The role of the skin in relation to the adaptation and the production of wool in the sheep.

Abstract

THE ROLE OF THE SKIN IN RELATION TO THE ADAPTATION AND THE PRODUCTION OF WOOL IN THE SHEEP (Invited Paper) H. B. CARTER* I. INTRODUCTION The domestication of the sheep in some form as a species may have occurred within the past 10,000 years but the use of wool as a textile fibre of any consequence may not have evolved much earlier than within the past 5,000 years. In broad terms the Neolithic Age was one that at best was an age of skins from the animal and, perhaps, also of fibres from the plant for early forms of textile fabrication. No animal fibres seem so far to have been associated with this early culture in any surviving textile form (Forbes 1956). In the Copper and Bronze Ages of the Mediterranean and Western Europe, the wool that did enter any form of fabrication was probably mostly reddish to chocolate-brown or black (at best parti-coloured) and not greatly different from the coarse hairy wools that even today account for the produce of about half the world' sheep (Clark 1952; Hawkes s and Woolley 1963). It was not until the countries of the Mediterranean basin had entered the Iron Age about 3,000 years ago that human civilization apparently had achieved the conditions where a technical need for fine white wools could be established. It is perhaps significant that, if there is any rational material basis for the legend of the Argonauts and Jason' quest for the Golden Fleece, this may s be the point in history to which it belongs. If any credence at all is to be given to the story, there is evidence (Bacon 1925) of a vessel setting out from ancient Thessaly to sail east through the Black Sea to the valley of the river Phasis and the mountains of the Caucasus and the lands near the Caspian Sea and the ancient East-West Trade routes. The most likely date for this event can be set about 1300 B.C. or one or two centuries before the conventional dating of the siege of Troy. This would not conflict with evidence gathering in other ways that about 1,000 B.C. we can detect the first signs of an important change in the wool textiles of the world from naturally-coloured dark wools to white fine fibres capable of being dyed in various colours and spun into soft fabrics to suit the tastes of a civilization becoming sophisticated and urbanised. If we couple this with the gradual northward and westward emergence of civilized man from the hotter and dryer south and east where light cotton and linen fabrics were more appropriate, then we have the conditions evolving for the cultivation of some such animal as the finewoolled prototype of what we call the Merino sheep today. We may even speculate with some good biological grounds that the ram with the Golden Fleece, whose *Agricultural Research Council, United Kingdom. 190 pelt Jason and his Argonauts sought, was symbolic of some such prototype recognised (not necessarily for the first time) as a mutation that Man could at last exploit to some effect. If this prototype became recognizable about 3,000 years ago, it probabl y did not become really significant until about 2,500 years ago during the period of the Phoenician and Greek colonization of the Mediterranean shores and islands. Nor, perhaps, did it become extensively valuable until the culmination of Roman imperialism about 2,000 years ago. But it was probably not until about 500 years ago in the Western Renaissance that fine white wool of good quality became an essential component of the economic progress of nations. As a highly significant element in world trade, such raw material from the skin of sheep may not be much older than 250 years. It is within this period then that all the present important variations among sheep have been effectively seized, numerically expanded and selectively exploited to meet the needs of a sophisticated agriculture and modern textile industry. It is within this period also that our knowledge of the skin as a structure and as a physiological organ of the sheep has grown slowly in step with advancing technology. It is, however, not much more than 100 years since we achieved any useful insight in terms that help us toward an understanding of the functions of the skin and its place as an organ of adaptation as well as of fleece growth and wool production. II. THE GENERAL CHARACTER OF THE SKIN AND PELAGE OF THE SHEEP Among the wild species of sheep, which today are essentially montane in their habitats, e.g. Ovis vignei, 0. ammon, 0. musimon, 0, canadensis, the general character of the skin and pelage is similar to that of many other wild genera in the sub-order Ruminantia. In this respect, the sub-family Caprinae, to which the sheep belongs, will bear a very close comparison with the family Cervidae, for example, in the main features of its skin and coat. The leading features are a skin of moderate thickness in which the primary and secondary follicles are sharply distinguished from each other in thickness, medullation and depth of implantation and are both seasonally decident, comparatively sparse and of relatively slow growth. The Cervidae may, however, possess almost twice the number of fine secondaries