372 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Makcii, 1906. 



The ColoroLtion of 

 MacmmaLls ©Lnd Birds. 



By J. Lkwis Honhotk, M.A., F.L.S., F./.S , M.B.u.L ., Ac. 



(Continued from page 343.) 



W'c will now turn to the second part of this paper, 

 which deals with the markings on mammals and birds, 

 cur object being to show that throughout these groups 

 certain si'OTS on the body will be found to differ in col- 

 our from the neighbouring p.u^ts, although in many 

 cases the differences will be either ;o slight or so transi- 

 tory in their nature that they cannot be said to serve 

 any of the purposes usually ascribed to the agency of 

 natural selection. 



The conditions instrumental in bringing on a moult 

 in mammals and birds are to my mind very obscure. A 

 high temperature is undoubtedly a stimulating cause, 

 as birds can be made to undergo a moult by merely 

 keeping them in a warm atmosphere, but it is also 

 necessary for a successful moult that the vigour of the 

 animal should not be at t(K> low an ebb, though, on the 

 other hand, it need not necessarily be very high. The 

 state of its " vigour " at the time of moult is visible 

 in the colour of the new coat, rather than in a suppres- 

 sion or suspension of the moult. .'\nd birds in captivity 

 in bad health will often show a tendency to become 

 white. Nevertheless, it cannot be denied that moult is 

 closely dependent on " vigour " ; for although the 

 moult will not be omitted if an animal's vigour be 

 very low, yet the attempt will [irove abortive 

 and the animal will die. So that an animal cannot 

 successfully moult without a certain amount of 

 "vigour," but a moult will be attempted at certain 

 fixed seasons, irrespective of that " vigour." 



Shortly before a moult takes place an alteration will 

 be visible in the colour of many mammals and birds; 

 this alteration is always in the same direction, namely, 

 a lightening or, as it has been termed " bleaching " 

 of the hairs or feathers, followed (and this is especi- 

 ally noticeable in the latter) by a breaking up and com- 

 plete disintegration of structure. 



I have referred to this matter in two previous papers,' 

 but it may well be dealt with more fully. It has been 

 generally assumed that this " bleaching " process is 

 gradual, and due to the action of weather and light 

 alone. This, as I hope to show^ below, is not so; the 

 actual disintegration may lie brought about by the 

 action of weather and light alone, but, if so, only after 

 the hair or feather has been physiologically disconnected 

 from the living tissues of the body. Tliis latter pro- 

 cess generally, but not always, takes place when a new 

 hair or feather is beginning to be formed, t 



One example of this is to be found in the so-called 

 " white tail " assumed in summer by our English 

 squirrel. Unlike the rest of its body, the tail is not 



* Ann. and Mag. Nat. Hist., 1900, Se.-. 7, Vol. 15, p. 492 

 Zoologist, 1901, p. 243. 



tThe leaves of deciduous trees ofler a similar, though not 

 precisely analogous case. They turn yellow in Autumn as soon as 

 the sap ceases to reach them and the coaling of bark tends to 

 disconnect them from the trees. No one supposes the yellow and 

 brown tints of Autumn to have been a gradual process going on 

 throughout the summer, it does not ccmmence until the leaf is 

 physiologically disconnected from the tree. Leaves on a broken 

 bough undergo a totally different change. 



moulted in spring, but shortly after the spring moult 

 the tail begins to turn to creamy white, beginning at 

 the tip and gradually spreading towards the base. 

 .Now, if this were merely due to the action of light, the 

 " bleaching " would go on equally all over the tail, 

 but this is not the case. 



Of this form of "bleaching," which usually pre- 

 cedes a moult, I could instance numerous examples. 

 In the case of the Anatidm, or ducks, in w hich group all 

 the primaries are moulted at the same time, the 

 bleaching of these takes place about a fortnight before 

 they are cast, the whole process taking ten days, or 

 at the most, a fortnight, and yet during the first eleven 

 months of their existence the change in colour was 

 hardly noticeable. 



This process of bleaching, however, need not always 

 be caused by an approaching moult. I had a striking 

 case some eighteen months ago exemplified by a Gad- 

 wall, which began to bleach about a month after he had 

 moulted. In the course of a fortnight his wings and 

 most of his body feathers w-ere of a light dirty brown, 

 and at the end of a month most of his feathers were 

 entirely disintegrated and in shreds. 1 need hardly 

 say that he was in very bad health. In the ordinary 

 course of events these feathers would have lasted ten 

 months practically unaltered, so that this disintegra- 

 tion cannot be put down solely to external causes. At 

 the follow ing moult the feathers were perfectly renewed. 



The so-called abrasion of the lips of hairs and 

 feathers is another cognate case, for, although it is 

 generally assumed that the tips gradually wear off 

 during the winter, I can, as regards birds, positively 

 state that this is not so. To take a single instance 

 out of many that suggest themselves. In the reed 

 bunting [E. scliocniclus) the black bases to the feathers 

 of the head will be obscured by the brown tips through- 

 out the whole winter down to the middle of March, and 

 yet, during the three weeks from the middle of March 

 to the end of the first week in April, those tips will 

 suddenly be lost and the whole bird will become 

 brighter, this change taking place without the pre- 

 sence of a moult. 



Is it probable that these tips, which have w-ithstood 

 the winter storms, should be washed away by April 

 showers, imless there was some active physiological 

 process behind it ? 



Many more instances could be quoted, but enough 

 has been said to make it clear that " bleaching " only 

 takes place when permitted to do so b\- physiological 

 agency. 



In my former papers, quoted above, several groups 

 of squirrels in which the " bleaching " is marked were 

 referred to, and it was pointed out that it was quite 

 absent in some species and present in varying degrees 

 in others very closely allied and inhabiting practically 

 the same country. 



Let us now carry the matter a step further and we 

 will endeavour to show that bleaching is the origin of 

 many of the markings and longitudinal stripes among 

 mammals and birds. 



The oriental genus of squirrels, known as Haittfa, 

 offers the best examples for a study of this question. 



The typical liatufa bicolor of the Malay Peninsula 

 is a large squirrel, deep glossy black above and light 

 yellowish below. The new coat when first assumed is 

 always black, but " bleaching " soon sets in, although 

 the extent to which it is carried varies grcativ among 

 different individuals. In some cases the whole body 

 becomes deep rufous bro\\n, the legs and feet remain- 

 ing black, while the hairs of the tail show a few red 



