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Revista de la Sociedad Entomológica Argentina

Print version ISSN 0373-5680On-line version ISSN 1851-7471

Rev. Soc. Entomol. Argent. vol.75 no.1-2 La Plata June 2016



Breaking the rule: multiple patterns of scaling of sexual size dimorphism with body size in orthopteroid insects

Quebrando la regla: multiples patrones alométricos de dimorfismo sexual de tamaño en insectos ortopteroides


Bidau, Claudio J. 1, Alberto Taffarel2,3 & Elio R. Castillo2,3

1 Paraná y Los Claveles, 3304 Garupá, Misiones, Argentina. E-mail:
2,3 Laboratorio de Genética Evolutiva. Instituto de Biología Subtropical (IBS) CONICET-Universidad Nacional de Misiones. Félix de Azara 1552, Piso 6°. CP3300. Posadas, Misiones Argentina.
2,3 Comité Ejecutivo de Desarrollo e Innovación Tecnológica (CEDIT) Felix de Azara 1890, Piso 5º, Posadas, Misiones 3300, Argentina.

Recibido: 14-I-2016
Aceptado: 17-III-2016


RESUMEN. El dimorfismo sexual de tamaño (SSD por sus siglas en inglés) es un fenómeno ampliamente distribuido en los animales y sin embargo, enigmático en cuanto a sus causas últimas y próximas y a las relaciones alométricas entre el SSD y el tamaño corporal (regla de Rensch). Analizamos el SSD a niveles intra- e interespecíficos en un número de especies y géneros representativos de los órdenes ortopteroides mayores: Orthoptera, Phasmatodea, Mantodea, Blattodea, Dermaptera, Isoptera, y Mantophasmatodea. La vasta mayoría de las especies mostraron SSD sesgado hacia las hembras, pero numerosas excepciones ocurren en cucarachas y dermápteros. La regla de Rensch y su inversa no constituyeron patrones comunes, tanto a nivel intraespecífico como interespecífico, con la mayoría de las especies y géneros mostrando una relación isométrica entre los tamaños de macho y hembra. En algunos casos, los patrones alométricos hallados podrían relacionarse con la variación geográfica del tamaño corporal. También demostramos que no todos los estimadores de tamaño corporal producen el mismo grado de SSD y que el dimorfismo puede estar influenciado por un gran número de condiciones de vida y patrones de desarrollo ninfal. Finalmente, discutimos nuestros resultados en relación a modelos actuales de la evolución del dimorfismo sexual de tamaño en animales.

PALABRAS CLAVE: Tamaño corporal; Blattodea; Dermaptera; Mantodea; Mantophasmatodea; Caracteres morfométricos; Orthoptera; Phasmatodea; Regla de Rensch; Alometría

ABSTRACT. Sexual size dimorphism (SSD) although a widespread phenomenon among animals, is both enigmatic as to its proximate and ultimate causes and the scaling relationships between SSD and body size (Rensch's rule). We analyzed SSD at the intra- and interspecific levels in a number of representative species and genera of the major orthopteroid orders: Orthoptera, Phasmatodea, Mantodea, Blattodea, Dermaptera, Isoptera, and Mantophasmatodea. The vast majority of the species showed female biased SSD but numerous exceptions occur in cockroaches and earwigs. Rensch's rule and its converse are not common patterns at both, intra-and cross-species level, most species and genera showing an isometric relationship between male and female body sizes. In some but not all cases, the demonstrated allometric patterns could be related to geographic body size variation. We also showed that not all body size estimators produce the same degree of SSD and that dimorphism can be strongly influenced by a number of living conditions and the patterns of nymphal development. Finally, we discuss our results in relation to current models of the evolution of sexual size dimorphism in animals.

KEY WORDS: Body size; Blattodea; Dermaptera; Mantodea; Mantophasmatodea; Morphometric traits; Orthoptera; Phasmatodea; Rensch's rule; Scaling



The length range of living systems is astonishing: it spans 17 orders of magnitude from DNA molecules to ecosystems; while organisms vary 7 orders of magnitude in length and 21 in mass (Ellers, 2001). Insects have an impressive body size range, from less than 0.2 mm in the parasitic wasp Dicopomorpha echmepterygis (Mymaridae) to ca. 360 mm in the stick-insect Phobaeticus chani (Phasmatidae). Body mass varies accordingly with females of the giant weta, Deinacrida heteracantha (Anostostomatidae) weighing more than 70 g (Björkman et al., 2009). The enormous amount of scientific literature relative to animal body size reflects the importance of this trait in biology. Almost every life history and ecological characteristic of animals is correlated with body size (LaBarbera, 1986, 1989; Calder, 1996; Smith & Lyons, 2013) and in turn body size is strongly affected by most ambient abiotic and biotic factors (Gaston, 1991; Chown & Gaston, 2010, 2013; Price et al., 2011). Thus, most physical, physiological, ecological, and evolutionary processes are highly dependent on size; these relationships are called scale effects or scaling. As defined by Barenblatt (2003), scaling "…describes a seemingly very simple situation: the existence of a power-law relationship between certain variables y and x, y = Axα, where A, αare constants." This so-called allometric equation is usually expressed in logarithmic form as log y = log A + αlog x. The concept of allometric scaling was initially developed by Otto Snell (1892), D'Arcy Wentworth Thompson (1917), and Julian Huxley (1932) and resulted in numerous theoretical and empirical investigations of the scaling laws regulating the allometric relationship of many organismic traits with body size (e.g. Schmidt-Nielsen, 1975, 1984; Brown & West, 2005; Hoppeler & Weibel, 2005).

