Effects of parity on pelvic size and shape dimorphism in Mus

The pelvis is a sexually dimorphic structure and although the causes of that dimorphism have long been studied, relatively little is known regarding the effects of partuitive events on the magnitude of that dimorphism. Here, we use a sample of Mus musculus domesticus to contrast dimorphism in body length and os coxae size and shape between males and parous and nulliparous females. We also test for correlations between relative litter size (L/M) and relative offspring size (O/M) with body length and os coxae size and shape in parous females. Males had greater body length than nulliparous females but were not different from parous females. Females as a whole had the largest os coxae, with parous females having the largest and males the smallest. Os coxae shape was also significantly different between groups and was most divergent between parous females and males than between nulliparous females and males. Os coxae shape differences between females are associated with differences in body length between females and O/M is correlated with os coxae shape in parous females such that females with the largest offspring have the most divergent shapes along the relative warp one axis. Pelvic shape differences between males and females were consistent with previous findings in other taxa which identify the pubo‐ischial complex as the primary region of dimorphism. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Multivariate morphometries and sexual dimorphism in the orb-web spider Metellina segmentata (Clerck, 1757) (Araneae, Metidae)

Sexual dimorphism in body size and leg length was investigated in a common orb-weaving spider of Ireland and northern Europe, Metellina segmentata (Clerck, 1757) (Araneae, Metidae). Univariate and multivariate analyses of sexual dimorphism revealed that a greater proportion of between sex variation (sexual dimorphism) was attributable to variation in shape than in size. Significant differences were found in the scores for males and females for the first two principal components. PCI (shape) accounted for 44.25% of the variation and PC2 (size) 13.01% of the variation. Although M. segmentata has been attributed with minimal sexual size dimorphism, females were markedly heavier, possibly a reflection of differential reproductive investment between the sexes, but males had markedly longer legs and broader prosoma. The results are discussed with regard to existing theories of natural and sexual selection, particularly those concerning sexual cannibalism and differential life history traits in males and females. Models that attempt to explain the evolution of sexual size dimorphism in spiders and of the web builders in particular, fail to account for the multivariate nature of dimorphism, especially with respect to shape.     

Study on long bones: variation in angular traits with sex, age, and laterality

The biological problems considered in this study are sexual dimorphism in angular osteometric traits, changes in these traits with aging and their bilateral asymmetry in the same individual. The sample comprises the right and left long bones of 200 skeletons (100 males and 100 females) of Sardinian adults (Frasetto collection) for whom the sex and age at death are known. As concerns the sexual dimorphism, some angular traits of upper limb (joint axis angle of the ulna in particular) and the lower limb (condylo-diaphyseal angle of the femur and retroversion angle of the tibia) are significantly different between males and females. Torsion angles of long bones (especially the femur) were found to change with age. In sides comparison revealed a clear right-left asymmetry in our sample. The asymmetry, sexual dimorphism and changes with age are discussed in relation to varying stresses of different living activities. Our data indicate that the angular traits of long bones could be very informative in biomechanical interpretations of human activities based on skeletal remains.     

Reproductive success of two male morphs in a free-ranging population of Bornean orangutans

The reproductive success of male primates is not always associated with dominance status. For example, even though male orangutans exhibit intra-sexual dimorphism and clear dominance relationships exist among males, previous studies have reported that both morphs are able to sire offspring. The present study aimed to compare the reproductive success of two male morphs, and to determine whether unflanged males sired offspring in a free-ranging population of Bornean orangutans, using 12 microsatellite loci to determine the paternity of eight infants. A single flanged male sired most of the offspring from parous females, and an unflanged male sired a firstborn. This is consistent with our observation that the dominant flanged male showed little interest in nulliparous females, whereas the unflanged males frequently mated with them. This suggests that the dominant flanged male monopolizes the fertilization of parous females and that unflanged males take advantage of any mating opportunities that arise in the absence of the flanged male, even though the conception probability of nulliparous females is relatively low.     

