How Arctic Animals Turn White
( Originally Published Ealry 1900's )
ALTHOUGH I have not the details of any one particular case before me, so many instances are chronicled in which the hair of human beings, under the influence of strong mental emotion due to terror or grief, has become suddenly blanched within a single night or some such period of time, that the occasional occurrence of such a phenomenon must apparently be accepted as a fact. Such a change is, of course, due to the bleaching of the pigment with which the hair is colored, although we need not stop to inquire by what particular means this bleaching is accomplished; all that concerns us on the present occasion being to know that the hair in man may turn white in this manner under abnormal circumstances. And there appears to be evidence that under equally abnormal conditions a similar change may take place suddenly in the hair of the lower animals. This is exemplified by the well-known experiment made considerably more than half a century ago by Sir John Richardson on an Arctic lemming — a small mouse-like rodent, which habitually turns white in winter, although dark-colored in summer. In this in-stance the little animal was kept in a comparatively warm room till winter was well advanced, when it was suddenly exposed to a lower temperature. In consequence of the conditions under which it had been kept, this lemming was still brown in midwinter, when it ought to have been white. As the result of its first night's exposure, the fur on the cheeks and a patch on each shoulder became completely white, and by the end of the first week the whole coat had turned white. On examination it was found that only the tips of some of the hairs had become blanched, and that these white-tipped hairs were longer than the rest of the coat, apparently owing to a sudden growth on their part in the course of the experiment. By clipping these long white-tipped hairs the animal was restored to its original brown condition.
Nothing is said with regard to any change of coat on the part of this lemming previous to the experiment, but it is probable that none occurred. It seems, however, to be clearly demonstrated that the tips of the hairs lost their color by bleaching, induced by sudden exposure to the cold, and that the hairs thus blanched increased considerably in length in a very short period.
In spite of the very obvious fact that these changes occurred under abnormal circumstances, it has been argued that Arctic mammals which turn white in winter do so normally by a similar blanching of the hair of the summer coat, and that the greater length of the winter, as compared with the summer dress of such white animals, is due to a lengthening of the individual hairs of the former. Moreover, it has been inferred that the color-change is directly under the control of the animals themselves. Quite apart from many other considerations, one weak point in this argument is that the hairs in the subject of the experiment were white only at their tips. It was doubtless assumed that, had the experiment been extended over a longer period, the white would have gradually extended downwards till the whole hair became blanched. But had this been the normal way in which the change from a black to a white coat is brought about, it is obvious that animals ought frequently to be captured in which the coat is in the same condition as that of the lemming. So far, however, as I am aware, no such condition has even been described.
Moreover, it is perfectly well known that, apart from those which turn white in winter, a large number of animals have a winter coat differing markedly in color, as well as in length, from the summer dress. The roe-buck, for instance, is of a brilliant foxy red in summer, while in winter it is grey-fawn with a large patch of pure white on the buttocks. And it is quite clear that the change from red to grey, and the development of the white patch behind, is due. to the shedding of the short summer coat and its replacement by the longer winter dress. Obviously, therefore, it is natural to expect that a similar change of coat takes place in the case of mammals which turn white in winter.
That the change in spring from a. white to a dark dress is due to a shedding of the fur seems to be admitted on all hands, for it would obviously be quite impossible for long hairs to become short, or for white ones to turn brown. And even in animals which do not alter their color in any very marked degree according to season, the spring change of coat is sufficiently obvious. For the winter coat, owing to the long time it is carried and the inclemency of the season when it is in use, becomes much faded and worn by the time spring comes, and the contrast between it and the fresh and brilliant summer coat is very striking indeed. On the other hand, the summer coat is only donned for a comparatively short season, and that at a time of year when it does not become much damaged by the effects of the weather. Consequently no marked change is noticeable as the long winter hairs grow up through it, and it has accordingly become a common article of belief that, whether there is a change of color or not, the long winter coat is produced by a lengthening of the summer dress.
Apart from the evidence of animals like the roebuck and many other deer as to the existence of an autumn change of coat, as deduced from a difference in color, the fact of such a shedding of the fur is demonstrated by the circumstance that in many species, as, for in-stance, the mountain hare, the individual hairs them-selves, as seen under a microscope, differ appreciably in calibre at the two opposite seasons of the year. In that species, for example, the hairs of the winter coat are of a much finer character than are those forming the short dress of summer, which are comparatively coarse and thick.
