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4 The relationship between log (male carapace width) and log(female carapace width) for the Orbiculariae and the RTA clade. The dotted line depicts a slope of 1. Also shown are the least-squares regression lines. For major-axis regression results see text and Table 7.1.
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This chapter uses data for 489 spider species from fifteen families to describe patterns of variation in sexual size dimorphism (SSD), and to evaluate hypotheses explaining these patterns. The direction and magnitude of SSD is found to depend strongly on the size measure chosen, and the use of carapace width is recommended because it is less affect...
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... Cheng and Kuntner 2014). Furthermore, they can be explained by the action of different forces driving the variation in SSD, such as the selection of fecundity in females (Monroe et al., 2015) whereby the need for greater allocation of resources to offspring, production of the number of offspring and transport of eggs can increase female body size, influencing female-biased SSD (Liao et al. 2013), or sexual selection in males, influenced by competition between males and by ecological factors that might favour smaller males (Carchini et al. 2000, Crompton et al. 2003, Blanckenhorn 2005, Foellmer and Moya-Larano, 2007, Husak and Fox 2008, Ceballos et al. 2013. ...
Sexual conflict is believed to be an important evolutionary force driving phenotypic diversification, especially sexual dimorphism. Males of diving beetles sometimes resort to coercive tactics to increase their chances of successful reproduction, which can impose costs on females. Sexual conflict can also drive sexual size dimorphism (SSD), particularly in species where males are larger than females. In this context, Rensch’s rule states that SSD tends to increase with body size in species with male-biased SSD and decrease with body size in species with female-biased SSD. The role of sexual conflict in driving the evolution of the allometric relationships between males and females remains unclear. We addressed whether sexual conflict in diving beetles might drive SSD. We found that dytiscids do not follow Rensch’s rule, whereby the SSD is isometric in relationship to species body size. Species with adhesive pads (Dytiscinae) showed a more pronounced SSD than other diving beetle species. These results suggest that the presence of adhesive pads might reduce the force necessary to control female movement during copulation and drive the evolution of smaller males. The findings of this study provide new insights into the role of sexual conflict in driving the evolution of SSD in animals.
... Some studies have found empirical evidence for both hypotheses; for example, Blanckenhorn et al. (1995) reported that smaller body size in males correlated with indicators of higher success in scramble competition, and De Mas and Ribera (2009) found that smaller body size in males correlated with higher mortality. Other studies have not found such correlations (see, e.g., Foellmer & Moya-Laraño, 2007). At any rate, an integrative understanding of the underlying mechanisms has remained elusive. ...
Sexual size dimorphism (SSD) is caused by differences in selection pressures and life‐history trade‐offs faced by males and females. Proximate causes of SSD may involve sex‐specific mortality, energy acquisition, and energy expenditure for maintenance, reproductive tissues, and reproductive behavior. Using a quantitative, individual‐based, eco‐genetic model parameterized for North Sea plaice, we explore the importance of these mechanisms for female‐biased SSD, under which males are smaller and reach sexual maturity earlier than females (common among fish, but also arising in arthropods and mammals). We consider two mechanisms potentially serving as ultimate causes: (a) Male investments in male reproductive behavior might evolve to detract energy resources that would otherwise be available for somatic growth, and (b) diminishing returns on male reproductive investments might evolve to reduce energy acquisition. In general, both of these can bring about smaller male body sizes. We report the following findings. First, higher investments in male reproductive behavior alone cannot explain the North Sea plaice SSD. This is because such higher reproductive investments require increased energy acquisition, which would cause a delay in maturation, leading to male‐biased SSD contrary to observations. When accounting for the observed differential (lower) male mortality, maturation is postponed even further, leading to even larger males. Second, diminishing returns on male reproductive investments alone can qualitatively account for the North Sea plaice SSD, even though the quantitative match is imperfect. Third, both mechanisms can be reconciled with, and thus provide a mechanistic basis for, the previously advanced Ghiselin–Reiss hypothesis, according to which smaller males will evolve if their reproductive success is dominated by scramble competition for fertilizing females, as males would consequently invest more in reproduction than growth, potentially implying lower survival rates, and thus relaxing male–male competition. Fourth, a good quantitative fit with the North Sea plaice SSD is achieved by combining both mechanisms while accounting for sex‐specific costs males incur during their spawning season. Fifth, evolution caused by fishing is likely to have modified the North Sea plaice SSD. The paper presents a model to evolutionarily explain the sexual size dimorphism with an eco‐genetic model adapted to detailed empirical data of North Sea plaice. Behavioral investments and diminishing fitness returns are considered as alternative explanations with its implication in individual energy allocation, and put into context with the ecology of the species.
