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.     

Intraspecific size variation in polyphagous parasitoids (Hym.: Parasitica), of leaf miners and its relation to host size

Wing and body lengths of polyphagous parasitoids of leafminers are analyzed in order to understand the relationship between host and parasitoid sizes. A distinct positive relationship was observed: within each parasitoid species, smaller individuals were reared from smaller hosts. There was no difference between idio-and koinobionts in sexual dimorphism, with females being significantly larger than males in both groups.     

Extra-pair paternity and sexual dimorphism in birds

Differences in the strength of sexual selection between males and females can lead to sexual dimorphism. Extra-pair paternity (EPP) can increase the variance in male reproductive success and hence the opportunity for sexual selection. Previous research on birds suggests that EPP drives the evolution of dimorphism in plumage colour and in body size. Because EPP increases the intensity of sexual selection in males, it should lead to increased dimorphism in species with larger or more colourful males, but decreased dimorphism in species with larger or more colourful females. We explored the covariation between EPP and sexual dimorphism in wing length and plumage colouration in 401 bird species, while controlling for other, potentially confounding variables. Wing length dimorphism was associated positively with the frequency of EPP, but also with social polygamy, sex bias in parental behaviour and body size and negatively with migration distance. The frequency of EPP was the only predictor of plumage colour dimorphism. In support of our prediction, high EPP levels were associated with sexual dichromatism, positively in species in which males are more colourful and negatively in those in which females are more colourful. Contrary to our prediction, high EPP rates were associated with increased wing length dimorphism in species with both male- and female-biased dimorphism. The results support a role for EPP in the evolution of both size and plumage colour dimorphism. The two forms of dimorphism were weakly correlated and predicted by different reproductive, social and life-history traits, suggesting an independent evolution.     

Dimorphism in flies with stalked eyes

Several series of measurements of eyestalks and of other parameters are presented graphically to demonstrate the forms of sexual dimorphism in five species of Diopsidae and one stalk-eyed species of each of the families Tephritidae and Platystomidae.
In the case of Pseudodiopsis detrahens (Walker) dimorphism of eyestalks is 'concealed' but in other cases it is clearly demonstrated; eyestalks of the males are, on average, longer than those of females, but there is generally considerable overlap in larger populations.
Owing to the extent of variability, length (actual or relative) can only be used as a taxonomic character with much greater caution than has been exercised in the past.     

Development of intraspecific size variation in black coucals,white-browed coucals and ruffs from hatching to fledging

Most studies on sexual size dimorphism address proximate and functional questions related to adults, but sexual size dimorphism usually develops during ontogeny and developmental trajectories of sexual size dimorphism are poorly understood. We studied three bird species with variation in adult sexual size dimorphism: black coucals (females 69% heavier than males), white-browed coucals (females 13% heavier than males) and ruffs (males 70% heavier than females). Using a flexible Bayesian generalized additive model framework (GAMM), we examined when and how sexual size dimorphism developed in body mass, tarsus length and bill length from hatching until fledging. In ruffs, we additionally examined the development of intrasexual size variation among three morphs (Independents, Satellites and Faeders), which creates another level of variation in adult size of males and females. We found that 27–100% of the adult inter- and intrasexual size variation developed until fledging although none of the species completed growth during the observational period. In general, the larger sex/morph grew more quickly and reached its maximal absolute growth rate later than the smaller sex/morph. However, when the daily increase in body mass was modelled as a proportion, growth patterns were synchronized between and within sexes. Growth broadly followed sigmoidal asymptotic models, however only with the flexible GAMM approach, residual distributions were homogeneous over the entire observation periods. These results provide a platform for future studies to relate variation in growth to selective pressures and proximate mechanisms in these three species, and they highlight the advantage of using a flexible model approach for examining growth variation during ontogeny.     

