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Sexual Selection

( Originally Published 1909 )

IN building up the theory of Natural Selection Darwin found that while this theory was sufficient to explain the useful in organic structures it did not sufficiently explain "that class of phenomena which go to constitute the Beautiful." Darwin, therefore, suggested an auxiliary theory to give a scientific explanation to these widespread phenomena. To quote Romanes : "Just as by his theory of natural selection he sought to explain the major fact of utility, so did he endeavor to explain the minor fact of beauty by a theory of what he termed Sexual Selection." Kellogg's exposition of this theory has been partly followed in the following account.

Every zoologist is familiar with the striking differences between the male and female individual of a single species. The reader will recall the feathers of many male birds which are in striking contrast to the sober plumage of their mates. These differences in size, color, general appearances, and various specific structural details in head, trunk, wings, feet, plumage, etc., are over and beyond those primary radical differences existing in all species in which the two sexes are differentiated. Some of these differences may, however, have obvious relation to the primary differences, in that they may be connected immediately with the act of pairing or with the work of rearing the young. The presence in male insects of complexly developed holding organs, and in female mammals of milk glands, exemplifies differences of this category. A great many sexual differences, however, have no such obvious direct relation to the function of producing and rearing the young.

Such are the metallic purple and bronze colors of the male grackles compared with the dull brown of the females; the long tails and brilliant coloration of the male pheasants, the great spreading, patterned tail of the peacock, the larger size or the winglessness of many female insects, etc. All these differences between male and female of the same species of animal, beyond or in addition to the differences between the actual primary reproductive organs, are known as secondary sexual differences, or the characters themselves, which may be characteristics of physiology and habit, as well as the more familiar ones of structure, are called secondary sexual characters. The layman may not readily appreciate the abundance and the great variety of these characters, but it is a fact that almost all species of animals, excepting those in the lower invertebrate branches, show them, and if one will try to recall the aspect of the two sexes in one after another of the species of animals with which one is familiar, mammals, birds, insects, etc., one will begin to realize how widespread and significant are these secondary sexual characters.

Many of these secondary sexual characters have uses which are of a kind directly helpful in the struggle for existence, as the strong antlers of the stags, useful in defense against attacking enemies; the brood sacs of the kangaroos and opossum, useful in caring for their helpless young; the milk glands and teats of all female mammals, and the protective colors and patterns of many in-sects and birds. But others of these secondary sexual characters are either of a kind apparently useless in the struggle for life, or even of a kind actually harmful.

Of apparent harmfulness are the conspicuous staring colors of many male birds, the long dangling plumes, the weighty crests and heavy, dragging tails of others; all these parts also usually being dangerously, conspicuously colored. The lively loud song of many male birds, and the dancing and leaping of numerous male spiders and some male birds must also involve some danger to the performers by attracting the attention of their enemies. In fact, most of those secondary sexual characters that are classified under the general head of 'exciting organs' are apparently of a sort that should be actually disadvantageous in the struggle for existence. They are of a character tending to make their possessors conspicuous, and thus readily perceived by their carnivorous enemies. How is to be explained the existence of so many and such highly developed structural and physiological characters of this kind, a condition that seems to stand in direct opposition to the theory of natural selection? Darwin's answer to this question is contained in his theory of sexual selection.

This theory, in few words, is that there is practically a competition or struggle for mating, and that those males are successful in this struggle which are the strongest and best equipped for battle among themselves, or which are most acceptable, by reason of ornament or _other attractiveness, to the females. In the former case, mating with a certain female depends upon overcoming in fight the other suitors, the female being the passive reward of the victor; in the second case the female is presumed to exercise a choice, this choice depending upon the attractiveness of the male. The actual fighting among males, and the winning of the females by the victor, are observed facts in the life of numerous animal species. But a special sexual selection theory is hardly necessary to explain the development of the fighting equipment antlers, spurs, claws, etc. This fighting array of the male is simply a special phase of the already recognised intraspecific struggle; it is not a fight for room or food, but for the chance to mate. But this chance often depends' on the issue of a life-and-death struggle. Natural selection would thus account for the development of the weapons for this struggle.

For the development, however, of such secondary sexual characters as ornament and song, and special odors, and 'love dancing,' the natural selection theory can in no way account; the theory of sexual selection was the logical and necessary auxiliary theory, and when first pro-posed by Darwin met with quick and wide acceptance.

