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In most fishes, the number of offspring increases with maternal body size. Although this size‐fecundity relationship often varies among species as a result of the coevolution of life‐history traits, the genetic basis of such size‐fecundity relationships remains unclear. We explored the genetic basis underlying this size‐fecundity relationship in two small medaka species, Oryzias latipes and O. sakaizumii. Our findings showed that O. sakaizumii has a higher fecundity than O. latipes, and quantitative trait locus analysis using interspecific F2 hybrids showed that chromosome 23 is linked to the size‐fecundity relationship. In particular, the genes igf1 and lep‐b in this region are known to be associated with life‐history traits, including somatic growth, gonad maturation, and progeny numbers in various taxa. Because O. sakaizumii is distributed at higher latitudes and has a shorter spawning season than O. latipes in the wild, we propose that the relatively high fecundity observed in O. sakaizumii is an adaptation to high latitudes. We also discuss the potential ecological ramifications associated with the evolution of increased fecundity in this species. [ABSTRACT FROM AUTHOR]

Female‐biased sexual size dimorphism stands as a widespread evolutionary pattern. Fecundity selection, favouring larger females with greater reproductive output, is a leading explanation. However, larger body sizes demand greater energy intake, potentially hindering the evolution of extreme female sizes. Thus, the evolution of more lucrative foraging tactics may allow for an increase in size. Hence, coupled with selection against larger males, fecundity selection should result in larger SSD in species with more lucrative foraging strategies. Crab spiders are sit‐and‐wait predators that hunt in several plant substrata. Species that forage on flowers or employ prey‐luring strategies likely have access to higher food intake than other species. We extracted body size measurements of 614 crab spider species from 43 genera and classified them based on their foraging strategy. Our findings show that foraging strategies that provide higher energy input (EFS) result in larger SSD. Statistical estimates indicate that females have a cephalothorax width 91% larger than males in EFS species, compared to 26% larger females than males in non‐EFS species. These differences possibly arise due to larger females and smaller males. The effects on male size reduction might result from scramble competition, whereas the increase in female size is likely due to fecundity selection. These results suggest that the shift towards more lucrative foraging strategies may have been a key event in body size and SSD evolution in crab spiders. [ABSTRACT FROM AUTHOR]

Abstract Female‐biased sexual size dimorphism stands as a widespread evolutionary pattern. Fecundity selection, favouring larger females with greater reproductive output, is a leading explanation. However, larger body sizes demand greater energy intake, potentially hindering the evolution of extreme female sizes. Thus, the evolution of more lucrative foraging tactics may allow for an increase in size. Hence, coupled with selection against larger males, fecundity selection should result in larger SSD in species with more lucrative foraging strategies. Crab spiders are sit‐and‐wait predators that hunt in several plant substrata. Species that forage on flowers or employ prey‐luring strategies likely have access to higher food intake than other species. We extracted body size measurements of 614 crab spider species from 43 genera and classified them based on their foraging strategy. Our findings show that foraging strategies that provide higher energy input (EFS) result in larger SSD. Statistical estimates indicate that females have a cephalothorax width 91% larger than males in EFS species, compared to 26% larger females than males in non‐EFS species. These differences possibly arise due to larger females and smaller males. The effects on male size reduction might result from scramble competition, whereas the increase in female size is likely due to fecundity selection. These results suggest that the shift towards more lucrative foraging strategies may have been a key event in body size and SSD evolution in crab spiders.

Sexual dimorphism can evolve in response to sex-specific selection pressures that vary across habitats. We studied sexual differences in subterranean amphipods Niphargus living in shallow subterranean habitats (close to the surface), cave streams (intermediate), and cave lakes (deepest and most isolated). These three habitats differ because at greater depths there is lower food availability, reduced predation, and weaker seasonality. Additionally, species near the surface have a near-even adult sex ratio (ASR), whereas species from cave lakes have a female-biased ASR. We hypothesized (a) a decrease in sexual dimorphism from shallow subterranean habitats to cave lake species because of weaker sexual selection derived from changes in the ASR and (b) an increase in female body size in cave lakes because of stronger fecundity selection on account of oligotrophy, reduced predation, and weaker seasonality. We measured body size and two sexually dimorphic abdominal appendages for all 31 species and several behaviours related to male competition (activity, risk-taking, exploration) for 12 species. Species with an equal ASR that live close to the surface exhibited sexual dimorphism in all three morphological traits, but not in behaviour. The body size of females increased from the surface to cave lakes, but no such trend was observed in males. In cave lake species, males and females differed neither morphologically nor behaviourally. Our results are consistent with the possibility that sexual and fecundity selection covary across the three habitats, which indirectly and directly, respectively, shape the degree of sexual dimorphism in Niphargus species. [ABSTRACT FROM AUTHOR]

