Dichogamy and sexual dimorphism in floral traits in the andromonoecious Euphorbia boetica

BACKGROUND AND AIMS: Euphorbia boetica (Euphorbiaceae) is a functional andromonoecious species that shows both intra- and interfloral dichogamy, hermaphrodite cyathia being protogynous. Sexual dimorphism of the cyathia of E. boetica is examined according to their gender and arrangement on the inflorescence. METHODS: Data were obtained from two natural populations, where the distribution of male and hermaphrodite cyathia in the inflorescence was recorded. The size, pollen production and viability, and nectar secretion were measured in both types of cyathia. KEY RESULTS: Most cyathia were male at the first levels of the inflorescence, then hermaphrodite cyathia predominated at the successive levels, although at the last levels the proportion of male cyathia increased. Male cyathia at basal positions lack ovaries, whereas those at distal positions showed vestigial ovaries. The size of the cyathia varied significantly depending on the level of the inflorescence where they were produced: those of the last levels were usually smaller. The hermaphrodites were significant bigger than males; however, these differences were due to the differential distribution of each cyathium type in the inflorescence. Male cyathia produced significantly more pollen and nectar than hermaphrodites. CONCLUSIONS: In Euphorbia boetica, basal male cyathia could be explained by the presence of protogyny, and apical male cyathia seem to respond to a preemption of resources. A true dimorphism affecting primary sexual characters and related to gender function appears at lower levels of the inflorescence, whereas an apparent size dimorphism due to positional effects occurs at upper positions. Longevity and distribution of cyathia, and their pattern of nectar production, could improve both male and female fitness.     

Sexual dimorphism and gynoecium size variation in the andromonoecious shrub Caesalpinia gilliesii

The degree of sexual dimorphism in flowers and inflorescences can be evaluated early in flower development through the study of floral organ size co-variation. In the present work, the gynoecium-androecium size relationship was studied to assess the degree of sexual expression in flowers and inflorescences of the andromonoecious shrub Caesalpinia gilliesii. The co-variation pattern of floral organ sizes was compared between small and large inflorescences, under the hypothesis that inflorescence size reflected differential resource availability. Also, staminate and perfect flowers were collected from three populations and compared on the basis of gynoecium, ovule length, filament length, pollen size and number. The obtained results indicated that staminate and perfect flowers differed only in the gynoecium and ovule length, whereas filament length, pollen size, and number varied across populations. The gynoecium size was smaller and its variability was much higher in staminate than in perfect flowers, as explained by a recent hypothesis about pollinator-mediated gynoecium size selection acting upon perfect flowers. The analysis of the gynoecium-androecium size relationship during flower development, revealed a dissociation of gynoecium growth relative to other floral structures in some buds. Lower gynoecium-androecium regression slopes and smaller gynoecia length characterized smaller inflorescences, thus reflecting the fact that sexual expression was more male-biased. This trend is in agreement with a differential resource-related response at the inflorescence level, however, post-mating resource allocation and the inclusion of other modular levels may also help us to understand the variation in sexual dimorphism in this species.     

Sexual dimorphism of the Weberian apparatus and pectoral girdle in Sundadanio axelrodi (Ostariophysi: Cyprinidae)

The miniature cyprinid fish, Sundadanio axelrodi , exhibits extreme sexual dimorphism in the skeleton of the Weberian apparatus, the fifth rib and pectoral girdle. Musculature associated with the fifth rib and Weberian apparatus also shows a high degree of sexual dimorphism. It is suggested that these modifications are responsible for the production of a croaking sound that seems to be restricted to males of the species, based on the lack of any corresponding anatomical specializations in females.     

Sexual dimorphism in weight among the primates: The relative impact of allometry and sexual selection

In this paper, we examine allometric and sexual-selection explanations for interspecific differences in the amount of sexual dimorphism among 60 primate species. Based on evidence provided by statistical analyses, we reject Leutenegger and Cheverud’s [(1982). Int. J. Primatol.3:387-402] claim that body size alone is the major factor in the evolution of sexual dimorphism. The alternative proposed here is that sexual selection due to differences in the reproductive potential of males and females is the primary cause of sexual dimorphism. In addition, we propose that the overall size of a species determines whether the dimorphism will be expressed as size dimorphism,rather than in some other form.     

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.     

