Dimorphism

DIMORPHISM (from Greek dis - double and morphe - form), the presence of two morphological forms of organization within a single animal species. In biology, sexual, age, and seasonal dimorphism are distinguished. Sexual dimorphism is spoken of in the case when individuals of the same species but of different sexes differ from each other. Since in dioecious animals males and females of any species differ at least in the structure of the gonads, it can be said (in general form) that sexual dimorphism is inherent in all dioecious animals. Often, however, the concept of D. is used in a narrower sense; among sexually dimorphic animals are included only those in which males and females differ from each other (besides the gonads and sexual ducts) also in external characteristics of organization. An example of clear sexual dimorphism can be man, in whom the man differs from the woman by larger body size, development of beard and mustache, lower voice, due to greater development of the vocal apparatus, and many other characteristics. In contrast to dimorphic species with clear differences between males and females in external characteristics, species in which males and females have the same external characteristics are called monomorphic (despite the difference between males and females in internal structure). Sexual dimorphism in external characteristics is a very widespread phenomenon, but it is especially sharply expressed in birds, for example in the family of gallinaceous birds (in domestic chickens, pheasants, turkeys, grouse), in many passerines, in African ostriches, etc. Sexual dimorphism is also very brightly expressed in many mammals (in some species of antelopes, bulls, deer). Among invertebrates, bright dimorphism characterizes many insects (especially from the group of beetles), crustaceans, etc. Sexual dimorphism often manifests itself in body size, coloration, and form of feather, hair, and scale, horn or chitinous cover, etc. In vertebrates, as a rule, the male is larger and more brightly colored than the female; often in invertebrates too the male is larger than the female and has various distinguishing marks in the form of outgrowths and appendages (there are exceptions, however, for example in many birds of prey the female is larger than the male; there are few birds in which the female carries a brighter plumage than the male, for example sandpipers-Rhyn-chaea, Phalaropus hyperboreus, etc.). Sexual dimorphism in vertebrates stands in a clearly established connection with the secretory activity of the gonads. Experience convinces us that sexual dimorphism in vertebrates in the form of unequal manifestation of characteristics in male and female develops only in the presence of gonads. In the case of removal of the gonads, as a result of surgical intervention or pathological underdevelopment of the gonads, male and female of normally dimorphic species develop similar characteristics, turning as it were into a monomorphic species. It is very characteristic that in birds (chickens, pheasants, ducks, etc.) castrated individuals develop characteristics that make them similar to a normal male. Thus, in castrated male and female chickens, plumage and spurs develop, similar to the plumage and spurs of a normal rooster. At the same time, castrated males of mammals (man, antelopes, bulls, deer, goats, rams, etc.) are similar in their coverings to a normal female. In insects (butterflies and orthopterans), connections between sexual dimorphism and gonads cannot be established; characteristics by which a normal male differs from a normal female develop also in castrated individuals.-Darwin developed a coherent theory of the origin of sexual dimorphism based on sexual selection. According to the theory of sexual selection, the brighter colors in the male's plumage are connected with the fact that females during pairing choose the 'best' males that leave offspring. The strong development of weapons of defense and attack in the male in the form of horns, teeth, fangs, etc., according to Darwin's view, is connected with the struggle of males for females. Darwin's views on sexual selection, however, meet many objections (especially with respect to insects) and cannot be considered generally accepted. Age dimorphism is spoken of in the case when individuals of the same species and sex have different characteristics at different ages. Thus, many birds in young age have a different coloration of plumage than adults of the same species; in the nilgau antelope, the calf has reddish fur, while the adult male has black-gray fur, etc. As a rule, the juvenile plumage is close (but not identical) in form and coloration to the plumage of the female. Age dimorphism, unlike sexual dimorphism, does not depend on the testes and ovaries, which is evident from experiments of early castration of ducks, pheasants and chickens (M. M. Zavadovsky). Early castrates still develop both juvenile and adult plumage. There is some reason to believe that age dimorphism depends on endocrine glands and primarily on the thyroid gland. Seasonal dimorphism is characterized by the fact that the animal carries two different plumages during the year: one in winter, another in summer. Thus, many male ducks, molting twice a year, put on in the autumn molt for winter and spring a bright, so-called 'nuptial' plumage, and in the spring molt put on instead of the discarded bright plumage a modest one, close to the plumage of the female. Hares-lemmings or arctic foxes carry a white hair coat in winter, and in summer a pigmented one. Seasonal change of plumage is manifested not only in coloration of the coat, but also in density of hair, partly in form of hair. Seasonal dimorphism (in ducks), as well as age dimorphism, does not depend on the gonad, since castrated individuals retain seasonal change of plumage. In the case when there are besides females two or more types of males or vice versa, or during the life of animals change their characteristics not once but several times, they speak of sexual or age polymorphism (see).

M. Zavadovsky. In protozoa, the phenomenon of D. is connected mainly with the sexual process and with the change of sexual and asexual generations. Thus for example in some infusorians (Vorticella) small male and large female individuals are formed, which conjugate. In the group Coccidiomorpha sexual dimorphism manifests itself in the development of special, mostly differentiated sexual individuals - anisogametes, for example in plasmodia (Pl. vivax and others). In some shelled rhizopods, shells of two kinds are formed: individuals dressed in a megalospheric shell (with a large central chamber) give rise to sexual individuals - isogametes, which copulate and form zygotes, from which grow individuals dressed in a microspheric shell (with a small central chamber); the contents of the latter multiply asexually, i.e. divide and form asexual forms, agametes, which grow and again turn into macrospheric forms. Changes in form of protozoa, connected with the influence of the environment or with age, are usually considered as phenomena of polymorphism.