Nature has its whims… It is not uncommon to see a black Gabar Goshawk, but you will rarely spot a white Red-faced Mousebird. Up to 25 percent of Gabar Goshawks are black – i.e. melanistic. Scientists don’t know the reason for this. A Red-faced Mousebird with pure white plumage, however, is very rare indeed. The red skin around the dark eyes is characteristic of the species. Black, i.e. melanistic specimens are also found in other birds of prey, such as the Ovambo Sparrowhawk (up to 2 percent of the Sparrowhawk population). White, i.e. leucistic individuals in the various species of birds are very seldom seen.
Just as rare are pure black or pure white mammals. A number of semi-melanistic Burchell’s zebras are found in Etosha National Park. They are not completely black but have a few white stripes and spots. Several years ago an almost pure leucistic black-faced impala was seen in the Namutoni area. It was an adult female with some brown hair in an otherwise white coat. The dark coloration on the rump and on the nose was still recognizable as well.
Leucism should not be confused with albinism. Leucistic animals do not have red eyes. Leucism is a harmless genetic disorder that causes the coat or feathers to be white and the skin beneath to be pink. Leucistic skin does not contain melanocytes, i.e. pigment producing cells. In albinism, by contrast, these cells are present but unable to produce the pigment melanin. As for leucism, the lack of pigmentation may show on the whole body of an animal or only in certain places, where isolated feathers or spots on the coat are white, while the rest of the body has the usual colour. Several Laughing Doves with white spots or single vaned feathers all over their body have been seen in Windhoek.
Melanism is an increase of the pigment melanin in the skin and fur of animals which causes their skin, hair, feathers or scales to darken. Melanism may be hereditary or can be the result of increased exposure to the sun, higher humidity, lower temperatures and other factors. The world’s best known melanistic animal is the black panther, a jaguar.
Deviation from their usual coloration tends to be fatal for most animals, because they lack the camouflage that is typical of their kind. A white bird or a white mammal stands out in its environment, as does a black animal in a sunlit landscape. But melanism can be an advantage for birds of prey when they hide in shady treetops from where they go hunting for prey.
Particularly striking was an almost white Black-necked Grebe which paddled about at the salt fields in Walvis Bay with its rather dark conspecifics. The bird clearly stood out from the others in their breeding dress.
Author: Dirk Heinrich