The evolution of sexual size dimorphism (SSD) is a long‐standing topic in evolutionary biology, but there is little agreement on the extent to which SSD is driven by the different selective forces. While sexual selection and fecundity selection have traditionally been proposed as the two leading hypotheses, SSD may also result from natural selection through mechanisms such as sexual niche divergence, which might have reduced resource competition between sexes.Here, we revisited the niche divergence hypothesis by testing the relationship between the sexual overlap in diet and SSD of 56 bird species using phylogenetic comparative analyses. We then assessed how SSD variation relates to the three main hypotheses: sexual selection, fecundity selection, and sexual niche divergence using phylogenetic generalized least squares (PGLS). Then, we compared sexual selection, fecundity selection and niche divergence selection as SSD drivers through phylogenetic confirmatory path analyses to disentangle the possible causal evolutionary relationships between SSD and the three hypotheses.Phylogenetic generalized least squares showed that SSD was negatively correlated with diet overlap, that is, the greater the difference in body size between males and females, the less diet overlap. As predicted by sexual selection theory, the difference in body size between sexes was higher in polygynous species. Confirmatory phylogenetic path analyses suggested that the most likely evolutionary path might include the mating system as a main driver in SSD and niche divergence as a result of SSD. We found no evidence of a role of fecundity selection in the evolution of female‐biased SSD.Our study provides evidence that sexual selection has likely been the main cause of SSD and that dietary divergence is likely an indirect effect of SSD. [ABSTRACT FROM AUTHOR]

The transition from terrestrial to aquatic life by hydrophiine elapid snakes modified targets of natural selection and likely affected sexual selection also. Thus, the shift to marine life also might have affected sexual dimorphism. Our measurements of 419 preserved specimens of six species of aipysurine snakes (genera Emydocephalus and Aipysurus) revealed sexual dimorphism in mean adult snout–vent length (SVL), body width relative to SVL, lengths and widths of heads and tails relative to SVL, and eye diameter relative to head length. Females averaged larger than males in all taxa, and generally were wider-bodied with shorter and wider tails and smaller eyes. For other traits, sexual dimorphism varied among species: for example, relative head length ranged from male-biased to female-biased, and head shape (width relative to length) was highly dimorphic only in A. laevis. The transition to marine life may have eliminated male–male combat (reducing selection for large males) and favoured visual rather than pheromone-based mate-searching (favouring larger eyes in males). Variation in head-size dimorphism may reflect intersexual niche partitioning, with different taxa following different trajectories. Repeated evolutionary transitions from terrestrial to aquatic life in snakes provide a powerful opportunity to explore selective forces on sexually dimorphic traits.

Masquerade, the resemblance of animals to inedible or inanimate objects, exists universally throughout the animal kingdom, especially in arthropods. However, masquerade has received little attention from biologists and is often misinterpreted as mimicry by the public and even by scientists, as a consequence of the lack of systematic biological information for masqueraders. Therefore, using the orchid mantis Hymenopus coronatus (Insecta: Mantodea), a classic masquerader, as the study species, we asked: (1) what is the population abundance and life cycle? (2) whether it closely coexists with specific plant or not? (3) how its colour morph changes across life stages? and (4) whether and how the key environmental factors affect its life cycle? Results suggested that the orchid mantis (1) had an extremely low wild population abundance; (2) did not coexist with specific plant; (3) exhibited colour morph diversity; (4) might match its reproductive and nymph developmental period with local seasonal fluctuations of temperature and precipitation. We then compared its life history differences between the two sexes. The results indicated that the two sexes can be remarkably different in development duration, growth rate, lifespan and body weight. This study is the first systematic investigation of the orchid mantis; the results provide useful natural history data for ecologists and evolutionary biologists to understand the adaptive strategies of elusive appearance of masqueraders. [ABSTRACT FROM AUTHOR]

Rensch's rule suggests that sexual size dimorphism (SSD) increases with species size when males are the larger sex, whereas it decreases when females are the larger sex. However, the process responsible for this pattern remains obscure. SSD can result from sexual selection, such as intrasexual competition for access to mates, or from natural selection, due to resource partitioning or fecundity selection. We studied SSD in Caribbean Eleutherodactylus frogs using phylogenetic comparative methods to investigate the influence of microhabitat, fecundity, and parental care. Our results show that in Caribbean Eleutherodactylus females tend to be larger and, contrary to Rensch's rule, dimorphism increases with species size. SSD was not related to microhabitat use. However, SSD was positively correlated with fecundity, mediated by a greater increase in female size. SSD was also influenced by parental care, suggesting that male care promotes larger male size and reduces the female bias in SSD. As suggested for other anurans, female‐biased SSD in Caribbean Eleutherodactylus results from fecundity selection, although the magnitude is countered by increased male size in species with paternal care. Our results highlight the importance of considering various selective forces that may act in concert to influence the evolution of SSD. [ABSTRACT FROM AUTHOR]

