Sexual selection and sexual size dimorphism in animals

Sexual selection is often considered as a critical evolutionary force promoting sexual size dimorphism (SSD) in animals. However, empirical evidence for a positive relationship between sexual selection on males and male-biased SSD received mixed support depending on the studied taxonomic group and on the method used to quantify sexual selection. Here, we present a meta-analytic approach accounting for phylogenetic non-independence to test how standardized metrics of the opportunity and strength of pre-copulatory sexual selection relate to SSD across a broad range of animal taxa comprising up to 95 effect sizes from 59 species. We found that SSD based on length measurements was correlated with the sex difference in the opportunity for sexual selection but showed a weak and statistically non-significant relationship with the sex difference in the Bateman gradient. These findings suggest that pre-copulatory sexual selection plays a limited role for the evolution of SSD in a broad phylogenetic context.     

Sexual bimaturation and sexual size dimorphism in animals with asymptotic growth after maturity

Many animal taxa exhibit a positive correlation between sexual size dimorphism and sex differences in age at maturity, such that members of the larger sex mature at older ages than members of the smaller sex. Previous workers have suggested that sexual bimaturation is a product of sex differences in growth trajectories, but to date no one has tested this hypothesis. The current study uses growth-based models to study relationships between sexual size dimorphism and sexual bimaturation in species with asymptotic growth after maturity. These models show that sex differences in asymptotic size would produce sexual bimaturation even if both sexes approach their respective asymptotic sizes at the same age, mature at the same proportion of asymptotic size and have otherwise equivalent growth and maturation patterns. Furthermore, our analyses show that there are three ways to reduce sexual bimaturation in sexually size-dimorphic species: (1) higher characteristic growth rates for members of the larger sex, (2) larger size at birth, hatching or metamorphosis for members of the larger sex or (3) smaller ratio of size at maturity to asymptotic size (relative size at maturity) for members of the larger sex. Of these three options, sex differences in relative size at maturity are most common in size-dimorphic species and, in both male-larger and female-larger species, members of the larger sex frequently mature at a smaller proportion of their asymptotic size than do members of the smaller sex. Information about the growth and maturation patterns of a taxon can be used to determine relationships between sexual size dimorphism and sexual bimaturation for the members of that taxon. This process is illustrated for Anolis lizards, a genus in which both sexes exhibit the same strong correlation (r 0.97) between size at maturity and asymptotic size, and in which the relative size at maturity is inversely related to asymptotic size for both sexes. As a result, sexually size-dimorphic species of anoles exhibit the expected pattern of a smaller relative size at maturity for members of the larger sex. However, for species in this genus, sex differences in the relative size at maturity are not strong enough to produce the same age at maturity for both sexes in sexually size-dimorphic species. Members of the larger sex (usually males) are still expected to mature at older ages than members of the smaller sex in Anolis lizards.     

Sexual size dimorphism and timing of spring migration in birds

Sexually selected traits are limited by selection against those traits in other fitness components, such as survival. Thus, sexual selection favouring large size in males should be balanced by higher mortality of larger males. However, evidence from red-winged blackbirds (Agelaius phoeniceus) indicates that large males survive better than small males. A survival advantage to large size could result from males migrating north in early spring, when harsh weather favours large size for energetic reasons. From this hypothesis we predicted that, among species, sex differences in body size should be correlated with sex differences in timing of spring migration. The earlier males migrate relative to females, the larger they should be relative to females. We tested this prediction using a comparative analysis of data collected from 30 species of passerine birds captured on migration. After controlling for social mating system, we found that sexual size dimorphism and difference in arrival dates of males and females were significantly positively correlated. This result is consistent with the hypothesis that selection for survival ability promotes sexual size dimorphism (SSD), rather than opposes SSD as is the conventional view. If both natural selection and sexual selection favour large adult males, then limits to male size must be imposed before males become adults.     

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 dimorphism in the andromonoecious Euphorbia nicaeensis: effects of gender and inflorescence development