Moreover, in spite of the natural tendency to believe in blanching on account of the aforesaid abnormal instances of turning white in a single night, there is abundant evidence to show that even in human hair the change from dark to white as age advances is brought about by the replacement of dark hairs by white ones, and not by the bleaching of the former. In this case, however, the change, instead of being seasonal and sudden, is gradual and due to age. If the change was due to blanching, we should, of course, find some hairs which were partially white and partially brown (or black, as the case may be). And here it may be remarked that if such partially blanched hairs were met with, we should naturally expect to find that it would be the basal half which was white, and the terminal half which retained its natural coloring — in other words, precisely the reverse of the condition obtaining in Sir John Richardson's lemming, thereby affording further presumptive evidence as to the abnormal condition of the change in that animal.
As a matter of fact, however, those of us who have reached an age when silver hairs have begun to make their appearance among the brown can easily satisfy themselves that such hairs are white throughout their en-tire length, and that a hair half white and half brown is quite unknown. From this we infer that the change from brown to white takes place in human beings by the gradual shedding of the dark hairs and their replacement by new ones from which pigment is entirely absent. So that normally there is no such thing as bleaching of individual hairs. The change is, indeed, precisely similar to the one which takes place at the approach of winter in mammals that habitually turn white at that sea-son, with the exception that, as a general rule, it is extremely slow and gradual, instead of being comparatively rapid, and also that the white hairs differ from their dark predecessors solely by the absence of coloring-matter. Unfortunately, there is no subsequent re-placement of the white hairs by dark ones !
The fact that the change from brown to white in the mountain hare is really due to a change of coat and not to bleaching was known at a very early period to the English naturalist Pennant ; and the existence of this change was likewise recognised by Macgillivray. It was not, however, till Dr. J. A. Allen, in a paper on the color-change in the North American variable hare published in the Bulletin of the American Museum of Natural History for 1894, demonstrated by actual experiment the truth of Pennant's statement, that the fact of the complete autumnal change of the coat in animals that turn white in winter was generally recognised by naturalists. So far as the spring change from the white to the brown dress is concerned, his conclusions are fully confirmed by Capt. G. E.. H. Barrett-Hamilton, who communicated some interesting notes on the change in the European mountain or variable hare to the Proceedings of the Zoological Society of London for 1899. The fact that the vernal color-change is due to the shedding of the coat seems, however, as already mentioned, to have been much more generally admitted than was the case with regard to the autumnal transformation.
Dr. Allen arrives at the conclusion that both the autumn and the spring change take place periodically and quite independently of the will of the animal, and also that they are but little affected by phases of the weather, although they may be somewhat retarded or accelerated by the prevailing atmospheric temperature.
So far as the fact of the seasonal change being normally beyond the control of the animal in which it occurs, Capt. Barrett-Hamilton is in full accord with the American writer; but he goes somewhat further, and believes that it is quite uninfluenced by temperature, or at least by such variations of the same as may be met with in different parts of the area of the British Islands ; and, as we all know, these are considerable!
As in the case of many other animals—deer, for instance—the change from the winter to the summer coat takes place very late in the season in the mountain hare in Scotland, specimens undergoing the change being often seen early in May. But the date of the spring change is no earlier in the south of Ireland, where the climate is much milder, although the extent of whiteness assumed in that district is very much less than in the north. This seems to demonstrate the contention that temperature has little or no influence on the change, so far as season is concerned.
That the animal has no control over the change from brown to white in autumn seems to be proved by in-stances referred to by Capt. Barrett-Hamilton, " in which variable hares transported from Scotland and from Irish mountains to southern and low-lying regions continued for some seasons to appear in the northern garb of snowy whiteness. This persistence of the habit of turning white, even in unsuitable conditions, together with the lateness of the moult, resulted frequently in the curious spectacle of a mountain hare running about in all its conspicuous Arctic livery under the bright rays of an April or May sun. After a few years such imported hares, or more probably their offspring, ceased to turn completely white, and the breed assumed the appearance of the ordinary hares of the southern locality to which they had been transported."
It would, of course, be extremely interesting to ascertain whether such transported individuals ever do give up the practice of turning white in winter, or whether it is only their offspring that do so; but, in any case, it is clearly demonstrated that the habit is very deep-seated and difficult to overcome.