Sexual dimorphism and sexual conflict in the diving beetle Agabus uliginosus (L.) (Coleoptera: Dytiscidae)

Sexual conflict can drive intersexual arms races, with female resistance and male persistence traits coevolving antagonistically. Such arms races are well documented in some diving beetles, although the extent of sexual conflict in this family remains unclear. The European dytiscid Agabus uliginosus has a strikingly dimorphic female; individuals from most regions are smooth and male‐like, whereas those from some populations have a strongly roughened dorsum, a trait that has attracted the name dispar. We demonstrate that rough and smooth females differ consistently in the development of dorsal surface microreticulation, and that these females are associated with males that differ in the development of their persistence traits. These findings extend the occurrence of pre‐insemination sexual conflict and associated intrasexual dimorphism in Dytiscidae, and suggest that such mating systems are relatively widespread in these beetles.     

Sexual dimorphism in a dioecious population of the wind-pollinated herb Mercurialis annua: the interactive effects of resource availability and competition

Background and Aims

Male-biased sex allocation commonly occurs in wind-pollinated hermaphroditic plants, and is often positively associated with size, notably in terms of height. Currently, it is not well established whether a corresponding pattern holds for dioecious plants: do males of wind-pollinated species exhibit greater reproductive allocation than females? Here, sexual dimorphism is investigated in terms of life history trade-offs in a dioecious population of the wind-pollinated ruderal herb Mercurialis annua.

Methods

The allocation strategies of males and females grown under different soil nutrient availability and competitive (i.e. no, male or female competitor) regimes were compared.

Key Results

Male reproductive allocation increased disproportionately with biomass, and was greater than that of females when grown in rich soils. Sexual morphs differentially adjusted their reproductive allocation in response to local environmental conditions. In particular, males reduced their reproductive allocation in poor soils, whereas females increased theirs, especially when competing with another female rather than growing alone. Finally, males displayed smaller above-ground vegetative sizes than females, but neither nutrient availability nor competition had a strong independent effect on relative size disparities between the sexes.

Conclusions

Selection appears to favour plasticity in reproductive allocation in dioecious M. annua, thereby maintaining a relatively constant size hierarchy between sexual morphs. In common with other dioecious species, there seems to be little divergence in the niches occupied by males and females of M. annua.     

Sexual dimorphism in intra- and interspecific competitive ability of the dioecious herb Mercurialis annua

Males and females of dioecious plant species often show different responses to competition with individuals of the same or opposite gender, but almost no data are available on the outcome of competition with members of other species. Here, we show that male and female individuals of the wind-pollinated herb Mercurialis annua are sexually dimorphic in both their intraspecific and interspecific competitive abilities. In a controlled experiment, we found that both sexes of M. annua were negatively affected by interspecific competition, but the sensitivity of males and females depended on the identity of their competitor species, with females tending to suppress the aboveground growth of competitor species more than males. Further, we found that intrasexual and intersexual competition affected the aboveground growth of males but not that of females: only males showed a significant reduction in growth when growing with conspecific competitors (male or female). We discuss our results with reference to related studies that suggest that males and females of M. annua have different resource requirements for reproduction, which in turn affect their competitive abilities.     

Sexual dimorphism in chelicerae size in three species of nuptial-gift spiders: a discussion of possible functions and driving selective forces

Positive allometric patterns observed for intersexual signalling characters are related to directional sexual selection, and supported by theoretical and empirical data. Recent models have shown that positive allometry may not hold as a rule if the influence of natural selection is added to the model. Here we tested these models applying traditional morphometrical techniques for the analysis of chelicerae sexual dimorphism and allometric patterns within the genus Paratrechalea : Paratrechalea azul , Paratrechalea galianoae and Paratrechalea ornata . Spider chelicerae are basically used for prey capture, but males of Paratrechalea also use the chelicerae to offer a nuptial gift during courtship, also presenting a clear size and colour sexual dimorphism supporting a possible role as a signal. Chelicerae size was male biased for all the variables studied and showed an isometric pattern, while females showed a higher variation. Our findings are in accordance with models of viability-related function for prey capture, questioning some statements proposed by the positive allometry model.     