Sexual dimorphism (SD) is a common phenomenon in the animal kingdom, especially in lizards. The mountain dragon (Diploderma micangshanensis) is a Chinese endemic that is widely distributed in montane regions of northern Sichuan, western Hubei Province, southern Gansu, Henan, and Shanxi Provinces. However, little is known about its morphology, ecology, behavior, physiology, and natural history. In this study, SD in size and shape in a population of D. micangshanensis from Henan Province, Central China was examined based on a sample of 35 males and 23 females. Sexual dimorphism was observed in a few body shape variables (e.g., head and trunk length) but not in body size (snout-vent length). Males had longer and wider heads and shorter trunks. Sexual dimorphism in these traits can be explained by the sexual selection, ecological divergence, and fecundity advantage hypotheses. Additional studies of the ecology, reproductive biology, natural history, and behavior of D. micangshanensis are needed to evaluate the relative contributions of these mechanisms to determining the patterns of SD observed in this population. [ABSTRACT FROM AUTHOR]

Abstract Natural selection favors animals that evolve developmental and behavioral responses that buffer the negative effects of food restrictions. These buffering responses vary both between species and within species. Many studies have shown sex‐specific responses to environmental changes, usually in species with sexual size dimorphism (SSD), less found in species with weak or no SSD, which suggests that sizes of different sexes are experiencing different selections. However, previous studies usually investigated development and behavior separately, and the balanced situation where males and females of sexually dimorphic species respond in the same way to food restriction remains little known. Here, we investigated this in Phintelloides versicolor (Salticidae) that presents sexual dimorphism in color and shape but weak SSD. We examined whether food restriction induced the same responses in males and females in development duration, adult body size and weight, daily time allocated to foraging, and hunting. We found food restriction induced similar responses in both sexes: both exhibited longer development duration, smaller adult body size and weight, higher probability of staying outside nests and noticing prey immediately, and higher hunting success. However, there were sexual differences regardless of food condition: females showed faster development, smaller adult body size, higher probability of staying outside of nests, and higher hunting success. These indicated the differential selection on male and female sizes of P. versicolor could be under a balanced situation, where males and females show equal developmental and behavioral plasticity to environmental constraints.

Rensch's rule relates to a pattern whereby sexual size dimorphism is more female‐biased in small‐sized species and more male‐biased in large‐sized ones. We collected literature and museum data on the body size of males and females belonging to 4032 lizard species, as well as data on their reproductive modes and clutch sizes. We used phylogenetic comparative analyses, and general linear mixed models, to test Rensch's rule and examined how reproductive mode and clutch size affect sexual size dimorphism. Sexual size dimorphism was independent of clutch size in lizard species with variable clutch sizes and in oviparous lizards. Large litters were associated with female‐biased sexual dimorphism in viviparous and in scincomorph lizards. Inference regarding Rensch's rule depended on the analytical method used to identify it. The widely used, but less conservative, reduced major axis regression usually support Rensch's rule while ordinary least squares regressions mostly show isometric relationships. The rule tended to apply more to oviparous than to viviparous lizards. We infer that Rensch's rule is, at best, a weak pattern in lizards. This is especially true in viviparous lineages where females reproduce infrequently and therefore evolve large sizes to maximise fecundity, resulting in female‐biased dimorphism. [ABSTRACT FROM AUTHOR]

Many influential mathematical models of sexual selection have stressed that mating preferences evolve due to correlations that build between mating preferences and preferred display traits - that is, through indirect selection. Nevertheless, there is a perception that indirect selection should generally be overwhelmed by direct selection, for example in the form of search costs. Recent work by Fry has used quantitative genetic models to argue that in many cases, including when there are direct benefits (a fecundity advantage to mating with the preferred male), direct and indirect selection may be of similar magnitude. Here I use population genetic models, in which the strength of the genetic correlation is an emergent property of evolution at mating preference and display trait loci, to assess the relative contributions of direct and indirect selection to the evolution of mating preferences. For the cases of direct benefits and of indirect benefits with fixed and frequency-dependent search costs, I outline parameter values of fecundity benefits, preference strengths, and search costs for which indirect selection on female preferences can potentially predominate. I also analyze male mate choice under polygyny, showing that direct selection will always outweigh indirect selection except when there are direct benefits., (© The Author(s) 2024. Published by Oxford University Press on behalf of The Society for the Study of Evolution (SSE). All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