Differences in body size between sexes (sexual size dimorphism, SSD) are pervasive in the animal kingdom and thus, a fundamental component of body size variation (e. g. Darwin, 1871; Andersson, 1994; Fairbairn, 2013). SSD is a controversial aspect of evolutionary biology for several reasons. On one side, although sexual selection has traditionally been assumed as the key process behind SSD, it is now well known that natural selection can also produce size differences between males and females and that both processes are not completely independent from one another (e.g. Isaac, 2005; Carranza, 2009). This problem includes the study of the adaptive significance of SSD, the genetic constraints to its evolution, and its proximate and ultimate causes (Fairbairn, 1997, 2007). Secondly, a problem which has not received a satisfactory explanation is that of the allometric scaling of SSD with body size. Bernhard Rensch (1950, 1960) proposed that in phylogenetically related species, SSD increases with general body size when males are larger than females and decreases when females are larger. This pattern was termed Rensch´s rule by Abouheif & Fairbairn (1997) but despite numerous studies in very diverse taxa (Fairbairn et al., 2007) there is little evidence to support this rule and no convincing mechanism for its operation has been proposed (Reiss, 1989; Webb & Freckleton, 2007; Bidau &Martí, 2008a; Martínez et al., 2014).

Further problems regarding the scaling of SSD with body size remain. In the first place, there is the question of the taxonomic level at which it is studied, and if Rensch's rule operates (if it does) in any taxonomic entity. Most studies of the scaling of SSD with body size either phylogenetically-based or not have been performed across species at different levels (Fairbairn et al., 2007), and only a few intraspecifically as for example, in insects, some grasshoppers and beetles (e.g. Bidau & Martí, 2008b; Stillwell & Fox, 2009; Blanckenhorn et al., 2007a,b). An additional problem is that of the appropriate measurements for analyzing SSD (Fairbairn, 2007). Is it the same using body mass or body length, or some other measurement (e.g. pronotum width, wing length) as a proxy for body size? Are SSDs for different measurements significantly correlated? (Martínez et al., 2014).

Orthopteroids do not only vary greatly in size (Nasrecki, 2004; Bell et al., 2007; Whitman, 2008; Brock & Hasenpusch, 2009) but also in the magnitude of SSD and in body shape (Hochkirch & Gröning, 2008; Bidau et al., 2013; Bidau, 2014). Furthermore, many species are fairly common, easy to collect, and have large geographic distributions that allow the sampling of several populations inhabiting different or even contrasting environments (Bidau et al., 2012). The latter is relevant because it has been suggested that in species showing intraspecific geographic variation in body size (e.g. Bergmann's rule [Bergmann, 1847]) there may exist a link between these patterns and the scaling of SSD with body size (Blanckenhorn et al., 2006). In this sense orthopterans are an excellent model for the comparative analysis of SSD and although a few studies have been performed (Bidau & Martí, 2008b), virtually nothing is known about patterns of SSD at the intraspecific level regarding the points mentioned in this Introduction.

The aim of this investigation is to analyze the magnitude of SSD, its scaling with body size, the comparison between different estimates of SSD, and the geographic variation of SSD in several species of Orthoptera belonging to the suborders Caelifera and Ensifera, as well as species of Mantodea, Phasmatodea, Blattodea, Isoptera and Dermaptera, using new data as well as published information.


1. Data collection

For the purposes of this paper we collected information from the published scientific literature on geographic variation of body measurements of several orthopteroid species from most of the orders usually included in the orthopteroid assemblage (Tables 1-9). We collected data for two purposes:

Table 1. Orthopteroid species for which scaling of sexual size dimorphism (SSD) was analyzed in this paper. In the column corresponding to Rensch's rule, values in parentheses are the slopes of RMA regressions (see text). All measurements correspond to adult individuals. In the case of the two Isoptera species, measurements correspond to alates.

Table 2. Mean índices of sexual size dimorphism (SSD= female size/male size) of different traits for all taxa shown in Table 1. References as in Table 1 except *.

Table 3. Male and female coefficients of variation (CV= s/*100) for morphometric traits used in the calculation of RMA regression slopes in species that follow Rensch's rule, its converse, or show isometric scaling. M= male; F= female; ∆CV= male CV - female CV. References as in Table 1.

Table 4. Scaling of sexual size dimorphism for several traits of seven species of melanopline grass-hoppers. References as in Table 1.

Table 5. A. Allometric scaling of 5 morphometric traits with body length in four species of melanopline grasshoppers. B. Correlations between SSD for body length and SSD for six linear traits, and the respective paired t-tests in the same species.

Table 6. Effects on sexual size dimorphism (SSD= female size/male size) of different living and rearing conditions in twelve species of orthopteroid insects.

Table 7. Sexual size dimorphism (SSD) of different traits in seven orthopteran species. SSD calculated as female size/male size is shown for each instar. Final (adult) SSD is indicated in bold type.

Table 8. Scaling of sexual size dimorphism with body size in assorted families and subfamilies of orthopteran insects. In columns 5, 6 and 7 mean male and female body lengths (BL) and average SSD are given. Values in parentheses represent the respective ranges. In the last column, the slope (β) of the RMA regression of log10 (male BL) on log10 (femaleBL) and the 95% confidence intervals (in parentheses) are shown. In bold type, taxa that follow Rensch's rule or its converse. The extensive bibliography consulted for the construction of this Table is readily available from the corresponding author.