... Locomotion efficiency via bridging and climbing are also affected by body size [52,53]. Furthermore, body size can be confounded with individual condition [54] as it varies due to food Sun-protected Sun-exposed Sun exposure roundness Figure 2. Sun-exposed orb-web spider species have more elongate abdomens (lower roundness) than Sun-protected species. Each box represents a Neotropical genus of the Araneidae family (Sun-protected, N = 551 species; Sun-exposed, N = 473 species). ...
Body temperature can strongly influence fitness. Some Sun-exposed ectotherms thermoregulate by adjusting body posture according to the Sun's position. In these species, body elongation should reduce the risk of heat stress by allowing the exposure of a smaller body area to sunlight. Therefore, selection should favour more elongated bodies in Sun-exposed than in Sun-protected species. Diurnal orb-web spider species that sit on their webs are more likely to be Sun-exposed, on average, than nocturnal or diurnal shelter-building species. We measured the body elongation of orb-web spiders (Araneae, Araneidae) across 1024 species and classified them as Sun-protected or exposed based on the literature. We found that Sun-exposed species evolved more elongate bodies than Sun-protected ones. Further, we built a model combining traditional heat transfer models with models of thermoregulatory postures in orb-web spiders and meteorological data. The model indicates that body elongation in large orb-web spiders decreases the risk of high body temperatures. Overall, our results suggest that Sun exposure influenced the evolution of body shapes of orb-web spiders.
... The molting process is a folding-extension mechanism that results in a determinate increase in size from one developmental stage to another. In addition, the body proportions may also be changed, and certain sensory organs may increase in number or may appear for the first time (Foelix et al. 2011). The number of molts depends on the ultimate body size, which is a key attribute of many adult organisms because it directly affects their ability for survival, competition, fecundity, and other components of fitness (Trabalon & Blais 2012). ...
... Therefore, the number and duration of the juvenile stages determine the age at maturity (Higgins & Rankin 1996); i.e. whereas small spiders need only a few molts (about 5), to reach the adult stage, large spiders pass through about 10 molts (Bonnet 1930). For most spiders, the last molt marks the transition to sexual maturity; only in some exceptional cases adult spiders still molt further (Foelix 2011). This particular case is the female tarantulas and some araneomorph spider families that continue to grow and molt over the sexually mature stage is reached (Beccaloni 2009;Kuntner et al. 2012;Nadolny 2019), whereas most spiders stop growing after reaching the mature stage. ...
... This particular case is the female tarantulas and some araneomorph spider families that continue to grow and molt over the sexually mature stage is reached (Beccaloni 2009;Kuntner et al. 2012;Nadolny 2019), whereas most spiders stop growing after reaching the mature stage. Consequently, females can live for more than 30 years, i.e., at least 3e4 years longer than the males, which generally die shortly after mating (Costa & P erez-Miles 2002;Foelix 2011;Padilla et al. 2018). ...