BACKGROUND AND AIMS: In andromonoecious taxa with separate floral types along the inflorescence, architectural or plastic effects can simulate floral sexual dimorphism. Both the primary and secondary sexual characteristics of the cyathia of the protogynous andromonoecious species Euphorbia nicaeensis were analysed according to their sex and arrangement on the inflorescence. METHODS: The production of male and hermaphrodite cyathia at each inflorescence level was surveyed in two natural populations. The longevity, size, pollen production and viability, and nectar secretion of both types of cyathia were checked between inflorescence levels and between sexes at the only level at which they occur together. This sampling method makes it possible to know whether differences between cyathia types are based on sex or are attributable to inflorescence development. KEY RESULTS: Male cyathia were produced predominantly at the first and second inflorescence levels, whereas at levels 3-5, the cyathia were almost exclusively hermaphrodite. Viable pollen production by male cyathia at the second inflorescence level was higher than that of hermaphrodite cyathia at the third level but, when males and hermaphrodites at the same level were compared, their pollen production was similar. Male and hermaphrodite cyathia were similar in size, irrespective of the inflorescence level, although the exclusively hermaphrodite cyathia of the last level were smaller. Both cyathium types produced similar amounts of sugar. However, male cyathia produced nectar during their whole lifespans, whereas hermaphrodites produced it exclusively during their male phase. Moreover, the nectary activity of male cyathia started earlier in the day than that of hermaphrodites. CONCLUSIONS: An apparent floral dimorphism exists in the primary sexual characteristics of Euphorbia nicaeensis because differences in pollen production between cyathium types are due to theirs positions. Similarly, differences affecting most secondary sexual characteristics are only apparent between the two cyathium types. However, E. nicaeensis shows a true but slight floral dimorphism in some of the secondary sex characters related to nectar secretion. The lack of nectar production by the female phase of the hermaphrodite cyathia of E. nicaeensis indicates that this is a deceit-pollinated species.     

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 selection on male size drives the evolution of male‐biased sexual size dimorphism via the prolongation of male development

Sexual size dimorphism (SSD) arises when the net effects of natural and sexual selection on body size differ between the sexes. Quantitative SSD variation between taxa is common, but directional intraspecific SSD reversals are rare. We combined micro‐ and macroevolutionary approaches to study geographic SSD variation in closely related black scavenger flies. Common garden experiments revealed stark intra‐ and interspecific variation: Sepsis biflexuosa is monomorphic across the Holarctic, while S. cynipsea (only in Europe) consistently exhibits female‐biased SSD. Interestingly, S. neocynipsea displays contrasting SSD in Europe (females larger) and North America (males larger), a pattern opposite to the geographic reversal in SSD of S. punctum documented in a previous study. In accordance with the differential equilibrium model for the evolution of SSD, the intensity of sexual selection on male size varied between continents (weaker in Europe), whereas fecundity selection on female body size did not. Subsequent comparative analyses of 49 taxa documented at least six independent origins of male‐biased SSD in Sepsidae, which is likely caused by sexual selection on male size and mediated by bimaturism. Therefore, reversals in SSD and the associated changes in larval development might be much more common and rapid and less constrained than currently assumed.     

昆虫体型及性体型二型性的地理变异

体型是昆虫基本的形态特性,它会影响到昆虫几乎所有的生理和生活史特性。同种昆虫不同地理种群在体型上常表现出明显的渐变,导致这些渐变的环境因素包括温度、湿度、光照、寄主植物、种群密度等,并且多种环境因素也会对昆虫种群内个体体型产生影响。雌雄个体的体型存在差异,称性体型二型性。性体型二型性也显示了地理差异。这些差异形成的途径已经得到详细的分析,其形成机制导致多个假说的提出,这些假说又在多种昆虫中得到验证。本文从同一种昆虫不同种群间、同一种群内、雌雄虫个体间3个水平,对种内昆虫体型变异的方式,影响昆虫种群间体型变异和种群内昆虫体型的变异的环境因素,以及昆虫性体型二型性及其地理变异的现象等方面的研究进行了综述,并对未来的相关研究提供了建议。     

Sexual size dimorphism in Ophisops elegans (Squamata: Lacertidae) in Iran

Twenty-three morphological features of 140 specimens of Ophisops elegans were analysed in order to identify sexual dimorphism in west and northwestern populations of Iran. Sexual dimorphism is significant (P<0.05) in nearly all metric features except for trunk length (TL) and length of widest part of belly (LWB), and in only two meristic characters, the number of dorsal scales around mid-body (DSN) and the number of femoral pores (FPN). Males have a relatively longer snout-vent length (SVL) than females and males have generally relatively larger heads compared to females.     

Sexual size dimorphism in species with asymptotic growth after maturity

If animals mature at small sizes and then grow to larger asymptotic sizes, many factors can affect male and female size distributions. Standard growth equations can be used to study the processes affecting sexual size dimorphism in species with asymptotic growth after maturity. This paper first outlines the effects of sex differences in growth and maturation patterns on the direction and degree of sexual dimorphism. The next section considers the effects of variation in age structure or growth rates on adult body sizes and sexual size dimorphism. Field data from a crustacean, fish, lizard and mammal show how information on a species' growth and maturation patterns can be used to predict the relationships between male size, female size and sexual size dimorphism expected if a series of samples from the same population simply differed with respect to their ages or growth rates. The last section considers ecological or behavioural factors with different effects on the growth, maturation, survival or movement patterns of the two sexes. This study supports earlier suggestions that information on growth and maturation patterns may be useful, if not essential, for comparative studies of sexual size dimorphism in taxa with asymptotic growth after maturity.     