Sexual dimorphism in steppe tortoises (Testudo horsfieldii): influence of the environment and sexual selection on body shape and mobility

Selective forces shape sexes differently, with male body proportions facing strong selection to enhance mate searching and male-to-male combat traits, and female fitness being influenced by the ability to assimilate large amounts of nutrients necessary for vitellogenesis (and/or gestation), and their ability to carry the eggs or embryos. We evaluated the sexual dimorphism of body proportion of more than 800 wild steppe tortoises (Testudo horsfieldii) in Uzbekistan. The thick, well-developed shell offers protection from predators but pronounced digging habits probably also constrain body shape (e.g. a shell that is dorso-ventrally flattened, although round from a dorsal view helps to penetrate into, and move within the soil). Thus, in this species, natural selection might favour a heavy and flat shell that is 'closed' with small openings for appendages. In males, these environmental influences appear to be countered by sexual selection. Compared to females, they weigh less (absolutely and relative to shell dimensions), have longer legs, have shell structure allowing wider movements for their legs, and they walk faster. Males were also able to right themselves more quickly than females did in experimental tests. This quick righting ability is critical because intra-sexual combats frequently result in males being flipped onto their backs and becoming prone to hyperthermia or predation. Females are heavily built, with wide shells (relative to male shells), which may provide space for carrying eggs. From our results, a number of simple hypotheses can be tested on a wide range of chelonian species.     

Sexual competition and N supply interactively affect the dimorphism and competiveness of opposite sexes in Populus cathayana

Several important dioecious species show sexual spatial segregation (SSS) along environmental gradients that have significant ecological effect on terrestrial ecosystem. However, little attention has been paid to understanding of how males and females respond to environmental gradients and sexual competition. We compared eco‐physiological parameters of males and females of Populus cathayana under different sexual competition patterns and nitrogen (N) supply levels. We found that males and females interacting with the same or opposite sex showed significant differences in biomass partition, photosynthetic capacity, carbon (C) and N metabolism, and leaf ultrastructure, and that the sexual differences to competition were importantly driven by N supply. The intersexual competition was enhanced under high N, while the intrasexual competition among females was amplified under low N. Under high N, the intersexual competition stimulated the growth of the females and negatively affected the males. In contrast, under low N, the males exposed to intrasexual competition had the highest tolerance, whereas females exposed to intrasexual competition showed the lowest adaptation among all competition patterns. Sexual competition patterns and N supply levels significantly affected the sexual dimorphism and competitiveness, which may play an important role in spatial segregation of P. cathayana populations.     

Predicting the pathway to wind pollination: heritabilities and genetic correlations of inflorescence traits associated with wind pollination in Schiedea salicaria (Caryophyllaceae)

The transition from biotic to abiotic pollination was investigated using Schiedea, a genus exhibiting a remarkable diversity of inflorescence architecture associated with pollination biology. Heritabilities and genetic correlations of inflorescence traits were estimated in gynodioecious Schiedea salicaria (Caryophyllaceae), a species that has likely undergone a recent transition to wind-pollination. Using a partial diallel crossing design, significant narrow-sense heritabilities were detected for inflorescence condensation (h2 = 0.56 to 0.68 in the two sexes) and other traits related to the extent of wind pollination in Schiedea species. Heritabilities were generally higher in hermaphrodites than in females. Strong genetic correlations may constrain the evolution of some inflorescence traits that facilitate wind pollination, such as simultaneous shortening of inflorescence length and elongation of the subtending internode. The presence of significant narrow-sense heritabilities for traits associated with wind pollination suggests, however, that selection for more effective wind pollination in the windy, pollinator-limited environments where S. salicaria grows could lead to the evolution of the highly condensed inflorescences characteristic of other wind-pollinated species of Schiedea.     

Sexual dimorphism in histological characteristics and contractility of the iliofibularis muscle in the lizard Sceloporus torquatus

The iliofibularis is a hindlimb muscle used in lizard locomotion that is composed of at least three types of fibres: fast‐twitch‐glycolytic (FG), fast‐twitch‐oxidative‐glycolytic fibre (FOG) and slow‐twitch‐oxidative (SO). The striated skeletal muscle is a highly plastic tissue undergoing phenotypic change in response to activity. The lizard Sceloporus torquatus has sexual differences associated with microhabitat use, which can be reflected in the physiology and anatomy of the muscle, and thus, in our study, we analysed the morphological and contractile characteristics of the iliofibularis muscle (IF) of S. torquatus males and females. We found a larger prevalence of FOG compared with FG and SO fibres in the muscle of both sexes. We also found that males show larger areas of the three types of fibres, develop greater strength but also faster fatigue than females, suggesting that strength is a key functional feature that enables males to perform faster movements (but for shorter periods), associated with the demands of territoriality.     

