their own melanin, which also contributes to coloration of bird skin and feathers. In birds, melanin is associated with ranges of blacks, browns, brownish-reds and pale yellow. However, some albino animals may also have some coloration from pigments that are not derived from melanin production. Melanin is not the only pigment affecting bird coloration.

Leucistic red-winged blackbird.

Leucistic red-winged blackbird.

Two additional pigments influence coloration in birds and plants: carotenoids and porphyrines. Birds acquire carotenoids from eating plants that produce the pigment in specialized cells (such as chloroplasts). Most people are familiar with the orange pigment of carrots, which is conferred by the carotenoid, carotene. Carotenoids can interact with melanins in birds to produce feather colors like the olive-green in several tanagers. The pigments are also physiologically important in animals, such as in their association with Vitamin A. Thus, pigments can serve more functions than just coloration. Even melanin contributes strength to feathers!

Porphyrins, like melanin, are amino acid derivatives but bind to metals and have characteristic light absorption patterns. The best known pigment is heme, which uses iron as a central functional group to form a matrix called hemoglobin that reflects red light. Because of their highly complex aromatic matrix, these pigments have very intense absorption bands in the visible region and may be deeply colored. Hence, they usually impart a brilliant coloration to bird plumage.

An animal, especially birds, with a complete absence of melanin may sometimes show some coloration due to these other pigments in the body that are unaffected by albinism. If the genetic aberration results in only reduced melanin production, referred to as an ‘albinoid’, an animal my exhibit an extremely lighter shade of those colors associated with melanin. Regardless, most albinos have pink eyes. Alternatively, an animal may show an absence of all types of skin pigments, including melanin. Or only pigments that are not melanin!  In both of these latter cases, it is a leucistic animal. An example is the rare white tiger with normal colored eyes.

Leucism, in contrast to albinism, originates more from cell abnormalities, such as in cell differentiation or migration from the neural crest (pool of undifferentiated cells in early fetus) to skin, hair, or feathers during development. A leucistic animal may lack pigmentation in its entire surface if all pigment cells fail to develop. Alternatively, if only patches on its body lack cells capable of making pigmentation, then the animal will display patchy white and color. This patchy lack of color is often referred to as ‘piebald’ and can be passed on to its progeny. Regardless, leucism can reduce all types of pigment production, whereas albinism results in the reduction of only melanin even though the melanocyte is still present.

Another difference between albinism and leucism is in eye color. When production of melanin is lacking in the retina and iris, albinos typically have red eyes due to the underlying blood vessels showing through. In contrast, most leucistic animals have normally colored eyes. With partial albinism, in which a single patch or patches of skin that lack melanin, the animal may still exhibit some coloration because of the presence of other pigments unaffected by albinism (melanin).

These two forms of aberrant coloration can be very difficult to verify in birds and reptiles. With albinistic birds, ruddy and yellow hues or other colors may be present on the entire body or in patches (common among pigeons), due to the presence of other pigments unaffected by albinism. To throw in another contribution to coloration, the structure of feathers adds to the diversity of avian colors. All animal coloration results from the way pigments interact with light. Most of the green and blue colors in birds are created through structural effects of feathers and refracted light. But that’s another future post. 🙂

Reading Bonta’s blog post brought back many memories. Back in the mid-1980′s, I bought acreage in the foothills of the Coastal Range of western Oregon, where I established an sheep and horse ranch. My first spring there, the ranch was inundated by people with binoculars standing on the side of the road pointing at the forest of Doug Fir beyond the pastures. A bit alarmed, I strolled down the driveway to investigate.

I was informed that an ‘albino’ hawk was a regular visitor to this area for several years and known to nest in the forest beyond these pastures. Every spring, local birders drive out with binoculars to view the bird.

As a field and molecular biologist, I was intrigued and watched for the hawk to confirm these reports. Sure enough, it made a showing. It was easily recognizable as a hawk, and the one lone ruddy tail feather suggested it was of the red-tail species. Otherwise, it was pure white. It’s full-colored mate confirmed the identity. This pair hunted and raised their young on the ranch for several years, where I often enjoyed watching and hearing the adults teach the young to fly.

I appropriately named my ranch ‘White Hawk Ranch’, which was also apropos since I raised colored sheep and had an interest in color genetics in both sheep and horses. It was a naturalist’s wild and domestic field lab.

Luecism derives from the Greek leuko meaning ‘white.’  Like many terms and words derived from the old classical languages, pronunciation varies with time, country and regional language. Many of the older biologists and ornithologists, especially from Europe, tend to use the older pronunciation lū-kizm.  On the other hand, younger generations, both professional and non, tend to pronounce it lū-sizm. Either is acceptable.
As Billie Holliday sang, “Tomāto, tomahto…”  We don’t have to call the whole thing off!

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