According to Rensch's rule, sexual size dimorphism (SSD) decreases with increasing body size in species where females are larger than males, whereas SSD increases with body size when males are larger than females. In this study, we examined body size from 15 populations of Bufo minshanicus in the Qinghai–Tibet plateau in China. We estimated the age of the individuals from eight populations to evaluate Rensch's rule and possible causes of variation in SSD. After or before correcting male and female body size for age differences, the patterns of SSD failed to obey Rensch's rule or its inverse, even though fecundity selection in toads tends to favor larger female body size. We also found that the degree of SSD was positively correlated with the operational sex ratio in the field across B. minshanicus populations. Thus, sexual and fecundity selection play an equal role driving the evolution of SSD within toads. In addition, sex‐specific growth rate and age structure explain part of the variation observed in SSD across populations. [ABSTRACT FROM AUTHOR]

Natural selection favors animals that evolve developmental and behavioral responses that buffer the negative effects of food restrictions. These buffering responses vary both between species and within species. Many studies have shown sex‐specific responses to environmental changes, usually in species with sexual size dimorphism (SSD), less found in species with weak or no SSD, which suggests that sizes of different sexes are experiencing different selections. However, previous studies usually investigated development and behavior separately, and the balanced situation where males and females of sexually dimorphic species respond in the same way to food restriction remains little known. Here, we investigated this in Phintelloides versicolor (Salticidae) that presents sexual dimorphism in color and shape but weak SSD. We examined whether food restriction induced the same responses in males and females in development duration, adult body size and weight, daily time allocated to foraging, and hunting. We found food restriction induced similar responses in both sexes: both exhibited longer development duration, smaller adult body size and weight, higher probability of staying outside nests and noticing prey immediately, and higher hunting success. However, there were sexual differences regardless of food condition: females showed faster development, smaller adult body size, higher probability of staying outside of nests, and higher hunting success. These indicated the differential selection on male and female sizes of P. versicolor could be under a balanced situation, where males and females show equal developmental and behavioral plasticity to environmental constraints. [ABSTRACT FROM AUTHOR]

Quantifying temporal variation in sex-specific selection on key ecologically relevant traits, and quantifying how such variation arises through synergistic or opposing components of survival and reproductive selection, is central to understanding eco-evolutionary dynamics, but rarely achieved. Seasonal migration versus residence is one key trait that directly shapes spatio-seasonal population dynamics in spatially and temporally varying environments, but temporal dynamics of sex-specific selection have not been fully quantified. We fittedmulti-event capture-recapture models to year-round ring resightings and breeding success data from partially migratory European shags (Phalacrocorax aristotelis) to quantify temporal variation in annual sex-specific selection on seasonal migration versus residence arising through adult survival, reproduction and the combination of both (i.e. annual fitness). We demonstrate episodes of strong and strongly fluctuating selection through annual fitness that were broadly synchronized across females and males. These overall fluctuations arose because strong reproductive selection against migration in several years contrasted with strong survival selection against residence in years with extreme climatic events. These results indicate how substantial phenotypic and genetic variation in migration versus residence could be maintained, and highlight that biologically important fluctuations in selection may not be detected unless both survival selection and reproductive selection are appropriately quantified and combined. [ABSTRACT FROM AUTHOR]

Rensch's rule is an allometric rule on sexual size dimorphism. It states that in small-sized species, females are larger than males, whereas in larger species, males are relatively larger than females. Several studies have explored this pattern, and its inverse in lizard species. China has a unique and high diversity of species, with a variety of ecological systems which shape diversity of phenotypes. In this study, sexual size dimorphism and Rensch's rule were determined using a dataset of Chinese lizard species. The findings show that Chinese lizards generally exhibit female-biased sexual size dimorphism. In addition, clutch size was positively correlated with sexual size dimorphism. Agamidae species were the only taxa that followed Rensch's rule (slope of males against females was steeper than 1). Clutch size was correlated with sexual size dimorphism in groups that do not follow Rensch's rule. This finding implies that strong fecundity selection limits application of Rensch's rule in these groups. [ABSTRACT FROM AUTHOR]