Sexual selection, sexual size dimorphism and Rensch's rule in Odonata

Odonata (dragonflies and damselflies) exhibit a range of sexual size dimorphism (SSD) that includes species with male-biased (males > females) or female-biased SSD (males < females) and species exhibiting nonterritorial or territorial mating strategies. Here, we use phylogenetic comparative analyses to investigate the influence of sexual selection on SSD in both suborders: dragonflies (Anisoptera) and damselflies (Zygoptera). First, we show that damselflies have male-biased SSD, and exhibit an allometric relationship between body size and SSD, that is consistent with Rensch's rule. Second, SSD of dragonflies is not different from unit, and this suborder does not exhibit Rensch's rule. Third, we test the influence of sexual selection on SSD using proxy variables of territorial mating strategy and male agility. Using generalized least squares to account for phylogenetic relationships between species, we show that male-biased SSD increases with territoriality in damselflies, but not in dragonflies. Finally, we show that nonagile territorial odonates exhibit male-biased SSD, whereas male agility is not related to SSD in nonterritorial odonates. These results suggest that sexual selection acting on male sizes influences SSD in Odonata. Taken together, our results, along with avian studies (bustards and shorebirds), suggest that male agility influences SSD, although this influence is modulated by territorial mating strategy and thus the likely advantage of being large. Other evolutionary processes, such as fecundity selection and viability selection, however, need further investigation.     

Sexual size dimorphism and assortative mating in Carolina Wrens

ABSTRACT.   Sexual size dimorphism (SSD) may be due to sexual and natural selection, but identifying specific mechanisms that generate such dimorphism in a species is difficult. I examined SSD in Carolina Wrens ( Thryothorus ludovicianus ) by examining (1) the degree of SSD in the population and between pairs using five morphometrics, (2) assortative mating patterns based on size and age, and (3) relationships between size and longevity. Analysis revealed that males were significantly larger than females in all body measurements. For example, mass, bill, and wing measurements yielded a canonical variable that permitted separation of the sexes and linear classification functions correctly determined the sex of 95% (238/250) of all wrens measured. No evidence was found to suggest that SSD was related to resource partitioning. However, assortative mating trends based on morphometrics (e.g., wing length), positive associations between longevity and morphometrics (e.g., wing length in females and body size in males), and intense male-male contests for territorial resources year-round provide evidence that sexual selection may contribute to SSD in Carolina Wrens.     

Sexual size dimorphism and reproductive ecology in relation to mating system in waders

The relationship between sexual size dimorphism, body-weight and different reproductive traits (e.g. clutch size, egg weight and incubation period) in relation to mating system and forms of parental care was studied in waders. Two hypotheses were examined. (1) Sexual size dimorphism is correlated with the intensity of sexual selection. (2) The degree of sexual size dimorphism is the result of an interrelationship between the reproductive strategy of the female and her body size. In the polygynous species the male was significantly larger than the female. This is consistent with the sexual selection hypothesis. However, among waders, a positive correlation exists between egg weight, clutch mass and body-weight. Selection for small eggs or a short incubation period may therefore have an influence on female body-weight. If the lack of paternal care reduces the female's possibility for producing large eggs or incubating a large clutch mass, we would expect a selection pressure for small female size among polygynous species. Thus, large sexual size dimorphism among polygynous waders may be a result of selection for small female size to lack of paternal care, or selection for large male size due to intramale competition or a female preference for large-sized males. In multiple-clutch species (viz. species in which the female regularly lays more than one clutch during the season) egg weight was low both for a given female and male body-weight. The low egg weight of multiple-clutch species is assumed to be a result of the constraints placed on the female from producing several clutches during a single breeding season.     

Sexual size dimorphism in Darevskia raddei (Sauria: Lacertidae) from northwestern Iran

We examined sexual size dimorphism of the rock-dwelling lizard Darevskia raddei (Boettger, 1892) with the help of 30 specimens that were provided from various sources. Eleven metric and seven meristic features were examined. Seven characters (gulars, length of basal tail, femoral pores, length of head, width of head, length of fore limb and length of hind limb) were identified as dimorphic between the two sexes. Some of these characters have important roles in copulation for males, especially the hind limb and the tail base. The number of femoral pores is important in the release of signal components because females release these components to attract males during the mating season. The length of the hind limb as locomotor performance plays an important role during mating, so that the male can grasp the female and adopt the correct position during copulation.     

