Patterns of sexual size dimorphism in cattle breeds support Rensch’s rule

Rensch’s rule describes the pattern of sexual size dimorphism (SSD) claiming that in taxa where males are the larger sex, larger species generally exhibit higher male to female body size ratios. Agreement with Rensch’s rule is manifested by the slope of the allometric relationship between male and female body size exceeding one. In this paper we have tested the hypothesis that recent rapid evolutionary changes of body size accompanying domestication process and morphological radiation of domestic breeds follow Rensch’s rule. We have analyzed literature data on adult body size of males and females in domestic cows, yaks, buffaloes and other bovines (315, 12, 24 and 2 breeds, respectively) and compared it with SSD in 18 wild species/subspecies of the subfamily Bovinae. Male to female body mass ratio in domestic cows (1.48) was fairly comparable to that found in other species of domestic and wild bovines except domestic buffaloes (1.19). In cows we have demonstrated clear positive allometry of male to female body mass ratio (slope 1.21) predicted by Rensch’s rule, however, no such clear relationship was found when body mass was replaced by shoulder height. These findings are in agreement with those we have previously reported in other livestock species, goats and sheep.     

Patterns of sexual size dimorphism in Chelonia

Macroevolutionary patterns of sexual size dimorphism (SSD) indicate how sexual selection, natural selection, and genetic and developmental constraints mold sex differences in body size. One putative pattern, known as Rensch's rule, posits that, among species with female‐larger SSD, the relative degree of SSD declines with species' body size, whereas, among male‐larger SSD species, relative SSD increases with size. Using a dataset of 196 chelonian species from all fourteen families, we investigated the correlation in body size evolution between male and female Chelonia and the validity of Rensch's rule for the taxon and within its major clades. We conclude that male–female correlations in body size evolution are high, although these correlations differ among chelonian families. Overall, SSD scales isometrically with body size; Rensch's rule is valid for only one family, Testudinidae (tortoises). Because macroevolutionary patterns of SSD can vary markedly among clades, even in a taxon as morphologically conservative as Testudines, one must guard against inappropriately pooling clades in comparative studies of SSD. The results of the present study also indicate that regression models that assume the x‐variable (e.g. male body size) is measured without statistical error, although frequently reported, will result in erroneous conclusions about phylogenetic trends in sexual size dimorphism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 108 , 396–413.     

Patterns of sexual size dimorphism in Chelonia: revisiting Kinosternidae

Rensch's rule, a macroevolutionary pattern in which sexual size dimorphism (SSD) increases with body size in male‐biased SSD species, or decreases with female‐biased SSD species, has been investigated in many vertebrates because it indicates whether SSD is being driven by sexual selection or a different force (i.e. fecundity or natural selection). Evidence in turtles has shown some conflicting results, which may be explained by the different phylogenies used in the analyses. Because the newly available well‐resolved phylogeny of family Kinosternidae provides evidence for the ancient monophyly of Staurotypidae and Kinosternidae and their recognition as separate families (previously Staurotypidae was considered as a subfamily within Kinosternidae) and introduced the genus Cryptochelys for the monophyletic leucostomum clade, we revisit the pattern of SSD and body size in Kinosternidae. By contrast to what had been proposed, we found that the Kinosternidae as formerly recognized (i.e. including Staurotypus and Claudius) and the restricted Kinosternidae both follow a pattern consistent with Rensch's rule. Our analysis with published body size data did not change our results, confirming the importance of the phylogeny used in macroevolutionary studies. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 806–809.     

Lack of support for Rensch's rule in an intraspecific test using red flour beetle (Tribolium castaneum) populations

Rensch's rule proposes a universal allometric scaling phenomenon across species where sexual size dimorphism (SSD) has evolved: in taxa with male‐biased dimorphism, degree of SSD should increase with overall body size, and in taxa with female‐biased dimorphism, degree of SSD should decrease with increasing average body size. Rensch's rule appears to hold widely across taxa where SSD is male‐biased, but not consistently when SSD is female‐biased. Furthermore, studies addressing this question within species are rare, so it remains unclear whether this rule applies at the intraspecific level. We assess body size and SSD within Tribolium castaneum (Herbst), a species where females are larger than males, using 21 populations derived from separate locations across the world, and maintained in isolated laboratory culture for at least 20 years. Body size, and hence SSD patterns, are highly susceptible to variations in temperature, diet quality and other environmental factors. Crucially, here we nullify interference of such confounds as all populations were maintained under identical conditions (similar densities, standard diet and exposed to identical temperature, relative humidity and photoperiod). We measured thirty beetles of each sex for all populations, and found body size variation across populations, and (as expected) female‐biased SSD in all populations. We test whether Rensch's rule holds for our populations, but find isometry, i.e. no allometry for SSD. Our results thus show that Rensch's rule does not hold across populations within this species. Our intraspecific test matches previous interspecific studies showing that Rensch's rule fails in species with female‐biased SSD.     