Sex,androgens, and whole‐organism performance in an Australian lizard

Understanding underlying physiological differences between the sexes in circulating androgens and how hormonal variation affects morphology–performance relationships may help clarify the evolution of sexual dimorphism in diverse taxa. Using a widely distributed Australian lizard (Eulamprus quoyii) with weak sexual dimorphism and no dichromatism, we tested whether circulating androgens differed between the sexes and whether they covaried with morphological and performance traits (bite force, sprint speed, endurance). Males had larger head dimensions, stronger bite force, faster sprint speed, and longer endurance compared to females. We found that the sexes did not differ in androgen concentrations and that androgens were weakly associated with both morphological and performance traits. Interestingly, high circulating androgens showed a nonlinear relationship with bite force in males and not females, with this relationship possibly being related to alternative male reproductive tactics. Our results suggest that androgens are not strongly correlated with most performance and morphological traits, although they may play an important organizational role during the development of morphological traits, which could explain the differences in morphology and thus performance between the sexes. Differences in performance between the sexes suggest differential selection on these functional traits between males and females. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 834–849.     

Sexual dimorphism in clonal growth forms and ramet distribution patterns in <Emphasis Type="Italic">Rumex acetosella</Emphasis> (Polygonaceae)

We investigated clonal traits in the dioecious herb Rumex acetosella to characterize sexual dimorphism in clonal forms and to correlate below-ground clonal patterns and above-ground ramet distributions. We recorded creeping root length, branching patterns, ramet and clump (caespitose ramets from the same position on the root) sprouting patterns, and biomass allocations in three females and males. We also estimated the patch size of flowering ramets within a quadrat. No sexual dimorphism was detected in the frequencies of branches and flowering ramets per root length. Male plants allocated proportionally more biomass to below-ground organs. Total root length did not differ between the sexes. Females sprouted more clumps with fewer flowering ramets per root length than males, which sprouted fewer clumps with more flowering ramets, which meant that clump sprouting patterns were phalanx-like in females and guerrilla-like in males. Flowering ramets were aggregately distributed in both females and males and patch sizes were similar between sexes, indicating that the spreader propagations were not found in the guerrilla-like males. We assumed that sexual dimorphism occurred in response to physiological integration for higher reproductive effort in females.     

Sexual size dimorphism and allometric growth of Morelet's crocodiles in captivity

Few studies have conducted morphological analyses of crocodilians, and little information exists on differences between size-classes and sexes in Neotropical crocodilians. In this study, we measured nine morphological traits in 121 captive Morelet's crocodiles Crocodylus moreletii (81 females and 40 males). Our results revealed that individuals < 2 m total length do not exhibit sexual dimorphism in morphometric characteristics. However, for crocodiles over 2 m in length, males were significantly larger than females in terms of dorsal-cranial length, cranial width, snout width and snout-ventral length. In general, morphological traits demonstrated a strongly significant relationship with total length at the smaller size class of 150-200 cm length. However, in the highest size class of 250-300 cm length (large adult males), morphological traits were no longer significantly related with total length. Male crocodiles demonstrated allometric growth of cranial morphology with significantly greater increase in cranial width, snout width, and mid-snout width relative to total length at higher size classes. Morphological dimorphism and allometric growth may be associated with adaptive strategies for reproductive success.     

Ontogenetic stage‐specific quantitative trait loci contribute to divergence in developmental trajectories of sexually dimorphic fins between medaka populations