Table 9. Scaling of sexual size dimorphism with body size in assorted genera of orthopteroid insects. In columns 5, 6 and 7 mean male and female body lengths (BL) and average SSD are given. Values in parentheses represent the respective ranges. In the last column, the slope (β) of the RMA regression of log10 (male BL) on log10 (femaleBL) and the 95% confidence intervals (in parentheses) are shown. In bold type, genera that follow Rensch's rule or its converse. The extensive bibliography consulted for the construction of this Table is readily available from the corresponding author.

a. Intraspecific analyses. The criteria used for including species were that at least 5 geographically separated populations were studied, and that data on body size (either body length, body mass or some adequate morphometric proxy) for both sexes were available for each population or sample. Also, we included unpublished data on three South American melanoplines (Orthoptera: Acrididae: Melanoplinae), Dichroplus fuscus (Thunberg) (17 populations, 193/133), Ronderosia bergii (Stål) (19 populations, 152/108) and Scotussa cliens (Stål) (6 populations, 56/58) (Table 1). Most studies of geographic variation of body size of orthopteroids are based on different linear measurements. However, different authors use different estimators of body size. For example, body length and length of hind femur are commonly used measurements but in some groups (e.g. Gryllidae and Proscopiidae) researchers tend to favor measurements of the head and the pronotum as proxies for body size. Body mass measurements are extremely rare in these insect groups thus, few cases of body mass SSD were included in this study. Some studies included only one measurement of body size while others, reported variation in male and female size of up to 10-plus linear characters. The latter were especially favorable because allowed the comparison of degrees of SSD and Rensch's rule in different traits. Data were obtained from published tables and, in a few cases, extrapolated from graphs provided in the publication. The study concentrates in species where data on a sufficient number of populations were available for statistical analyses, although some cases included only a few populations and this is indicated in the text. Whenever the information was available, geographic data of each population (latitude, longitude and elevation) were recorded. The raw data in all analyses were male and female population means for each trait. These values were log-transformed for the purpose of statistics. Mean body length values for each species as shown in Tables 1, 6, 8, and 9 were obtained from the published literature usually averaging data from several populations. In the case of those species reported here for the first time, six linear measurements were obtained: body length, hind femur length, hind tibia length, length of tegmina, pronotum length and pronotum height.

b. Cross-species (interspecific analysis): In order to put each of the intraspecific analyses within the context of the higher-order taxa to which the species belong regarding body size and sexual size dimorphism, we collected data on body length of a large number of species of Orthoptera, Phasmatodea, Mantodea, Blattodea, and Dermaptera to produce cross-species analyses of the scaling of SSD with body size, and graphs representing the variation of SSD within each higher-order taxon (Tables 8, 9; Figs. 1, 2). A graph for the Orthoptera was not included because a comprehensive one has been recently published (Hochkirch & Gröning, 2008). Only data on male and female body length of each species were considered, and only length measurements from tip of head to tip of abdomen were included. Measurements were obtained from primary and secondary published sources on the basis of availability and the meeting of our standard criterium for length measurement.

Fig. 1. Distribution of sexual size dimorphism for body size (female body length/male body length) in a. Phasmatodea. b. Mantodea. Arrows mark mean values.

Fig. 2. Distribution of sexual size dimorphism for body size (female body length/male body length) in a. Blattodea. b. Dermaptera. In b. White columns represent the distribution of sexual dimorphism for forceps length. Arrows mark mean values.

2. The analysis of sexual size dimorphism and testing of Rensch's rule

Because SSD is practically always female-sex biased in the studied species as is the rule in most orthopteroids, we used the simplest SSD estimator which is the ratio of the arithmetic means of female size and male size that produces SSD indices higher than 1.0. Three exceptions occurred within our sample: length of pronotum and wing length SSD are male-biased in the mole-cricket Scapteriscus borelli and the katydid Metrioptera roeselii (it could be possible that, in the last case, sampling bias is the cause of the observation), respectively, and head width in the cricket Velarifictorus micado (Table 2). The scaling of SSD with body size was analyzed using a Model II regression method: Reduced Major Axis (RMA) regression; ordinary least-squares (OLS) regression is inadequate for this type of analysis. The use of RMA regression of log10 (male size) on log10 (female size) is also justified because RMA is symmetric which means that a single regression line defines the bivariate relationship independently of which variable is X and which is Y, and this is the case for SSD comparisons: Rensch's rule is supported when the slope βRMA is significantly > 1.0, while slopes < 1.0 signal its reversion. Slopes not significantly different from 1.0 indicate sexual isometry. We run the regressions using the software of Bohonak & van der Linde (2004). One-delete Jacknife estimates of a, β and r2 were obtained and 95% confidence intervals were calculated by bootstrapping 10000 times over cases. The RMA slope is significantly different from 1.0 when the former value is not included within the calculated 95% confidence intervals. In a few cases although the 1.0 values were included in the CI but so close to one of the limits, the difference of the slope was considered significant. RMA regression was also employed to investigate allometries between body parts. Simultaneous autoregressions (SARs) between body size and SSD variables (different SSD indexes using various body size estima tors), and geographic coordinates and elevation, were performed in SAM v.4.0 (Rangel et al., 2010). Taxonomy follows Beccaloni (2015), Brock (2015), Eades et al. (2015), Hopkins et al. (2015), and Otte et al. (2015). Final retrieval dates are indicated.

3. A test of the differential variability hypothesis for Rensch's rule

An expected condition for the operation of Rensch's rule is that males are more variable in body size than females. If this condition holds, we expect that in those species showing the converse Rensch's rule, females should display the highest variability while in those cases where the relationship between male and female body size is isometric, both sexes should be equally variable. In order to test this hypothesis, we calculated male and female coefficients of variation (CV = s/*100 where s= standard deviation, and = arithmetic mean) of body size to produce a measure of the differential intersex variability (∆CD).

4. SSD and Rensch's rule in different traits

We tested Rensch's rule for different linear morphometric characters using the above described methodology in 7 species in order to test if differences of SSD for each character affected the scaling of sexual dimorphism with body size. Also, a large scale comparison was performed in our Dermaptera sample to high-light a higher-order pattern of sexual dimorphism divergence comparing body length and forceps length sexual dimorphism.

5. Intraspecific allometric scaling

Because differences in static allometry within species are important factors in determining SSD when different traits are used, we explored these allometric patterns within four melanopline grasshopper species using Ordinary Least Squares (OLS) regressions and paired-samples t-test comparisons.