Big-bodied males help us recognize that females have big pelves

Schultz ([1949] Am. J. Phys. Anthropol. 7:401-424) presented a conundrum: among primates, sexual dimorphism of the pelvis is a developmental adjunct to dimorphism in other aspects of the body, albeit in the converse direction. Among species in which males are larger than females in body size, females are larger than males in some pelvic dimensions; species with little sexual dimorphism in nonpelvic size show little pelvic dimorphism. Obstetrical difficulty does not explain this relationship. The present study addresses this issue, evaluating the relationship between pelvic and femoral sexual dimorphism in 12 anthropoid species. The hypothesis is that species in which males are significantly larger than females in femoral size will have a higher incidence, magnitude, and variability of pelvic sexual dimorphism, with females having relatively larger pelves than males, compared with species monomorphic in femoral size. The results are consistent with the hypothesis. The proposed explanation is that the default pelvic anatomy in adulthood is that of the female; testosterone redirects growth from the default type to that of the male by differentially enhancing and repressing growth among the pelvic dimensions. Testosterone also influences sexual dimorphism of the femur. The magnitude of the pelvic response to testosterone is greater in species that are sexually dimorphic in the femur than in those that are monomorphic.     

Sexual size dimorphism and selection in the wild in the waterstrider Aquarius remigis: Body size,components of body size and male mating success

Sexual size dimorphism is assumed to be adaptive and is expected to evolve in response to a difference in the net selection pressures on the sexes. Although a demonstration of sexual selection is neither necessary nor sufficient to explain the evolution of sexual size dimorphism, sexual selection is generally assumed to be a major evolutionary force. If contemporary sexual selection is important in the evolution and maintenance of sexual size dimorphism then we expect to see concordance between patterns of sexual selection and patterns of sexual dimorphism. We examined sexual selection in the wild, acting on male body size, and components of body size, in the waterstrider Aquarius remigis, as part of a long term study examining net selection pressures on the two sexes in this species. Selection was estimated on both a daily and annual basis. Since our measure of fitness (mating success) was behavioral, we estimated reliabilities to determine if males perform consistently. Reliabilities were measured as ? statistics and range from fair to perfect agreement with substantial agreement overall. We found significant univariate sexual selection favoring larger total length in the first year of our study but not in the second. Multivariate analysis of components of body size revealed that sexual selection for larger males was not acting directly on total length but on genital length. Sexual selection for larger male body size was opposed by direct selection favoring smaller midfemoral lengths. While males of this species are smaller than females, they have longer genital segments and wider forefemora. Patterns of contemporary sexual selection and sexual size dimorphism agree only for genital length. For total length, and all other components of body size examined, contemporary sexual selection was either nonsignificant or opposed the pattern of size dimporhism. Thus, while the net pressures of contemporary selection for the species may still act to maintain sexual size dimorphism, sexual selection alone does not.     

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that, overall, body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the lab, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to, and to pair with a receptive female compared with males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.     

奇台沙蜥生长过程中的两性异形

研究奇台沙蜥Phrynocephalus grumgrzimailoi头、尾、腋胯距大小在个体发育过程中的变化.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,雌性具有较大的腋胯距.头、尾、腋胯距大小的两性异形在幼体就已存在,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部、尾部、腋胯距随SVL呈异速增长,表现为两性头部的增长速率在个体发育过程中逐渐增大,尾部的增长速率逐渐减慢,腋胯距在雌性蜥蜴中增长速率逐渐增大,在雄性中逐渐变小.     

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that overall body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the laboratory, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to and to pair with a receptive female compared to males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.     

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection

Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male-male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.     

Reduction of sexual dimorphism in stream-resident forms of three-spined stickleback Gasterosteus aculeatus

Sexual dimorphism in geometric body shape and external morphology was compared between marine and stream-resident forms of three-spined stickleback Gasterosteus aculeatus collected from North America and Japan. Some aspects of sexual dimorphism were shared between ecotypes: males had larger heads than females with no significant effect of ecotype on the magnitude of sexual dimorphism. By contrast, a significant sex-by-ecotype interaction was found for body depth. Males tended to have deeper bodies than females in both forms, but the magnitude of sexual dimorphism was reduced in stream-resident forms. Although females were generally larger in standard length and had larger pelvic girdles, significant sexual dimorphism in these traits was not consistently found across populations or ecotypes. These results suggest that some aspects of sexual dimorphism were shared between ecotypes, while others were unique to each population. The results further suggest that ecology may influence the evolution of sexual dimorphism in some external morphological traits, such as body depth.     

