the sexual selection theory as applied to birds
where the r?le of the sexes is reversed—polygamy and how it is brought about—coloration and courtship—instinctive actions—the importance of landed possessions—the meaning of “display”—the springs of “behaviour”—a new light on the wild-duck—the “display” of the great-crested grebe—some neglected factors.
the significance of the varied behaviour of birds—more especially of the males—during the period of reproductive activity must now be more minutely analysed. but before this analysis can be profitably begun, it will be necessary to recall the fact that there are several cases known wherein the r?le of the sexes is largely reversed. herein the females do the “courting,” and fight one another as rivals for the males; while the males perform the duties of incubation and brooding, and feeding the young. this is really very remarkable, and demands more attention than it has yet received.
what factors have brought about this curious reversal? in any search for an explanation it must be borne in mind that in all such cases polyandry is the rule, and in all such cases the female is larger and more vividly coloured than the male. here, then, we have exactly the opposite to what obtains in cases of polygamy. what 135is the reason for this preponderance of males? why is it that when the males are in excess of the females the latter should be the more brilliantly coloured and the more amorous? these questions at present are unanswerable. when polygamy obtains it seems always to be assumed that it is explained by the excessive pugnacity of the males, which, after fierce contests for the mastery, take forcible possession of as many females as may be captured and held in durance; the same argument seems never to have been applied when polyandry obtains. there can be no doubt but that it applies in neither case.
when polygamy obtains, as we have already pointed out, the females are not seized and captured by the males, they are not victims of a lecherous lord. on the contrary, they seek the males, and the intensity of the desire to satisfy their natural cravings extinguishes any feeling of jealousy.
the same interpretation must obtain where the numerical values of the sexes is reversed. failure to appreciate this accounts for one of the many futile suggestions made for the suppression of the rabbit plague in australia, which was that large hauls of these pests should be made by netting, and that the females should be slain and the males released. this, it was held, would lead to the speedy reduction of the latter, which would kill one another in their fights for the remaining females. the plan was impracticable, but the suggestion demonstrated the prevalent belief as to the attitude of the male in this respect. had it been well founded, surely polyandrous species, whether of birds or beasts, would never have existed; for, by the reduction of the males, monogamy would speedily have been restored. how, then, are 136we to explain polyandry? how are we to explain the fact, as it seems to be the fact, that the excess of males has brought about such a complete reversal in behaviour—the males, instead of the females, requiring the aphrodisiac? the solution of this problem probably lies with the physiologist. we now know that the problem of sex does not rest merely in the complete development of the primary sexual organs; we know that fertile unions do not depend merely on the act of pairing, but on the functional activity of those ancillary glands already referred to. and it may well be that some change in the character of the secretions has not only altered the numerical values of the sexes, but reversed the normal r?le of coloration and behaviour. that is to say, neither polygamy nor polyandry among the lower animals, at any rate, has been brought about or is maintained by the excessive death-rate due to combats for possession of mates, but must be explained as demonstrating inherent changes in the germ-plasm, disturbing the relative proportions of the sexes and correlated with a profound transformation, not only in the behaviour of the sexes during the period of reproductive activity, but also in their physical characteristics.
the action of the primary sexual glands and of the ancillary glands has, then, to be allowed for in all attempts to interpret behaviour in sexual matters. no less so must this be the case in regard to the development of coloration and other forms of ornament, and the genesis of weapons of offence. but at present we are, in this direction, dealing with an unknown quantity. the recognition of this, however, should not deter us from attempting to solve the riddle of sex from the phenomena 137which have so far been surveyed.
to-day the interpretation which holds the field is darwin’s theory of “sexual selection.” but this was framed rather to account for the existence of conspicuous secondary sexual characters—the antlers of deer, the train of the peacock, and so on; it did not take cognizance of the unarmed, and the soberly-clad individuals. but whatever shortcomings we may discover, real or imaginary, in this theory, we must never forget that he had not only to analyse and present his facts, but he had first to collect them. this, in his case, was a more laborious task than most people seem to suppose. our criticisms to-day are based, not so much on the revelations of new facts, as on the harvests of his gleaning. yet when all is said and done, the theory of “sexual selection” remains, though perhaps in a new setting.
to attempt to epitomize this theory is to essay a very difficult task. but, in a condensed form, it may be said to be a theory which accounts for the development of secondary sexual characters, on the one hand through the agency of conquest by battle, whereby rival males strive for the possession of one or more females, who have no choice in the matter, or who may deliberately elect to follow the victor: and on the other by display of conspicuous ornamentation, or of more or less grotesque antics, or of some form of music, using this term in a very wide sense. wherever display is the agent, however, its purpose seems to be to win the affections of the female to whom such attentions are addressed. she is supposed to elect to mate with the finest performers of a number of suitors. in this way, it is assumed, the intensity of the display, whatever its nature, has been gradually increased.