Very curious is the circumstance that the mode in which the coat is changed in the variable hare at the two seasons of the year differs in toto as regards the parts of the animal first affected. On this subject, with one verbal change in the first sentence, I quote from Dr. Allen, who writes as follows : —
" In the fall the change begins with the feet and ears, the sides of the nose and the front of the head, which often become radically changed before the body is much affected ; while as regards the body, the change begins first at the base of the tail and extreme posterior part of the back, and at the ventral border of the sides of the body, working thence upward towards the middle line of the back, and from behind anteriorly, the crown of the head and a narrow median line over the shoulders and front part of the back being the parts last changed. In the spring the order of change is exactly the reverse, the moult beginning on the head and along the median line of the anterior half of the dorsal region, extending laterally and gradually to the ventral border of the sides of the body and posteriorly, and then later to the ears and down the limbs to the feet, which are the parts last affected, and which often remain but little changed till the head and body have pretty completely assumed the summer dress."
It is very hard indeed to conjecture any satisfactory reason for this remarkable, difference.
The American variable hare ranges, at ordinary levels, about as far south as Massachusetts—that is to say, nearly to the latitude of Madrid, and throughout the whole of this extensive tract it turns white in winter. On the other hand, owing to the much milder climate of Western Europe, no color-change takes place in the mountain hares of Ireland, while it is reported that in those introduced into Ayrshire and the neighboring counties of south-western Scotland the change is much less complete and regular than in those inhabiting the northern parts of the country.
An impression appears to be prevalent that in the more northern portion of their range both the mountain hare and the ermine (or stoat) are white at all seasons, but this does not seem to be authenticated.
Observations are wanting as to whether the changes of coat and color in the mountain hare bear any relation to the appearance and disappearance of snow, or whether they occur regularly at the same season of the year. In the case of the ermine in the Adirondack region of New York, Dr. C. H. Merriam tells us that in this animal the white livery is assumed only after the first fall of snow, while the resumption of the brown coat does not take place till the snow begins to melt. Unfortunately, he says nothing in regard to change of coat. The late Dr. Coues stated, however, that in the case of the ermine the bi-annual change of coat takes place at the same sea-son, but that it depends upon the condition of the temperature at the time whether the new coat differs in color from its predecessor. In other words, the change from brown to white might be due either to shedding the coat or to bleaching of the hair subsequent to such shed-ding. The case of the mountain hare is, however, strongly suggestive that the color-change is in all in-stances coincident with the shedding of the coat.
It is, of course, quite evident that the assumption of a white winter livery by mountain hares and ermines living in regions where the snow lies on the ground for a considerable portion of the year is for the purpose of rendering such animals as inconspicuous as possible when in their native haunts. And, so far as we know, such a change is universal among the species named when dwelling in high northern latitudes.
There is, however, another animal inhabiting the North Polar regions of both hemispheres in which the change to a pure white winter dress is limited to certain individuals. The species in question is the Arctic fox, of which the beautiful fur, in both the white and the blue phase, is now much affected by ladies. That both the white and the blue individuals of this species are in the winter dress will be evident to every one who examines such furs carefully, the length and thickness of the hair being quite decisive on this point.
With the single exception of Iceland, where they are always blue, it appears that the white and the blue phase are met with throughout the habitat of the species.
What makes the matter so puzzling is this : if blue foxes are able to thrive during winter in a snow-clad country, what necessity is there for their fellows—and, indeed, for any species—to turn white at that season of the year?
Important information with regard to the manner in which hair bleaches has been afforded by a communication from Mr. E. Metchnikoff, published in the Proceedings of the Royal Society for 1902. It is there stated that the all-devouring cells known as phagocytes are the cause of the mischief. These cells, which frequently have amoeba-like processes, are developed in the central or medullary part of the hair, whence they make, their way into the outer or cortical layer, where they absorb, and thus destroy, the pigment-granules. Numbers of these phagocytes may be seen in hair which is commencing to turn white.
" The part played by phagocytes," writes the author, " in the whitening of the hair explains many phenomena observed long ago, but not as yet sufficiently under-stood." Thus the phenomenon of hair turning white in a single night, or in a few days, may be explained by the increased activity of the phagocytes, which remove the pigment within an abnormally short period.