Sex and Weapons: Contrasting Sexual Dimorphisms in Weaponry and Aggression in Snapping Shrimp

Sexual dimorphisms in weaponry and aggression are common in species in which one sex (usually males) competes for access to mates or resources necessary for reproduction – sexually dimorphic weaponry and aggression, in other words, are frequently the result of intrasexual selection. In snapping shrimp, the major chela (snapping claw) can be a deadly weapon, and males of many species have larger chelae than females, a pattern readily interpreted as resulting from intrasexual selection. Thus, males might be expected to show more sex‐specific aggression than females, and be more aggressive overall. We tested these predictions in two species of snapping shrimp in a territorial defense context. Neither of these predictions was supported: in both species, females, but not males, engaged in sex‐specific aggression and females were more aggressive than males overall. These contrasting sexual dimorphisms – larger weaponry in males but higher aggression in females – highlight the importance of considering the function of weaponry and aggression in contexts other than direct competitions over mates. In addition, species differences in the degree of sexual dimorphism in chela size were due to differences in female, not male, chela size, and the species with greater sexual dimorphism in weaponry was significantly less aggressive overall; also, while paired and solitary males did not differ in residual chela size, for the species with greater sexual dimorphism, females carrying embryos had smaller residual chela sizes. These results suggest that understanding the sexual dimorphisms in weaponry and aggression in snapping shrimp requires understanding the relative costs and benefits of both in females as well as males.     

It is not always about body size: evidence of Rensch's rule in a male weapon

In many species, sexual dimorphism increases with body size when males are the larger sex but decreases when females are the larger sex, a macro-evolutionary pattern known as Rensch''s rule (RR). Although empirical studies usually focus exclusively on body size, Rensch''s original proposal included sexual differences in other traits, such as ornaments and weapons. Here, we used a clade of harvestmen to investigate whether two traits follow RR: body size and length of the fourth pair of legs (legs IV), which are used as weapons in male–male fights. We found that males were slightly smaller than females and body size did not follow RR, whereas legs IV were much longer in males and followed RR. We propose that sexual selection might be stronger on legs IV length than on body size in males, and we discuss the potential role of condition dependence in the emergence of RR.     

The ontogeny of cranial sexual dimorphism in two old world monkeys:Macaca fascicularis (Cercopithecinae) andNasalis larvatus (Colobinae)

Allometric and heterochronic approaches to sexual dimorphism have contributed much to our understanding of the evolutionary morphology of the primate skull and dentition. To date, however, extensive studies of sexual dimorphism have been carried out only on the great apes and a few cercopithecine monkeys. To fill this gap, representative dimensions of the skull were collected among ontogenetic series of two dimorphic Old World monkeys:Macaca fascicularis (Cercopithecinae) andNasalis larvatus (Colobinae). The ontogeny of cranial sexual dimorphism was evaluated with least-squares bivariate regression, analysis of covariance (ANCOVA), and analysis of variance (ANOVA). Results indicate that within each species the sexes typically exhibit nonsignificant differences in ANCOVAs of ontogenetic trajectories, except for bivariate comparisons with bicanine breadth. AmongMacaca fascicularis, ANOVAs between males and females of common dental ages show that adult, and frequently subadult, males are significantly larger than females, i.e., sexual dimorphism develops via time and rate hypermorphosis (males primarily grow for a longer time period as well as faster). AmongNasalis larvatus, however, comparisons between males and females of common dental ages indicate that only adult males are significantly larger than females, i.e., sexual dimorphism develops primarily via time hypermorphosis (males grow for a longer time period). Within both species, females appear to exhibit an early growth spurt at dental age 2; that is, many cranial measures for females tend to be larger than those for males. Measures of the circumorbital region (e.g., browridge height), body weight, and bicanine breadth exhibit typically the highest sexual dimorphism ratios. The fact that postcanine toothrow length and neurocranial volume (less so inNasalis) demonstrate very low dimorphism ratios generally supports assertions that postnatal systemic growth (and associated selective pressures thereon) exerts a greater influence on facial, but not neural, dental, or orbital, development (Cochard, 1985, 1987; Shea, 1985a,b, 1986; Shea and Gomez, 1988; Sheaet al., 1990). Additional consideration of ontogenetic differences between species generally supports previous functional interpretations of subfamilial differences in cranial form related to agonistic displays in cercopithecine monkeys (Ravosa, 1990).     

