THE SEXUAL SELECTION THEORY AS APPLIED TO BIRDS
Where the Rôle 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 rôle 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 is 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 we 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 rôle 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 which 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.
Wallace 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 under similar
circumstances, are scrupulously careful to remove the fæces 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 fæcal 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 fæces
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 fæcal 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
internal 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 the 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.
[Illustration: Plate 22.
From a drawing by H. Grönvold.
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 the 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 the 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.
[Illustration: Plate 23.
_From a drawing by H. Grönvold._
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.]
[Illustration: 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.]
[Illustration: _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 and 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, in 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 this 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
to 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 means 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 the 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 are 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 that 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 more 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 witnessed
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.
[Illustration: Plate 25.
_From a drawing by H. Grönvold._
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.
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 stimulating
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.
[Illustration: 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 characteristic 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.
[Illustration: 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.]
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, H.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 point 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.
[Illustration: 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.]
[Illustration:
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 _C. 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 wither. 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.