as the wild Caprinae. In both, the primaries are consistently associated with a tubular gland and the secondaries not. In both, the acinous glands are present though in neither are they well-developed except as adjuncts to the primary follicles. Today, however, the wild species of the genus Ovis are numerically insignificant beside the vast populations of the domestic species, Ovis aries, within which is found perhaps a more diverse range of pelage structure and integumental variation than is to be seen in any mammalian order. It is therefore an advantage to consider for the species as a whole, both wild and tame, the range in certain significant values which in various combinations, appear to characterize the adult skin and the pelage. Thus, skin thickness may range from 0.5 to more than 3 -0 mm; hair follicle population density from 5 to more than 150 per mm2; the ratio of secondary : primary follicles from 1 to more than 40; fibre length growth rate from 0.1 to more than l-0 mm per day; fibre thickness from 8 to more than 191 300 p if both primary and secondary fibres are considered together; incidence of fibre medullation, over the range of both fibre types, from 0 to 100% ; fibre decidence from 0 to 100%) varying from seasonal and non-seasonal patterns to permanent and continuous growth; tubular (apocrine) glands, associated with each primary follicle, from 1 to more than 5 per mm2, varying in size and apparent activity by a factor of about six; lobulated acinous (sebaceous) glands from l-2 to lOO+ per mm2, with an apparent size and activity range similar to the tubular glands; for the skin as a whole keratin fibres may be extruded at rates from about 1 to 30 g/ma/day (15.8-16.8 % nitrogen); the ether-soluble wax excretions, largely from the sebaceous glands, may be extruded at rates from 0.1 to 30 g/me/day (wax M.P. = 30-35 OC; total cholesterol = ca. lo-60% dry wax); the watersoluble suint excretion, mainly inorganic salts of K and Na largely from the tubular glands, at rates from 0.1 to more than 4-O g/mz/day (K = ca. 20.0% ; Na = ca. 1025%; N = ca. 4.0% dry suint); and for these three gross fractions of the fleece a range in moisture uptake (within the range R.H. 30.100%) of: wool keratin = ca. 7-32%; wax = ca. O-9-4.0%; suint -= ca. 14-600%. From this array of characters alone it is not difficult to visualise the range of the structural and physiological complexities which may be encountered in studying the skin of the sheep and the adaptations of which it is capable in achieving fitness, as a species, to the natural environments, or as a farm animal to meet the selective demands of Man. All these are quantitative characters whose variation may reflect in one degree or another the effects of some environmental complex or component operating within the limits of some gene-controlled system or arising from some inter-action with it. III. THE ENVIRONMENTAL RANGE OF THE SHEEP As a species comprising both the wild and the domesticated forms it is doubtful if there is another animal useful to Man whose environmental range is greater and, therefore, whose mechanisms of adaptation as a whole have been called into greater play. With a world population for the species somewhat less than 1,000 million, it must be accounted one of the most successful of the larger mammals in a biological sense, certainly among the Ungulates. Undoubtedly this is due to the wide dispersal and vast increase in the domestic forms of the species as a symbiote under the exploitation of Man. In this long evolutionary process, however, the sheep has been perhaps the most important and efficient of the animal species used by Man as an agent in his own struggle for biological dominance and civilized fitness. Under the selective and often conflicting pressures of human demands and natural environments, the sheep has displayed great resiliency and few species can have been cast into a greater variety of moulds or bent to so many purposes, alive or dead. The long story of its natural and economic adaptation still goes on. More than ever the skin and all that constitutes the emergent fleece ranks high in the heirarchy of our selective demands on the domesticated species. The reservoir of adaptive variation still seems able to supply new forms to meet the situations we create. As a frontier species the sheep may now have reached the limits of the environments in which we wish to operate, but as an established species within the spectrum of agricultural patterns we continue to create, there is yet no easil y defined limit to its future. Even in the wild state, as Cowan (1940) has shown, a 192 single species, 0. canadensis, has been able to range from the Arctic Circle to the Tropic of Cancer, about 40-50' of latitude, from ambient temperatures of -12 to -15 ' up to +5O'C, and within a precipitation range of 500-1000 mm o f rainfall per annum, from the desert conditions of Lower California to the high montane environment of the Canadian Rocky Mountains. In the domestic state the sheep has been carried by Man at some time within the historic past to every habitat he has himself occupied. Today, the sheep as a species ranges from about 66'N and 55' Latitude to