138wallace strongly opposed this, contending that it assumed too much, that it assumed a common and uniform standard of perfection shared by all the females concerned in the selection, which is indeed assuming too much. but his own theory was no more satisfactory. indeed it was very much less so, for he contended that these various exaggerations of colour and form are to be regarded simply as evidences of a superabundant vitality, though there is no evidence that “superabundant vitality,” if it exists, is a transmissible character.
the revised version of the sexual selection theory advanced in these pages is largely inspired by the work of mr. h. eliot howard who, in his monograph on the british warblers, has not only added very materially to our knowledge of the life-histories of these birds, during the reproductive period, but has also done much—both in the direction of destructive, and constructive criticism, of generally accepted conceptions on this head—to set us on the right track for further research.
a study of his work leaves one with the conviction that, while these birds exhibit what we may call a nascent intelligence, their actions, on the whole, may be described as instinctive, or congenitally definite. that is to say, they follow one another in definite sequence. hence we must regard each new phase in the chain of events appertaining to the reproductive cycle, as following one another in a definite sequence, so that any break therein throws the orderly performance of the necessary acts out of gear. there is no realization of what reproduction means, no deliberate striving to achieve that end. each new phase brings its own set of associations and sets a new train of actions in motion, which are performed mechanically. for instance, these warblers, like hosts of other species 139under similar circumstances, are scrupulously careful to remove the f?ces of their young from the nest; thereby preserving it in a sanitary condition. it is certain that any neglect to do this would speedily end in the death of the young. this act is “instinctive”; it is not performed because the parents have evolved any views on sanitation, and any strain in whom this instinct was defective would speedily become eliminated. mr. howard has demonstrated the mechanical character of this sanitary measure by placing leaves in nests of young. the parents, having fed their offspring, at once seized upon the leaf and commenced to dispose of it after their usual fashion, first by trying to swallow it and then by carrying it away. they did not, evidently, realize the difference between the texture of the leaf and the milk-white, jelly-like envelope which always encloses the f?cal matter of the nestling. we shall probably never know how this most vitally important instinct came into being; nor can we hope to discover what chain of happenings begot the instinct, which each parent displays, to gently stimulate the cloacal lips of their offspring in order to induce the discharge of the f?ces when this does not immediately follow the stimulus of swallowing food.
we cannot credit these birds with notions on the importance of the regular discharge of the evacuations. equally mysterious is the development of the envelope enclosing the f?cal matter. this is jelly-like in substance, and of considerable thickness, and is enclosed within a very delicate skin or pellicle, enabling one to lift the whole in the fingers without soiling them. how and where it is formed should not long evade discovery. but how it has come to be is another matter. we can, at any rate, vaguely account for responses of the organism to 140internal stimuli reacting directly on the individual, but here is an elaborate mechanism evolved in response to extra-personal needs: and which cannot be regarded as of exactly the same configuration as the instinct to feed the young.
a return must be made to the nature of the early phases in the procession of the reproductive instincts. mr. howard’s study of the warblers seems to show conclusively that these first manifest themselves in an overmastering desire to seize upon territory large enough to ensure an abundance of food for the offspring that are yet to be. to this end the males arrive from their far-distant winter quarters at least a week in advance of the females. since each returns approximately to the scene of last year’s nursery, the arrivals are fairly distributed at the first; but nevertheless this distribution inevitably brings a conflict of interests between one or more males, perchance young birds about to start in life, and having therefore no definite objective. but whatever the reason, the competition is there. the strongest male remains in possession, and immediately commences to express the ecstasy of feeling which possesses him in continuous outbursts of song. such, doubtless, answer to the bellowing of the male stag. they advertise the presence of a male to the female, who, as she arrives, would seem to be already stirred by the rising storm of sexual desire, for having once discovered a male in possession of the all-necessary site for the nest, and the equally necessary domain, each settles down to conjugal bliss: within twenty-four hours the task of building has begun. there is evidently here no sexual selection in darwin’s sense: no choice from among a number of males of the individual which most excites desire within her; but the mating of the most mettlesome, most virile males has been determined before her arrival and by a double sieve. in 141the first place, the duller-witted birds fail to secure suitable territory, and in the second, the territory, having been taken, must be held by force, so that only the strongest males remain to mate when the females eventually arrive. so far as one can see, selection is less exacting in the case of the females, which apparently need do little more than respond to the advances of the males.
plate 22.
from a drawing by h. gr?nvold.
fighting for territory.
two black-caps are here seen fighting for their annual breeding territory. a chiff-chaff has been unable to resist the excitement of conflict.
face page 140.