Spiders are perfect model for developmental stage and growth studies because the juvenile period is broken into instars. Theraphosidae family, are largest and longest-lived spiders and females continue to grow and molt over the sexually mature stage is reached. However, their development and growth are still unknown. Thus, our objective was describe in detail the development of the juveniles of Grammostola vachoni until reaching the adulthood, estimate the growth of somatic and spermathecae dimensions, and analyze any possible allometric relation between the spermathecae and body size during the development. The mortality of individuals was 10.7% per year; and both sexes molted between once and twice by a year with a peak in the molt frequency in the third year of life. The intermolt interval was similar between sexes and tended to increase during development. The spermathecae appeared in the immature females more frequently in the 8th molt (4.03 years) and males reached adulthood frequently at the 11th molt (7.03 years). Both sexes had similar growth percentage (38%) and its was constant throughout their whole development. In contrast, the spermathecae growth percentage was 98% and always was higher than the percentage of body growth. The somatic characters did not show any differences between females and males. The spermathecae measurements showed positive allometric growth related to the body size. The results of our study completes the baseline biology information about the development, growth and its relationship, aspects mostly unknown in tarantulas.
... The difference in the number of intakes of prey or variety of food items is usually attributed to differences in size, either total size or specific structures such as chelicerae or carapace (Foellmer & Moya-Larano, 2007). Body size dimorphism may be the result of selection for many factors, such as reproductive success, hyperpredation, or dispersal capacity among others (Crawley, 2009). ...
Arachnids are the most abundant land predators. Despite the importance of their functional roles as predators and the necessity to understand their diet for conservation, the trophic ecology of many arachnid species has not been sufficiently studied. In the case of the wandering spider, Phoneutria boliviensis F. O. Pickard‐Cambridge, 1897, only field and laboratory observational studies on their diet exist. By using a DNA metabarcoding approach, we compared the prey found in the gut content of males and females from three distant Colombian populations of P. boliviensis. By DNA metabarcoding of the cytochrome c oxidase subunit I (COI), we detected and identified 234 prey items (individual captured by the spider) belonging to 96 operational taxonomic units (OTUs), as prey for this wandering predator. Our results broaden the known diet of P. boliviensis with at least 75 prey taxa not previously registered in fieldwork or laboratory experimental trials. These results suggest that P. boliviensis feeds predominantly on invertebrates (Diptera, Lepidoptera, Coleoptera, and Orthoptera) and opportunistically on small squamates. Intersex and interpopulation differences were also observed. Assuming that prey preference does not vary between populations, these differences are likely associated with a higher local prey availability. Finally, we suggest that DNA metabarcoding can be used for evaluating subtle differences in the diet of distinct populations of P. boliviensis, particularly when predation records in the field cannot be established or quantified using direct observation.
... It is well-known-almost a truism-that differences between sexes are common in nature and determine primary and secondary sex characters (Barrett and Hough 2013). Sexual dimorphism (SD) is observed in many organisms, such as humans, cervids (Geist and Bayer 2009), birds (Owens and Hartley 1998), spiders (Foellmer and Moya-Laraño 2007;Inkpen and Foellmer 2010) and other animals, while SD in plants is much less widely appreciated (Geber et al. 1999). Meanwhile, focusing attention on the sex of plant specimens and the effects of the differences between the sexes may have significant ecological and practical significance. ...
Knowledge of the impacts of sex on plant mortality and biomass production has scientific and practical importance. In the case of willows, we know relatively little about such effects. The main objective of this study was to evaluate whether the sex of individuals of different willow species determines their biomass and mortality. An additional goal was to determine whether the secondary sex characteristics, such as leaf traits, depend on sex. The experiment was conducted from 2011 to 2014 with 8100 plants comprising 150 willow genotypes, including 8 species, 16 interspecies hybrids, cultivars, and specimens differentiated by sex. Statistical analysis of the leaf traits revealed their relationship to sex. On average, male specimens have longer and wider leaves. They also have longer petioles. Males of the studied Salix genotypes were characterized by higher biomass and showed a greater survival rate than females but only under better site conditions; when the site conditions were poorer, males had higher mortality than females.
