The experimental analysis of feather pattern in the Amherst Pheasant, Chrysolophus amherstiae (Leadbeater).
Authors: Vevers Henry Gwynne 
Published: August 2010

Summary.

A brief account is given of the physical and chemical basis of colour in feathers, and of the process of feather development.

Analysis was made of the colour patterns and growth characteristics of normal male and female feathers in the pendent, back and saddle tracts of the Amherst Pheasant, Chrysolophus amherstiae (Leadbeater). Besides pigmentation particular attention was paid to the iridescent median barbules, which were described in terms of the lengths and widths of their reflecting surfaces. For comparison non‐iridescent barbules were described in similar terms. Other characters which affected the feather patterns were final feather size, barring, growth rate, and growth period.

Feathers of the pendent, back and saddle tracts were grown on male Amherst Pheasants receiving different intramuscular injections of oestrogenic and androgenic substances (oestrone, oestradiol dipropionate, stilboestrol, testosterone propionate, and testosterone dipropionate).

In all three tracts feathers grown with injections of oestrogens or androgens were smaller than their normal counterparts. This disparity in size was most marked in the pendent tract. The experimental feathers thus tended to approach the size of the normal female feathers.

Pigmentation of all the experimental male genotype feathers was markedly different from that of the normal male feathers. In the pendent tract the originally white vane became pigmented with dark melanin in the presence of oestrogens and androgens. With increased dosage of oestrogen light brown bars were also deposited in the feathers of this tract. Where a light brown bar crossed an iridescent dark melanin‐pigmented bar the colour of the latter was inhibited but its iridescent structure remained.

In the back tract regular light brown transverse bars were deposited in the presence of oestrogens, and there were traces of such bars in feathers grown with a large dose of testosterone dipropionate. In the saddle tract the yellow colour of the feather tip was lost in the presence of the hormones, and in the remainder of the vane light brown melanin was deposited as well as dark brown melanin.

Changes in barbule structure with hormone dosage were followed in greatest detail in the iridescent median barbules of the pendent subterminal bar. In general the hormones reduced the size of these barbules; with graded injections of oestrone the amount of reduction was approximately proportional to the dosage and to the reduction in shaft length. Also in the pendent tract there is evidently some interaction between the injected hormones and the factors responsible for the deposition of the subterminal iridescent bar. For although the size of the iridescent median barbules of this bar was reduced in the presence of oestrogenic and androgenic hormones the torsion of these barbules and its associated iridescence was never completely lost. Furthermore the experimental pendent feathers and the normal male pendent feathers showed clearly that barbule torsion can exist in the presence of light brown pigmentation and even in the absence of all pigmentation. This observation runs contrary to Rensch's (1925 a) postulation of a causal connection between barbule torsion and heavy melanin pigmentation.

In the back tract the iridescent structure of the median barbules was completely lost in the presence of the oestrogens. In the saddle tract hormone‐treated male feathers never lost the iridescent barbules of the subterminal bar, although there was some reduction in their size and number.

Barring patterns in the feathers studied were of two kinds: (a) those appearing in normal feathers of the male or of both sexes, and (b) those appearing only in female feathers and in male feathers grown with oestrogen injections.

In the pendent tract terminal and subterminal iridescent bars were well developed in the male and were present in reduced form in the female. In male feathers of this tract grown with oestrogens or androgens these bars were reduced in proportion to the reduction of the whole feather, but were never lost completely. The persistence of the torsion of the barbules of these bars has already been mentioned ((6) above).

In the saddle tract the subterminal iridescent bar of the male persisted in the presence of oestrogens. This bar did not occur in female genotype feathers.

In the back tract barring only occurred in normal female feathers and in male genotype feathers receiving injections of oestrogens. Differences in the angles shaft/bars were observed between the patterns of normal female and male genotype experimental feathers.

Analysis has shown that this difference is dependent upon the angle at which the barbs join the shaft, and it is considered that this angle is determined in the early feather germ, and that, in Chrysolophus, it is a sexual character unaffected by the hormonal environment.

The growth curves of the normal male feathers did not differ greatly from tract to tract except in the length of the growth period and the final size attained. The female feathers, on the other hand, showed greater differences in the rate of growth between the tracts. With injections of oestrogens and androgens the growth curves of the male feathers became similar to those of the normal female.

Some feathers showed a higher threshold of response to hormones than others. This high threshold was correlated not only with faster growth, but also with a longer growth period, greater final size, and larger cell size as measured in barbule cells. It is suggested that one of the ways in which oestrogens reduce the thresholds for these attributes is by increasing the rate of cell division and differentiation; since the growth period of the experimental feathers is shorter this would result in the production of smaller feathers.

Barbules and barbule cells near the shaft had a lower threshold of response to hormones than those more distant from it, and they were also smaller in size. It is suggested that the low threshold of response of the barbules near the shaft is correlated with the small size of the barbule cells.

In general, feathers with distal iridescent features had a low initial rate of growth, and it is possible that the morphogenesis of iridescence in feathers only occurs during periods of relatively slow growth.

A series of transplants between sexes was made with skin from the back and saddle regions of Amherst Pheasant chicks. Male skin grown on a female host yielded feathers similar in all characters to those of male feathers grown on a male bird receiving heavy doses of oestrogen. Female back skin grown on a male host produced dark almost black feathers, without the iridescence characteristic of a normal male feather, but with a little of the light brown melanin pigmentation of the type found in normal female back feathers. The pigmentation of these feathers (female skin on male host) suggests that the female feather germ may respond to the slight “feminizing” effect of the male hormones more readily than the male genotype feather germ, but the evidence is not conclusive.