with the advent of the females the amorous instincts of the male speedily gather force; but for their satisfaction it is imperative that the female should be possessed by a like desire. to provoke this, for it is essential to the well-being of the race that offspring should be produced as early as possible, some form of aphrodisiac seems to be necessary. this fact has never been properly realized, though it is implied in darwin’s theory of “sexual selection.” here, however, it was used to account for the evolution of resplendent coloration, eccentric postures, and dances which, it was assumed, enabled or induced the female to choose the most mettlesome males. what obtained among sombre-clad species, appears to have excited no curiosity among the students of the evolution theory. hence it comes somewhat as a surprise to find that the soberly-clad warblers behave exactly as though they too wore coats of many colours. after what has been said in the last chapter on this head it will be unnecessary to describe these displays among the warblers in detail, more especially as my friend mr. howard has kindly allowed me to use some of the illustrations from his book. these show convincingly enough that the wings and tail are made to play the same part as though they bore all the hues of the rainbow. to bring this fact home compare 142the figures of some of these small birds clad in sober russet and black with that of the sun bittern (eurypyga helias) in like mood, whose wings and tail when spread, and only then, display bands of vivid chestnut-red, contrasting with bands of black, on a background of grey and buff, variegated with delicate mottlings and vermiculations of black and brown, and streaks of white. in the case of the warblers, it is to be remarked, the male, in these ecstatic moods, will commonly hold a leaf, or a piece of stick, in his beak, as if suggesting the work of nest-building and its delightful sequence. this, or its equivalent, is a common phase, for the great crested grebe, for example, in these paroxysms will dive and bring up weed, the nest material of the species, as an offering to his mate, or as a stimulant to her yet slumbering passion.
it seems clear, then, that the evolution of colour is not the stimulant to display, for this is present where conspicuous colours are wanting. yet it can readily be understood how the association of ideas in regard to colour and display arose, for there are cases where this interpretation seems inevitable. such are afforded by certain sea-birds like the kittiwake, guillemot, fulmar and cormorant, wherein the inside of the mouth is of a lurid orange-red in the case of the first-mentioned, and of flaming gamboge yellow in that of the others. during moments of sexual ecstasy the mouth is widely opened, as if to charm the beholder with its gaudy hue. both sexes have the same colouring, and both behave alike. but it is doubtful whether either is conscious that its own mouth is like that exposed to its gaze: the action is sympathetic. no doubt it may play its part in stimulating desire, but we cannot contend from this that it has been evolved by sexual selection, that is to say, that 143the hues have undergone a process of gradual intensification owing to the deliberate rejection of the less gaily-coloured suitors. the tendency to develop colour in the mouth would appear to be latent in all birds.
plate 23.
from a drawing by h. gr?nvold.
the display of the grasshopper warbler.
the behaviour of this bird under the stimulus of sexual excitement is precisely similar to that of the sun-bittern and the kagu, yet it has no brilliant colours to exhibit by such actions.
face page 142.
plate 24.
the display of the sun-bittern.
quite inconspicuous in repose, this bird, in its moments of exaltation, becomes banded and blotched with vivid colours, revealed by spreading the wings and tail.
photos copyright, d. seth-smith.
the kagu in display.
what is true of the sun-bittern is true also of the kagu.
it is significant that whenever bright colours appear, they do so first in the males, the females and young retaining the dress common, up to this time, to the species at all ages. in the majority of instances, at any rate, it would seem that this accession of colour appears with the seasonal re-awakening of the reproductive activities: it forms a “nuptial” dress, and is discarded after the breeding season is over for a livery indistinguishable from that of the female, this forming the so-called “winter plumage.” but if all the available facts are taken into consideration there seems good reason to believe that the nuptial plumage tends to be assumed earlier and to be retained later, as this disposition to develop ornament gathers force, till finally only the head and neck go into “eclipse,” as in the case of the black-cock, jungle-fowl and partridge.
in the pheasant we have an instance—one of hundreds—where the resplendent dress is worn throughout the year. the next phase in the direction of the growth of colour occurs when the female, towards old age, develops a more or less well marked tendency to assume the hues of her lord, and this accession of colour makes its appearance earlier and earlier in succeeding generations, till finally the adults of both sexes are coloured alike, save that, as a rule, the female lacks the intensity of coloration which her mate displays. the original sombre dress is now only worn by the young. in due course the resplendent dress is assumed also by the young, as witness the numerous instances among the kingfishers 144and among the parrots, where adults and young are all habited in the same vivid hues. there are infinite variations of these changes which cannot be discussed here, for obvious reasons. all that matters now is the fact of such sequences, which inevitably raise the questions: why, in so many cases, do the females show no disposition to assume resplendent colours? and to what factors can such coloration, when it occurs, be attributed? the second only of these questions is germane to the present discussion, and to this no very satisfactory answer can be returned.
