Albinism

Albinism is the 'congenital absence of any pigmentation or coloration in a person, animal or plant, resulting in white hair, feathers, scales and skin and pink eyes in mammals, birds, reptiles, amphibians and fish and other small invertebrates as well.' Varied use and interpretation of the terms mean that written reports of albinistic animals can be difficult to verify. Albinism can reduce the survivability of an animal; for example, it has been suggested that albino alligators have an average survival span of only 24 hours due to the lack of protection from UV and their lack of camouflage to avoid predators. Albino animals have characteristic pink or red eyes because the lack of pigment in the iris allows the blood vessels of the retina to be visible. Familiar albino animals include in-bred strains of laboratory animals (rats, mice and rabbits), but populations of naturally occurring albino animals exist in the wild, e.g. Mexican cave tetra. Albinism is a well-recognized phenomenon in molluscs, both in the shell and in the soft parts. It has been claimed by some, e.g. that 'albinism' can occur for a number of reasons aside from inheritance, including genetic mutations, diet, living conditions, age, disease, or injury. However, this is contrary to definitions where the condition is inherited. In plants, albinism is characterised by partial or complete loss of chlorophyll pigments and incomplete differentiation of chloroplast membranes. Albinism in plants interferes with photosynthesis, which can reduce survivability. Some plant variations may have white flowers or other parts. However, these plants are not totally devoid of chlorophyll. Terms associated with this phenomenon are 'hypochromia' and 'albiflora'. Biological pigments are substances produced by living organisms that have a colour resulting from selective colour absorption. What we perceive as 'colour' are the wavelengths of light that are not absorbed by the pigment, but instead are reflected. Biological pigments include plant pigments and flower pigments. Animals can appear colored due to two mechanisms, pigments and structural colors. Animals may have both biological pigments and structural colors, for example, some butterflies with white wings. Many animal body-parts, such as skin, eyes, feathers, fur, hair, scales and cuticles, contain pigments in specialized cells called chromatophores. These cells are found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, have a class of cells called melanocytes for colouration. The term chromatophore can also refer to coloured, membrane-associated vesicles found in some forms of photosynthetic bacteria. Chromatophores are largely responsible for generating skin and eye colour in poikilothermic animals and are generated in the neural crest during embryonic development. Mature chromatophores are grouped into subclasses based on their colour under white light: Animals can also appear coloured due to structural colour, the result of coherent scattering perceived as iridescence. The structures themselves are colourless. Light typically passes through multiple layers and is reflected more than once. The multiple reflections compound one another and intensify the colours. Structural colour differs according to the observer's position whereas pigments appear the same regardless of the angle-of-view. Animals that show iridescence include mother of pearl seashells, fish, and peacocks. These are just a few examples of animals with this quality, but it is most pronounced in the butterfly family. The primary function of pigments in plants is photosynthesis, which uses the green pigment chlorophyll along with several red and yellow pigments including porphyrins, carotenoids, anthocyanins and betalains. Definitions of albinism vary and are inconsistent. Whilst they are clear and precise for humans and other mammals, this is because the majority of mammals have only one pigment, melanin. Many animals have pigments other than melanin, and some also have structural colours. Some definitions of albinism, whilst taking most taxa into account, ignore others. So, 'a person or animal with very pale skin, white hair or fur, and pink eyes caused by a medical condition that they were born with' and 'a person or animal with white skin and hair and pink eyes' do not include feathers, scales or cuticles of birds, fish and invertebrates. Nor do they include plants. Some definitions are too broad to be of much use, e.g. 'an animal or plant with a marked deficiency in pigmentation'