The genetic and the operative evidence relating to secondary sexual characters

by Thomas Hunt Morgan

There are a number of observations on ducks. Several cases have been recorded where in old age the female assumed the male plumage (Darwin, Shattock, and Sellheim). Also a few cases in which the testes were removed. Those of Goodale are the most complete and striking. The male duck has two characteristic plumages, one called the nuptial, also called the summer or breeding plumage that is assumed at the molt in the autumn, and the other the eclipse plumage, which is not identical with but much like that of the female. Here, then, we find a new situation, and one that invites comparison with the condition in Sebrights, in so far as the male becomes hen-feathered at certain seasons.

Throughout the greater part of the year the Rouen drake has the nuptial plumage. The head is green and the breast is claret. Two median tail feathers are strongly curved; the next two are also often curved. These four are called the sex feathers. At the close of the breeding-season (July) both sexes molt. The male now has the same coat as the female, or nearly so. The green head becomes brown to buff; the sex feathers are straight. The change back again to the nuptial plumage begins at the end of summer and is completed early in October. Thus in the race of Rouens the eclipse plumage lasts only a very short time. In the mallard it lasts longer. The eclipse plumage develops, therefore, only when the testes are active, or, as Goodale puts it, “the presence of the active testis is necessary for the drake to assume this plumage.” Conversely, the nuptial plumage comes on in the late summer, when mating is over, and when the testes have shrunken and are not active, at least as far as the sex-cells are concerned. In some respects the situation is like that in the fowls, for in both the testes are not necessary for the development of the full plumage, but in other respects the situation is different, because at the time in the ducks when the testes are active the eclipse plumage develops. Are we to suppose that at the time of sexual activity a substance is produced analogous to that produced by the ovary of the female? This seems the most plausible assumption, for we know that if the testis is removed the eclipse plumage does not appear. Such a situation suggests a comparison with the Sebright, where it has been shown that the testis must actively produce some substance which, like that in the ovary, keeps down cock-feathering. It is plausible, even if it can not be established, that the substance in the duck and the inhibitory substance in the male Sebright are the same as that produced in the female.

Goodale’s results with females (ducks) are not so clear cut, because the ovariotomized females turned out to be of two sorts. One sort is almost identical with the male, the other is more intermediate. There are sufficient reasons for thinking, he says, that these differences are not due to defective operations. Goodale suggests a genetic difference in the females used, but this is apparently even to Goodale himself not a very satisfactory solution. For our present purpose the important fact is that the ovariotomized female may assume the perfect male plumage. Evidently the ovary produces some substance which, as in the hen, suppresses the potential plumage of the male. One such female known to have had all the ovary removed never assumed the summer (eclipse) plumage of the drake. On the other hand, another female developed first the nuptial plumage, but this was replaced by the summer coat “of the male of this variety.” Again, in the summers of 1914 and 1915 the change to the eclipse plumage was followed in the autumn by a return to the nuptial plumage.

How can we explain the apparent discrepancy of Goodale’s results? In one case, the nuptial plumage was molted to nuptial plumage; in the other case an eclipse plumage appeared at the breeding-season. Goodale regards the latter case as a more perfect approach to the male than the former, but this view undoubtedly offers serious theoretical difficulties. It seems to me possible to suppose that in those cases where the summer plumage appeared there was in reality enough ovarian tissue (or related tissue) left after the operation to produce an effect at the normal season for such ovarian tissue to become most active. It might then suffice to eclipse the male plumage sufficiently to make it very similar to the eclipse of the normal male. At any rate, on this basis we have a consistent explanation of the entire complex of phenomena.

What bearing have these results relating to castration and transplantation on the theory of sexual selection? Granting, of course, that selection takes the materials as it finds them, there may still be restrictions imposed on the theory by the kind of material offered. For instance, the development of the plumage of the cock is independent of the condition of his testes. Hence, if the female selected the more vigorous male, she would not necessarily obtain one more ornate than his less vigorous rivals. If the taste of the hen has built up the plumage of the cock, it has been carried out then independently of the vigor resulting from the greater activity of the testis. In a word, the more vigorous male is not necessarily the most highly colored one. Darwin concedes that these two conditions, high color and vigor, must go together to insure success, or at least that the most vigorous and therefore the most highly colored male will have more offspring. Wallace’s contention that the greater vigor of the male accounts for his greater development of plumage gets scant support from the facts of castration. One might rather contend that the female must be more vigorous, since she is obliged to suppress plumage that is allowed to run riot in the male.