to say that the development of brilliance in species hitherto sombrely clad is due to “changes in the metabolism” is only an affectation of wisdom. what we want to know is what induces the changes? time was when no more than a guess could be hazarded as to this: a suggestion that ornament, of whatever kind, was one of the many modes of the expression of that instability of the organism which is characteristic of living things: that it was one of the outward and visible signs of that inward, intangible tendency to vary which is so familiar. later research seemed to show, fairly conclusively, that ornament was one of those “secondary sexual characters” which was dependent on the stimulating juices, or “hormones,” emanating from the primary sexual glands. to-day it is manifest that this is only partly true, for it is certain that these glands are not alone concerned and they may only participate indirectly. it seems to have been clearly demonstrated that the thyroid and pituitary glands, or the “hormones” therefrom, play a large part in this matter of the “secondary sexual characters.”
castration, it is true, profoundly affects these characters. in the case of deer it inhibits the growth of antlers, 145in cattle the horns are increased in length but reduced in thickness—they are longer than those of the female, but resemble them in appearance, and further, the whole stature is greatly increased, but it is at the same time conspicuously less massive, particularly at the neck and fore-quarters. in eunuchs it results in immense stature and the loss of the more characteristic male features, such as the beard and the bass voice. the removal of the testes in birds is always a difficult operation and is rarely successfully performed. hence the accounts of changes in plumage consequent on this operation are inconclusive. it has generally been supposed that whenever, either by removal or by disease, the testes are rendered inoperative the plumage, when normally of a resplendent type, assumes the coloration of the female. this is probably an erroneous supposition, but what happens is a failure to secrete the more intense pigments and the more specialized forms of feathers, so that the resultant dress answers to the juvenile male dress. it is not a case of “reversion” to this livery, but a failure to assume the latest acquirements of the species. these, as has already been shown, are only very gradually developed. the intensity of pigmentation, or concentration of pigmentation, which results in sharply defined areas of colour, is a cumulative process. as it loses in intensity at any given moult, so the individual tends to reproduce the phases of the earlier and vanishing livery. sooner or later, however, this earlier livery disappears more or less completely: is eliminated from the system, so to speak: and what is commonly called lack of “vigour” results, not in a return to the earlier, sombre dress, but in the later-acquired, resplendent plumage lacking intensity. the seasonal, temporary secondary sexual character has become, as some say, a “somatic” character. highly probable as 146this view appears, it ought, it may be argued, to receive support from nestling plumages. young gulls, for example, should occasionally revert from the mottled to the earlier striped livery. but we have no evidence of this; and it does not follow that this sequence of events should occur. the conditions of control are different.
what exactly are the factors which govern the evolution of resplendent plumage is not known. but they would seem to be more complex than was supposed. that the primary sexual glands play an important part, through the juices or “hormones” which they liberate, there can be no doubt but these are only partial factors. the “hormones” of the pituitary and thyroid glands are also necessary contributors, controlling as they do both fertility and the more superficial characters, such as colour and ornament. evidence, indeed, is slowly accumulating to show that the problem of the behaviour of animals during the period of sexual activity, as well as the peculiarities of structure and coloration which they develop at this time, are all largely governed by the action of these secretions.
these, in their turn, are undoubtedly inhibited, or increased, by the control of the nervous system, though this control is of course involuntary. this much seems clear from the fact that birds will display when under the excitement of fear, though the character of that display is never the same as that in moments of sexual exaltation. if the nervous system, through the eye, by “suggestion,” played no part, there could be no use for display, but it is equally certain that for the realization of the sexual activities a number of other factors have 147to contribute.
the existence of this nexus of conditions is commonly overlooked, but it is extremely important. normally, not only among birds, but other animals higher and lower in the scale of life, “suggestion” does not suggest until the “hormones” concerned with the sexual activities have, as it were, saturated the system and rendered it, so to speak, highly inflammable. even then it commonly happens that, with the male at any rate, this inflammable state bursts into flame of its own accord. but for this, indeed, how could the consummation—of the period of sexual activity ever be realized? in many cases the sexes are sundered far apart. what, but the merest accident, could bring them together if it were not for this consuming fire of desire which impels each sex to seek out the other? this stage is manifested in the case of the deer, where, we have seen, the stag wanders far and wide bellowing to advertise his errand and listening for a response to his call. he is possessed by a “male-hunger” which eventually attains to a state of frenzy. here no “suggestion” is needed, but the necessity for this stimulus, for some form of aphrodisiac, occurs with him after the first relief of his pent-up state has been attained. this stimulus is applied, both through the eye and the sense of smell, by the females of his herd. the same conditions apply in the case of the birds. but it is to be noted that with the females, as in the case of mammals, sexual desire is commonly less intense than in the males, and hence, in their case the need for “suggestion” by display of some sort. but apart from this, a “display” of some kind is necessary. how else can desire be indicated? and here is “sexual selection.” for males, mate-hungry as they might be, which resorted to no 148means of expressing their condition would go mateless: and the same is true, though perhaps in less extent, with the females; hence, then, it is clear display is a product of sexual selection.
