The manifestation of sexual dimorphism in the preimago phases of 2 species of the genus Hyalomma (Ixodoidea, Ixodidae)

Experiments on laboratory cultures of Hyalomma asiaticum and H. anatolicum have shown that at the nymphal phase sexual dimorphism manifests itself in statistically reliable differences between general sizes and mass of the body of male and female individuals. Larger nymphs moult mostly into females, small ones mostly into males. Reliable differences have been noted in the sizes of scutum, gnathosoma and its appendages in male and female nymphs of both species. At the larval phase sexual dimorphism does not manifest itself in the sizes of individuals.     

The correlation variability of the morphometric traits in 4 species of ixodid tick species (Ixodidae)]

Correlation analysis of 10 characters was run in three species of Dermacentor, D. nuttalli, D. ushakovae, D. niveus, and in Ixodes persulcatus, separately for each phase and sex. Correlation coefficients are very low in larvae of all species. In male nymphs correlation coefficients are on the same level in all species while the correlation coefficient level in female nymphs reflects the degree of independence of the development of characters. This defines the degree of manifestation of sexual dimorphism at the nymphal phase: the more rigid the connections between the characters the less number of characters shows sexual differences. At the phase of imago the lowest correlation coefficients are in I. persulcatus. This species displays most distinct differences in the morphology of females and males. On the basis of comparison of correlation and variability coefficients the functional role of the characters is discussed.     

Sexual dimorphism of the nymphal phase of ixodid tick species in the genus Dermacentor

In four studied Dermacentor species at preimaginal phases sexual dimorphism hardly reveals itself in body size and mass of individuals. The formation of sexual distinctions at the nymphal phase concerns different characters as in different species so in populations of one species. In D. niveus male and female nymphs differ in the length of II-III palpal joints and width of gnathosoma, in D. ushakovae in the length of scutum and its proportions, in the width of gnathosoma and hypostome and in the diameter of peritreme. The sex of D. silvarum nymphs can be identified by the width of gnathosoma, length of hypostome and diameter of peritreme. Female and male nymphs of D. marginatus from the Stavropol Territory and Armenia differ in the scutum proportions and populations from the western Pamirs in the length of scutum and gnathosoma.     

Sexual dimorphism of the larvae and nymphs of the taiga tick--Ixodes persulcatus

Manifestations of sexual dimorphism at the preimaginal phase was studied on the laboratory culture of Ixodes persulcatus. It has been found out that larger engorged larvae produce, in general, larger nymphs and smaller larvae produce smaller nymphs. The classification of engorged nymphs into large (over 2.9 mm) and small ones (less than 2.9 mm) has made it possible (with 100% probability) to obtain females from the former and males from the latter. Differences in linear sizes of scutum, gnathosoma and its appendages in male and female nymphs were determined that has made possible the identification of sex in hungry nymphs.     

Sexual behaviour and evolution of sexual dimorphism in body size in Jaera (Isopoda Asellota)

Individuals of the genus Jaera do not mate at random. In the species from the Mediterranean group, J. italica and. J. nordmanni, large males and medium sized females are at an advantage and their sizes are positively assorted. These effects are attributable to sexual competition between males. In the Ponlo-caspian species J. istri, no advantage of large males exists, but sexual selection could be the cause for a long passive phase prior to copulation and for normalizing selection upon female size at pairing. In the Atlantic species, J. albifrons, no selection can be ascertained.
Differential mating success in males appears as one of the causes of the evolution of sexual dimorphism in body size, which makes males larger, of equal size, or smaller than females according to the species. The reason for this reversal in dimorphism seems to differ in the two sexes. Sexual selection provides an explanation for the evolution of male size, while the interspecific changes in female length are more likely due to ecological factors.     