Sexual dimorphism in the tusked frog, Adelotus brevis (Anura:Myobatrachidae): the roles of natural and sexual selection

At least two adaptive processes can lead to the evolution of sexual dimorphism: sexual selection (e.g. male-male combat) or natural selection (e.g. dietary divergence). We investigated the adaptive significance of a distinctive pattern of sexual dimorphism in a south-eastern Australian frog, Adelotus brevis. Male Adelotus grow larger than female conspecifics, have larger heads relative to body size, and have large paired projections (‘tusks’) in the lower jaw. All of these traits are rare among anurans. We quantified the degree of dimorphism in Adelotus, and gathered data on diets and mating systems of this species to evaluate the possible roles of sexual selection and dietary divergence in favoring die evolution of these sexually dimorphic traits. Analysis of prey items in alimentary tracts revealed significant sex differences in prey types. For example, females ate proportionally more arthropods and fewer molluscs than did males. However, this difference is likely to be a secondary consequence of habitat differences between the sexes (due in turn to their different reproductive roles) rather than a selective force for the evolution of sexual dimorphism. Calling males spend their time in moist habitats where pondsnails are abundant, whereas females are more often encountered in the drier arthropod-rich woodlands. A three-year behavioural ecology study on a field population revealed that reproductive males engage in agonistic interactions, with the sexually dimorphic tusks used to attack rivals. Larger body size contributed to male reproductive success. Small males were excluded from calling sites and, among the calling males, larger animals had higher reproductive success (numbers of matings) than did smaller individuals. Hence, the atypical pattern of sexual dimorphism in Adelotus brevis seems to have resulted from sexual selection for larger body size and tusk size in males, in the context of male-male agonistic behaviour, rather than natural selection for ecological divergence between the sexes.     

Sexual size dimorphism,prey morphology and catch success in relation to flight mechanics in the peregrine falcon: a simulation study

In common with many other raptors, female peregrine falcons Falco peregrinus are about 50% heavier than males. Their sexual dimorphism is thought to allow breeding pairs to exploit a wider range of prey through a division of labor: the male being able to catch more maneuverable prey species; the female capable of carrying larger ones. Given the difficulty of assessing the catch success and load carrying capacity of both sexes of falcon in the field, we here adopt a novel approach to test the division‐of‐labor theory by using a detailed physics‐based flight simulator of birds. We study attacks by male and female peregrines on prey species ranging from small passerines to large ducks, testing how catch success relates to the flight performance of predator and prey. Males prove to be better than females at catching highly maneuverable prey in level flight, but the catch success of both sexes improves and becomes more similar when diving, because of the higher aerodynamic forces that are available to both sexes for maneuvering in high‐speed flight. The higher maximum roll acceleration of the male peregrine explains its edge over the female in catching maneuverable prey in level flight. Overall, catch success is more strongly influenced by the differences in maneuverability that exist between different species of prey than between the different sexes of falcon. On the other hand, the female can carry up to 50% greater loads than the male. More generally, our detailed simulation approach highlights the importance of several previously overlooked features of attack and escape. In particular, we find that it is not the prey's instantaneous maximum centripetal acceleration but the prey's ability to sustain a high centripetal acceleration for an extended period of time that is the primary driver of the variation in catch success across species.     

Sexual size dimorphism in shorebirds, gulls, and alcids: the influence of sexual and natural selection