the Equator and from sea-level to above 3,000 metres. S It is true that neither the range nor the species numbers would be possible without the guiding hand of agricultural man and the protection of his husbandry. In a wild state in its various present forms, at least half the world population of sheep would, if released immediately, probably suffer extensive decimation, if not extinction, even where feral predators are not a major factor. The general range of habitats occupied by domestic sheep in one form or another is shown in Table 1. IV. THE PLACE OF THE SKIN IN THE GENUS OVZS AS AN ORGAN OF ADAPTATION The general importance of the skin of the sheep and its fleece has been well reviewed by Hutchinson and Wodzicka-Tomaszewska (1961) and by Blaxter (1962) in relation to the major physiological questions of adaptation to the environment, both experimental and natural. We may note, however, that in all work so far reported concerning the insulating or general adaptive significance of the skin and the pelage of the sheep in relation to the environment, little account seems to have been taken of the presence in varying degree of deposited constitutents of animal origin other than the keratin fibres whose properties so far have had the major claim on our attention. Whatever the subtleties of their physiological origins and biophysical and biochemical attributes, it is undeniable that there is among sheep a very great variation in the absolute and relative amounts of the etherand water-soluble components which may so readily be demonstrated by simple methods of fractionation. No discussion of the physiological place of the skin and fleece in relation to the adaptation of the sheep is well balanced if we do not keep firmly in mind that the follicles with their variously associated tubular and acinous glands within the context of their neuro-vascular relations are an integrated system. Whether we consider the extruded products of these structures as secretions of adaptive significance or as inevitable end-products of metabolism, there is no gainsaying that the growing fleece contains certain organic waxy and inorganic hygroscopic compounds in widely varying amounts and proportionsaccording to age, environmental conditions and genotype. The study of these complex physio-genetic relations has rarely, if ever, been made experimentally in any well-integrated fashion as expressing or modifying the adaptations of the animal to its physical environment. Without such an approach whole areas of our appreciation of the ecology of the sheep lie dormant. Some impression of the scope of this intra-specific variation is shown in Table 2. To the physiologist -and indeed to many other scientists-the sheep is a sheep as long as it bleats, ruminates and grows something called 'wool'. Occasionally it may appear that it is something more specific, such as a Merino or a 193 TABLE 1 Basic Environments. Mean Annual Mean Annual TABLE 2 Wool, wax and suint in the fleece sf various types of sheep. Blackface. More often, in experimental studies it seems to be some obscure cross of uncertain age and sex with a very doubtful period of fleece growth, emerging from some unmentionable past into a politely vague present. Lee (1961) has recently, and rightly, stressed not only the technical but also the philosophical inadequacies of the marriage custom in many research laboratories which so uneasily and often so fruitlessly mates beautiful and precise bench expertise with the chance sweepings of the sale yard or the fortuitous by-blow of the butcher. Nowhere is this more evident than in the, as yet, desultory studies of the physiology of the ovine skin and pelage. It may reasonably be assumed that for any given physiological question we are likely to ask in the immediate future, there is a precise and useful procedure ready and available to assist us in finding an answer by experimental methods. What is not so sure is that the experimenter will seek out the appropriate range of suitable animals, especially with regard to definable genotype and immediate pre-experimental history, from which answers of sufficiently general application may be derived. For example, the admirable studies of Blaxter (1962) have yet to be extended both in principle and detail before we shall have useful answers by which to assess the true adaptive place of the skin and fleece as insulating barriers against heat disposal from the animal or of heat gain from the environment. The differences in the thermodynamic or psychrometric properties of both skin and pelage which may arise from differences in the activities of tubular and acinous glands and their accumulated by-products within the growing fleece of one kind or another have yet to be assessed. It is not enough to assume that the fleece consists of keratin fibres alone in the matter of insulation, nor that evaporative cooling from the skin depends solely on the immediate activity of the tubular glands. The thermal conductivity of the fleece may well be modified by the degree of impregnation with organic waxy components and the limits of evaporative cooling possible may well be extended by the extra-cutaneous 195 storage of hygroscopic