Sexual size dimorphism (SSD) is a widespread phenomenon that can profoundly affect the evolution of reproductive strategies and life history (Andersson 1994; Hartcourt et al. 1995a; Fairbairn 1997). Many researchers have debated the underlying causes for the evolution of SSD and the resulting consequences for individual fitness (e.g., Darwin 1871; Shine 1989; Kraushaar and Blanckenhorn 2002; Cox et al. 2003). Because most morphological characters increase with size, species that exhibit extreme SSD may face a functional conflict because of the different sizes of the reproductive organs of males and females. Consequently, one would predict co-evolution of male and female genitalia size as the sexes diverge in size to ensure successful coupling of genitalia. While some studies have examined the scaling of reproductive characters within females or males of the same species (e.g., Hartcourt et al. 1995a; Eberhard et al. 1998; Schmitz et al. 2000; Uhl and Vollrath 2000; Berstein and Berstein 2002), we are aware of no studies that have examined the interspecific scaling of both female and male genitalia across different species. Here, we examine the interspecific relationship of female to male genitalic size (sexual genitalic dimorphism or SGD) in relation to SSD using spiders as a model system. The models we test can be applied to many other animal groups, therefore, this study has broad implications for understanding the evolution of SSD and life-history characteristics.

Three possible evolutionary models exist to explain how SSD and SGD are related among species. The “constant genitalic dimorphism” model posits that SGD remains constant as SSD increases. In this case, natural selection favors a constant ratio of female to male genitalic size as body sizes diverge. If so, as females become larger than males (SSD increases), male genitalic size will increase at a much faster rate than for females (higher slope). This model predicts a positive matching (slope of 1) between the absolute sizes of genitalia of males and females across species across species diverging in SSD. The “positive genitalic divergence” model posits that SGD increases with SSD. As females become progressively larger than males (SSD increase), female genitalic size increases relatively more than in males (SGD increase). A prediction of this model is a mismatch (lack of correlation) between the absolute sizes of the genitalia of males and females across species differing greatly in SSD. Further, if male and female genitalia scale isometrically (i.e., proportionally the same, slope =1), or even hypoallometrically (i.e., slope <1), then “positive genitalic divergence” will occur simply because the size of one sex (e.g., females) changes faster than the size of the other (e.g., males). The third, an unlikely possibility, is “negative genitalic divergence”: SGD decreases evolutionarily as SSD increases. This model predicts a negative relationship between the absolute sizes of the genitalia of males and females across species differing greatly in SSD. Genitalic size would scale negatively or hypoallometrically with female size but positively in males, however to our knowledge such a phenomenon is yet unknown. We tested these three models by examining SSD and SGD in a clade of 16 spider species.

Web-building spiders are an excellent model system to investigate SSD because the range from monomorphism to dimorphism is dramatic (e.g., Hormiga et al. 2000; Fig. 1). Phylogenetic analysis implies that extreme SSD has arisen independently in various instances through several different evolutionary pathways, including female size increase and male size decrease (Hormiga et al. 2000). Large size may favor females by enhancing fecundity (Head 1995; Coddington et al. 1997; Prenter et al. 1998; Hormiga et al. 2000), and small size may favor males by enhancing their ability to climb while searching for sedentary females (Moya-Laraño et al. 2002), or by accelerating maturation (Ghiselin 1974; Vollrath and Parker 1992). Selection for effective functional coupling should be especially strong in animals whose genitalia are sclerotized and hard, like spiders, because their copulatory organs cannot mold to each other during copulation. Spider sex organs are complex (Foelix 1996) and precisely fit and orient to each other during copulation (Kraus 1984). Long and sometimes convoluted female uptake ducts precede the paired sperm storage sacs, and the male embolus (tip of the of the sperm-transferring blind duct) must fit snugly to transfer sperm efficiently (Foelix 1996).

We examined aspects of genitalic morphology for 16 araneoid spider species to test the three above models for how genitalia size changes for males and females in relation to SSD. We employed three primary analyses: (1) we tested whether SSD and SGD showed a general pattern of increase with one another and if this relationship was different between monomorphic and dimorphic species; (2) we tested how the absolute value of male and female body size and genitalia size relate between the sexes across species; (3) we examined the scaling of genitalic morphology within males and within females. Finally, we conduct two other analyses to understand (4) generally the evolution of genitalic shape (as opposed to size) and (5) also to understand how genitalic morphology differs between dimorphic and monomorphic spiders.