Wallace’s argument in favor of natural selection holding down the plumage in the female as a protection to her while nesting might appear to fit the facts better were it not that the quest for an explanation of the male’s plumage is thereby abandoned. It should not be forgotten in this connection that the nest is generally only partly concealed, that bright color at rest need not be conspicuous, and that the male, exposed as he is through a considerable part of the year, still manages to maintain himself in about equal numbers with the female. Suppose, however, for the sake of argument, that natural selection has kept under the full possibilities of the female. The modus operandi would be competition between the least adorned females, suppression being brought about by the activity of the ovary; while the male is left therefore to exhibit the full possibilities of the genetic complex of his race without restraint. The facts in the case are that the plumage of the male is the direct result of his genetic composition; the female has the same genetic composition (the sex-linked characters are duplex), but the ovary produces a substance that holds them in restraint. Put in this way, there is nothing further to be explained, unless we insist on finding an explanation as to how the species came to have its genetic constitution. In other words, if we are not satisfied with the statement as to the actual situation, we must explain it by a utilitarian appeal to a relation between the plumage and the world outside of the individual or the species. To those who feel unsatisfied to leave the case as it stands on a physiological basis, there is another hypothetical means of escape. It may be assumed that the genetic factors that are instrumental in producing the secondary sexual characters have also other but unknown influences in the economy of the species, color and ornamentation being by-products of these factors whose utility in other directions accounts for their presence. Such a philosophy has perhaps one redeeming feature, since it suggests the possibility of searching for other influences—influences that only incidentally give the striking coloration and ornamentation of the males.

At first sight the absence of cock-feathering in the Sebright may seem to furnish the occasion for such a quest. It might appear that since only one or two genetic factor differences are responsible for the “nuptial” plumage of the male, that this plumage may have originated in one or two genetic changes. Such an argument is fallacious, however, for very many genetic factors may historically have been necessary to build up the nuptial plumage of the male. The breeding experiment shows no more than that one or two other factors have appeared that counteract the effect of all that the others are capable of producing; the experiment throws no light upon how many or how few these other factors may be. That the nuptial complex is still present in the Sebright is evident after castration. Castration shows only that the testes in the Sebright produce some material that keeps down the effects of all the other factors combined. This conclusion, it is true, somewhat simplifies the problem for those who appeal to natural selection as suppressing in the female the feathering of the cock, because it shows that this could have been accomplished by one or two Mendelian factors that appeared of such a kind that they caused the ovary to produce a substance antagonistic to the influences coming from the genetic complex of the species.

With this by way of provisional exposition, let us return to the question as to whether the Sebright-game cross throws any other light on the possibly useful character of the genetic factor or factors that produce cock-feathering. It is obvious that the evidence gives us no clue at all, for with the exception of the normal allelomorphs of the dominant factor for hen-feathering, all the other factors are still present in the Sebright. The normal allelomorph in question need not have had any relation to the other complex; in fact, it seems not to have any, because the castrated Sebright (with both normal allelomorphs replaced by genes for hen-feathering) still develops the characteristic cock-feathering.

The outcome in the duck with its double male plumage is still more puzzling when we attempt to analyze the situation in the light of the selection theory. At the height of the breeding-season, when his testes are enlarged and functioning actively, a substance is being produced that leads to the eclipse of the nuptial plumage. If the male were selected by his partner for his plumage, he would be chosen for a plumage that develops in the absence of the functioning testes. If the male is chosen because of his greater aggressiveness or “activity” or “vitality” due to the development of his testes, the result would be to select males that would probably develop a better eclipse plumage. The case is interesting because it gives an opportunity to distinguish between a plumage that develops under the influence of the sexual organs and one that does not; and the latter is paradoxically the nuptial plumage. It is true that the male might be selected for his nuptial suit, and, theoretically at least, female choice might still be made responsible for this plumage, but this merely shifts the problem, for it leaves “unexplained” the appearance historically of the effect of the activity of the testes in suppressing this plumage for a short time after maturity. No doubt an attempt might be made to show that natural selection comes in at this time of the year in giving a protective color to the male, but so long as any evidence is lacking as to the need of this protection the argument serves rather to further complicate an already difficult situation.

Goodale has written to me that there is an account, in the Agricultural Journal, Union of South Africa, IV, 1912, of the effects of the removal of the ovary of the female ostrich. I have not been able to see the account, but according to my informant such female individuals assume the male secondary characters.