The evolution of sexual size dimorphism in cottontail rabbits (Sylvilagus,Leporidae)

In mammals, ‘female‐biased’ sexual size dimorphism (SSD), in which females are larger than males, is uncommon. In the present study, we examined Sylvilagus, a purported case of female‐biased SSD, for evolutionary correlations among species between SSD, body‐size, and life‐history variables. We find that: (1) although most species are female‐biased, the degree and direction of SSD vary more than was previously recognized and (2) the degree of SSD decreases with increasing body size. Hence, Sylvilagus provides a new example, unusual for a female‐biased taxon, in which allometry for SSD is consistent with ‘Rensch's Rule’. As a corollary to Rensch's Rule, we observe that changes in SSD in Sylvilagus are typically associated with larger, more significant changes in males than females. Female‐biased SSD could be produced by selection for larger females, smaller males, or both. Although larger female size may be related to high fecundity and the extremely rapid fetal and neonatal growth in Sylvilagus, we find little evidence for a correlation between SSD and various fecundity‐related traits in among‐species comparisons. Smaller male size may confer greater reproductive success through greater mobility and reduced energetic requirements. We propose that a suite of traits (female dispersion, large male home ranges, reduced aggression, and a promiscuous mating system) has favoured smaller males and thus influenced the evolution of SSD in cottontails. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 141–156.     

A test of Rensch's rule in varanid lizards

In a model group of giant reptiles, we explored the allometric relationships between male and female body size and compared the effects of sexual and fecundity selection, as well as some proximate causes, on macroevolutionary patterns of sexual size dimorphism (SSD). Monitor lizards are a morphologically homogeneous group that has been affected by extreme changes in body size during their evolutionary history, resulting in 14‐fold differences among the body sizes of recent species. Here, we analysed data concerning the maximum and/or mean male and female snout–vent lengths in 42 species of monitor lizard from literary sources and supplemented these data with measurements made in zoos. There was a wide scale of SSD from nearly monomorphic species belonging mostly to the subgenus Odatria and Prasinus group of the Euprepriosaurus to apparently male‐larger taxa. The variable best explaining SSD was the body size itself; the larger the species, the higher the SSD. This pattern agrees with the currently discussed Rensch's rule, claiming that the relationship between male and female body size is hyperallometric, i.e. the allometric exponent of this relationship exceeds unity and thus SSD increases with body size in the case of male‐larger taxa. All our estimates of the reduced major axis regression slopes of this relationship ranged from 1.132 to 1.155. These estimates are significantly higher than unity, and thus unequivocally corroborate the validity of Rensch's rule in this reptilian group. In spite of our expectation that the variation in SSD can be alternatively explained by variables reflecting the strength of sexual selection (presence of male combat), fecundity selection (e.g. clutch size and mass) and/or proximate ecological factors (habitat type), none of these variables had consistent effects on SSD, especially when the data were adjusted to phylogenetic dependence and/or body size. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 293–306.     

Intraspecific variation in body size and sexual size dimorphism,and a test of Rensch's rule in bats

The magnitude and direction of sexual size dimorphism (SSD) may vary considerably within and among taxa, and the primary causes of such variation have not been thoroughly elucidated. For example, the effect of abiotic factors is frequently attributed to explain intra‐ and interspecific variation in SSD. Rensch's rule, which states that males vary more in size than females when body size increases, has rarely been tested in bats. Therefore, whether bats follow Rensch's rule remains unclear, particularly when females are larger than males. We investigated whether four bat species presented SSD, as well as whether their body sizes varied within each sex across localities, testing the hypothesis that intraspecific SSD varies substantially depending of sampling localities. We finally examined whether bats followed Rensch's rule by simultaneously using intraspecific and interspecific approaches. Although SSD was not observed for most bat species within each locality, the females of three of the four captured species exhibited differences in body size between particular localities. Usually the females varied more in size than did males across localities, mostly exhibiting a female‐biased SSD. Significant differences in SSD were observed (i.e. mean values of the sexual dimorphism index), even though Rensch's rule was not followed.     

Does ‘gliding while gravid’ explain Rensch's rule in flying lizards?

Among species with sexual size dimorphism (SSD), taxa in which males are the larger sex have increasing SSD with increasing body size, whereas in taxa in which females are the larger sex, SSD decreases with body size: Rensch's rule. We show in flying lizards, a clade of mostly female‐larger species, that SSD increases with body size, a pattern similar to that in clades with male‐biased SSD or more evenly mixed SSD. The observed pattern in Draco appears due to SSD increasing with evolutionary changes in male body size; specifically divergence in body size among species that are in sympatric congeneric assemblages. We suggest that increasing body size, resulting in decreased gliding performance, reduces the relative gliding cost of gravidity in females, and switches sexual selection in males away from a small‐male, gliding advantage and toward selection on large size and fighting ability as seen in many other lizards. Thus, selection for large females is likely greater than selection for large males at the smaller end of the body size continuum, whereas this relationship reverses for species at the larger end of the continuum. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 270–282.     