inorganic salts within the staple itself. So, too, the character of the sheep as a 'black body' in relation to the solar heat load and its dissipation may well be changed by a heavy and relatively immobile wax impregnation modifying conductivity as well as the emissivity of the outer surface. In either case, the physiological consequences may under certain conditions be adaptively critical and the physical computations much changed (Blaxter 1962; Macfarlane 1958). Conversely, excessive wax may under cold conditions perhaps enhance heat loss by conduction. It may conceivably also impede heat loss in certain types of fleece structure in extreme cold by reducing moisture uptake within the staple. B y adding rigidity to the staple, it may also minimise the convective and evaporative sources of heat loss by reducing fleece deformation in strong winds. All these are as yet experimental mysteries but their importance is paramount if we are to reason clearly on questions which invoke the skin and the fleece as structures of adaptive importance. In general, as an organ of adaptation the skin is probably best considered as a complex of three excretory systems: (i) that of the hair follicle or fibrous protein excretion; (ii) that of the crcinous glclnd or 'steroid' excretion; (iii) that of the tubular gland or electrolyte excretion. Linked by the neuro-vascular systems to the body as a whole, the general integument and its function in the adaptive responses of the sheep to the exigencies of natural environments must be studied in terms of these three systems collectively and simultaneously, pursued to their physio-genetic origins. V. THE PLACE OF THE SKIN IN THE GENUS OVIS IN RELATION TO FLEECE GROWTH AND WOOL PRODUCTION Under domestication, the genus Ovis has been selected by Man to evolve striking deviations from the character of pelage growth that may be found as viable phenotypes in the wild species or, indeed, among any other of the Ungulata, In other words, the physiological capacity of the sheep' skin for fleece growth has s been exploited over several thousand generations to yield oddities that as yet defy explanation. Some few varieties of the species under domestication display the well-marked seasonality of fleece growth and decidence characteristic of the wild species. Fully half the species shows some form of intermediate partial seasonality. But perhaps more than one-third- and, productively the most significant-of the extant world population is distinguished by a pronounced and well-sustained continuity of fibre growth. This is dominated especially by the widespread use of the Merino genotype and the subsequent intermingling with the long-wool genes to 'improve ' various indigenous 'breeds ' within the past two hundred years. On the eve of the Agricultural Revolution about 10,000 B.C., there may have been between 2 and 20 million humans all living as hunters and food-gatherersalmost certainly, however, less than the population of Australia today (Cipolla 1962). On the eve of the Scientific Revolution about 1700 A.D., the world population stood in the vicinity of 750 millions of whom perhaps 50 million were on the verge of entering the industrial age as we know it now, and thus at the point of demanding sophisticated changes in the sheep' fleece for the imminent advances s in textile technology. On the eve of the Atomic Revolution about 1950 A.D. the human population of the world stood at about 2500 millions, of whom about 1000 196 million were industrialized as distinct from agricultural social groups. Such figures may be used as broad measures of the growth in economic interest which has inevitably focussed on the sheep and its fleece during the 10,000 years of its domestication, i.e. during the turn-over of perhaps 2500 sheep generations. By the year 2000 A.D. we are assured the human population of the world could be 3000 million but no more than another 10 sheep generations perhaps remain to us in which, by that date, we may change or adapt the skin of the sheep to further compromises between the natural environments of the earth and the textile needs of Man. What kind of changes do we need or can we make? It is perhaps salutary to recall that while the skin and its pelage may have an obvious adaptive function to perform in the survival and growth of any sheep, the integument is not the sole nor even the main barrier against unfitness or loss. It is also salutary to remember that neither are all sheep kept by Man for the products of their integument. Of those that are, however, it is very probable that most would become extinct without Man's husbandry or would rapidly change under the pressures of the natural environment or in the absence of specific human selection. In considering the present distribution of the sheep throughout the world, it may also be further worth recalling that perhaps the most important agency in natural selection is the nature of Man 's thinking and that perhaps nothing reflects this more clearly than the character of the skin and the fleece. Whatever the innate character of the responses of the sheep to a given set of environmental conditions in the short or the long term, these are always likely