Sexual dimorphism can evolve when males and females differ in phenotypic optima. Genetic constraints can, however, limit the evolution of sexual dimorphism. One possible constraint is derived from alleles expressed in both sexes. Because males and females share most of their genome, shared alleles with different fitness effects between sexes are faced with intralocus sexual conflict. Another potential constraint is derived from genetic correlations between developmental stages. Sexually dimorphic traits are often favoured at adult stages, but selected against as juvenile, so developmental decoupling of traits between ontogenetic stages may be necessary for the evolution of sexual dimorphism in adults. Resolving intralocus conflicts between sexes and ages is therefore a key to the evolution of age‐specific expression of sexual dimorphism. We investigated the genetic architecture of divergence in the ontogeny of sexual dimorphism between two populations of the Japanese medaka (Oryzias latipes) that differ in the magnitude of dimorphism in anal and dorsal fin length. Quantitative trait loci (QTL) mapping revealed that few QTL had consistent effects throughout ontogenetic stages and the majority of QTL change the sizes and directions of effects on fin growth rates during ontogeny. We also found that most QTL were sex‐specific, suggesting that intralocus sexual conflict is almost resolved. Our results indicate that sex‐ and age‐specific QTL enable the populations to achieve optimal developmental trajectories of sexually dimorphic traits in response to complex natural and sexual selection.     

Reproduction and sexual dimorphism in two populations of the polymorphic spiny lizard Sceloporus minor from Hidalgo,México

Male‐biased sexual dimorphism is extensive in New World spiny lizards (Sceloporus: Phrynosomatidae) and is particularly prominent in the polymorphic minor lizard Sceloporus minor. However, the possible relationship between patterns of sexual dimorphism and gonadal development is little known for most species. In this study, we explored aspects of sexual dimorphism in noncolor morphological traits in S. minor and characterized the gonadal cycle in males and females from each of two sites (El Enzuelado and La Manzana) in Hidalgo, México, differing in key ecological aspects linked to life‐history trait expression in other lizards. Males were generally larger than females in each population and expressed larger forms of several other morphological traits, although not all comparisons attained statistical significance. Both sexes attained reproductive maturity at a larger size in El Enzuelado relative to La Manzana, and females from El Enzuelado had larger litters. Gonadal cycles differed substantially between the two populations and suggest that reproductive activity of males and females was synchronous at El Enzuelado and asynchronous at La Manzana, an unusual pattern. Geographic variation in sex‐specific responses to environmental variables may be at least partly responsible for the exceptional species diversity of spiny lizards in México.     

Asymmetrical expression of transsexual phenotypes in hummingbirds

I present evidence for asymmetry in the expression of transsexual traits in adult hummingbirds. Among females, individuals with male-like plumage are common and define a continuous range of variation. Among males, individuals with female-like plumage are rare and define discontinuous plumage morphs. Quantitative characters also distinguish transsexuals from other members of their sex, but the characters involved differ for male-like females (bill length) and female-like males (bill, wing and tail length). Gonadal development is correlated with transsexuality only in males; female-like males have significantly smaller testes than male-like males. Both sexes demonstrate a significant negative association between plumage brightness and bill length. This association suggests an ecological basis for transsexuality because differences in plumage and bill morphology are associated with differences in foraging behaviour within and between hummingbird species. Morphological differences between transsexuals and non-transsexuals imply that plumage sexual dimorphism is more likely to evolve through changes in the frequency of female, rather than male, transsexual variants.     

The evolution of sexual size dimorphism in the house finch. III. Developmental basis

Sexual size dimorphism of adults proximately results from a combination of sexually dimorphic growth patterns and selection on growing individuals. Yet, most studies of the evolution of dimorphism have focused on correlates of only adult morphologies. Here we examined the ontogeny of sexual size dimorphism in an isolated population of the house finch (Carpodacus mexicanus). Sexes differed in growth rates and growth duration; in most traits, females grew faster than males, but males grew for a longer period. Sexual dimorphism in bill traits (bill length, width, depth) and in body traits (wing, tarsus, and tail length; mass) developed during different periods of ontogeny. Growth of bill traits was most different between sexes during the juvenile period (after leaving the nest), whereas growth of body traits was most sexually dimorphic during the first few days after hatching. Postgrowth selection on juveniles strongly influenced sexual dimorphism in all traits; in some traits, this selection canceled or reversed dimorphism patterns produced by growth differences between sexes. The net result was that adult sexual dimorphism, to a large degree, was an outcome of selection for survival during juvenile stages. We suggest that previously documented fast and extensive divergence of house finch populations in sexual size dimorphism may be partially produced by distinct environmental conditions during growth in these populations.     

