2.3 Skin pigmentation and pattern
Most domesticated livestock differ from their wild ancestors in the colour and pattern of the skin, hair or feathers.
SAQ 10
What gene-associated factors determine the colour of feathers in modern birds?
Answer
Melanin and other pigments are synthesised and become incorporated into the feather early in its development. Different-coloured feathers can emerge from the same follicle in successive moults, showing that this process is modulated by hormones especially, but not exclusively, sex hormones.
Most mammals, including extant descendants of the wild ancestors of domesticated species, are brown or black (Figure 5), or have markings that conform to consistent patterns (Figure 6). Many domesticated mammals and birds are white or piebald or skewbald (see Figures 2 and 3): pigs, sheep, horses, cattle (Figures 1 and 7) and dogs and cats (Figure 8) often have irregular and asymmetrical spots and blotches but very rarely the regular, symmetrical patterns seen in wild mammals (Figure 6). (Piebald (= black-and-white) and skewbald (= any other colour and white) are mostly frequently applied to horses, but as this section explains, very similar patterns are found in a wide variety of other domesticated species. These terms are still correct only for livestock; the equivalent term for wild animals is pied, e.g. pied wagtail.)
Such irregularly patterned hair and plumage has appeared spontaneously among many lineages of livestock. Farmers and pet fanciers selectively bred preferred animals, producing ‘pure’ strains of livestock. For example, sheep have been bred so that the fleece is usually a uniform creamy white (Figures 1d and Figure 9a), but the occasional exceptions (Figure 9b) are still common and viable, and the faces and legs of those with white fleeces are often black (Figure 9c) or mottled (Figures 1d and Figure 9d). Note the similarity in the speckled patterns of these sheep and that of some dogs (Figure 8d). The patterns of ‘belted’ pigs and cattle (Figure 7 a and b) are also remarkably similar.
Mottled and white feathers are also common in birds that have been bred under domestication (Figure 10). Domestic fowl are order Galliformes, related to peafowl and pheasants; the breeding plumage of male galliforms is almost always colourful and elaborate, but that of females is speckled brown because they and they alone incubate the eggs and protect the chicks. For similar reasons, the plumage of mallards and many other ducks (order Anseriformes) is sexually dimorphic, at least during the breeding season.
Question 3
Why would white plumage in poultry (Figure 10a) and peacocks (Figure 10c) during the breeding season be eliminated by natural selection in (a) males and (b) females?
Discussion
(a) Males without the species-specific breeding plumage would attract very few females and would be unlikely to succeed in male-male rivalry.
(b) Conspecific males may be unable to recognise white females as members of their own species. Predators would more easily notice white females brooding eggs on a nest than well camouflaged hens.
Question 4
What features of the plumage of the peacocks (Figure 10c) show that the only difference between the wild and domesticated forms is pigmentation, i.e. the mechanism of formation of the feathers is unaltered?
Discussion
The crown of feathers on the head and the long tail feathers are the same sizes and shapes in both the wild-type and white birds. Feather size and shape are determined by elaborately controlled proliferation, maturation and death of keratinocytes. A distinct lineage of cells, the pigment cells, synthesise various coloured molecules, or none, and may or may not be incorporated into the developing feather. If the pigment cells are absent or inactive, the feathers are white.