that sexual desire is less intense in the case of the females is to be regarded as another result of this form of selection. if they displayed the same intensity of passion the males would speedily become exhausted, for it is well known that the gratification of the sexual emotions is far more enervating in the case of the male. it may well be that polyandry has arisen from this transference to the females, or development by the females, of increased sexual hunger.
the fact that birds will repeat, albeit imperfectly, the phases of the sexual display under the stimulus of fear, or anger, and when no females are present, must be regarded as an indication, for we can scarcely call it a proof, that exaggerated movements have become the normal concomitants of great excitement, at any rate during the season of reproductive activity. they are purely nervous responses to external conditions. it must not be forgotten that, at this time, fear begets other movements, equally striking, such as feigning lameness, and death, which have no part in the sexual display.
interpreted in this light one can understand that to the female not as yet sexually “ripe” or sexually “hungry,” these movements, when not interpreted as signs of fear or anger, fail to produce any response. so soon, however, as this period of “ripeness” arrives, the stimulus through the nervous system produces the desired response, begetting a complementary stimulus through the secretions of the sexual glands, by what we may call 149the flow of the hormones; just as the sight of food stimulates the flow of saliva, or “makes the mouth water” before we are conscious of feeling hungry. in due time hunger will assert itself without the stimulus of the nervous system through the senses. but there must in any case be some form of display, some form of communicating and stimulating desire between the sexes, to secure the consummation of the reproductive acts. how else could intimation of sex hunger be indicated and satisfied?
that the desire for sexual congress is inherently more avid, more intense, in the male than in the female is often called in question; and more especially so by those who imagine that they have a mission to carry on “social reforms” and to regulate the relations between the sexes of the human race. such aims and ambitions are commonly those of the arrogantly ignorant. there are few people who possess a sufficiently wide knowledge of this theme, or of the factors which underlie it, to qualify them to become the mentors of their fellow-men in these matters. however much we may choose to seek refuge in sophistry, the fact remains that man is still an animal, and if the human race is to continue he must always remain so.
a lurid light has just been shed on the fierceness of the sexual passion in the male by mr. julian huxley, who relates some facts pregnant with meaning to all who have understanding, in regard to what obtains among birds. these facts are primarily concerned with the mallard (anas boscas). this bird is ostensibly monogamous, and, on the whole, seems to be a fairly considerate mate. the normal period of pairing having passed, and the duties of incubation having begun, the female ceases to harbour any further desire for sexual intimacy. her whole energies 150are devoted to nursing her embryonic young into life. not so the male. he is yet far from satiated; in him the sexual fever still burns fiercely, but somehow he seems never to make any attempt to provoke in his mate a like condition, as in the days before brooding began. on the other hand, he does not scruple to savagely pursue every other female who ventures abroad in his neighbourhood. so soon as a duck takes wing for a brief relaxation from the arduous work of brooding she is pursued by ten or a dozen already mated males, till at last she is obliged to descend on the water, and with her descend her pursuers, now to mob her without mercy. commonly at least half of these infuriated males will eventually succeed in treading her; leaving their victim only after she has become completely exhausted or killed outright. this is no unusual occurrence. on the reservoirs at tring, where every spring from one thousand to one thousand two hundred pairs congregate to breed, from seven per cent, to ten per cent, of females are annually killed in this way.
it is just possible, however, that an error may have crept into these observations. one cannot help asking, may it not be possible that these pursuing males were actually unmated birds? the chief argument against this is the fact that there is no sort of attempt to “display” apparent with these birds, simply an overmastering, ravenous desire to satisfy the craving which possesses them.
evidence is not wanting that the evolution of pigment intensification and the consequent development of vividly coloured liveries, or the equivalent development of ornament, has been accompanied by an intensification of the reproductive instincts. for there can be no doubt but 151that the display of species which are conspicuous for their ornamentation is more animated than those of duller hues. as an argument in favour of this view the case of the display of the great crested grebe may be cited, wherein each sex has developed both colour and ornament to a high degree, and are distinguishable only to the expert.