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.     

A biogeographic reversal in sexual size dimorphism along a continental temperature gradient

The magnitude and direction of sexual size dimorphism (SSD) varies greatly across the animal kingdom, reflecting differential selection pressures on the reproductive and/or ecological roles of males and females. If the selection pressures and constraints imposed on body size change along environmental gradients, then SSD will vary geographically in a predictable way. Here, we uncover a biogeographical reversal in SSD of lizards from Central and North America: in warm, low latitude environments, males are larger than females, but at colder, high latitudes, females are larger than males. Comparisons to expectations under a Brownian motion model of SSD evolution indicate that this pattern reflects differences in the evolutionary rates and/or trajectories of sex‐specific body sizes. The SSD gradient we found is strongly related to mean annual temperature, but is independent of species richness and body size differences among species within grid cells, suggesting that the biogeography of SSD reflects gradients in sexual and/or fecundity selection, rather than intersexual niche divergence to minimize intraspecific competition. We demonstrate that the SSD gradient is driven by stronger variation in male size than in female size and is independent of clutch mass. This suggests that gradients in sexual selection and male–male competition, rather than fecundity selection to maximize reproductive output by females in seasonal environments, are predominantly responsible for the gradient.     

Sexual dimorphism of body size and shell shape in European tortoises

Adult body size and shape were examined in almost 1400 individuals of the tortoises Testudo graeca , T. hermanni and T. marginata from Greece. The size at maturity was greater in females than in males in all three species. Maximum and mean adult sizes were also greater in females than in males in T. graeca and T. hermanni . Males grew to a larger size than females in T. marginata , and mean adult size was similar in the sexes in this species. Sexual dimorphism of shape (adjusted for size covariate) was shown in most of the characters examined, and the degree of this dimorphism differed significantly among the three species. Differences were related to their contrasting courtship behaviours: horizontal head movements and severe biting in T. marginata , vertical head bobs and carapace butting in T. graeca , and mounting and tail thrusting in T. hermanni . There was no difference in the frequency of observations of courtship or fighting among the three species, but courtship was about 10 times more common than combat in males. All species showed greatest courtship activity in autumn; copulation was rarely observed in T. hermanni (only 0.36% of courting males) and not seen in the other species in the field. Observations made throughout the activity season indicated that feeding was equally common in males and females in all three species. Differences in shape were more likely to be the result of sexual selection than of natural selection for fecundity. Detailed predictions are made for sexual dimorphism of other characters in these species.     

A comparative study on the evolution of reversed size dimorphism in monogamous waders

Sexual dimorphism in size is common in birds. Males are usually larger than females, although in some taxa reversed size dimorphism (RSD) predominates. Whilst direct dimorphism is attributed to sexual selection in males giving greater reproductive access to females, the evolutionary causes of RSD are still unclear. Four different hypotheses could explain the evolution of RSD in monogamous birds: (1) The ‘energy storing’ hypothesis suggests that larger females could accumulate more reserves at wintering or refuelling areas to enable an earlier start to egg laying. (2) According to the ‘incubation ability’ hypothesis, RSD has evolved because large females can incubate more efficiently than small ones. (3) The ‘parental role division’ hypothesis suggests that RSD in monogamous waders has evolved in species with parental role division and uniparental male care of the chicks. It is based on the assumption that small male size facilitates food acquisition in terrestrial habitats where chick rearing takes place and that larger females can accumulate more reserves for egg laying in coastal sites. (3) The ‘display agility’ hypothesis suggests that small males perform better in acrobatic displays presumably involved in mate choice and so RSD may have evolved due to female preference for agile males. I tested these hypotheses in monogamous waders using several comparative methods. Given the current knowledge of the phylogeny of this group, the evolutionary history of waders seems only compatible with the hypothesis that RSD has evolved as an adaptation for increasing display performance in males. In addition, the analysis of wing shape showed that males of species with acrobatic flight displays had wings with higher aspect ratio (wing span/²wing area) than non-acrobatic species, which probably increases flight manoeuvrability during acrobatic displays. In species with acrobatic displays males also had a higher aspect ratio than females although no sexual difference was found in non-acrobatic species. These results suggest that acrobatic flight displays could have produced changes in the morphology of some species and suggest the existence of selection favouring higher manoeuvrability in species with acrobatic flight displays. This supports the validity of the mechanisms proposed by the ‘display agility’ hypothesis to explain the evolution of RSD in waders.     

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.