Materials and Methods

Results

Discussion

Based on a small but broad sample of araneoid spider species varying greatly in SSD, we have shown several striking trends. (1) SGD showed a general pattern of increase with SSD among species, although these two variables do not change at the same rate (Figs. 4, 5, 7). (2) Male and female body size, as well as male and female genitalia size, are decoupled evolutionarily (Fig. 6, 7, Table 3). This latter finding is particularly striking: In other words, the absolute size of female genitalia for a given species is not a significant predictor of the size of male genitalia for that same species. (3) Within males and within females, genitalic morphology scales negatively; three of the four genitalic variables (male pedipalp volume, female spermathecal volume, and female epigastric furrow length) scale substantially lower than expected isometric slopes (Table 2). (4) For genitalic shape (i.e., size-adjusted analysis), we found that female genitalic characteristics are evolutionarily decoupled from one another, but show an evolutionary correlation with male genitalic volume (Table 3). (5) Both female spermatheca volume and female epigynal area were generally larger in dimorphic compared to monomorphic species (Figs. 5, 7, Table 1).

Our findings support the “positive genitalic divergence” model which posits that as females and males diverge in size, they will also diverge in the absolute dimensions of their genitalia, resulting in a “mismatch” of male and female genitalia size. Thus, our results suggest that extreme size divergence in genitalia could impair effective mating, especially in species with hard or sclerotized genitalia. The “positive genitalic divergence” model generally implies that SGD can constrain SSD in animal species (see Fig. 7).

In our sample, female and male body size appear to be decoupled in evolutionary time. We suggest that the lack of co-evolution of male and female size is a consequence of our broad macroevolutionary comparisons of spider species varying dramatically in SSD, in contrast to previous studies (e.g., Prenter et al. 1998) which show co-evolution between male and female body size among spider species that diverge less in SSD. By contrast, there is little correspondence between male and female body size within highly dimorphic species (e.g., Head 1995). The nonsignificant correlation between the absolute sizes and male and female genitalia also suggests a general evolutionary mismatch between male and female spiders varying greatly in SSD, thus supporting the “positive genitalic divergence” model.

Some key size components of the female genitalic structures appear to be largely decoupled in evolutionary time. Although relative epigynum area and epigastric furrow length were correlated with each other, neither variable was correlated with spermathecal volume. Epigynal area and epigastric furrow length (external female genitalia), which may be characteristics related to genital coupling or stimulation, respond to different selective pressures than spermatheca volume (internal female genitalia), which strictly concerns sperm storage capacity. As female spiders appear efficient in the activation and use of sperm for fertilization (Berendonck and Greven 2002) and may not be sperm limited (Snow and Andrade 2005), spermathecal volumes need not be large. Selection might act to limit the size of organs such that they show a functional fit. Large female epigyna relative to small female spermathecae may reflect stimulatory interactions between males and females during copulation. In contrast, male genitalic shape (relative pedipalp volume) tracks both female epigynum area/epigastric furrow length and female spermatheca volume. Large male pedipalps are probably more efficient at engaging and/or interacting with large female epigyna, and filling large female spermathecae.

Rapid and divergent evolution of genitalia is widespread among animals but its causes remain poorly understood (Eberhard 1985; Arnqvist 1998; Arnqvist and Danielsson 1999; Hosken and Stockley 2004). The “lock-and-key” hypothesis is controversial (Eberhard 1985; Ware and Opell 1989), particularly for explaining interspecific evolutionary relationships, but may still hold value for understanding trends within species. Leading alternative hypotheses include-cryptic female choice (Eberhard 1985, 1996) and sexual conflict (for a recent review, see Hosken and Stockley 2004). In spiders, cryptic female choice might be particularly important. Copulation is complex both behaviorally and mechanically, and release of sperm is not necessarily linear in relation to copulatory duration (Bukowski and Christenson 1997; Snow and Andrade 2005). Copulation prior to insemination can be lengthy (van Helsdingen 1965; Elgar 1998) and stimulation during copulation influences uptake of sperm (Bukowski and Christenson 1997). We agree with Eberhard (2004) that sexual conflict related to natural selection and female fecundity should be rare in spiders, other things being equal, because the large size and cannibalistic tendencies of the female should overrule the effects of male manipulation. We note that the large size of females would not necessarily preclude chemical manipulations by males, however. The species most prone to sexual conflict would be those in which the female has a much larger genitalic volume than a single male can fill. Females would therefore benefit from multiple copulations, whereas the male would always try to monopolize the female. However, according to our data, these are the species with greatest SSD and in which the size advantages of the female are most pronounced.