the latest and the best exponent of the behaviour of this species under the spell of sexual exaltation is mr. julian huxley, whose observations, in a condensed form, are now to be surveyed. the most conspicuous features in this bird are the great elizabethan ruff of bright chestnut and dark vandyke brown, and the long dark-brown tufts of feathers, or “ears,” which surmount the head. but the satin-like sheen of the white breast and the fore part of the neck and face add not a little to the general effect. these ornaments are worn only during the breeding season. so soon as the fires within begin to burn, the parade of this finery commences, and it would seem that a somewhat protracted dalliance takes place before any actual pairing. during the early phases of these performances much play is made with ruffs and “ears.” the courting pair will frequently face one another on the water, and go through a strange ceremony of head-shaking. to this is soon added a sort of ghost dance, wherein the male suddenly dives, leaving his mate swinging excitedly from side to side. in a moment or two, however, he appears, not suddenly, as usual, but arising gradually out of the water. he seems to “grow” out of the water. first his head appears, with ears and ruff extended, and beak pointed downwards; then his neck, and finally the body arises into view, till only the extreme tail end remains submerged, so that he looks 152more like a penguin than a grebe! all the while he is turning on his long axis, as it were, till he gradually displays before his mate the dazzling white sheen of his breast and neck, set off by the rich red chestnut and brown of his face and frills. a moment more and both subside into their normal attitude, shake their heads at one another, and then proceed to feed as if nothing had happened.
but these quaint antics are only the preliminaries to still stranger. a pair of birds, engaged, apparently, solely in fishing and feeding, will suddenly approach one another and begin head-shaking, each striving to outdo the other. then the ears, till now erect, are thrust out laterally, and the ruff is still further erected till it forms, with the ears, a common disc. then the hen dives: immediately after down goes the cock. after some fifteen seconds or so she appears at the surface again, speedily followed by the cock, who breaks out about five-and-twenty yards off. each crouches low over the water, and each will be seen bearing a tuft of weed in the beak. as each sights the other a tremendous rush is made, as if they intend to charge. but when about a yard apart each springs up and assumes the penguin position, save that the beak, instead of pointing downwards, is now held horizontally and bears its burden of weed. still approaching, they eventually touch one another, treading the water and swaying in a sort of ecstasy, all the while shaking their heads from side to side. then they gradually settle down into the normal swimming pose, though still keeping up the head-shaking; then this, too, subsides, the weed is dropped, and the performers drift apart and begin feeding. but no actual pairing accompanies these strange performances. this final rite is associated with a quite different ceremonial, and was 153witnessed more than once by mr. huxley. on the particular occasion which he describes he was watching a male swimming along near the reeds, apparently on the look-out for something, and turning his eyes in the direction of the course, he saw, at some distance off, what he supposed was a dead grebe lying hunched up in the water, with outstretched neck, and ruff and ears depressed. presently the male swam alongside the body and bent down his head as if to examine it. then he swam to the tail end, and suddenly scrambled out of the water on to the body; and there, with bowed head and depressed ears and crest, he seemed to stand a moment. then he waddled forward over its head and into the water. instantly the supposed corpse raised its head and neck, gave a sort of jump, and was swimming by the side of its mate. they had been pairing on a half-made nest, whose surface lay level with the water.
plate 25.
from a drawing by h. gr?nvold.
a male-savi’s warbler
—in one of his “courtship” attitudes. note the leaf held in the beak.
face page 152.
mr. edmund selous seems to have witnessed some almost incredible behaviour on the part of the owners of a nest he had under observation, inasmuch as, on more than one occasion, he declares the male lay prone upon the nest and the female assumed the position of the male. after this pantomime both would leave the nest, but commonly the female would speedily return and pairing would be duly performed.
this brief summary of mr. huxley’s observations, which he was generous enough to give me the privilege of seeing in manuscript, taken in conjunction with many other facts of a like kind given in these pages, seems to lend support to the view that an excessive amorousness is commonly associated with conspicuous ornamentation, as if these stood in the relation of cause and effect.
154
finally, it is contended, the facts garnered during recent years show that the theory of sexual selection, as darwin propounded it, especially in so far as birds are concerned, is no longer tenable: but it is not an exploded theory, it has only undergone modification. so far as the evidence goes, it would seem that the first of the series of events in the sexual cycle is performed by the already avid male, when he proceeds to secure a “territory” large enough for his needs. in insectivorous and carnivorous species this area is fairly extensive. no other male will be allowed within its confines. the perfection of this instinct is vitally important, if sufficient food for the offspring that are to be is to be assured. where the food is inexhaustible, as with the auk-tribe, only a ledge large enough to hold the egg is required. only avid males will develop and respond to this stimulus. the second stage occurs with the arrival of a female in the area. she does not at once proceed to “select” her mate, passing on if he fails to provoke her admiration. her sexual condition is apparently as yet but half awakened: to rouse this, the male supplies an aphrodisiac in some form of display to which, in the normal course of things, she responds, often also with some form of display, or indication of the desire which has been aroused. the intensity of the performance seems to vary with the intensity of the sexual passion, which appears to be greater in some species than others, and especially so with such as have conspicuously ornamental plumage. there is, indeed, a variation in the sexual appetite as there is in the ornamentation. the two are reciprocal, and are determined in degree by the stimulatory qualities of the hormones of the sexual glands. where these have been developed in like intensity by the females, they also display. diminution in the quality and quantity of the 155stimulating secretions of the ancillary sexual glands, the hormones of the pituitary and thyroid, or the primary glands—testis and ovary—decreases fertility, or induces sterility. where these stimulants are lacking there will be no desire, no display, and no pairing, and consequently an end to this defective strain. here then is sexual selection.
plate 26.
photo copyright by d. seth-smith.
another aspect of the kagu’s “display.”
herein two birds are seen facing one another with the great head-crest fully erected. while in this mood these birds will strut up and down with mincing gait and drooping wings. this is a posture commonly assumed during momentary excitement, whereas the posture shown in plate 24 is apparently only assumed during moments of sexual excitement.