Sexual dimorphism and sexual selection in a montane scincid lizard (Eulamprus leuraensis)

Sex‐based divergences in body sizes and/or shapes within a species imply that selective forces act differently on morphological features in males versus females. That prediction can be tested with data on the relationship between morphology and reproductive output in females, and between morphology and realized paternity (based on genetic assignment tests) in males. In a sample of 81 field‐collected adult Blue Mountains water skinks (Eulamprus leuraensis), males and females averaged similar overall body sizes (snout–vent lengths (SVLs)). Reproductive success (based on 105 progeny produced by the females) increased with SVL at similar rates in both sexes (as expected from the lack of sexual size dimorphism). Multiple paternity was common. Males had larger heads than females of the same body size, and (as predicted) reproductive success increased with relative head size in males but not in females. Males also had relatively longer limbs and shorter trunks than females, but we did not detect significant sex differences in selection on those traits. Reproductive success in both sexes was increased by relatively longer hind limbs. Our data clarify mating systems in this endangered species, and suggest that mating systems are diverse within the genus Eulamprus.     

Meek Males and Fighting Females: Sexually-Dimorphic Antipredator Behavior and Locomotor Performance Is Explained by Morphology in Bark Scorpions (Centruroides vittatus)

Sexual dimorphism can result from sexual or ecological selective pressures, but the importance of alternative reproductive roles and trait compensation in generating phenotypic differences between the sexes is poorly understood. We evaluated morphological and behavioral sexual dimorphism in striped bark scorpions (Centruroides vittatus). We propose that reproductive roles have driven sexually dimorphic body mass in this species which produces sex differences in locomotor performance. Poor locomotor performance in the females (due to the burden of being gravid) favors compensatory aggression as part of an alternative defensive strategy, while male morphology is coadapted to support a sprinting-based defensive strategy. We tested the effects of sex and morphology on stinging and sprinting performance and characterized overall differences between the sexes in aggressiveness towards simulated threats. Greater body mass was associated with higher sting rates and slower sprinting within sexes, which explained the greater aggression of females (the heavier sex) and, along with longer legs in males, the improved sprint performance in males. These findings suggest females are aggressive to compensate for locomotor costs of reproduction while males possess longer legs to enhance sprinting for predator evasion and mate finding. Sexual dimorphism in the metasoma (“tail”) was unrelated to stinging and sprinting performance and may best be explained by sexual selection.     

Sexual dimorphism in the pelvic bones of Svalbard reindeer, Rangifer tarandus platyrhynchus

The determination of sex of carcasses of reindeer and caribou (Rangifer tarandus) is complicated because both sexes normally carry antlers. Growth curves of males and females diverge sufficiently so that the length of long bones can often indicate the sex of a carcass if its age at death is known; results, however, are not always conclusive. Pronounced sexual dimorphism was found in the breadth of the ventro-medial border (rim) of the acetabulum of adult Svalbard reindeer. Measurement of the rim of the acetabulum thus provides a quick and conclusive means for determining the sex of carcasses of these animals. Similar results have been found in several other species of ungulates. Reasons for the dimorphism and for the non-normal distribution of the data for both sexes in reindeer are discussed.     

Function‐related Drivers of Skull Morphometric Variation and Sexual Size Dimorphism in a Subterranean Rodent,Plateau Zokor (Eospalax baileyi)