Intraspecific genitalic size generally scales with negative allometry (also termed hypoallometry) in insects and spiders (e.g., Cohn 1990; Eberhard et al. 1998; Schmitz et al. 2000, but see Pérez-Miles 1988; Green 1999). We found the same pattern interspecifically (Table 2), implying a lower evolutionary size increase in genitalic dimensions than isometry would predict. Considering the high variability observed in the genitalic structure in both males and females, even in closely related species (Eberhard 1985; Pitnick et al. 1999; Hosken and Stockley 2004), it is remarkable to detect decreased variability in genitalic size. The most obvious hypothesis to explain this hypoallometry is that isometry would result in functional inefficiencies during mating. If true, stabilizing selection may even level or equate genitalia size within a lineage.

Despite this hypoallometry within each sex, it is intriguing that SGD nevertheless changes so dramatically across species (Figs. 4, 6), and that male and female genitalia are partly decoupled. We suggest the underlying reason for these patterns is that female spiders more in body size than males on a broad macroevolutionary scale. For example, in the sample of 536 species from Hormiga et al. (2000), female body length varied from 1.6 to 45 mm (28×), whereas males ranged from 1.4 to 17.5 mm (13×). Our smaller (and less random) sample shows about 25% greater variation in female size compared to males. Spiders are an exception to Rensch's rule (i.e., SSD increases with body size in species in which males are larger, and decreases with body size in species in which females are larger) as female body size diverges more over evolutionary time than male body size, and SSD increases with female body size (Head 1995; Prenter et al. 1995, 1997,1998; but see Prenter et al. 1999). This often occurs in species in which the female is the larger sex (Fairbairn 1997). All genitalic variables increase significantly with size (Table 3), and as female size varies much more than in males, female genitalic size also outpaces that of males, but not as much as isometry would predict. Evolutionary forces that stabilize genitalic size may balance sex-specific size-determining factors, such as increased fecundity or enhanced mate-searching ability (e.g., Vollrath 1980; Head 1995; Prenter et al. 1998; Moya-Laraño et al. 2002). Although we did not observe any evidence for the constant genitalic ratio model, such a ratio might apply to the components of male and female genitalia that interact during mating, thus scaling differently than gross genitalic volume. Individual species could be observed during courtship and copulation to identify the appropriate genitalic structures.

The dramatic shift in SGD with SSD increase may influence sperm competition in all animals where SSD is extreme. Previous work on sperm competition has focused on the size of male testes in relation to sperm competition, or the likelihood that females will repeatedly mate (Short 1979; Birkhead and Møller 1992; Hartcourt et al. 1995b; Stockley et al. 1997). In male spiders, the gonads (abdominal) are disconnected from the two copulatory organs or pedipalps (modified appendages). Sperm must be transferred to the exterior of the body and then picked up with the pedipalps for successful insemination. Thus, the amount of sperm a pedipalp contains at any given time is limited. If one assumes that sperm storage capacity correlates with male pedipalpal volume, then the relatively large pedipalpal volumes in monomorphic species may signal stronger sperm competition. This assumption might not be entirely valid, because in some species there is not a 1:1 correlation between pedipalp volume and male sperm storage capacity. Several authors have suggested that there may be preconditions which intensify sperm competition in spiders (Austad 1984; Elgar 1998), such as polyandry, and time delays between copulation and fertilization. Our data suggest that the relative volumes of male and female reproductive organs may also influence sperm competition. In monomorphic species, one male may fill a female's spermathecae, whereas in dimorphic species one male would be incapable of filling her spermathecae during a single mating. If the positive genitalic divergence model is correct, females of dimorphic species in general should mate with more males than females of monomorphic species. This prediction suggests that evolutionary changes in SGD as a consequence of SSD may help to explain variation in animal social and mating systems.

Finally, evolutionary changes in the relationship between SGD and SSD clearly has functional consequences. The spider Tidarren sisyphoides is highly size dimorphic (the male is 1% of the total female body weight) and male palps are large enough (about 10% of their total body weight) that they substantially hinder locomotion (Ramos et al. 2004). Such large pedipalps may be maintained because they ensure a fit to the much larger female copulatory apparatus (Vollrath 1998). If so, these spiders appear to have resolved the conflict by self-amputating one of their two overly large pedipalps, and thus considerably enhancing both maximum speed and endurance (Ramos et al. 2004). In sum, different and even opposing scaling trends in sexual genitalic and size dimorphism could have profound consequences for reproduction and life history of spiders, and perhaps other taxa as well. Further research that examines whether genitalic size changes in a similar manner for males and females in other taxa with high SSD might shed light on the generality of the positive genitalic divergence model. Finally, we note that further comparisons of both the body sizes and genitalia sizes of spider species would also be informative for testing Rensch's rule (Fairbairn 1997), which we were unable to consider because of our relatively small sample of spiders.