[face page 154.
instances of such impotency on the part of either sex are wanting, and we can only speculate as to how such cases would be met. would a female who had chanced to settle in the territory of a male whose sexual impulses carried him no further than seizing territory remain with him throughout the mating season, held by an imperfectly roused, ill-defined, sexual instinct? or, eventually becoming mate-hungry, and failing to stimulate him to perform his part, would she desert him and seek another mate? on the other hand, would a male, failing to arouse response in the female he had secured, drive her away and supplant her?
in other words, are we then justified in postulating differential effects in regard to display: a minimum of intensity to ensure mating? a display of some sort is essential. it may be feeble as compared with that of another species—that of the sparrow, for instance, compared with that of the peacock—but it must be sufficiently good of its kind to effect its purpose, which is to “hustle” up the production of offspring. a phlegmatic but virile male, or a too feeble performer, is almost as certainly doomed to extinction as an impotent male; for his offspring will probably be eliminated by the adverse conditions of existence to which their late appearance exposed them. where a female settles down with a male which does not attain to the standard of display 156characteristic of his race, it is conceivable she may sooner or later seek a mate elsewhere, deserting the phlegmatic bird as if under the impression that she had made the mistake of settling down with one of her own sex. there is no need that the female should have to “select” the best performer of a number of males displaying at the same time and place as a number of rivals.
finally, the ornamental crests and frills, and the vivid hues which so many birds display have not arisen, as is generally supposed, as a direct result of the selection, by the females, of the most vividly coloured, or ornamented, from among a number of suitors presenting varying degrees of intensity in ornamentation. such “frills and furbelows” are to be regarded as “expression points” of internal variations in the germ-plasm, which have been free to develop along their own lines because they have not proved in disharmony with the conditions of the birds’ environment. their development is to be traced to the stimulating action of the “hormones” which control both pigmentation and structure, as is shown by the fact that both are modified by any interference with the glands in question. such ornamental features then are the concomitants not the results of sexual selection.
the development of ornament, whether of colour or structure, may be taken then as an index of specialization, and as one of the many manifestations of that variation which is going on in every part of every living organism.
so long as the continued increments in the development of these characters do not hamper their possessors in the struggle for existence, they are free to go on developing. sexual selection, other things being equal, operates by according the greatest number of descendants to the most amorous, and not necessarily to those of the brightest hues.
plate 27.
photos copyright, g. herring.
some strange accompaniments of courtship.
the white-headed bell-bird.
this species is remarkable for the enormous, erectile wattles which arise from the base of the beak of the male at the courting season.
the umbrella-bird.
the crest which adorns the crown of the head has many counterparts, but the long-feather clad wattle which depends from the fore-part of the breast is unique.
[face page 156.
157
plate 28.
skull of the american white beaked pelican.
the beak of this bird develops at each breeding season an irregular horny plate which falls off at the end of this period, it is difficult to regard this as a sexual “ornament,” yet it comes under this category.
photos copyright, g. herring.
head of a puffin, showing the moulting of the beak sheath.
at the breeding season, in both sexes, a triangular horny plate is developed over the eye, an oblong plate below it, while the sides of the beak become deepened by means of larger triangular horny plates. all these embellishments are highly coloured, and they are shed at the end of the breeding season. a further ornament is developed at the gape, in the shape of a fleshy rosette of a bright orange colour.
face page 156.]
but sexual selection does not begin, and end, with the evolution of frills and furbelows. “behaviour” counts for more than is generally supposed. this is as specific as “structure,” that is to say, it is as constant for each species as is its coloration, and it is also as variable. that evolution may be determined by variation in behaviour, no less than through structural variations, is a possibility which has received but little consideration at the hands of students of evolution.
the singular history of the australian bower-birds lends additional support to this view, and at the same time provides an additional argument against the generally accepted opinion that bright colours have been evolved by reason of the preference shown by the females for the most vividly coloured of their suitors. for while the males affect all the tricks and turns which are the common accompaniment of courtship, they, in addition, introduce very extraordinary features in the shape of “bowers” cunningly constructed and often gaily decorated, as will be seen presently. eight of the total number of species of this group exhibit this behaviour, and while they differ very conspicuously in coloration among themselves, they agree very closely in the type of the bower they build. if the coloration is determined by the female, then in this they display very different standards, and if they do select, each according to the standard of the species, then we must suppose that they also must exercise a choice in regard to the character of the bower, the favoured male being the best builder. but why, in this case, is there not as much diversity in the form of the bowers as in the coloration of the feathers? a survey of the facts will perhaps make this 158point clear.