Aim: Body size explains most of the variation in fitness within animal populations and is therefore under constant selection from ecological and reproductive pressures, which often promote its evolution in sex‐specific directions, leading to sexual size dimorphism (SSD). Several hypotheses have been proposed to explain the vast diversity of SSD across species. These hypotheses emphasize: (a) the mate competition benefits to larger male size (sexual selection); (b) the benefits of larger female size for fecundity (fecundity selection); (c) the simultaneous benefits of niche divergence for males and females to reduce intersexual competition for ecological resources (natural selection); and (d) the underlying impact of geographical variation in climatic pressures expected to shape large‐scale patterns of SSD in synergy with the above selection pressures (e.g., intensification of fecundity selection as breeding seasons shorten). Based on a new, global‐scale amphibian dataset, we address the shortage of large‐scale, integrative tests of these four hypotheses. Location: Global. Time period: Extant. Major taxa studied: Class Amphibia. Methods: Using a > 3,500 species dataset spanning body size, ecological, life‐history, geographical and climatic data, we performed phylogenetic linear models to address the sexual, fecundity, ecological and climatic hypotheses of SSD. Results: Evolution of SSD is discordant between anurans and salamanders. Anuran SSD is shaped by climate (male‐biased SSD increases with temperature seasonality) and by nesting site. In salamanders, SSD converges across species that occupy the same types of microhabitats ("ecodimorphs"), whereas reproductive or climatic pressures have no effects on their SSD. These contrasts are associated with latitudinal gradients of SSD in anurans, but not in salamanders. Main conclusions: Amphibian SSD is driven by ecological and climatic pressures, whereas no roles for sexual or fecundity selection were detected. We show that macroevolutionary processes determined by different forms of selection lead to latitudinal patterns of trait diversity, and the lack of them. [ABSTRACT FROM AUTHOR]

Abstract Background Comparing sexual size dimorphism (SSD) in the light of the phylogenetic hypothesis may help to understand the phenotypic evolution associated with sexual selection (size of whole body and of reproduction-related body parts). Within a macroevolutionary framework, we evaluated the association between the evolution of SSD and the evolution of reproduction-related phenotypic traits, and whether this association has favored female fecundity, considering also variations according to reproductive modes. We focused on the lizard species that inhabit the Chaco Domain since this is a natural unit with a high diversity of species. Results The residual SSD was related positively with the residuals of the reproduction-related phenotypic traits that estimate intrasexual selection and with the residuals of inter-limb length and, according to fecundity selection, those residuals were related positively with the residuals of clutch size in oviparous species. Lizards of the Chaco Domain present a high diversity of SSD patterns, probably related to the evolution of reproductive strategies. Conclusions Our findings highlight that the sexual selection may have acted on the whole-body size as well as on the size of body parts related to reproduction. Male and female phenotypes evolutionarily respond to variations in SSD, and an understanding of these patterns is essential for elucidating the processes shaping sexual phenotype diversity from a macroevolutionary perspective.

The evolution of sexual dimorphism has long fascinated evolutionary biologists and theory suggests that variation in sexual dimorphism is a consequence of selective forces acting differently on morphological traits in males versus females. Here, we analyzed sexual differences in size and shape of the Boulenger's lazy toad, Scutiger boulengeri , based on the intersex variation pattern of sixteen morphometric traits including body size. The results suggested that sexual dimorphism was apparent in body size and some body shapes (e.g., head length and width, internasal space, interorbital space, diameter of lower arm and tibia width) of this toad. The bigger body size in females may be relevant to fecundity selection, a larger head in males as well as a broader internasal and interorbital space may be subject to male-male competition in combination with ecological selection, and both robust forelimbs and hindlimbs in males may be related to mating and competitive behaviors. These results are discussed with respect to the above selection procedures and possible sex differences in life history traits. [ABSTRACT FROM AUTHOR]

The evolution of sexually dimorphic ornamentation should reflect sex differences in a form of parental care, incubation behavior, but empirical support is scarce after controlling for phylogenetic inertia. Here, using phylogenetic comparative methods in swallows and martins (Aves: Hirundinidae), we studied the evolution of sexual dimorphism in tail length in relation to two incubation types, female-only and biparental incubation. We found that species with biparental incubation were more likely to have sexually monomorphic tails compared to species with female-only incubation. Furthermore, an analysis of evolutionary pathways suggested interdependent evolution of biparental incubation and sexual tail dimorphism; transitions to biparental incubation and sexual tail dimorphism were less likely to occur than transitions from that state. This finding is consistent with the hypothesis that selection on male ornamentation matters during the incubation period. We also confirmed that species with biparental incubation had significantly lower rates of extra-pair paternity compared to species with female-only incubation in this clade, providing additional support for the importance of selection on male ornamentation during the incubation period. The current study is one of few phylogenetic comparative studies demonstrating the correlated evolution of biparental incubation and sexual dimorphism in ornamentation. [ABSTRACT FROM AUTHOR]