6.  Testing the effects of different rearing and ecological conditions on SSD

Since it is well-known that body size is highly affected by environmental conditions (e.g. Whitman, 2008) it is only reasonable to expect that SSD will be similary affected. To test this hypothesis we obtained body size data of males and females of several orthopteroid species that were laboratory-reared in different ambient conditions or that were studied in different ecological scenarios

7. Testing the effects of ontogenetic allometry on final SSD

Assuming that differential rates of development and number of nymphal stages affect the degree of adult SSD, we performed comparisons of SSD during nymphal development of 10 species of orthopteroid insects for which accurate measurements of different traits were performed at each nymphal instar and adult stage.


1. Body size and sexual size dimorphism in the studied species

Because SSD could be influenced by body size (see below) we tried to cover the widest possible range of sizes among the study species (Table 1). Within our sample, the smallest caeliferans were Phaulacridium vittatum (Sjöstedt) (Catantopinae) and Myrmeleotettix maculates (Thunberg) (Gomphocerinae), and the smallest ensiferans, the tettigoniid Conocephalus spartinae (Fox) (Conocephalinae) and the cricket Polionemobius taprobanensis (Walker) (Nemobiinae) (Tables 1, 4). The smallest species were the termites Reticulitermes speratus Kolbe and Nasutitermes corniger (Motschulsky) (Table 1). The largest species are represented by Romalea microptera (Palisot de Beauvois) (Roma leidae) and the acridids Xanthippus corallipes (Haldeman) (Oedipodinae) and Ornithacris turbida (Walker) (Cyrtacanthacridinae) within the Orthoptera, and the Japanese mantid Tenodera angustipennis Saussure (Tables 1, 6). It must be kept in mind that the mean lengths are averages of many individuals and populations; most species and especially those with large geographic distributions, show high variability in body length.

SSD was calculated for all available measurements of each species. Most measurements are linear and in only a few cases, body mass or dry weight were available for male/female comparison (Table 2). For the Orthoptera, female/male size ratios were in general higher in caeliferans than in ensiferans as it has been previously reported (see discussion). However, interspecies variation is high even between closely related species. For example, within the genus Dichroplus, D. pratensis Bruner shows a body length SSD of 1.04-1.08 while its sister species D. vittatus Bruner and other congener, D. fuscus (Thunberg) are much more dimorphic (SSD= 1.27) (Table 2). Within the melanoplines studied here, the most dimorphic species was Podisma sapporensis Shiraki (Table 2). While the Melanoplinae show low to moderate SSD (see discussion) other caeliferans are characterized by higher levels of dimorphism as is the case of the Gomphocerinae represented in this work by several truxaline species all of them showing relatively high SSD values (Tables 1, 2, 6).

In the few cases where body mass was available for calculating SSD, values were significantly higher than those for linear measurements (Tables 2, 6). For example, the oedipodine Xanthippus corallipes has a mean SSD index of 1.44 when body length is considered, but SSD= 3.33 for body mass (Table 2) reflecting the different dimensionality of the employed measurements. However, the difference beween both indexes are not always as high: in the katydid Metrioptera roeselii (Hagenbach) BL SSD= 1.20 and BM SSD= 1.47, and in the cockroach Eupolyphaga sinensis (Walker) 1.28 and 1.76, respectively (Table 2). In one case, the cricket Velarifictorus micado (Saussure), SSD for head width and body mass showed opposite directions (Table 2 and see Discussion).

In order to illustrate the wide variation in SSD in our study organisms, we contructed Figs. 1 and 2. The bar graphs clearly show the extent and range of variation of SSD in the different polyneopteran orders discussed in this paper.

2. Scaling of SSD with body size follows different patterns

Table 1 shows the results of RMA regressions between log10 (male size) and log10 (female size) for several orthopteran species, a cock-roach, a praying mantis, and two termites. For each species, the regression slope (βRMA) and the 95% confidence intervals are shown. In all cases when it was possible, the slope for the regression of male body length on female body length is shown; in the rest of cases, the slope corresponds to the regression using the first measurement shown in Table 2. Of the 45 analyzed cases, Rensch's rule (as indicated by a slope significantly > 1.0) occurred in 12 (26.7%) thus, SSD decreases as body size increases. In eight cases (17.8%) scaling of SSD with body size followed a converse trend (βRMA<1.0) in which dimorphism increases with body size. In the rest (55.5%), male and female body sizes scaled isometrically (βRMA =1.0). Interestingly enough, all three patterns were observed in three closely related species of a single genus, Dichroplus Stål (Tables 1, 2).

A further interesting observation pertains to the scaling of SSD with body size within hybrid zones. As a whole, D. pratensis follows Rensch's rule but the rule was not verified within a hybrid zone between two chromosomal races which differ in body size and the degree of SSD (Table 1). Melanoplus sanguinipes (Fabricius) also follows Rensch's rule but the analysis of a hybrid zone between this species and M. devastator showed the converse pattern (Table 1). Finally, Pseudochorthippus parallelus parallelus (Zetterstedt) complies with Rensch's rule and the samepattern was observed among populations of a hybrid zone with the subspecies Pseudochorthippus parallelus erythropus (Faber) (Table 1).

3. SSD scaling and morphometric variability

Because it has been considered that one of the preconditions for Rensch's rule is a higher variability of body size in males with respect to females, we calculated the coefficients of variation (CV) of the body size estimators of both sexes (Table 3). In all cases where Rensch´s rule was verified, males were more variable than females. In the case of those taxa following the converse to Rensch's rule, females were more variable than males in five species. Those cases in which a higher CV was observed in males could be attributed to low sample size (Aeropedellus clavatus (Thomas) and Poecilimon luschani birandi Karabag) or to the fact that measurements were taken from populations within a hybrid zone (Melanoplus devastator Scudder). Those species where the scaling of SSD with body size was isometric did not show any consistent pattern of body size variation in males and females. Furthermore, both sexes of these taxa showed very similar levels of variability (Table 3).