Wallace, in particular, took up the theory and applied it to explain many cases of remarkable plumage and pattern development among birds. Later, as he analyzed more carefully his cases, and those proposed by others, he became doubtful, and finally wholly skeptical of the theory.

A few extracts from the 'Origin of Species' will present the theory in Darwin's own words. "This form of selection," he explains, "depends, not on a struggle for existence in relation to other organic beings, or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession: of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many cases victory depends not so much on general vigor as on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, might surely give indomitable courage, length to the spur, and' strength to strike in the spurred leg, in nearly the same manner as does the brutal cockfighter by the careful selection of his best cocks.

"Among birds the contest is often of a peaceful character. All those who have attended to the subject believe that there is the severest rivalry between the males of many species to attract, by singing, the females. The rock thrush of Guiana, birds of paradise, and some others, congregate; and successive males display with the most elaborate care, and show off in the best manner, their gorgeous plumage; they likewise perform strange =ties before the females, which, standing by as spectators, at last choose the most attractive partner. If man can in a short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect."

Many difficulties in the way of the application of this theory have been advanced in recent years as a result of experimental work and more widespread observation. Morgan lists twenty such objections. Space will allow mention of only a few of the most important criticisms of the theory. These are outlined by Kellogg essentially as follows.

The theory can be applied only to species in which the males are markedly more numerous than the females, or in which the males are polygamous. In other cases there will be a female for each male, whether he be ornamented or not; and the unornamented males can leave as many progeny as the ornamented ones, which would prevent any cumulation of ornamental variations by selection. As a matter of fact, in a majority of animal, species, at least among the vertebrates, males and females exist in approximately equal numbers.

Observation shows that in most species the female is wholly passive in the matter of pairing, accepting the first male that offers.

"Ornamental colors," moreover, as Kellogg points out, "are as often a characteristic of males of kinds of animals in which there is no real pairing as among those which pair. How explain by sexual selection the remark-able colors in the breeding season of many fishes, in which the female never, perhaps, even sees the male which fertilizes her dropped eggs?

"Choice on a basis of ornament and attractiveness implies a high degree of esthetic development on the part of the females of animals for whose development in this line we have no (other) proof. Indeed, this choice demands esthetic recognition among animals to which we distinctly deny such a development, as the butterflies and other insects in which secondary sexual characters of color, etc., are abundant and conspicuous. Similarly with practically all invertebrate animals. Further, in those groups of higher animals where esthetic choice may be presumed possible we have repeated evidence that preferences vary with individuals. Besides, even if we may at-tribute fairly a certain amount of esthetic feeling to such animals as mammals and birds, is this feeling to be so keen as to lead the female to make choice among only slightly differing patterns of songs? Yet this assumption is necessary if the development of ornament and other attracting and exciting organs is to be explained by the selection and gradual cumulation through generations of slight fortuitously appearing fluctuating variations in the males.

"Even if the females do choose among the males on a basis of attractiveness, how are the characters of the more attractive males to become especially fostered and accumulated by selection? Do such males produce more offspring or more vigorous ones than the other males, which, tho rejected by the first females, find their mates among the females not already mated? Are we to attribute to the more ornamental males a particular vigor? If so, may not that very vigor be the cause of the extra-production of color or plumage or wattles, etc.?

"Darwin admits, in order to explain the beginnings of color and ornament development, a certain degree of difference between the male and female in regard to their reaction to environment influences. If so, may not these admitted differences be really sufficient to account for even a pretty high degree of difference in development of secondary sexual characters?

"The special display of colors, tufts, plumes, spreading tails, and other secondary sexual characters by the males at mating time is an observed fact; the 'dances' of cranes and storks, the serenades of the song birds, the evolutions of the male spiders, are all familiar phenomena in the mating season of these animals. And they probably do exercise an exciting effect on the females, and are probably actually displayed for this purpose. But does this in any way prove, or even give basis for, a reasonable presumption for belief in a discriminating and definitive choice among the males on the part of the female? And it is this actual choosing which is the necessary basis for the theory of sexual selection.

"How explain the well-known cases of a similar extra-development of plumage in the nuptial season by both males and females, as in certain herons and other birds? And what of those cases in which it is the female that is the brighter-colored individual of the pair? To explain the latter case Darwin assumes that in these cases the males have done the selecting, but even this rather too easy removal of the situation postulated as a fundamental generalization of the theory does not explain the first of the questions in this paragraph. Do both sexes among' the herons do selecting?"

