SEXUAL DIMORPHISM IN RELATION TO WINTER FORAGING AND TERRITORIAL BEHAVIOUR OF THE THREE-TOED WOODPECKER PICOIDES TRIDACTYLUS AND THREE DENDROCOPOS SPECIES

A comparison of morphological characters shows that Picoides tridactylus is more markedly sexually dimorphic than Dendrocopus leucotos, D. major or D. minor. This is correlated with intersexual differences in foraging behaviour in P. tridactylus, lacking in the Dendrocopus spp. Apparently as a consequence, the sexes are able to share winter territories among P. tridactylus but not among D. major or D. minor; conclusive observations of D. leucotos were not possible. It is suggested that these differences may reflect the distributional history of the species, and the extent of interspecific competition.     

CORRELATED EVOLUTION OF SEXUAL DIMORPHISM AND MALE DIMORPHISM IN A CLADE OF NEOTROPICAL HARVESTMEN

Secondary sexual traits increase male fitness, but may be maladaptive in females, generating intralocus sexual conflict that is ameliorated through sexual dimorphism. Sexual selection on males may also lead some males to avoid expenditure on secondary sexual traits and achieve copulations using alternative reproductive tactics (ARTs). Secondary sexual traits can increase or decrease fitness in males, depending on which ART they employ, generating intralocus tactical conflict that can be ameliorated through male dimorphism. Due to the evolutionary forces acting against intralocus sexual and tactical conflicts, male dimorphism could coevolve with sexual dimorphism, a hypothesis that we tested by investigating these dimorphisms across 48 harvestman species. Using three independently derived phylogenies, we consistently found that the evolution of sexual dimorphism was correlated with that of male dimorphism, and suggest that the major force behind this relationship is the similarity between selection against intralocus sexual conflict and selection against intralocus tactical conflict. We also found that transitions in male dimorphism were more likely in the presence of sexual dimorphism, indicating that if a sexually selected trait arises on an autosome and is expressed in both sexes, its suppression in females probably evolves earlier than its suppression in small males that adopt ARTs.     

EVOLUTION OF SEXUAL SIZE DIMORPHISMS IN EMYDID TURTLES: ECOLOGICAL DIMORPHISM, RENSCH'S RULE, AND SYMPATRIC DIVERGENCE

The origin of sexual size dimorphisms (SSD) has long been a central topic in evolutionary biology. However, there is little agreement as to which factors are most important in driving the evolution of SSD, and several hypotheses concerning SSD evolution have never been tested empirically. Emydid turtles include species with both male and female-biased SSD, and some emydids exhibit among the most extreme SSD in tetrapods. Here, we use a comparative phylogenetic approach in emydids to analyze the origins of SSD and test several hypotheses for the evolution of SSD, some for the first time. We test the Fairbairn–Preziosi hypothesis for the origin of Rensch's rule, and support it in lineages with male-biased SSD but not those with female-biased SSD. We also find support for the secondary ecological dimorphism hypothesis, which proposes that selection for ecological divergence between sexes exaggerates preexisting SSD. Finally, we find only equivocal support for the Bolnick–Doebeli hypothesis, which relates intersexual ecological divergence to interspecific ecological divergence. Our results also illustrate how global analyses of SSD may mislead in groups in which the factors that drive the evolution of SSD vary among clades.     

金丝猴肩带和前肢的性二型

本文研究了两种金丝猴(Rhinopitheous bieti和R.brelichi)肩带及上肢的62项线性变量及指数的性二型,其中64.5%达到了显著性差异水平。结果表明,两性在运动行为上存在一些差异。在所分析的变量中,性别间的差异主要由于成熟时间的不同所引起(雄性比雌性具更长的生长时期),其次是生长速率的不同(雄性比雌性具有更明显的正异速生长率)。对一些指数及不同部位的性二型分析表明,在上肢利用和尺、桡骨的前旋及后旋活动方面,两性也具一些不同。     

SEXUAL DIMORPHISM IN CYPRAEIDAE

EVIDENCE of sexual dimorphism has been found in two species,Cypraeagracilis Gaskoin and Umbilia hesitata Iredale. In theformer the observed difference lies in the colour of the livinganimals, and in the latter in the length of the shell, the malebeing the longer. (Received 10 June 1960;     

蜘蛛的性二型现象及其进化

对蜘蛛的性二型现象进行了初步的概括,并试图以食物对种群繁衍的影响为线索说明其进化机制。蜘蛛的性二型现象主要表现在体形大小上,一般雌性大于雄性;食物的数量和分布制约着蜘蛛性二型现象的进化。     