Sexual dimorphism is prevalent in most living organisms. The difference in size between sexes of a given species is generally known as sexual size dimorphism (SSD). The magnitude of the SSD is determined by Rensch's rule where size dimorphism increases with increasing body size when the male is the larger sex and decreases with increasing average body size when the female is the larger sex. The unique underground environment that zokors (Eospalax baileyi) live under in the severe habitat of the Qinghai‐Tibetan Plateau (QTP) could create SSD selection pressures that may or may not be supported by Rensch's rule, making this scientific question worthy of investigation. In this study, we investigated the individual variation between sexes in body size and SSD of plateau zokors using measurements of 19 morphological traits. We also investigated the evolutionary mechanisms underlying SSD in plateau zokors. Moreover, we applied Rensch's rule to all extant zokor species. Our results showed male‐biased SSD in plateau zokors: The body‐ and head‐related measurements were greater in males than in females. Linear regression analysis between body length, body weight, and carcass weight showed significant relationships with some traits such as skull length, lower incisor length, and tympanic bulla width, which might support our prediction that males have faster growth rates than females. Further, the SSD pattern corroborated the assumption of Rensch's rule in plateau zokors but not in the other zokor species. Our findings suggest that the natural underground habitat and behavioral differences between sexes can generate selection pressures on male traits and contribute to the evolution of SSD in plateau zokors.     

Sexual and male horn dimorphism in Copris ochus (Coleoptera: Scarabaeidae)

Copris ochus (Coleoptera: Scarabaeidae), an endangered species, is the largest dung beetle in Japan. In C. ochus, males have a long head horn, while females lack this long horn (sexual dimorphism). Very large males of C. ochus have disproportionately longer head horns than small males, suggesting male horn dimorphism, although the dimorphism has not been investigated quantitatively. To clarify sexual and male horn dimorphism in C. ochus quantitatively, we examined the scaling relationship between body size (prothorax width) and head horn length in 94 females and 76 males. These beetles were captured during July 1978 from a natural population on Mt. Aso in southwestern Japan using a light trap. Although the horn length of the females and males scaled with prothorax width, the scaling relationship differed between the sexes, i.e., the relationship was linear in females and nonlinear in males. Statistical tests for dimorphism in male horn length showed a significant discontinuous relationship, thus indicating distinct sexual and male dimorphism in head horns. Long- and short-horned C. ochus males may have different reproductive behaviors, as described in other horned dung beetles.     

Wind pollination, sexual dimorphism, and changes in floral traits of Schiedea (Caryophyllaceae)

Both changes in sex allocation and pollination mode may promote the separation of sexes in plant populations. Simultaneous evolution of wind pollination and dimorphism has occurred in Schiedea, where species with different female frequencies provide an opportunity to observe the effect of wind pollination on sex allocation and floral morphology. Differences among species in the ratio of anther to ovary volume were not the result of sex allocation trade-offs, but instead resulted from production of vestigial stamens in females; there were no changes in ovary volume in males and hermaphrodites (MH) of dimorphic species. Relative to hermaphroditic species, dimorphic species had more condensed inflorescences, a pattern often associated with wind pollination. Within dimorphic species, MH had longer filament lengths than females, and females had longer stigmas than MHs. These traits are characteristic of wind pollination, but there was no relationship between the degree of sexual dimorphism and female frequency. Ovary volume and ovule number and size had positive phenotypic correlations between females and MH of dimorphic species, making sex specialization more difficult. In dimorphic Schiedea species, selection for wind pollination may have a greater effect on floral traits than trade-offs in allocation between male and female function.     

Reproductive constraints,not environmental conditions,shape the ontogeny of sex‐specific mass–size allometry in roe deer