Variation in the degree of sexual size dimorphism (SSD) among taxa is generally considered to arise from differences in the relative intensity of male–male competition and fecundity selection. One might predict, therefore, that SSD will vary systematically with (1) the intensity of sexual selection for increased male size, and (2) the intensity of fecundity selection for increased female size. To test these two fundamental hypotheses, we conducted a phylogenetic comparative analysis of SSD in fish. Specifically, using records of body length at first sexual maturity from FishBase, we quantified variation in the magnitude and direction of SSD in more than 600 diverse freshwater and marine fish species, from sticklebacks to sharks. Although female-biased SSD was common, and thought to be driven primarily by fecundity selection, variation in SSD was not dependent on either the allometric scaling of reproductive energy output or fecundity in female fish. Instead, systematic patterns based on habitat and life-history characteristics associated with varying degrees of male–male competition and paternal care strongly suggest that adaptive variation in SSD is driven by the intensity of sexual selection for increased male size. [ABSTRACT FROM AUTHOR]

We study a spatial branching model, where the underlying motion is d-dimensional (d≥1) Brownian motion and the branching rate is affected by a random collection of reproduction suppressing sets dubbed mild obstacles. The main result of this paper is the quenched law of large numbers for the population for all d≥1. We also show that the branching Brownian motion with mild obstacles spreads less quickly than ordinary branching Brownian motion by giving an upper estimate on its speed. When the underlying motion is an arbitrary diffusion process, we obtain a dichotomy for the quenched local growth that is independent of the Poissonian intensity. More general offspring distributions (beyond the dyadic one considered in the main theorems) as well as mild obstacle models for superprocesses are also discussed.

We explore the effect of different mechanisms of natural selection on the evolution of populations for one- and two-locus systems. We compare the effect of viability and fecundity selection in the context of the Wright-Fisher model with selection under the assumption of multiplicative fitness. We show that these two modes of natural selection correspond to different orderings of the processes of population regulation and natural selection in the Wright-Fisher model. We find that under the Wright-Fisher model these two different orderings can affect the distribution of trajectories of haplotype frequencies evolving with genetic recombination. However, the difference in the distribution of trajectories is only appreciable when the population is in significant linkage disequilibrium. We find that as linkage disequilibrium decays the trajectories for the two different models rapidly become indistinguishable. We discuss the significance of these findings in terms of biological examples of viability and fecundity selection, and speculate that the effect may be significant when factors such as gene migration maintain a degree of linkage disequilibrium.

Persististrombus deperditus (Sowerby) from the Lower Miocene of Kutch, Gujarat, western India is represented by two size classes in our collection. Statistical analyses discriminate the size morphs. Large size variations generally result from either (1) sexual differences or (2) ecophenotypic causes. All the living species of the family Strombidae, wherever examined, are characterized by sexual size dimorphism (SSD). Persististrombus deperditus shares all the characters of SSD in these recent species. Size variations due to difference in ecological factors generally occur in allopatric populations. Similar variations are known to characterize sympatric sub-populations of molluscs living only in the intertidal zone, where upper and lower shorefaces differ significantly in physico-chemical and biological properties. Persististrombus deperditus comes from a stable shelf setting that received less siliciclastic input in response to transgression. Hence, its size dimorphism is considered to have sexual origin. This is the first report of SSD in a fossil strombid gastropod. It is argued that fecundity selection was the primary driving force behind the evolution of SSD in this gonochoristic gastropod species. Hence, the larger morph is the female.

The transition from terrestrial to aquatic life by hydrophiine elapid snakes modified targets of natural selection and likely affected sexual selection also. Thus, the shift to marine life also might have affected sexual dimorphism. Our measurements of 419 preserved specimens of six species of aipysurine snakes (genera Emydocephalus and Aipysurus ) revealed sexual dimorphism in mean adult snout-vent length (SVL), body width relative to SVL, lengths and widths of heads and tails relative to SVL, and eye diameter relative to head length. Females averaged larger than males in all taxa, and generally were wider-bodied with shorter and wider tails and smaller eyes. For other traits, sexual dimorphism varied among species: for example, relative head length ranged from male-biased to female-biased, and head shape (width relative to length) was highly dimorphic only in A. laevis . The transition to marine life may have eliminated male-male combat (reducing selection for large males) and favoured visual rather than pheromone-based mate-searching (favouring larger eyes in males). Variation in head-size dimorphism may reflect intersexual niche partitioning, with different taxa following different trajectories. Repeated evolutionary transitions from terrestrial to aquatic life in snakes provide a powerful opportunity to explore selective forces on sexually dimorphic traits., Competing Interests: We declare we have no competing interests., (© 2023 The Authors.)