... Sexual size dimorphism (SSD) is a phenomenon widely analyzed in several biological groups, such as mammals (Mitani et al., 1996), birds (Fairbairn and Shine, 1993), amphibians (Schäuble, 2004) and invertebrates (Foellmer and Moya-Larano, 2007). Reptiles also show SSD, both at the group level (i.e., turtles, lizards, snakes) and among species and populations , mainly in body size (snout-vent length;Fitch, 1981;Shine, 1989). ...
Sexual dimorphism in lizards is determined by ecological and environmental factors. Broadly distributed species may show variation in patterns of sexual dimorphism toward either sex, as well as exhibiting variation in morphological dimensions. In the present study, sexual dimorphism in size and shape attributes was evaluated in three populations of the lizard Sceloporus variabilis from different environments in Mexico. We evaluated the size attributes of ten morphological variables: snout-vent length (SVL), tibia length (TL), femur length (FeL), forearm length (FoL), interaxial distance (ID), head length (HL), head width (HW), head height (HH), jaw length (JL), and jaw width (JW). We also evaluated the attributes of shape (relative dimensions of the ten morphological variables). In the size attribute, sexual dimorphism was found, with males being larger than females. In the case of shape, sexual dimorphism was found, with the females being larger in relative dimensions of ID and JW. Also, the males showed larger relative dimensions in TL, FeL and FoL. Differences were found between populations in the dimension of the variables analyzed in each sex. The pattern in size can be explained by sexual selection, where the males of each population maintain larger dimensions to compete for territory and access to females. In shape, females can be favored if they have larger relative ID and JW, as it promotes maintenance of clutch sizes, and use of microhabitats and different consumption of prey types than males. In the case of males, relative dimensions of TL, FeL and FoL may be functioning as important traits for escape from predators. The present study shows the importance of incorporating size and shape variables into analyses of sexual dimorphism among populations of a single species with a wide distribution. These types of studies help to identify the causes that promote sexual dimorphism, as well as the degree of difference among populations that inhabit different environments.
... This review focuses on sexual size dimorphism (SSD) in spiders, particularly female-biased extreme SSD (eSSD, i.e., female to male body length ≥2.0). Spiders exhibit the greatest eSSD among terrestrial animals, and eSSD females may be 3-10 times larger than males (33,43,90,117,143); Nephila constricta females reach 11.44 times the size of males (86), and Arachnura logio females reach 14.8 (60). However, SSD itself is probably not selected as such (except in particular, rare situations), but is rather the gendered outcome of natural or sexual selection acting on each sex. ...
... Prior to the 1990s, spider SSD research used verbal arguments or evolutionary models that did not account for phylogeny. While some literature emphasizes small male size (30,33,43,107,110,143), multiple pathways involving change in either sex could lead to an eSSD ratio. Male dwarfism, female gigantism, and combinations of these are all feasible (Figure 1a). ...
... Fecundity selection (Figure 2) usually explains large female size in arthropods in general (14), and spiders in particular (43,54,118). In spiders, female body size and fecundity are tightly linked as in Stegodyphus (101) and in Trichonephila (58). ...
Sexual size dimorphism is one of the most striking animal traits, and among terrestrial animals, it is most extreme in certain spider lineages. The most extreme sexual size dimorphism (eSSD) is female biased. eSSD itself is probably an epiphenomenon of gendered evolutionary drivers whose strengths and directions are diverse. We demonstrate that eSSD spider clades are aberrant by sampling randomly across all spiders to establish overall averages for female (6.9 mm) and male (5.6 mm) size. At least 16 spider eSSD clades exist. We explore why the literature does not converge on an overall explanation for eSSD and propose an equilibrium model featuring clade- and context-specific drivers of gender size variation. eSSD affects other traits such as sexual cannibalism, genital damage, emasculation, and monogyny with terminal investment. Coevolution with these extreme sexual phenotypes is termed eSSD mating syndrome. Finally, as costs of female gigantism increase with size, eSSD may represent an evolutionary dead end.