Sexual dimorphism in longnose gar Lepisosteus osseus

The present study of Lepisosteus osseus dimorphism expands upon previous work by examining a broader array of morphometric characters while removing the bias associated with overall body length. A stepwise discriminant function analyses (swDFA) found that five characters best distinguish the sexes: head width, mid-snout width, anal-fin height, anal-fin width and prepectoral-fin length. Discriminant function analyses (DFA) with the five characters yields misclassification rates of 23·5% for females and 9·7% for males. Subsequent DFA using these five characters plus standard length yields misclassification rates of only 8·8% for females and 6·2% for males. The data presented here reveal differences in head and anal-fin shape between male and female L. osseus that may have evolved to enhance predatory abilities or competitive abilities during reproduction. This study is the first to find that L. osseus exhibits sexual dimorphism in characters without the biases of overall size.     

Sexual dimorphism in leg length among orb-weaving spiders: a possible role for sexual cannibalism

The degree and direction of sexual dimorphism across different species is commonly attributed to differences in the selection pressures acting on males and females. The extent of these differences is especially apparent in species that practise sexual cannibalism, where the female attempts to capture and eat a courting male. Here, we investigate the relationship between sexual dimorphism in size and leg length, sexual cannibalism and courtship behaviour in three taxonomic groups of orb-weaving spiders, using morphological data from 249 species in 36 genera. Females are larger than males in all three taxonomic groups, and males have relatively longer legs than females in both the Araneinae and Tetragnathidae. Across genera within each taxonomic group, male body size is positively correlated with both female body size and male leg length, and female body size is positively correlated with female leg length. Sexual size dimorphism is negatively correlated with relative male leg length within the Araneinae, but not within either the Tetragnathidae or the Gasteracanthinae. There was no negative correlation between sexual size dimorphism and relative female leg length in any taxonomic group. We argue that the relationship between sexual size dimorphism and relative male leg length within the Araneinae may be the result of selection imposed by sexual cannibalism by females.     

Sexual size dimorphism in parasitoid wasps

Sexual dimorphism in body length and proportion of overlap between the ranges of body length for males and females were estimated for 361 species of parasitoid wasps from 21 families. In most species, females are generally larger than males, though the range of male and female sizes overlap. Species in the family Ichneumonidae differ significantly from species in other families in three ways: (1) ichneumonids on average are larger, (2) in most species, females are generally smaller than males, and (3) on average, proportion overlap between the ranges of body length for males and females is greater. At present, there is a paucity of life history data on parasitoid wasp species for which size dimorphism is known. Thus it is not clear why ichneumonids differ from species in other families. Possible evolutionary explanations for variation in dimorphism among parasitoid wasp species are discussed.     

Sexual size dimorphism in a landlocked Pacific salmon in relation to breeding habitat features

Many animal species exhibit size dimorphism between sexes. Sexual selection, whereby male–male competition favors larger body sizes, has been considered a likely cause of sexual size dimorphism. Habitat features in breeding areas could affect the outcome of male–male competition, yet few attempts have been made to relate breeding habitat features with interpopulation variation in sexual size dimorphism. In this study, we examined interpopulation variation in sexual size dimorphism by studying the landlocked amago salmon (Oncorhynchus masou ishikawae) at a microgeographic scale. We found that female body size was independent of stream size but that male body size decreased with smaller stream sizes. A likely explanation is that the relationship between reproductive success and the size of males is influenced by the availability of refuges that are only available to small-bodied males. Sexual differences in body size increased with decreasing stream sizes, supporting the hypothesis that the reproductive success of larger males is reduced in smaller streams. In contrast, the maturation-length threshold increased with stream size for both sexes. The stream-size-based interpopulation variation in sexual size dimorphism and size at maturity in landlocked amago salmon may therefore have arisen through a combination of sexual and natural selection.     