Sexual size dimorphism (SSD), the difference in body size between males and females, is a widespread phenomenon and commonly attributed to variation in sex-specific patterns of selection. Using data from three populations of Polypedates megacephalus, we tested the hypotheses that sexual selection favoured large male body size and that fecundity selection favoured large female size. Females had a larger body size than males in all populations and thus exhibited a female-biased sexual size dimorphism. Standardized selection gradients showed that sexual selection for large male size was weak at best and hence unlikely to be an important driving force for SSD. As fecundity selection favouring large females was weak as well, it could not explain the female-biased SSD patterns found. However, a faster growth rate in females favoured a larger size, providing an explanation for female-biased SSD. Our findings suggest that female-biased SSD does not result from a phenotypic selection of female by size in P. megacephalus. [ABSTRACT FROM AUTHOR]

ABSTRACT Fitness results from an optimal balance between survival, mating success and fecundity. The interactions between these three components of fitness vary depending on the selective context, from positive covariation between them, to antagonistic pleiotropic relationships when fitness increases in one reduce the fitness of others. Therefore, elucidating the routes through which selection shapes life history and phenotypic adaptations via these fitness components is of primary significance to understanding ecological and evolutionary dynamics. However, while the fitness components mediated by natural (survival) and sexual (mating success) selection have been debated extensively from most possible perspectives, fecundity selection remains considerably less studied. Here, we review the theoretical basis, evidence and implications of fecundity selection as a driver of sex-specific adaptive evolution. Based on accumulating literature on the life-history, phenotypic and ecological aspects of fecundity, we ( i) suggest a re-arrangement of the concepts of fecundity, whereby we coin the term 'transient fecundity' to refer to brood size per reproductive episode, while 'annual' and 'lifetime fecundity' should not be used interchangeably with 'transient fecundity' as they represent different life-history parameters; ( ii) provide a generalized re-definition of the concept of fecundity selection as a mechanism that encompasses any traits that influence fecundity in any direction (from high to low) and in either sex; ( iii) review the (macro)ecological basis of fecundity selection (e.g. ecological pressures that influence predictable spatial variation in fecundity); ( iv) suggest that most ecological theories of fecundity selection should be tested in organisms other than birds; ( v) argue that the longstanding fecundity selection hypothesis of female-biased sexual size dimorphism ( SSD) has gained inconsistent support, that strong fecundity selection does not necessarily drive female-biased SSD, and that this form of SSD can be driven by other selective pressures; and ( vi) discuss cases in which fecundity selection operates on males. This conceptual analysis of the theory of fecundity selection promises to help illuminate one of the central components of fitness and its contribution to adaptive evolution. [ABSTRACT FROM AUTHOR]

El dimorfismo sexual es una característica extendida en lacertilios y se relaciona con la adecuación de los individuos. En este estudio se analizó si Aspidoscelis costata costata (Squamata) presenta dimorfismo sexual al sur del Estado de México, México, especialmente en relación a su forma corporal altamente conservada dentro de la familia Teiidae. La población estudiada habita a 1500 msnm. Se utilizaron individuos adultos que fueron capturados y liberados durante estudios ecológicos. Para cada individuo se registró la longitud hocico-cloaca, el peso corporal, largo de la cabeza, ancho de la cabeza, alto de la cabeza, longitud del fémur y distancia interaxilar. Los análisis realizados indicaron que los machos fueron más grandes en longitud hocico-cloaca y peso; además presentaron dimensiones más grandes en cabeza y fémur que hembras de la misma longitud hocico-cloaca. La distancia interaxilar fue mayor en hembras que en machos de la misma longitud hocico-cloaca. El dimorfismo sexual de A. costata costata puede explicarse por selección sexual y selección por fecundidad.Sexual dimorphism is a widespread characteristic in lizards, and it has been related to the individual fitness. Sexual dimorphism was investigated in Aspidoscelis costata costata (Squamata) to the south of the State of Mexico, Mexico, relating to its body proportions highly similar within the Teiidae. Study area was located at 1500 m. Adult individuals from mark-recapture study were used. Morphometric characteristics were measured in individuals of A. costata costata: snout-vent length, mass, head length, head width, head height, femur length and interlimb length. Males were larger than females for SVL and mass. At the same body, males had longer heads and femur and shorter interlimb length than did conspecific females. The sexual dimorphism of A. costata costata can be explained by selection sexual selection and fecundity selection.

Rensch's rule describes a pattern of allometry whereby sexual size dimorphism (SSD) increases with body size when males are the larger sex, and whereby SSD decreases with body size when females are larger by intraspecific comparison. In groups of related species or sets of conspecific populations, Rensch's rule has so far largely been confirmed with females being the larger sex in small-sized populations and males the larger sex in large-sized populations. The toad-headed lizard Phrynocephalus vlangalii is a small viviparous agamid endemic to China. It exhibits either male- or female-biased SSD depending on the population, and is therefore a good model species to test Rensch's rule relative to SSD direction. Using morphological data from 38 populations across the species' range, we studied whether populations with male- and female-biased SSD both exhibit isometric relationships consistent with Rensch's rule. For populations with male-biased SSD, we reject the hypothesis that SSD consistent with Rensch's rule is driven by allometries in tail length (in this species the tail is used for visual signalling during territory defence). In populations with female-biased SSD we find significant evidence for fecundity selection favouring large females, but Rensch's rule is not supported in this species. Our findings suggest that the underlying evolutionary forces (sexual and fecundity selection) do not promote the direction of SSD consistent with Rensch's rule in both male- and female-biased SSD across populations within a species. [ABSTRACT FROM AUTHOR]