4. The use of different measurements may produce different estimates of SSD and scaling patterns

The vast majority of measurements employed in this study are linear since these are the most frequently used by biologists when analyzing body size variation in orthopteroid insects. Body mass measurements are rare and for reasons that will be discussed later, probably not the best for studying SSD (however see below). We calculated SSD for all available measurements in all taxa and the results are shown in Table 2. It can be seen that, considering that all SSD estimates were calculated using the same individuals and populations, a considerable variation in SSD indices occur in many of the species. For example, although in some species different linear measurements produced practically identical SSD estimates (e. g. Melanoplus sanguinipes (Fabricius), Dichroplus pratensis), in others strikingly different indexes were obtained. One clear case is that of Podisma sapporensis where SSD ranges from a male biased 0.85 for the length of the tenth abdominal tergum, to 1.61 in the case of pronotum length while body length produced 1.31 (Table 2).

Furthermore, variation in the scaling patterns of SSD with body size also occurs when different measurements are used in regression analyses. Table 4 shows values of βRMA in seven melanopline species for which several linear measurements were available in a variable number of populations. It is evident that while some species show a remarkable consistency regarding the scaling pattern (e.g. D. fuscus, D. vittatus, D. pratensis, R. bergii, and S. cliens) others do not (e.g. P. sapporensis and Neopedies brunneri (Giglio-Tos)) (Table 4).

It is worth noting that despite the fact that different linear traits may show different degrees of sexual dimorphism, SSD tends to be highly correlated although exceptions do occur. However, the distribution of SSD values of different traits are usually significantly different as demonstrated by paired t-test comparison (Table 5).

If the growth of different body organs were isometric in both sexes we would not expect differences in SSD for different linear traits. However, most structures show allometric growth and, if differential sexual allometry occurs, then unequal SSD estimates could be obtained for different body parts. To analyze this problem we studied static allometry in relation to SSD of six linear morphometric characters in 4 grasshopper species using individual (not population averages) measurements. Results are shown in Table 5. It can be seen that for most traits, males and females have different patterns of allometric growth (as shown by significant differences between the slopes of OLS regressions of log10 [trait] on log10 [body length]) and the degree of variation in SSD estimates is associated with these differences.

A more dramatic case of the disparity between sexual dimorphism for body size and specific body parts is found in earwigs. Differently from other orthopteroid orders, the Dermaptera show a large number of species with little or no SSD with respect to body length and almost equivalent numbers of species with male-biased and female-biased SSD (see Discussion and Fig. 2b). However, earwigs have conspicuous forceps-like cerci which can be extremely dimorphic in size and form. As shown in Fig. 2b sexual dimorphism of forceps length follows a completely different distribution from that of SSD for body length. Furthermore, average body length SSD is, in our sample, 1.04, and sexual dimorphism for forceps length, 0.85. Both SSDs are not significantly correlated (R2= 0.005; p= 0.235). A second selected example of this situation is that of the bark mantid genus Liturgusa Saussure, which as all Mantodea possesses highly specialized hunting forelegs (see Table 9). Mean SSD (range) for six morphometric characters (data from Svenson, 2014) were: body length, 1.29 (1.14-1.63); prothoracic femur (F1) length, 1.26 (1.12-1.55); mesothoracic femur (F2) length, 1.15 (1.04-1.27); metathoracic femur (F3) length, 1.14 (1.02-1.29); and pronotum (P) length, 1.24 (1.15-1.44). The converse Rensch's rule was verified for body length (βRMA= 0.70 [0.51-0.94]), and F1 (βRMA= 0.59 [0.49-0.81]) while the other three characters showed sexual isometry; F2 (βRMA= 0.85 [0.69-1.01]), F3 (βRMA= 0.999 [0.76-1.14]), and P (βRMA= 0.91 [0.74-1.08])

5. Geographic variation of SSD within species

Many of the species studied by us showed significant clinal variation in body size along latitudinal, elevational and/or longitudinal geographic gradients (Table 1). The most frequent trends involved a decrease in size towards higher latitudes or elevations although other patterns or the lack of a pattern, were observed (Table 1). Because SSD could be affected by these body size clines, we analyzed if significant SSD geographic clines also existed. As shown in Table 1, in at least 19 cases, SSD clines along the geographic coordinates and elevation were observed usually in coincidence with body size clines although the existence of the latter did not always imply SSD clines.

6. Temporal Variation of SSD

The vast majority of species analyzed by us are univoltine. However, there is an exception represented by the mole cricket Neoscapteriscus borellii (Giglio-Tos) which has more than one generation per year. It is most interesting that this species follows Rensch's rule not spatially (data used in this study come from the same population at different times of the year) but temporally since different generations show different body sizes and SSDs (Table 1). Many bi or multivoltine grasshoppers also show differences in size and SSD between generations as is the case of Oedaleus senegalensis (Krauss) in which two consecutive adult generations showed a 12% increase in body size of both sexes and a slight but significant increase of SSD in all studied characters (Table 6).

7. Different ecological and/or rearing conditions can change SSD

Different living conditions such as different diets or rearing temperatures can modify adult body size of insects, thus potentially altering SSD. We analyzed different situations in which orthopteroid populations experienced divergent living conditions (Table 6). In almost all studied cases, changes in diet, rearing temperature or environment produced modifications of  final adult size of males and females and changes in the degree of SSD. However, these changes seem to be species specific as shown by the effects of rearing in isolation or crowded conditions in four orthopteran species. While SSD decreases in crowded conditions in Schistocerca pallens (Thunberg) and two strains of Locusta migratoria (Linnaeus), it has the opposite effect in Ornithacris turbida while no differences in SSD were observed in a study of Acheta domesticus (Linnaeus). Regarding diet, opposite effects were obtained when locusts, Schistocerca shoshone (Thomas) from two different populations were made to feed on two different plant species (Table 6). Increasing rearing temperature produced parallel effects in four species of gomphocerine grasshoppers, that is, an increase of SSD while no significant differences were observed in the cockroach Blatella germanica (Linnaeus) (Table 6).