海南变色树蜥个体发育中形态和食性的变化

研究变色树蜥（Calotes versicolor)头,尾大小和食性在个体发育过程中的变化,以及头,尾大小两性异性的个体发生,成体体长（SVL）无显著的两性差异,两性异形主要表现为雄性个体有较大的头部（头长和头宽）和尾部。头,尾大小的两性异形在初生幼体就已存在,并随个体发育的进行变得更加显著。不同年龄组两性个体头长以及雌体头宽随SVL呈同速增长;雄性头宽随SVL呈异速增长,表现为雄性头宽的增长速率在个体发育过程中逐渐增大。头,尾部的相对大小在个体发育过程中有显著的变化,初生幼体头部相对较大,尾部相对较小,这种形态特征是胚胎优先保证生态学意义更为显著的部分（如头部）生长的结果,有利于初生幼体的早期生存和生长,不同性别和年龄组的变色树蜥摄入食物的种类及各种食物在摄入食物中所的比例有一定程度的差别,因而食物生态位宽度和重叠度有一定程度的差别,然而没有直接的证据表明头部大小的两性异形能导致两性食物生态位的明显分离。     

Male-biased size dimorphism in ichneumonine wasps (Hymenoptera: Ichneumonidae) – the role of sexual selection for large male size

Abstract.  1. Sexual differences in body size are expected to evolve when selection on female and male sizes favours different optima.
2. Insects have typically female-biased size dimorphism that is usually explained by the strong fecundity advantage of larger size in females. However, numerous exceptions to this general pattern have led to the search for selective pressures favouring larger size in males.
3. In this study, the benefits of large size were investigated in males of four species of ichneumonine wasps, a species-rich group of parasitoids, many representatives of which exhibit male-biased size dimorphism.
4. Mating behaviour of all ichneumonine wasps are characterised by pre-copulatory struggles, in the course of which males attempt to override female reluctance to mate. A series of laboratory trials was conducted to study the determinants of male mating success.
5. A tendency was found for larger males as well as those in better condition to be more successful in achieving copulations. Size dimorphism of the species studied, mostly male-biased in hind tibia length but female-biased in body weight, indicates that sexual selection in males favours longer bodies and appendages rather than larger weight.
6. The qualitative similarity of the mating patterns suggests that sexual selection cannot completely explain the considerable among-species differences in sexual size dimorphism.
7. The present study cautions against using various size indices as equivalents for calculating sexual size dimorphism.
8. It is suggested that female reluctance in ichneumonine wasps functions as a mechanism of female mate assessment.     

青藏高原褐背拟地鸦表型特征的性别差异与地理变异

将数值分类用于鸟类分类学研究时,对于雌雄的形态特征差异没有被重视,尤其是对于雌雄同形的鸟类。本文以雌雄同形的褐背拟地鸦Pseudopodoces humilis为材料,运用SPSSl0．0FORwINDOwS统计分析软件对108号褐背拟地鸦标本(51♀♀,57♂♂)的数量性状(体长、跗跖长、翅长、尾长、嘴宽、嘴高、上喙长、下喙长、嘴裂、第3趾爪长、第1趾爪长)的原始数据进行分析,结果表明雌、雄性状在翅长(n＝51,P＝0．012)和嘴高(n＝57,P＝0．043)上有明显差异,但在体长、跗跖长、尾长、嘴宽、上喙长、下喙长、嘴裂、第3趾爪长、第1趾爪长等数值特征上却没有明显的差异。所以在以后对于褐背拟地鸦的地理种群变异和亚种分化的研究中,翅长和嘴高两特征应该根据雌雄分别讨论。通过对不同性状量度和纬度的相关回归分析,发现在测量标本所涉及的采集地范围内,即主要在青藏高原东南部地区,褐背拟地鸦体长和雄乌的翅长在地理分布上随纬度的增加而变小,而其它性状特征没有明显的地理分布纬度上的变化。     

条纹短攀鲈的雄性个体大小、雌性偏好以及雄性之间的竞争

本研究考察了条纹短攀鲈(Trichopsis vittata)雄性个体的身体大小对雌性个体性偏好的影响以及对雄性之间竞争的影响。本研究设立了两种处理来研究雌性个体的性偏好以及雄性之间的竞争。在一种处理中,放入同样大小的雄性个体,而在另一种处理中,放入大小不同的雄性个体。结果表明,雄性个体的大小不影响雌性条纹短攀鲈的性偏好。在对雄性之间竞争的研究中,发现个体较大的个体获胜的次数比较多。在个体大小相同的处理中,雄性之间打斗的持续时间比另一种处理中的打斗持续时间长。对这两种处理中不同对抗行为的比较表明,个体大小相同处理组中的发声、咬、以及总体对抗行为发生得更为频繁。对个体大小不同处理组中的大、小个体的进一步观察也揭示出体型大的个体比体型小的个体有更多的追逐行为和更少的逃脱行为。这些结果表明,在雄性个体比雌性个体大的物种中,体型的性两型可能与雄性之间的竞争有关,而不是与雌性偏好有关。     

