Sexual Dimorphism in Mammals

Abstract

Summary 1. Life expectancy and mortality rates from diseases arising in various organs vary with sex because of differential exposure to external hazards and because of essential differences between males and females in aspects not directly connected with reproduction. This review attempts to collate data about the structural and functional dimorphism of mammals exclusive of the genital organs and psychological aspects. 2. The primary sex ratio is not certain and like the secondary and tertiary may vary with species. In many mammals more males are aborted and born than females. Later a higher mortality of males, due to sex‐linked congenital diseases and greater exposure to external hazards, shifts the balance in favour of females at the time of sexual maturity. The average life span of females is longer than that of males, except in hamsters and in inbred strains of mice with a high incidence of mammary tumours. 3. Chromosomes as well as gonadal hormones are responsible for the development of male and female characteristics. The Y‐chromosome initiates the differentiation of the testis, but gonadal hormones control the subsequent differentiation of the genital tract and other organs. In embryos the testicular secretion precedes that of the ovary. The Y‐chromosome is devoid of, but the X‐chromosome retains structural genes. The random heterochromatization of a paternal or a maternal X‐chromosome in the somatic cells of female embryos equalizes the genetic information for both sexes and produces a mosaicism of female somatic cells except in the kangaroo where the paternal X‐chromosome is selectively inactivated. Deficient genes on the X‐chromosome become manifest in hemizygous males, in homozygous females and can be detected in heterozygous women in half of the somatic cell population in some conditions. 4. The testis grows faster than the ovary and starts to secrete earlier, but the maturation of female gonocytes precedes that of males. Spermatogenesis starts at puberty and is maintained throughout life, while multiplication of oogonia ceases in the perinatal period (except in lemurs), when the stage of the first meiotic division is reached. The stock of oocytes dwindles during life. 5. In many mammals the male grows faster than the female before and after birth, but is less mature. Puberty tends to start earlier in females and the associated growth spurt does not last as long as in males. Testosterone has a direct anabolic effect, promotes growth and delays differentiation. Oestrogens are considered katabolic, but promote growth indirectly by stimulating the production of growth hormone in the pituitary. Progesterone has an anabolic and slight androgenic effect. 6. A female pattern of differentiation of the hypothalamus, the pituitary and the pineal gland, manifested at puberty by cyclical activities of the reproductive organs requires the absence of androgens during a critical phase of ante‐ or perinatal development. Oestrogens given to males at that period produce effects similar to castration. Antiandrogens ind...

Sexual dimorphism in Hucul horses using discriminant analysis

“…Last but not least, this study has shown that indices of length and width of the head do not play a major role in discriminant functions for different age groups of Hucul horses, which are at odds with claims by Glucksmann (1974) that a stallion's head is proportionally larger than that of a mare. Graphical classification into gender groups for all age groups (I to VI) together, based on a linear regression model using the force index (G) and the metacarpus circumference index (A).…”

“…E-mail: hapu@tlen.pl mares, whereas chest width and metacarpus circumference are greater in stallions. Glucksmann (1974) observed the head to be proportionally larger in stallions. While analysing biometric traits of Arabe-Barbe horses, Boujenane et al (2008) noted gender dimorphism in this breed to be strongly pronounced in most parameters other than head dimensions, which is likely related to differences in work performed by stallions and mares.…”

“…The term 'sexual dimorphism' denotes differences in dimensions and proportions of the body between males and females (Glucksmann, 1974) of a species of horses. This phenomenon has been described at length for many animal species; but again it has not been investigated in depth in horses, primitive breeds in particular.…”

Bisoniana LXIV. Biometric analysis of some internal organs of hybrids between the European bison and domestic cattle

“…An analysis of the structural sexual dimorphism of mammals carried out by Glucksmann (1974) shows that in most species males are larger and have higher absolute weights of organs than females. The differences in the absolute weights of organs than females.…”

“…At the same time Glucksmann (1974) emphasizes that the differences in the weight of organs may result from fat deposition, especially in females, and from the development of sustentacular tissue in the organs of males. In the considerations of the structural sexual dimorphism in hybrids of the European bison and cattle the body weight ratio between females and males was assumed, as in Glucksmann (1974), as the basis for comparison and the weight ratio of the internal organs under study was compared with it. If the value obtained exceeded the body weight ratio, the given organ was relatively larger in the male.…”