Abstract. Charadrii (shorebirds, gulls, and alcids) have an unusual diversity in their sexual size dimorphism, ranging from monomorphism to either male-biased or female-biased dimorphism. We use comparative analyses to investigate whether this variation relates to sexual selection through competition for mates or natural selection through different use of resources by males and females. As predicted by sexual selection theory, we found that in taxa with socially polygynous mating systems, males were relatively larger than females compared with less polygynous species. Furthermore, evolution toward socially polyandrous mating systems was correlated with decreases in relative male size. These patterns depend on the kinds of courtship displays performed by males. In taxa with acrobatic flight displays, males are relatively smaller than in taxa in which courtship involves simple flights or displays from the ground. This result remains significant when the relationship with mating system is controlled statistically, thereby explaining the enigma of why males are often smaller than females in socially monogamous species. We did not find evidence that evolutionary changes in sexual dimorphism relate to niche division on the breeding grounds. In particular, biparental species did not have greater dimorphism in bill lengths than uniparental species, contrary to the hypothesis that selection for ecological divergence on the breeding grounds has been important as a general explanation for patterns of bill dimorphism. Taken together, these results strongly suggest that sexual selection has had a major influence on sexual size dimorphism in Charadrii, whereas divergence in the use of feeding resources while breeding was not supported by our analyses.     

Sexual dimorphism and synchrony of breeding: variation in polygyny potential among populations in the common brushtail possum, Trichosurus vulpecula

Male-biased sexual dimorphism in both size and body mass isa common characteristic of many polygynous mammals and is oftenattributed to sexual selection favoring large males. The degreeof dimorphism is thought to be related to the potential forsome males to monopolize access to estrous females, which isin turn related to the distribution of receptive females inspace and time. In the present study, we investigated the relationshipbetween the temporal distribution of breeding females and thedegree of mass dimorphism among 11 populations of the commonbrushtail possum from northern Australia. Breeding patternsvaried from complete aseasonality in some populations to a seasonalconcentration of births within 2 or 3 months in others. We predictedthat in populations in which mating opportunities were distributedthroughout the year, dominant males would be able to monopolizeaccess to larger numbers of estrous females than in populationsin which matings were more seasonal, and in such populations,large body size in males would be favored. We found that dimorphismwas related to seasonality of breeding, being greatest in populationswith a more aseasonal pattern. Mean body mass of male possumsalso decreased with increasing population density. Populationdensity may influence the degree of breeding synchrony withinpopulations, particularly in locations with a more seasonalclimate. The present study is the first to demonstrate plasticityin mass dimorphism in response to local variation in the synchronyof breeding in a mammal species.     

Effects of variation in inflorescence size and floral rewards on the visitation rates of traplining pollinators ofAralia hispida

Summary In field experiments withAralia hispida inflorescences, the following variables were manipulated: number of umbels per inflorescence, number of flowers per umbel, and amounts of pollen and nectar per flower. Visitation rates by bumble bees, the principal pollinators, were then observed. In the reward-variation experiments, bees appeared to learn the positions of nectar-rich shoots, and visited them significantly more often than nectar-poor shoots. They did not respond to similar variation in pollen production. The nectar preferences developed slowly after the treatments were imposed, and bees continued to favor sites that had been occupied by nectar-rich shoots even after the treatments were discontinued. Visitation rate was approximately proportional to flower number, making it unlikely that increases in inflorescence size produced a disproportionate gain in male reproductive success (a necessary condition in certain models for the evolution of dioecy). For a fixed number of flowers per inflorescence, bees preferred inflorescences with more umbels. In pairwise choice tests of male-phase and female-phase umbels of various sizes, bees preferred male-phase umbels and larger umbels; the preference for male-phase umbels is stronger in bees that had previously fed on male-phase umbels.     

Low siring success of females with an acquired male function illustrates the legacy of sexual dimorphism in constraining the breakdown of dioecy

Dioecy has often broken down in flowering plants, yielding functional hermaphroditism. We reasoned that evolutionary transitions from dioecy to functional hermaphroditism must overcome an inertia of sexual dimorphism, because modified males or females will express the opposite sexual function for which their phenotypes have been optimised. We tested this prediction by assessing the siring success of monoecious individuals of the plant Mercurialis annua with an acquired male function but that are phenotypically still female‐like. We found that pollen dispersed by female‐like monoecious individuals was ~ 1/3 poorer at siring outcrossed offspring than pollen from monoecious individuals with an alternative male‐like inflorescence. We conclude that whereas dioecy might evolve from functional hermaphroditism by conferring upon individuals certain benefits of sexual specialisation, reversion from a strategy of separate sexes to one of combined sexes must overcome constraints imposed by the advantages of sexual dimorphism. The breakdown of dioecy must therefore often be limited to situations in which outcrossing cannot be maintained and where selection favours a capacity for inbreeding by functional hermaphrodites.