Sexual Dimorphism of the Jilin Clawed Salamander,Onychodactylus zhangyapingi,(Urodela:Hynobiidae:Onychodactylinae) from Jilin Province,China

Sexual dimorphism in size and shape is common in many organisms, and is a key evolutionary feature. In this study, we analyzed morphometric data of the Jilin clawed salamander Onychodactylus zhangyapingi, an endemic Chinese salamander, to examine sexual size and shape dimorphism. The morphometric data included 14 characteristics of 13 females and 11 males and was analyzed using univariate and multivariate methods. Our results showed that sexual dimorphism occurs not only in body size, but also in body shape. Males have a longer snout-vent length than females, a rarely reported pattern of male-biased sexual size dimorphism. Females have a larger space between the axilla and groin than males, while males have longer and larger tails compared to females. The sexual dimorphism in body size and shape can be explained by existing theories, but there is little data for the mating system, behavior, reproduction, or ecology of O. zhangyapingi, so further studies are required.     

Is natural selection promoting sexual dimorphism in the Green-backed Firecrown Hummingbird ( Sephanoides Sephaniodes )?

In many hummingbird species there is an opposite pattern of sexual dimorphism in bill length and other morphometric measures of body size. These differences seem to be closely related with differences in foraging ecology directly associated with a different resource exploitation strategy. The aim of this study was to assess if natural selection is acting on wing length and bill size in hummingbird males and females with different resource exploitation strategies (i.e., territorial males and non-territorial females). If competition for resources promotes sexual dimorphism as a selective pressure, males should be subjected to negative directional selection pressure for wing length and no selection pressure over bill size, while females should undergo positive directional selection pressure for both bill size and wing length. The morphometric data we collected suggests that there is no selection for wing length and bill size in male hummingbirds. In contrast, our females exhibited positive directional selection for both wing length and bill size. Although we cannot reject sexual selection acting on sexually dimorphic traits, this study suggests that natural selection may promote sexual dimorphism in traits that are closely related with hummingbird foraging ecology and resource exploitation strategies.     

Precopulation Sexual Selection in Nysius huttoni White (Heteroptera: Lygaeidae) in Relation to Morphometric Traits

Nysius huttoni White is a polygamous bug, endemic to New Zealand, and an important pest of wheat and brassicas. This bug has a female-biased sexual size dimorphism but relative to body length, males have longer antennae, suggesting that the allometric scales of antennal–body relationships may be highly selective in sexual selection. Body weight and most morphometric traits measured have no effect on mating success of either sex. Males significantly preferred mating with females having thicker abdomens, more mature eggs, and longer ovipositors. This result suggests that males may select their mates on the basis of immediate reproductive benefit: fertilizing more eggs and ensuring better survival of these eggs. Males with large genital structures have mating advantages over those with small ones, suggesting that precopulation sexual selection in this species act on male genital traits rather than body weight and nonsexual traits. Finally, females significantly preferred males with greater slopes for the antennal-body relationship for mating. The allometry in the male antennal length may be an indicator of male reproductive fitness.     

Perspective: female remating,operational sex ratio,and the arena of sexual selection in Drosophila species

Abstract.— As commonly observed among closely related species within a variety of taxa, Drosophila species differ considerably in whether they exhibit sexual dimorphism in coloration or morphology. Those Drosophila species in which male external sexual characters are minimal or absent tend, instead, to have exaggerated ejaculate traits such as sperm gigantism or seminal nutrient donations. Underlying explanations for the interspecific differences in the presence of external morphological sexual dimorphism versus exaggerated ejaculate traits are addressed here by examining the opportunity for sexual selection on males to occur before versus after mating in 21 species of Drosophila . Female remating frequency, an important component of the operational sex ratio, differs widely among Drosophila species and appears to dictate whether the arena of sexual selection is prior to, as opposed to after, copulation. Infrequent female mating results in fewer mating opportunities for males and thus stronger competition for receptive females that favors the evolution of male characters that maximize mating success. On the other hand, rapid female remating results in overlapping ejaculates in the female reproductive tract, such that ejaculate traits which enhance fertilization success are favored. The strong association between female remating frequency in a given species and the presence of sexually selected external versus internal male characters indicates that the relationship be examined in other taxa as well.     