8. SSD and nymphal development

Final adult size of insects, and thus SSD, is determined during development. As hemime-tabolus insects, orthopteroids reach adulthood after a number of nymphal stages which varies among species. We studied SSD for several characters during nymphal development of seven orthopteran species (Table 7). The number of nymphal stages varied widely (4-11) in the studied species. What all analyzed cases have in common is that during a large part of development nymphs show no SSD or reversed SSD and that final female-biased dimorphism is reached during the final developmental stages. In some species, this occurs mainly because females add a further instar (e.g. Bryophyma deblis (Karsch), Chorthippus brunneus (Thunberg), Eyprepocnemis plorans meridionalis Uvarov, and Atractomorpha sinensis sinensis Bolívar) not present in males, while in other species where males and females share the same number of nymphal stages (e.g. Phymateus leprosus (Fabricius), Deinacrida White spp.), there is a fixed moment when female-biased SSD starts to incresase until reaching the adult value (Table 7). The situation is further complicated in cases such as the mantid Psudomantis albofimbriata Stål where although males experience one extra nymphal stage adults nevertheless reach high female-biased SSD (Table 7). In another praying mantis the developmental outcome is even more complicated because nymphs of both sexes may experience a variable number of instars in the same population; thus, the degree of SSD depends on what categories of males and females are compared (Table 7).


Sexual dimorphism is arguably the most pervasive characteristic of bisexual organisms and was the main inspiration of Darwin's (1871) theory of sexual selection. Sexual dimorphism has multiple manifestations as differences between males and females in secondary sexual characters. Although the latter have been difficult to define precisely (e.g. Darwin, 1871; Cunningham, 1900; Morgan, 1919) a simple definition would be "Differences between males and females of a species in size, structure, color, ornament, or other morphological trait(s), not including the sex organs" (Broughman, 2014), although dimorphism is also manifested in behavioral or biochemical traits. One of the most conspicuous types of sex dimorphism is constituted by differences in size between males and females. Sexual size dimorphism (SSD) can be slight and barely perceptible, or spectacular with members of one sex many times larger or heavier than the other (Fairbairn, 2013). Additionally, both sexes frequently differ in the size of specific body parts (sexual body component dimorphism or SBCD) which are sometimes used to estimate the degree of SSD (Fox et al., 2015): SSD may be male-biased or female-biased which is the case of the majority of invertebrates including insects (although exceptions do occur; see below) (e.g. Andersson, 1994; Faibairn et al., 2007; Fairbairn, 2013), while SBCD may not always follow the same direction as SSD (Fox et al., 2015).

Despite the enormous quantity of studies of SSD in all kinds of species since Darwin´s time, the phenomenon remains largely an enigma (Fairbairn, 2007, 2013). The studies of SSD roughly involve two main problems (Reiss, 1986, 1989; Andersson, 1994; Fairbairn et al., 2007). One is that of the ultimate causes of SSD where both sexual selection and natural selection have been variously favored since Darwin's time. While Darwin (1871) proposed sexual selection as the main (but not unique) mechanism behind SSD and other forms of sexual dimorphism, Wallace (1889) considered that the vast majority of cases could be explained essentially by classic natural selection. Sexual selection operates via two processes: intrasexual selection where individuals of one sex compete in various ways for the access to individuals of the opposite sex, and intersexual or epigamic selection that involves choice of the members of one sex by members of the other sex (Darwin, 1871; Andersson, 1994; Kokko et al., 2006; Clutton-Brock, 2009). A further proposed cause for SSD especially apt for female-biased SSD is fecundity selection (Honek, 1993; Reeve & Fairbairn, 1999) although a positive relationship between body size and fertility has also been documented for male insects (e.g. bush crickets; Wedell, 1997). Natural selection could be the cause of SSD in cases of niche partitioning between males and females (sexual segregation) (Shine, 1989; Isaac, 2005). However, the effects of sexual and natural selection are frequently very difficult to discriminate, hence, some authors have proposed to eliminate the distinction between both forms of selection and concentrate on "contrasts in the components, intensity and targets of selection between males and females'' (Clutton-Brock, 2010). In this sense, the "differential equilibrium hypothesis" of SSD proposes that males and females are differential targets of opposing selective forces that shape SSD (Blanckenhorn, 2005; Hochkirch & Grõning, 2008). A further complication is represented by the multiple proximate mechanisms that can determine differences in size between the sexes: in insects for instance protandry may favor smaller males (e.g. Morbey & Ydenberg, 2001; Bidau & Martí, 2007a, b; Blanckenhorn et al., 2007a, b) while a greater number of larval or nymphal stages and longer development may produce larger females (e.g. Teder & Tammaru, 2005; Esperk et al., 2007; Tammaru et al., 2010; Teder, 2014). The other problem that has generated a proffuse literature is that of the scaling of SSD with body size essentially derived from Rensch's hypothesis (Rensch, 1950, 1960) later termed Rensch's rule (Abouheif & Fairbairn, 1997). However, as Reiss (1986, 1989) has pointed out, Rensch's original data are not statistically significant. Furthermore, many studies have failed to prove an allometric scaling of SSD with body size in the sense of Rensch's rule especially when females are larger than males (e.g. Webb & Freckleton, 2007; Bidau et al., 2013) but also when SSD is male-biased (Lindenfors et al., 2007; Martínez et al., 2014; Martínez & Bidau, 2016). This is particularly true for insects (Blanckenhorn et al., 2007b). Thus far, no convincing explanatory mechanism for Rensch's rule (at least in the cases in which it seems to operate) has been postulated (Reiss, 1986, 1989; Martínez et al., 2014).