to be modified by the intervention of human notions preconceived or otherwise. In this category are such old and often widespread views as these: that sheep do not sweat; that wool fibres are all hollow tubes; that from such tubes the 'grease' or 'yolk' is extruded to 'protect' the fleece; that wool fibres do not grow in the winter; that the fleece becomes 'thicker' in the winter; that 'wool' fibres become 'hair' fibres in hot climates; that a fine dense fleece is incompatible with survival in a hot climate; that a coarse shaggy fleece is essential to survival in cold climates; that, indeed, cold climates produce such fleeces; that hilly or mountainous terrain is incompatible with heavy fleeces; that fine wool cannot be grown on rich pastures -and so on. Some of these, and various ideas of the same general kind in relation to other aspects of adaptation in the sheep, may have their roots in direct and contemporary observation; others may have emerged from some such roots of fairly recent origin; yet others are lost in the misty past and arise from circumstances that once were valid but now no longer apply. Whether such concepts have a sound basis or not, the situation only too often arises that they are taken as the basis for some husbandry practice (or malpractice); or as the directing idea in some pattern of selection in breeding over a long period of years; or in making some initial selection or rejection of some particular genotype on which to build a flock; or in establishing some sheep population in a new district or another country. Whether right or wrong, the survival of any particular concept for a sufficient length of time at any period or place may create the semblance of the phenomenon it purports to demonstrate. Thus we undoubtedly do find sheep with coarse shaggy fleeces in cold climates as also we find sheep with short kempy pelages in hot climates. But we also find examples in such climatic extremes which are quite the reverse. So also we may find sheep that to all appear197 ante have fleeces that may have almost ceased temporarily to grow during the winter. But so also we may find, on the same pastures even, other sheep whose pelages have every appearance of growing continuously. It is tempting to generalise to the extent of saying that for every alleged adaptation in sheep involving some presumptive relation between the type of skin and pelage and the ambient environment there is also an exception. Morevover, so numerous are these exceptions that at times one is also tempted to wonder whether there are in fact any rules. In other words, it would seem that Man has at some time or other in the long course of his domestication of the sheep and its application to his various economic needs found the means of securing its adaptation to the range of environments he himself has penetrated. Of this, perhaps, the supreme example is the Merino genotype which for so many centuries was regarded as the peculiarl y specific property of the Spanish environment and its human tenants. Within 200 years, this variant of the species has been carried to every major climate of consequence to human pastoralism and agriculture and there succeeded within the context of the economics of its time and place. However, this does not mean that it has been equally successful everywhere nor that it has not been the victim of the kind of misconception of which several examples have been given above. Less dramatic but no less illustrative of the resiliency of yet other variants is the widespread use of the breeds of British sheep, of which, in relation to the production of wool, the Lincoln and the Border Leicester may be taken as particular pioneering examples with a wide climatic range, and sharing with the Merino a long history of misconception wherein it was assumed that they, too, were the peculiar and specific property of British genius and the British environment. It is worth noting also that whereas preoccupation with the concept of specific breed fitness for particular environments has often deprived us of valuable extensions of the genus into new territory there have been other cases of unfit genotypes entering new territory in a thoughtless drive for wool production per se. Thu s within the Merino genotype, the types elaborated in the northern and eastern U.S.A. under the conditions of winter feeding in snow-bound barns about the mid-nineteenth century, with a fleece dominated by wax and a skin remarkable for its pleats and folds, were suddenly transferred to the semi-arid hot interior of Australia with disastrous results in many areas. The wrinkled skin and wax y fleece which may (or may not) have suited the winter cold of Vermont and its neighbouring states was perhaps lethal or semi-lethal on the treeless plains at high temperatures in western and north-western N.S.W. and Queensland. From all this we may fairly say that, in relation to the exploitation of the sheep as a wool-growing machine, Man has done more than temper the wind to the shorn lamb. He has intervened extensively in modifying the whole environment and in modifying the whole sheep the one in relation to the other. 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