中国兔猴的单一物种及其两性差别

观察到以前描述的中国兔猴不同种的特征在禄丰的全部中国兔猴标本中交叉地存在、并通过连续的中间表现型而衔接,本文建议石灰坝中国兔猴是厚齿中国兔候的同种异名,因而前一名称应予取消。以犬齿和下第四前臼齿为材料研究了中国兔猴的两性差别。本文使用双变量分布确定两性重叠分布的测量性状的性别差异取得较好的效果。     

SEXUAL DIMORPHISM IN MAMMALS

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 induce in males a cyclical pattern of function in the hypothalamus and the pituitary, enlargement of the breasts and formation of nipples in the rat and a female type of sexual behaviour. There is no complete sex reversal in mammals comparable to that of fish and amphibians. 7. With some exceptions (hamsters, rabbits, guinea-pigs) males are larger than females. Gender differences in weight of organs and in other parameters must be assessed as proportion to male or female weight, surface and activities. The relatively greater amount of fat in female and of connective tissue in male organs in relation to the active parenchyma complicate comparisons. 8. The head and shoulder region is proportionately larger in males and the pelvic region in females. Men and male mice have heavier bones, muscles, hearts, lungs, salivary glands, kidneys and gonads in proportion to body weight, while females have proportionately heavier brains, livers, spleens, adrenals, thymus, stomach and fat deposits. 9. The basal metabolic rate in women is lower than in males. A great variety of metabolic parameters, levels of enzyme activity, location of fat deposits, sensitivity to drugs is sexually dimorphic and responsive to the action of androgens, oestrogens and progestagens. 10. Males tend to have more red blood corpuscles, haemoglobin and erythropoietin per unit volume of blood than women, cows, mares, sows, bitches, female cats and hamsters, but there is no sex difference in this respect in rats, rabbits, goats or sheep. Females tend to have more granulocytes and a proportionately larger lymphomyeloid complex (bone marrow, spleen, thymus, lymph nodes and lymphoepithelial tissues) and greater immunological competence than males. The cortical epithelium of the thymus in mice and rats is sexually dimorphic, responsive to castration and treatment with sex hormones and varies with the oestrous cycle. 11. The kidney is proportionately larger in male mice, rats, cats and dogs, is reduced by castration and enlarged by treatment with testosterone. The kidneys of hamsters and guinea-pigs do not differ in size with sex, nor do they respond to castration or to androgens. The proportion of tubules to glomeruli is greater in the male than the female kidney. The tubular mass increases with androgenic medication, but not the juxtaglomerular apparatus. The parietal epithelium of Bowman's capsule, the histochemistry of the kidney and the composition of the urine vary with gender and respond to sex hormones according to species and strain. The bladder of male mice is proportionately larger than that of females. Some pheromones are present in the bladder urine of intact male mice and of spayed females given testosterone, but absent from that of castrated males. 12. Boars, male elephants, mastodons, horses, deer and monkeys have larger canines than the females. The submaxillary gland of male mice, rats and pigs is proportionately larger than in females, but smaller in hamsters. The proportion of mucous to serous acinar cells in female rodents is greater than in males; female hamsters produce more sialic acid. The secretory tubules of male rats and mice are larger than in females and produce a nerve- and an epidermal-growth factor. Apart from amylase the levels of enzyme activity vary with sex. The liver is sexually dimorphic as regards size, content and metabolism of glycogen, fat, vitamin A, levels of enzymatic activity, phagocytic activity and in its response to castration, sex hormones, to toxic agents, drugs and carcinogens. Sex hormones affect the production of insulin by the pancreas in vivo and in vitro. 13. The male larynx which enlarges and induces voice changes in many mammals at puberty or the onset of the breeding season, is affected by castration and by sex hormones. Male lungs are proportionately larger than female ones with a greater vital and maximal respiratory capacity. Breathing rate and manner varies with sex and is related to differences in the muscular development of the diaphragm. 14. The epidermis and dermis of males are thicker, but the subcutis thinner than in females. The skin is sexually dimorphic in respect of dermatoglyphics, the replacement of vellus by terminal hair and pigmentation of specific regions, the colour of the face and of the sexual skin in monkeys, the development of antlers and horns. The synchrony of the hair cycle and the growth wave of the hair coat in mice and rats depend on the sex of the animals. The X-chromosome mosaicism in the hair follicles of female mice accounts for the mosaicism in pigmentation. Apart from a genetic disorder, the sweat glands are not sexually dimorphic, but the apocrine, the sebaceous glands and their specialized forms are. The embryonic development of mammary glands depends on the absence of androgens and can be induced in male rats and guinea-pigs by antiandrogens. 15. An intact cerebral cortex is necessary for the performance of reproductive functions in male, but not in female rats, cats, rabbits and guinea-pigs. Removal of the olfactory bulb impairs reproduction in female, but not in male mice. Pinealectomy prevents the testicular atrophy of hamsters kept in the dark. The reproductive cycles in females are regulated by the hypothalamus through the control of the ratio of FSH to LH release in the pituitary. This in turn acts on the ovary and thus affects the activity of the thyroid, thymus and lung. In males FSH and LH act synergistically and their secretion is not controlled separately. Oestrogens are more effective than androgens in inhibiting pituitary functions. Sexual dimorphism in cytology, enzyme levels and oestrogen-binding is manifest in the preoptic area, the hypothalamus and the nucleus medialis amygdalae. The female brain is proportionately larger than the male with equal relative amounts of grey and white matter, but a bigger hypothalamic-pituitary-pineal complex. The pineal gland is more prone to tumour formation in boys than in girls and retains its cellularity longer in women than in men. Colour blindness is manifested less in heterozygous women than in hemizygous men. Mature women are more sensitive to the smell of synthetic musk than girls or men. Male rats and mice are more susceptible to audiogenic seizures than females. 16. The activity of the thyroid gland varies at different phases of the oestrous cycle in rats, mice and guinea-pigs. Female mice release more thyroid hormone into the blood than males or spayed animals. Oestrogens increase the level of thyroxin-binding protein. The concentration of TSH in the blood of mature women is double that of men and of menopausal women. The incidence of non-endemic thyroid disorders in women considerably exceeds that in men. 17. The adrenals of females are much larger than those of males except in hamsters. The gland of the female mouse contains more lipid than that of the male. The juxtamedullary X-zone of mice involutes at puberty in males and during the first pregnancy in females. Castration induces an X-zone in male mice, voles, hamsters and cats and an enlargement without stratification in rats. ACTH controls the secretion of glucocorticoids and since its formation is promoted by oestrogens and inhibited by androgens, sex hormones influence indirectly the size and activity of the adrenal cortex. Hepatic inactivation of glucocorticoids is 3 to 10 times greater in intact females than in males. 18. The implications of species variations in sexual dimorphism for the survival and the evolution of mammals are discussed.     