The large assemblage of Neopteran insects referred to as "orthopteroids" shows a striking amplitude of body sizes from tiny (less than 5 mm long) antinquiline crickets and termites to giant stick insects exceeding 300 mm in total body length (e.g. Prete et al., 1999; Bell et al., 2007; Whitman, 2008; Brock & Hasenpusch, 2009; Bignell et al., 2011). Also, the vast majority of species in all orthopteroid orders shows SSD. As in most insects, SSD is frequently female-biased but cases of male-biased SSD also occur in some orders (e.g. Blanckenhorn et al., 2007a, b; Hochkirch & Gröning, 2008; Chown & Gaston, 2010). The distribution of SSD within orthopteroid orders has seldom been analyzed (Sivinski, 1978; Hochkirch & Gröning, 2008; Bidau et al., 2013). In the Orthoptera the Caelifera are generally more dimorphic than Ensifera: the former average 1.37 SSD in body length ranging from 0.83 to 2.45, while the latter show a mean SSD of 1.09 (0.77-1.44) (Hochkirch & Gröning, 2008). However, different families and subfamilies within each suborder show marked differences in degrees of SSD (Table 8). The same is true for different related genera (Table 9 and Bidau et al., 2013). Sometimes, extreme differences in SSD occur in very closely related species as is the case of two recently evolved species of the pamphagid genus Purpuraria Enderlein from the Canary Islands that, despite their very close relationship and morphological similarity, show a dramatic difference in SSD: while Purpuraria magna López & Oromi and Purpuraria erna Enderlein show females of similar size (average body lengths, 42.41 and 43.48 mm respectively) males of the first species average 25.2 mm in length and those of the second species, 16.17 mm producing SSDs of 1.68 and 2.79 respectively (López et al., 2013) which largely exceeds the range observed in many genera containing large numbers of species (Table 9).

Very few studies of Rensch's rule have been performed in orthopteroid insects either at the interspecific (Bidau et al., 2013) or intraspecific (Bidau & Martí, 2007a, 2008a, b) levels. From Tables 8 and 9 it can be seen that the vast majority of orthopteroid taxa analyzed show isometric scaling of SSD with body size demonstrated by RMA slopes not different from 1. Of course, these results could be different if a phylogenetic approach is used but comprehensive phylogenies for these groups are not available. However, in a number of nonorthopteroid cases, SSD has been shown to lack phylogenetic signal and Rensch's rule is not veri fied with or without the phylogenetic approach (e.g. Martínez & Bidau, 2014; Martínez et al., 2014) while in others hyperallometry has been unquestionably demonstrated (e.g. Frýdlová & Frynta, 2015). At the intraspecific level, however, orthopteroids showed a variety of responses when scaling of SSD was analyzed (Table 1). Only a fourth of the cases showed a response consistent with Rensch's rule (SSD decreasing with increasing body size) and interestingly enough, 17% of the species showed the converse trend which according to Rensch's rule original formulation should be expected when males are larger, not smaller, than females. Furthermore, more than half the species displayed isometric scaling indicating that Rensch's rule is not a common pattern in orthopteroids at the intraspecific level. In fact, these results suggest that the scaling of SSD with body size is a rather idiosyncratic phenomenon in these insects with each species following its particular trend. The latter is reflected in cases such as the five closely allied grasshopper species belonging to the tribe Dichroplini of the Melanoplinae (Dichroplus fuscus, D. pratensis, D. vittatus, Ronderosia bergii, and Scotussa cliens) one of which follows Rensch's rule, two its converse and two, isometric scaling (Table 1). Nevertheless, one thing seems to be true: it has been considered a precondition for Rensch's rule that male size variability is higher than that of females (Fairbairn, 1997) which was substantiated by our results. As a confirmation, in most species showing converse Rensch's rule, females were more variable in size than males while those species with isometric scaling showed practically the same degree of variability in both sexes.

However, this kind of results must be evaluated cautiously. This is because different characters used to evaluate SSD could yield different estimations of dimorphism and produce discordant scaling patterns, thus the election of such characters is of utmost relevance (Fairbairn, 2007; Fox et al., 2015). In insects, body mass data are hard to come by, so that in most cases, size and SSD are analyzed using linear measurements of body length or other body structures such as legs, wings, pronotum, head, etc. The election of such a character would not be problematic if the length of these different structures scaled isometrically with general size but this is rarely the case: most structures show allometry and the allometric scaling is frequently different in males and females (see Table 5). Many examples are clear from our results: in Dichroplus pratensis which follows Rensch's rule independently of the character used for its testing, SSD for body length (1.33) is much lower than that estimated for all other five linear characters. This is a direct consequence of the different degrees of male and female allometry of these structures (allometric equations not shown in this paper; see Table 4 in Bidau & Martí, 2008b). Dichroplus vittatus that also showed a considerable variation in SSD for different characters, produced converse Rensch's patterns in all cases except for pronotum length which is, significantly, a structure frequently used in orthopteroid insects as a proxy for body size. Conversely, Podisma sapporensis exhibited a converse pattern for pronotum length but isometry for all other traits (see Table 4). Even the length of the third femur, also used frequently to estimate size in orthopteroid insects, can produce discordant results: the four species of the romaleid genus Brachystola Scudder show moderate (for the family) female-biased SSD for body length (1.1-1.2) and pronotum length (1.1-1.3) but surprisingly (and uniquely) the larger females have shorter - in absolute length- hind legs than males producing male-biased SSDs ranging from 0.77 to 0.88. However, allometries are not inevitable (Clutton-Brock et al., 1977) but when they occur differentially in both sexes it is not unreasonable to infer different selective pressures on the same structure in males and females. This is probably the case in to examples described in the Results section. The dramatic difference between the degree of SSD for body length and SBCD for cerci (forceps) in ear-wigs could be a result of the multiple functions that these structures perform in males, such as: male-male aggressive interactions, weapons, sexual display, and clasping of females (Briceño & Eberhard, 1995). In the other example, that of the bark mantises of the genus Liturgusa (Svenson, 2014) femurs of the forelegs show a degree of sexual dimorphism comparable to that of body length, which largely exceeds that of the femurs of meso and metathoracic legs, and also exhibit differential allometry respect to body length. It is worth noting that size of foreleg femurs and tibiae are essential in determining optimum prey size in mantids (Holling et al., 1976). Interestingly in this genus, a converse Rensch pattern is obtained when using body length and prothoracic femur length as estimators of body size, while sexual dimorphism for second and third femur length and pronotum length display sexual isometry.