虎斑颈槽蛇肾脏两性异型性的结构特点

目的了解虎斑颈槽蛇肾脏两性异型性的结构基础。方法虎斑颈槽蛇(Rhabdophis tigrinus)为性成熟个体,共19条,雄性12条,雌性7条。于2005～2008年的7月份对蛇及其肾脏进行测量和称重,应用光镜和电镜对肾脏进行观察。结果雄性蛇肾脏不仅颜色呈黄色,而且相对长度和重量也均大于雌性;雄性蛇肾脏富含由远曲小管膨大而成的肾性节,而雌性则没有。肾性节细胞呈高柱状,具有典型的分泌性细胞的超微结构特点。结论虎斑颈槽蛇肾脏的两性异型性是由于雄性的肾脏富有特化的肾性节所致。     

Ontogeny of sexual dimorphism in the Long-tailed Manakin Chiroxiphia linearis: long maturation of display trait morphology

Males of dimorphic species often show ornaments that are thought to have evolved through female choice or/and male–male competition. The sexual differentiation of similar morphologies occurs during ontogeny, resulting in differential sex and age-specific selection. The Long-tailed Manakin is a dimorphic species with a highly skewed mating system, the males of which delay plumage maturation over 3 to 4 years. We describe ontogenetic changes in feather morphology in this species through sexual maturity. Males showed a significant increase in length of the central rectrices with age, hence their degree of sexual dimorphism increased from zero in 1-year-old males to 189.5% in adults. In contrast, male tail length decreased with age. Wing length did not vary significantly with age, but females had relatively longer wings than males. Wing loading was greater in females and decreased with age in males. In adults, rectrix length was positively correlated with testis volume, supporting the hypothesis that secondary sexual characters can signal the condition of primary sexual characters. Rectrix length showed positive allometry with body size in males less than 4 years old, whereas older males showed negative allometry and females showed isometry. Wing area and wing loading shifted from negative to positive allometry in males of 2 to 3 years of age. Changes in male morphology during ontogeny in the Long-tailed Manakin appeared to be associated with their specific display behaviours. Age-related changes in allometric growth of rectrices in the Long-tailed Manakin suggested that young males invest disproportionately more in the length of this trait relative to their body size. This investment could act as a signal of competitive ability to move status position in their orderly queue.     

Relationship between size and shape in the sexually dimorphic beetle Prosopocoilus inclinatus (Coleoptera: Lucanidae)

Male characters that are used for male−male combat are often developed and exaggerated, whereas female equivalent characters are vestigial or vanished. In order to assess whether the characters common to both sexes share the same phenotypic variability due to common genetic architecture, we compared males and females of the stag beetle Prosopocoilus inclinatus using recently developed geometric morphometric methods. Elliptic Fourier analysis was used to compare shape variation between male characters (including exaggerated mandibles) and developmentally homologous female characters. A significant positive correlation was found between the size or between the weight of different body parts in both sexes, but a conspicuous difference was detected in the frequency distribution of the weight of all the body parts. Elliptic Fourier analysis demonstrated that there was marked discontinuous variation in mandibles in males, whereas such a discontinuity was not clear in females. The shape of a character in males exhibited some similarity with that of other characters, but this was not found in females. In a character, growth trajectory of shape was significantly affected by both size and weight in males, whereas size and shape tended to vary independently in female characters. These results support the hypothesis that a large sexual dimorphism in variation in shape is due to alleles accumulating in tight linkage with a sex-determining gene.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 219–233