one of the best known of these bowers is that of the satin bower-bird (ptilonorhynchus violaceus). on either side of a platform of small twigs a fence of similar twigs is reared, sloping inwards to form a more or less complete tunnel. at the entrance to this is placed a platform of sticks, which is strewn with a miscellaneous assortment of brightly coloured feathers, bleached bones, and occasionally flowers. the work of construction is almost entirely performed by the male: it is indeed a little curious, having regard to the circumstances, that the female should bear any share in its construction at all.
really this is a more wonderful piece of architecture than would appear from the mere description of its main features: for it represents psychical activities which are difficult to fathom. it does not take the place of display, but is an extension of this. during his amorous moments the cock becomes greatly excited, chasing his mate in and out of the bower, carrying the while, in his beak, a brightly coloured feather or a leaf.
at the same time he sets all his feathers on end and every now and then drops first one wing, then the other, accompanying these actions with curious whistling notes and pretences of picking up food.
the regent-bird (sericulus melinus) differs conspicuously from the satin bower-bird, for while this is of a uniform, deep, metallic steel-blue, the regent-bird is jet black, with a golden yellow crown and hind-neck and a great blaze of golden yellow on the wing. yet the bowers of the two species—which belong to different genera—are practically identical, save that brightly coloured berries are used more frequently by the regent-bird.
plate 29.
photo by w. p. dando.
the satin bower-bird and its bower.
the “bowers” of the “bower-builders” are the most remarkable variants on “secondary sexual characters” yet brought to light.
photo by l. medland.
the “bower” of the bower-bird.
the “bower” must not be confused with the nest, which is placed in a tree and bears no sort of likeness to the bower.
[face page 156.
the spotted bower-birds (chlamydodera maculata and 159c. nuchalis) are quite dull-coloured species save for a vivid semicircular crest of pink and mauve feathers which arise from the nape of the neck. their bowers differ from those just described in having a longer run and for the immense quantities of shells which are deposited at each end of the run. some of them are brought from long distances, as is shown by the large number of sea shells which are to be found in the collections made by birds living far from the sea.
by far the most remarkable of all are the bowers of newton’s bower-bird (prionodura newtoni) and the gardener bower-bird (amblyornis inornata). the first of these, a native of the mountains of queensland, is somewhat strikingly coloured, at any rate so far as the male is concerned, for he is of an oil green above and has a small yellow crest, while his breast is of a bright yellow; the female, on the other hand, is brown above and grey below.
the gardener bower-bird, on the other hand, is of a sombre olive-brown, but the male boasts an enormous crest of a flaming orange yellow. yet, widely dissimilar as are these two species, in the matter of their bowers they display much in common.
that of the gardener bower-bird takes the form of a hut-like structure of twigs, arranged around a central support, commonly a very young sapling. as a rule the thin stems of an orchid (dendrobium) are used in the construction of this curious hut, whose diameter is about three feet. before the entrance is a carpet of moss, which is kept clear of leaves or debris of any sort, and on this the most vividly coloured fruit, seed-pods, fungi, and flowers are laid, being constantly replaced as they 160wither. newton’s bower-bird, in like manner, forms a hut around a central column: a hut which may attain to a height of as much as six or even eight feet, and the walls of the pyramid thus raised are generally gaily decorated with flowers and fruit. around the central a number of subsidiary huts are not infrequently found, and in and out of these the birds pursue one another in ecstasies of excitement.
we have in these facts some extremely puzzling features, which at present, at any rate, permit of no more than a very rough analysis. probably the whole of these bower-building instincts have their origin in the habit, which the males of so many birds exhibit, of carrying a leaf in the beak when under the excitement of love-making. this is suggestive of nest-building, and in many species this is actually begun before the arrival of a female in the breeding territory, while others build what are known as “cock-nests” which are never used. among the bower-birds these “cock-nests” have taken a new and more elaborate form, and are placed on the ground instead of in the trees, the normal site for the nest in all these birds. furthermore, stages in the evolution of such strange fabrications can be found. these are furnished by the tooth-billed bower-bird (scenopaeetes dentirostris), the cat-bird (aeluredus viridus) and the gorgeous lawe’s bird of paradise (parotid lawesi)—which is not perhaps a bird of paradise. these build no bowers, but are content with clearing a patch of ground, of about ten feet in diameter, on which to disport themselves. but while the “displays” of these birds closely resemble one another, in the matter of coloration and ornament they present the most striking contrasts.