Expected final online publication date for the Annual Review of Entomology, Volume 65 is January 7, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... Spiders (Araneae) are unusual among terrestrial animals in that some species exhibit extreme sexual size dimorphism, with females being orders of magnitude more massive and several times longer than conspecific males . This phenomenon has been explored in a phylogenetic context within orb-weavers, with the family Araneidae Clerck, 1757, having some of the most exceptional examples (Abouheif and Fairbairn 1997;Hormiga et al. 2000;Foellmer and Fairbairn 2005;Foellmer and Moya-Laraño 2007;McLean et al. 2018). The family Araneidae is one of the most speciose groups of spiders, with over 3100 described species in 176 genera (World Spider Catalog 2019). ...
... Carapace width is often used as a proxy for spider size given its correlation with mass (Prenter et al. 1995;LeGrand and Morse 2000;Foellmer and Moya-Laraño 2007;Foelix 2010;Cheng and Kuntner 2015). The carapace is the dorsal sclerite of the prosoma (Fig. 1), and is generally more sclerotized and conserved in shape than the abdomen in most spiders, though some spectacular exceptions exist, such as the male cephalic modifications of some members of Linyphiidae Blackwall, 1859 (Hormiga et al. 2000). ...
... Carapace width also tends to be more conservative than other single measures-such as total length-used for shape and size, which are less reliable predictors of dimorphic differences Cheng and Kuntner 2015). Despite this, intersexual differences between male and female carapaces are less explored than measures related to the abdomen and legs (Eberhard et al. 1998;Uhl et al. 2004;Foellmer and Moya-Laraño 2007;Fernández-Montraveta and Marugán-Lobón 2017), which are often invoked in discussions concerning the evolution of dimorphism (e.g., Moya-Laraño et al. 2002;Brandt and Andrade 2007a, b;Moya-Laraño et al. 2007). Given its relationship with size, carapace measures are appropriate metrics for investigations of Rensch's Rule, which states that sexual size dimorphism increases in a lineage when males are the larger sex and decreases when females are the larger sex (Rensch 1950). ...
Sexual size dimorphism in orb-weaving spiders is a relatively well-studied phenomenon, and numerous works have documented evolutionary variation in interspecific size and degree of dimorphism. To date, these studies have been largely limited to assessing the evolution of a single or few linear measurements correlated with body size. While the descriptive and comparative literature is rich with qualitative and linear comparisons that distinguish the sexes and characterize species, the extent to which interspecific or dimorphic variation in size correlates with morphological shape remains relatively unexplored. The carapace of spiders is generally conserved in shape, especially among members of the same family, but is neither well-characterized as a potential facet of spider sexual dimorphism nor as a variable structure overall. Here, we use geometric morphometric techniques to quantify differences in carapace shape among members of the family Araneidae and test for allometric influences on interspecific and dimorphic shape differences across orb-weavers. We show that females and males differ in shape, occupying overlapping but distinct areas of morphospace, with males having more piriform carapaces than females. Araneid spider subfamilies overlap substantially in morphospace, though interspecific differences in shape are generally greater than those distinguishing males and females of a species. Furthermore, we show that female carapace shape shows phylogenetic signal and is more conserved than is male shape. Carapace shape differences made evident from canonical variates analysis are congruent with the more mobile lifestyle adopted by males, as a broader carapace may support more robust leg musculature.
... The family therefore figures prominently in popular works (e.g. McCook, 1889;Nielsen, 1932;Kaston, 1948;Bristowe, 1958;Brunet, 1994;Forster and Forster, 1999;Bradley, 2012;Brunetta and Craig, 2012) and its species have been the target of considerable research on sexual size dimorphism (SSD) (Elgar et al., 1990;Elgar, 1991;Hormiga et al., 2000;Foellmer and Moya-Laraño, 2007;Cheng and Kuntner, 2014), behaviour (e.g. Herberstein et al., 2000;Hesselberg, 2015;Xavier et al., 2017), ecology (Turnbull, 1973), material science (e.g. ...