Body size, age and paternity in common brushtail possums (Trichosurus vulpecula)

Sexual selection should produce sexual size dimorphism in species where larger members of one sex obtain disproportionately more matings. Recent theory suggests that the degree of sexual size dimorphism depends on physical and temporal constraints involving the operational sex ratio, the potential reproductive rate and the trade-off between current reproductive effort and residual reproductive value. As part of a large-scale experiment on dispersal, we investigated the mating system of common brushtail possums inhabiting old-growth Eucalyptus forest in Australia. Paternity was assigned to 20 of 28 pouch-young (maternity known) genotyped at six microsatellite loci. Male mating success was strongly related to body size and age; male body weight and age being highly correlated. Despite disproportionate mating success favouring larger males, sexual size dimorphism was only apparent among older animals. Trapping and telemetry indicated that the operational sex ratio was effectively 1 : 1 and the potential reproductive rate of males was at most four times that of females. Being larger appeared to entail significant survival costs because males 'died-off' at the age at which sexual size dimorphism became apparent (8-9 years). Male and female home ranges were the same size and males appeared to be as sedentary as females. Moreover, longevity appears to be only slightly less important to male reproductive success than it is to females. It is suggested that a sedentary lifestyle and longevity are the key elements constraining selection for greater sexual size dimorphism in this 'model' medium-sized Australian marsupial herbivore.     

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.     

Inter- and intrasexual dimorphism in the diving beetle Hydroporus memnonius Nicolai (Coleoptera: Dytiscidae)

Sexual conflict can drive rapid intersexual arms races, and lead to pronounced sexual dimorphism. Such dimorphism is frequent in diving beetles, where males typically possess expanded front and middle tarsi, supplied with adhesive setae to grasp females during mating, and females often have rough dorsal surfaces which hinder male attachment. In a number of species, females are dimorphic, being either smooth and male-like, or heavily sculptured dorsally. Smooth and sculptured females often have distinct biogeographies, and may be expected to be associated with specific counter-adaptations in males. The European diving beetle, Hydroporus memnonius Nicolai, includes a smooth male-like female, and a matt morph, var. castaneus Aubé, which are largely allopatric in distribution. We show that the two morphs differ in the density and intensity of their surface microreticulation, and that matt females are associated with morphologically distinct males, which have developed specific countermeasures on their tarsi, including a greater number of large adhesive setae, individually larger in area. Such males are expected to be more successful in pairing with both matt and shining females, and it is suggested that a process of population replacement, partly driven by sexual interactions, may occur where the two forms overlap in range.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 685–697.     

Sexual Dimorphism in the Hindlimb Muscles of the Asiatic Toad(Bufo gargarizans)in Relation to Male Reproductive Success

In many anurans, the forelimb muscles of males are used to grasp females and are often heavier than those of females despite the larger female body size. Such sexual dimorphism in forelimb musculature is thought to result from sexual selection. In addition, the hindlimbs of frogs and toads play an important role in the reproductive process as amplectant males can expel rivals with robust hindlimbs through kicking. In this study, the sexual dimorphism in dry mass for six hindlimb muscles of the Asiatic toad(Bufo gargarizans) was investigated. The results showed that, when controlled for body size, the hindlimb muscle mass of males significantly exceeded that of females for every muscle. The hindlimb muscle mass of amplectant males was also significantly larger than that of non-amplectant males. These results suggested that if strong hindlimb muscles could improve mating success of males, sexual selection would promote the evolution of dimorphism in this character.     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.