Litter size and body size of animals may coevolve under the concomitant pressures of sexual selection, fecundity selection, and viability selection. Studies have demonstrated the positive relationship between body size and litter size, consistent with Darwin's fecundity advantage of large body size. However, the counterbalancing selection hypothesis predicts fecundity selection would result in a positive correlation between litter size and body size initially, whereas the opposite pressure by viability selection would decrease litter size with further increasing body size beyond a threshold size. Moreover, the reproduction senescence hypothesis predicts that litter size of old, large females would decline with deteriorating body conditions. In this study, we tested the predictions of the counterbalancing selection hypothesis and the reproduction senescence hypothesis concerning the quadratic relationship between body size and litter size in Daurian ground squirrels ( Spermophilus dauricus ). Litter size increased initially with increasing carcass weight of females, and then decreased with further increases in carcass weight, supporting the prediction of the counterbalancing hypothesis. However, litter size was not related to body condition index of females, and body conditions improved with increasing body weight, suggesting that the reproduction senescence hypothesis alone cannot explain the observed quadratic relationship between litter size and carcass weight of female S. dauricus . [ABSTRACT FROM AUTHOR]

In insects with sex role reversal in mating, in which females actively court males, large and nutritious ejaculates are a common direct benefit to females. Such ejaculates are costly for males to produce and their size and composition can depend on male condition. However, the fitness effects to males and females of such condition-dependent provisioning are less clear. Here, we studied the effects of phenotypic condition on mating behaviour, ejaculate size and reproductive output in honeylocust beetles, Megabruchidius dorsalis . Our experimental design allowed us to disentangle the independent effects of juvenile resource acquisition in both sexes (as reflected by body size) and resource acquisition by adult males (feeding). We show that phenotypic condition of both sexes had sizeable independent and interactive effects on mating and reproductive output. In males, resources accrued during the juvenile phase had significant but relatively marginal effects on male mating and reproduction. Male adult feeding, in contrast, had sizeable effects on almost all aspects of male and female reproduction, through the nutritional effects of ejaculates in females. We discuss our findings in light of the reversal of both sex roles and sexual size dimorphism exhibited by this species, relative to related species. Our results highlight the importance of testing the interaction of male and female condition on components of fitness to understand the evolution and maintenance of mating systems. [ABSTRACT FROM AUTHOR]

Variation in sexual size dimorphism (SSD) is a widespread phenomenon and is commonly attributed to variation in sex-specific patterns of selection. According to Rensch's rule, SSD increases with increasing body size when males are the larger sex, and decreases when females are the larger sex. Using data from 17 populations of Andrew's toad ( Bufo andrewsi), we tested whether the patterns of SSD conform to Rensch's rule. Using field experiments, we also evaluated the hypothesis that sexual selection favours large male body size and that fecundity selection favours large female body size. The results revealed that the degree of SSD increased with increasing mean size in females, consistent with the inverse of Rensch's rule. Although experiments revealed evidence for a large-male mating advantage, selection for large male size was weak at best, and hence unlikely to be an important source of variation in SSD. However, fecundity selection favouring large females was evident, and likely to explain the observed inverse of Rensch's rule. After correcting male and female body size for age differences, the patterns of SSD remained the same, suggesting that the intra- and interpopulational variation in SSD is not driven by sex differences in age structure. Hence, these findings suggest that the strong fecundity selection favouring large females drives the evolution of female-biased SSD in B. andrewsi, providing an explanation for the inverse of Rensch's rule. As such, the study provides an important addition to the small body of literature that uses an intraspecific approach to demonstrate the inverse of Rensch's rule. [ABSTRACT FROM AUTHOR]

We collected 60 adult males and 85 adult females of Trapelus lessonae De Filippi, 1863 in early May 2008 from central Iranian plateau to study sexual dimorphism in morphological traits as well as female reproductive traits. Size-related sexual dimorphism occurred in all the compared body dimensions. The largest female was 83.88 mm in snout-vent length, and the largest male was 73.12 mm. As well, adult males were larger in head size and tail length, whereas females were larger in body size and abdomen length. Males and females also presented differences in color pattern, and in the presence of preanal pores. The results suggest that, in Trapelus lessonae, sexual dimorphism in size is determined by sexual selection, competition between males and fecundity selection, fecundity advantage for large female size. A pattern generated by fecundity advantages enjoyed by large females.