| Summary: | The hairs and the follicles in which they arise constitute the mammalian pilary system. It is a dynamic biological system in which the hair follicles undergo cyclically partial regression and redevelopment, producing simultaneously new hairs to replace the old which are due to be shed.
The pelage is important in maintaining the animals thermal equilibrium, protection from abrasion and in display etc. Nonetheless, knowledge of cervid pilary system is extremely limited. The present study covers its development, morphology, moult and the annual hair cycle. Effects on the cervid pelage of experimentally induced adverse nutrition, as well as varying habitat, are also studied.
Morphogenesis of follicles and development stages of the hair follow the general mammalian pattern. The details of anatomy and development have been recorded. Large guard hair follicles, which during development grow faster and attain larger dimensions, have been recorded for the first time in ungulates. They are related to "Tylotrichs" referred to by Straile (1960) and produce longer hair.
Both primary and secondary follicles are present. The first formed secondaries are larger and, like the primaries, possess a sweat gland, sebaceous gland and arrector pilii muscle. Like primaries they too produce medullated hairs. The later secondaries form non medullated woolly underhairs. Paired and branched follicles have been recorded amidst secondaries.
The primaries give rise to the overcoat and the secondaries to the undercoat.
The percentage of non medullated hairs in the birth coat of the fawn is less than in other coats. The hair follicles in which they arise continue to form postnatally and become fully functional in fawn winter coat. Adult winter coat differs from adult summer coat in colouration, length and diameter, here there is a greater development of medulla, and a well developed woolly undercoat is present. The latter is functionally lacking in adult summer coat. The autumn moult (adult) begins on the flanks and spreads cephalad and caudad. In the summer coat moult is caudad. The fawn birth coat moult is caudad. Their details have been documented.
The winter coat is greyish in colour while the summer coat is reddish yellow. The winter colouration is a product mostly of the colour zone in the top 10 mm of hair length. The winter ciat guard hairs stand more erect, because of the padding provided by the woolly undercoat. The adult summer coat has more sloping hairs and its colouration is the product of the top 20 mm of the total hair length. The characteristics of hair scale are recorded for representative hair types.
A morphometric study of hair samples from identical regions in the coats of black tail deer has been attempted and the features recorded. The hair increases in coarseness and diameter from fawn birth coat up to adult winter coat. The hair length increases up to adult summer coat but decreases in adult winter coat. The winter coat exhibited a consistent length to diameter relationship but this was lacking in summer coat. The hairs in white-spots of the fawn birth coat differed only in respect of colour from the adjoining non-spotted area and not in any other external morphological respect.
Fortnightly skin biopsy samples were taken by means of a trephine; their histological study provided data on the annual hair cycle. The overcoat hairs moult twice while the undercoat hairs are shed only once. The statement by Lyne (1966) that "the rate of follicle development is inversely proportional to the size of the mature follicle" Is true for guard hairs. First formed secondaries, however, reach resting stages earlier than do the later formed secondaries. Rate of growth is greater in larger follicles, which though not much advanced developmentally, reach larger size.
Effects of adverse nutrition on pelage have been studied experimentally, on normally growing hairs as well as those induced artificially by plucking resting hairs. Hair length and diameter were reduced in underfed animals and initiation of new hair growth was delayed. Reduction in width of medulla appeared to be responsible for most of the reduction in hair diameter. The medulla is an important insulator, as can be seen from its greater extent in Alaskan forms compared with those from Southern California living in dry hot habitats. Regression analysis indicated that in underfed animals, though actual hair diameter was less, relative increase in diameter for a unit increase of length was greater than that in well fed animals. Thus when nutrients are limited, growth in hair diameter (in turn dependent on size of medulla) enjoys a priority over growth in length.
Finally it is felt that to comprehend properly the phenomenon of hair growth in cervids future investigations could fruitfully concentrate on the biology of cells in the follicle base and the dermal papilla. The role of dermal papilla in hair growth and the process of differentiation of different follicle layers are of great significance but need to be well understood. === Science, Faculty of === Zoology, Department of === Graduate
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