In Kenya, a white reticulated giraffe and her equally white calf have attracted a lot of attention because such coloration is so rare in this species. These giraffes are not albino, though they have been repeatedly and erroneously identified as such. In fact, they are leucistic. Leucism and albinism are frequently confused because both cause white coloring in species in which that is not the norm. Despite similarities in the appearance of leucistic and albino animals, they are not the same.
To understand these two different conditions in mammals, you must know about the development of normal mammalian pigmentation. Long before birth, a structure called the neural crest develops in the area that will later become the spine. Cells from the neural crest migrate within the developing embryo and will later form a variety of different cell types, including the melanocytes, which are the cells that produce melanin.
Albinism is the absence of melanin and is caused by one of many possible genetic mutations that results in a lack of functional tyrosinase—a protein that’s essential for the production of melanin. In albino individuals, the melanocytes migrate to their proper position but are unable to produce melanin. In animals with leucism, at least some melanocytes fail to migrate to their proper location, leading to an absence of pigments in those areas. Because some or all of the tissue may be affected, sometimes leucistic animals are all white and sometimes the color is patchy.
One common way to tell an albino animal from a leucistic one is to look at their eyes. Albino mammals typically have reddish or pinkish eyes. The eyes themselves are not this color. Rather, the red blood cells in the retina show through the iris because it lacks any pigment to mask them. As melanin plays a role in the development of many parts of the eye—iris, optic nerve, rod cells, eye muscles and the retina—albino individuals typically have compromised vision. Eye color is normal in leucistic animals because the pigments that give eyes their color are derived from the neural tube (not the neural crest) and are therefore not affected by the developmental irregularities responsible for leucism.
Albinism and leucism occur in other animals besides mammals. In mammals, the absence of melanin is responsible for the pale coloration, as that is the only pigment present in this group of animals. Albinism is more complicated in fish, reptiles, birds and amphibians because they have additional pigments.
Although the many leucistic and albino animals include pythons, gorillas, salamanders, sharks rats, alligators, peacocks and lions, perhaps the most famous white animal of all time is the white whale of literary fame: Moby Dick. Author Herman Melville was inspired by a real-life white whale named Mocha Dick that sank many ships, had been harpooned multiple times and was finally killed after years of attempts by whalers. Though these two whales are usually assumed to be albino, it is impossible to know for certain which condition led to their unusual coloration.