In polygynous mammals, sex‐specific patterns of body growth are linked to divergent selection pressures on male and female body size, resulting in sexual dimorphism (SD). For males, reproductive success is generally linked to body size, hence, males should prioritise early growth. For females, reproductive success is linked to resource availability, so they may adopt a more conservative growth tactic. Using longitudinal monitoring of known‐age animals in two contrasting populations and an allometric approach to disentangle the relative contribution of structural size and physiological condition to SD, we addressed these issues in the weakly polygynous roe deer. Despite very different environmental conditions, we found remarkably similar patterns in the two populations in the mass–size allometric relationship at each life history stage, suggesting that relative allocation to structural size and physiological condition is highly constrained. SD in structural size (indexed by hind foot length) involved sex‐specific growth trajectories governed by a single mass–size allometric relationship during the juvenile stage, such that males were both bigger and heavier than females. In contrast, SD in physiological condition (indexed by the allometric relationship between body mass and hind foot length, expressed as body mass for a given body size) developed markedly during the sub‐adult stage in relation to sex differences in the timing of first reproduction. Among adults, males were heavier for a given size than females, suggesting that, relative to females, males express a capital breeder tactic, accumulating fat reserves to offset reproductive costs. By the senescent stage, SD in physiological condition had disappeared, with both sexes governed by a single allometric relationship, suggesting more rapid senescence in males than females. Individuals born into poor cohorts were generally lighter for a given size, indicating growth priority for skeletal size over physiological condition in both sexes. However, sex differences in cohort effects among sub‐adults resulted in lower size‐specific SD in poor cohorts, indicating that body condition of sub‐adult females is buffered against environmental harshness. We conclude that sex‐differences in reproductive tactics impose constraints on the ontogeny of SD in roe deer, leading to sex‐specific trajectories in structural size and physiological condition.     

Shape,size, and maturity trajectories of the human ilium

Morphological traits of the ilium have consistently been more successful for juvenile sex determination than have techniques applied to other skeletal elements, however relatively little is known about the ontogeny and maturation of size and shape dimorphism in the ilium. We use a geometric morphometric approach to quantitatively separate the ontogeny of size and shape of the ilium, and analyze interpopulation differences in the onset, rate and patterning of sexual dimorphism. We captured the shape of three traits for a total of 191 ilia from Lisbon (Portugal) and London (UK) samples of known age and sex (0–17 years). Our results indicate that a) there is a clear dissociation between the ontogeny of size and shape in males and females, b) the ontogeny of size and shape are each defined by non‐linear trajectories that differ between the sexes, c) there are interpopulation differences in ontogenetic shape trajectories, which point to population‐specific patterning in the attainment of sexual dimorphism, and d) the rate of shape maturation and size maturation is typically higher for females than males. Male and female shape differences in the ilium are brought about by trajectory divergence. Differences in size and shape maturation between the sexes suggest that maturity may confound our ability to discriminate between the sexes by introducing variation not accounted for in age‐based groupings. The accuracy of sex determination methods using the ilium may be improved by the use of different traits for particular age groups, to capture the ontogenetic development of shape in both sexes. Am J Phys Anthropol 156:19–34, 2015 © 2014 Wiley Periodicals, Inc.     

Sexual dimorphism in the human bony pelvis, with a consideration of the Neandertal pelvis from Kebara Cave, Israel.

Sexual dimorphism of the human pelvis is inferentially related to obstetrics. However, researchers disagree in the identification and obstetric significance of pelvic dimorphisms. This study addresses three issues. First, common patterns in dimorphism are identified by analysis of pelvimetrics from six independent samples (Whites and Blacks of known sex and four Amerindian samples of unknown sex). Second, an hypothesis is tested that the index of pelvic dimorphism (female mean x 100/male mean) is inversely related to pelvic variability. Third, the pelvic dimensions of the Neandertal male from Kebara cave, Israel are compared with those of the males in this study. The results show that the pelvic inlet is the plane of least dimorphism in humans. The reason that reports often differ in the identification of dimorphisms for this pelvic plane is that both the length of the pubis and the shape of the inlet are related to nutrition. The dimensions of the pelvis that are most dimorphic (that is, female larger than male) are the measures of posterior space, angulation of sacrum, biischial breadth, and subpubic angle. Interestingly, these dimensions are also the most variable. The hypothesis that variability and dimorphism are inversely related fails to be supported. The factors that influence pelvic variability are discussed. The Kebara 2 pelvis has a spacious inlet and a confined outlet relative to modern males, though the circumferences of both planes in the Neandertal are within the range of variation of modern males. The inference is that outlet circumference in Neandertal females is also small in size, but within the range of variation of modern females. Arguments that Neandertal newborns were larger in size than those of modern humans necessarily imply that birth was more difficult in Neandertals.