禄丰古猿的两性差别

本文首先讨论了粗壮池猿化石标本的性别判断问题,然后把粗壮池猿和禄丰西瓦古猿分别与有关的现生猿类的两性差别进行比较研究,得出这两个进化系统性别差异的时代变化的结论。这样的结论支持禄丰西瓦古猿的系统地位的论证。     

SEXUAL SIZE DIMORPHISM AND EGG-SIZE ALLOMETRY IN BIRDS

We tested the hypothesis that egg size should evolve in sexually dimorphic birds to reduce costs associated with more rapid growth by nestlings of the larger sex. Consistent with this hypothesis, we found that in species in which males were larger, females laid proportionately larger eggs as sexual size dimorphism increased. However, this result was also consistent with the hypothesis that egg size varied allometrically with both male and female body size. Furthermore we found that in species in which females were larger, relative egg size decreased as size dimorphism increased, which is consistent with the “allometry hypothesis” but not the “cost-reduction hypothesis. That male body size contributes to the allometric relationship between egg size and body size suggests that the basis for the allometric relationship is not wholly a mechanical one stemming from the physical requirements of developing, transporting, and laying an egg of a particular size. Rather, the relationship seems likely to be tied more directly to body size itself the tact that male body size influences a female trait suggests that egg size–body size relationships otter some scope for investigating the basis for allometric relationships in general.     

SEXUAL DIMORPHISM IN BIOMASS ALLOCATION AND CLONAL GROWTH OF XANTHOXYLUM AMERICANUM

Reproduction can have a high resource cost. It has been suggested that greater investments in sexual reproduction by female dioecious plants leads to a lower rate of vegetative growth in females than in males. In this study, we investigated sexual dimorphism in biomass allocation and genet growth of the dioecious clonal shrub, northern prickly ash (Xanthoxylum americanum). The allocation of biomass over the course of one growing season to reproductive tissue, leaves, and growth of aboveground first-year wood, was compared in 18 clones growing in fields and six clones in woods in southeastern Wisconsin during 1985 and 1986. In addition, the number of shoots per clone, and weight of nonfirst-year wood (accumulated biomass) above- and below-ground were estimated. In open field sites, male clones allocated more biomass to new wood and less to reproduction than females, although males allocated more to flowers alone. Accordingly, male clones had significantly more shoots and more accumulated biomass both above- and below-ground than female clones. In the woods, where fruit set was near zero, there were few significant differences between male and female clones in either biomass allocation or accumulated biomass. These results support the hypothesis that the high resource investment in fruit production by females reduces their vegetative growth relative to males.     