To the objection that choice on the part of the female assumes her possession of an esthetic ideal and a power of selection in accordance with that ideal which is contrary to our knowledge of animal psychology, Lloyd Morgan answers that the choice is not one of deliberation but rather of impulse; it is simply a definite response to an adequate stimulus. "She accepts that mate which by his song or otherwise excites in sufficient degree the pairing impulse; if others fail to excite this impulse they are not accepted. It is a choice from impulse, not the result of deliberation; but it is a choice which is deter-mined by the emotional meaning of the conscious situation. And it is the reiterated revival of the associated emotional elements which generates an impulse sufficiently strong to overcome her instinctive coyness and reluctance It is a perceptual choice arising from impulse rather than an ideational choice due to motive and volition."

The final line of criticism is that experimental evidence is strongly opposed to the theory of sexual selection. Mayer, director of the Tortugas laboratory of the Carnegie Institute, has proved by many careful experiments that the striking differences between the wings of male and female promethea moths, 'Callosamia promethea,' are absolutely without meaning in relation to sexual selection. The animals mate normally when painted, or after the wings have been cut off and others glued on in their place. Mayer tried to test the selective action of the female. The male promethea has blackish wings, while the females are reddish-brown. In accordance with the theory of sexual selection, the peculiar coloration of the male should be due to the selection of dark-colored males, so that under this influence the males would be-come, in successive generations, darker until the present coloration has been attained. Mayer's own account of his experiments and conclusions to test the preferences and selective action of the females is as follows:

"In order to test this hypothesis I cut off the wings of a number of females, leaving only short stumps, from which all the scales were carefully brushed. Male wings were then neatly glued to the stumps, and thus the female presented the appearance of a male. Under these circumstances the males mated with the female quite as readily as they would have done under normal conditions.

"I then tried the experiment of gluing female wings upon the male. Here, again, the mating seemed to occur with normal frequency, and I was unable to detect that the females displayed any unusual aversion toward their effeminate-looking consorts.

"It is also interesting to note that normal males pay no attention to males with female wings.

"In another series of experiments the wings were cut entirely off of males and females and the scales brushed off their bodies, and yet these shabby males were readily accepted by normal females; nor could I see that normal males displayed any aversion to mating with wingless females."

Mayer's next experiments were directed to the end of determining if the males found the females by sight or by smell. By enclosing females in numerous jars variously arranged, and covered or uncovered, it was readily determinable that males never pay any attention to females enclosed in transparent jars so closed as to pre-vent the escape of any odors from the female, while to females enclosed in boxes, or wrapped in cotton so as to be invisible, but yet capable of giving odor off into the air, males came promptly and hovered about. To locate the organs of scent in the female, Mayer cut off abdomens from various females and then placed abdomens and abdomenless females at some little distance apart. Males came to the abdomens and not to the thorax plus wings, legs, and head parts. It was readily proved by experiments with males whose antennae were covered with shellac, photographic paste, etc., that the sense of smell is seated in the antennae. Males with antennae covered with photographic paste did not find females, while the same males with this paste dissolved off did.

All this evidence showed quite clearly that it was odor rather than color which served to attract the males to the females. In lizards, too, in which sexual dimorphism is conspicuous, females showed no preference for particular patterns exhibited by the males in breeding coat. Many such experiments, with like results, seem to make the rejection of Darwin's theory of sexual selection necessary.

But if it is rejected other explanations of the origin of the secondary sexual characters are needed. Such theories have been advanced. Kellogg says of them that the theories proposed to account for secondary sexual characters "mostly rest on one or both of two principal assumptions; first, that the secondary sexual characters are produced as the result of the immediate stimulus (naturally different) of the sexually differing primary reproductive organs, this stimulus being usually considered to result from an internal secretion of the genital organs acting on certain tissues of the organism; and second, that the males, in most species, possess an excess of energy, which manifests itself in extra-growths, extra-development of pigment, plumage, etc., and that displays by the males of special movements, sound making, etc., are direct effects or manifestations of sexual excitation."

It thus appears that the sexual selection' theory, as a special application of natural se' action, is far from being in good standing with present ay biologists. The truth is that most of the work recently done has been destructive, and there is, as yet, no satisfactory replacing theory.

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