Latitudinal, elevational, and longitudinal size clines related to variation in biotic and abiotic factors are frequent in insects and have been relatively well studied in orthopteroid insects. The most frequent pattern is one in which body size decreases towards higher latitudes or elevations (the converse Bergmann's rule) and it is most satisfactorily explained by a shortening of the developmental time as seasonality increases and temperature decreases. Because SSD has been also shown to vary geographically in many species, it has been suggested that Bergmann's (or converse Bergamnn's) rule and Rensch´s rule may overlap in the analysis of body size variation (Blanckenhorn et al., 2006; Bidau & Martí, 2007a). The possible correlation is a logical one since Rensch's rule depends on body size which in turn shows geographic clinal variation. The majority of species shown in Table 1 presented clinal patterns of geographic body size variation mainly of the converse Bergmannian type. In most cases, body size variation is accompanied by a corresponding clinal change in the degree of SSD along the same spatial coordinates. In some cases (e.g. Dichroplus pratensis and D. vittatus) that show strong latitudinal and altitudinal patterns this correlation, although expected, was not found but this is due to confounding effects of elevation within the latitudinal patterns that span many degrees of latitude (Bidau & Martí, 2008b).

Since in most insects larger sizes occur in more favorable conditions, an explanation of Rensch's rule and its converse could be produced if we assume differential sensitivity of males and females to environmental factors as suggested by Teder & Tammaru (2005).

If females are more sensitive, as conditions improve, they could achieve their optimal size more readily than in poorer conditions producing an increase in SSD (converse Rensch's rule). The reverse would occur if males are the most sensitive sex. Thus, in this hypothetical scenario Rensch's rule and its converse (not SSD per se) would be subproducts of body size variations related to environmental conditions, specially in species with large geographic distributions.

The effects of external conditions on adult body size and SSD of orthopteroid insects have been extensively studied experimentally. Again, as shown by the examples summarized in table 6 responses to variation in living conditions, diet, temperature, etc. are largely idiosyncratic. Increase or decrease of body size may be accompanied by different and contrasting responses of SSD. For example, the effects of crowding may produce increased SSD (e.g. Ornithacris turbida), decreased size and SSD (e.g. Schistocerca pallens, Melanoplus differentialis (Thomas), and different strains of Locusta migratoria), while no size or SSD changes were observed in similar experiments with the common cricket Acheta domesticus. These and other experiments with varying living conditions strongly suggest species-specific responses of body size to external factors which, if translated to nature could explain the diversity of SSD patterns observed in orthopteroid insects. Furthermore, while estimations of SSD are usually performed at the adult stage, external factors act during the whole period of development. One of the proximate causes that have been invoked to explain size differences between males and females in insects is the higher number of larval instars shown by females of most species (Esperk et al., 2007). Although this phenomenon occurs frequently in orthopteroid insects (see Table 7), it cannot be the sole cause of female-biased SSD. For example, both giant weta Deinacrida species and the pyrgomorphid Phymateus leprosus showh high female-biased SSD but equal number of instars for both sexes, and in the praying mantis Pseudomantis albofimbriata males, not females, undergo an additional nymphal stage (Table 7). The problem is further complicated in that most species do not show significant SSD until the more advanced developmental stages or even only after the final moult (Stilwell et al., 2010; Table 7). Furthermore, other factors such as size at hatching, growth rate, size-dependent survival, and phenotypic plasticity of the characters under study greatly influence adult SSD (Stilwell et al., 2010). Sex differences in plasticity resulting from varying degrees of stabilizing and directional selection on body size or the size of specific characters, are probably the source of much of the observed variation of SSD and is a promising field of study to start disentagling the proximate and ultimate causes of SSD (Stilwell et al., 2010).


  1. Orthopteroid insects show several different scaling patterns of SSD with body size and this is expressed at intra and interspecific levels.
  2. Scaling patterns may differ significantly within taxa depending on the body size estimators considered for analysis and even closely related species may show completely contrasting patterns.
  3. Rensch's rule is just one of the possible modes of scaling of SSD with body size and it can hardly be regarded as a proper "rule".
  4. Numerous environmental factors affect SSD both in nature and experimental studies suggesting the role of differential plasticity between males and females in shaping SSD and its variation.
  5. The study of plasticity and the comparison of sexual dimorphism for different characters that may be under different selective pressures are needed in the future for an understanding of the proximate and ultimate causes of SSD.

General conclusion: Despite the pervasive nature of sexual size dimorphism and the enormous wealth of studies devoted to understand its evolutionary significance and the mechanisms responsible for its enormous variation among widely different organisms as shown in this paper, we still remain confronted with the enigma of intersex size differences. One of the multiple intriguing problems is the relationship between SSD and body size and why in comparative studies, the latter usually shows strong phylogenetic signal while SSD usually does not. This may in part reflect the fact that SSD is not a classic organismal property such as body mass or form, but an adimensional measurement of a difference. This special characteristic of SSD strongly suggests that novel methods for its study must be developed in the future.


This paper honors Prof. Dr. Axel O. Bachmann, superb entomologist and excellent teacher on the occasion of his 89th birthday. Alberto Taffarel and Elio Rodrigo Castillo acknowledge the continuous support of CONICET (Argentina). We are grateful to two anonymous reviewers and the section editor for suggestions that improved the original manuscript.


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