NATURAL SELECTION AND SEXUAL SIZE DIMORPHISM IN RED-WINGED BLACKBIRDS

Patterns of overwinter mortality in the sexually dimorphic red-winged blackbird (Agelaius phoeniceus) were examined to test the predictions of the sexual-selection hypothesis that male size is limited by directional selection favoring small males and that female size is maintained by stabilizing selection wherein extreme phenotypes experience higher mortality. Museum specimens collected from Ontario over a 95-yr period were used to compare the sizes of males and females collected in fall and spring. In a separate field study, body sizes of returning and nonreturning male and female red-winged blackbirds were compared over a 6-yr period. Overall, there was no evidence of higher overwinter mortality among larger males. Among adult (ASY) males, large individuals appeared to have higher survival than small individuals, although among subadult (SY) males, large size may have been disadvantageous. Weak evidence of stabilizing selection on female body size was found. Among adults, sexual size dimorphism seemed more pronounced after winter than before winter. Our results do not support the hypothesis that body size in male red-winged blackbirds is limited by selective mortality outside the breeding season. It is possible that size selection occurs earlier in life, when males are still in the nest. Our results suggest that caution should be exercised when interpreting interspecific evidence showing higher adult male than female mortality in sexually dimorphic species. Such patterns could arise as a cost to males of sexual selection and yet provide no insight into how natural selection opposes sexual selection for increased male size.     

SEXUAL SELECTION AND THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN THE WATER STRIDER,AQUARIUS REMIGIS

Sexual size dimorphism (SSD) is often attributed to sexual selection, particularly when males are the larger sex. However, sexual selection favoring large males is common even in taxa where females are the larger sex, and is therefore not a sufficient explanation of patterns of SSD. As part of a more extensive study of the evolution of SSD in water striders (Heteroptera, Gerridae), we examine patterns of sexual selection and SSD in 12 populations of Aquarius remigis. We calculate univariate and multivariate selection gradients from samples of mating and single males, for two sexually dimorphic traits (total length and profemoral width) and two sexually monomorphic traits (mesofemoral length and wing form). The multivariate analyses reveal strong selection favoring larger males, in spite of the female-biased SSD for this trait, and weaker selection favoring aptery and reduced mesofemoral length. Selection is weakest on the most dimorphic trait, profemoral width, and is stabilizing rather than directional. The pattern of sexual selection on morphological traits is therefore not concordant with the pattern of SSD. The univariate selection gradients reveal little net selection (direct + indirect) on any of the traits, and suggest that evolution away from the plesiomorphic pattern of SSD is constrained by antagonistic patterns of selection acting on this suite of positively correlated morphological traits. We hypothesize that SSD in A. remigis is not in equilibrium, a hypothesis that is consistent with both theoretical models of the evolution of SSD and our previous studies of allometry for SSD. A negative interpopulation correlation between the intensity of sexual selection and the operational sex ratio supports the hypothesis that, as in several other water strider species, sexual selection in A. remigis occurs through generalized female reluctance rather than active female choice. The implications of this for patterns of sexual selection are discussed.     

QUANTITATIVE GENETICS OF SEXUAL DIMORPHISM IN HUMAN BODY SIZE

A classical data set is used to predict the effect of selection on sexual dimorphism and on the population means of three characters—stature, span, and cubit—in humans. Given selection of equal intensity, the population means of stature and of cubit should respond more than 60 times as fast as dimorphism in these characters. The population mean of span should also respond far more rapidly than dimorphism, but no numerical estimate of the ratio of these rates was possible. These results imply that sexual dimorphism in these characters can evolve only very slowly. Consequently, hypotheses about the causes of sexual dimorphism cannot be tested by comparing the dimorphism of different human societies. It has been suggested that primate sexual dimorphism may be an allometric response to selection for larger body size. We show that such selection can indeed generate sexual dimorphism, but that this effect is too weak to account for the observed relationship between dimorphism and body size in primates.