Function‐related Drivers of Skull Morphometric Variation and Sexual Size Dimorphism in a Subterranean Rodent,Plateau Zokor (Eospalax baileyi)

Sexual dimorphism is prevalent in most living organisms. The difference in size between sexes of a given species is generally known as sexual size dimorphism (SSD). The magnitude of the SSD is determined by Rensch's rule where size dimorphism increases with increasing body size when the male is the larger sex and decreases with increasing average body size when the female is the larger sex. The unique underground environment that zokors (Eospalax baileyi) live under in the severe habitat of the Qinghai‐Tibetan Plateau (QTP) could create SSD selection pressures that may or may not be supported by Rensch's rule, making this scientific question worthy of investigation. In this study, we investigated the individual variation between sexes in body size and SSD of plateau zokors using measurements of 19 morphological traits. We also investigated the evolutionary mechanisms underlying SSD in plateau zokors. Moreover, we applied Rensch's rule to all extant zokor species. Our results showed male‐biased SSD in plateau zokors: The body‐ and head‐related measurements were greater in males than in females. Linear regression analysis between body length, body weight, and carcass weight showed significant relationships with some traits such as skull length, lower incisor length, and tympanic bulla width, which might support our prediction that males have faster growth rates than females. Further, the SSD pattern corroborated the assumption of Rensch's rule in plateau zokors but not in the other zokor species. Our findings suggest that the natural underground habitat and behavioral differences between sexes can generate selection pressures on male traits and contribute to the evolution of SSD in plateau zokors.     

Sexual size dimorphism and Rensch's rule in Canidae

The size variation between males and females of a species is a phenomenon known as sexual size dimorphism (SSD). The observed patterns of variation in SSD among species has led to the formulation of Rensch's rule, which establishes that, in species showing a male size bias, SSD increases with an increase in the body size of the species. However, for species in which there is a female size bias, the SSD would decrease when the body size of the species increases. In the present study, we examined the variation in body size and SSD of 33 species of canids from estimates of body mass and body length. We studied its relationship with life‐history characteristics and tested Rensch's rule using phylogenetic generalized least squares and phylogenetic reduced major axis regressions, respectively. We observed the existence of correlation between body mass and body length, although the SSDs from these estimators are uncorrelated. SSD did not show the pattern predicted by Rensch's rule. SSD also did not show any correlation with life‐history traits. It is likely that the low SSD observed in canids is related to the monogamy observed in the family, which is a rare situation in mammals.     

Allometry of Sexual Size Dimorphism in Domestic Dog

Background

The tendency for male-larger sexual size dimorphism (SSD) to scale with body size – a pattern termed Rensch''s rule – has been empirically supported in many animal lineages. Nevertheless, its theoretical elucidation is a subject of debate. Here, we exploited the extreme morphological variability of domestic dog (Canis familiaris) to gain insights into evolutionary causes of this rule.

Methodology/Principal Findings

We studied SSD and its allometry among 74 breeds ranging in height from less than 19 cm in Chihuahua to about 84 cm in Irish wolfhound. In total, the dataset included 6,221 individuals. We demonstrate that most dog breeds are male-larger, and SSD in large breeds is comparable to SSD of their wolf ancestor. Among breeds, SSD becomes smaller with decreasing body size. The smallest breeds are nearly monomorphic.

Conclusions/Significance

SSD among dog breeds follows the pattern consistent with Rensch''s rule. The variability of body size and corresponding changes in SSD among breeds of a domestic animal shaped by artificial selection can help to better understand processes leading to emergence of Rensch''s rule.     

Sexual size dimorphism in musteloids: An anomalous allometric pattern is explained by feeding ecology

Rensch's rule states that sexual size dimorphism (SSD) increases with body size in taxa where males are larger, and decreases when females are larger. The dominant explanation for the trend is currently that competitive advantage for males is greater in larger individuals, whereas female size is constrained by the energetics of rearing offspring. This rule holds for a variety of vertebrate taxa, and opposing trends are rare. We examine the allometry of SSD within the Musteloidea and demonstrate a hypo‐allometry contrary to Rensch's rule, with lower SSD associated with larger body size. We provide evidence that feeding ecology is involved. Where diet promotes group‐living, the optimal strategy for the males of larger species is often not to attempt to defend access to multiple females, obviating any competitive advantage of relatively greater size. We conclude that the effect of feeding ecology on mating systems may be a hitherto neglected factor explaining variation in SSD.     

Sexual dichromatism in wing pigmentation of New World dragonflies follows Rensch's rule

Many animal taxa that display sexual size dimorphism (SSD) exhibit a positive allometric relationship in which the degree of dimorphism increases with body size. This macroevolutionary pattern is known as Rensch's rule. Although sexual selection is hypothesized to be the main mechanism causing this pattern, body size is influenced by several selective forces, including natural and sexual selection. Therefore, by focusing exclusively on SSD one cannot ascertain which of these selective forces drives Rensch's rule. If sexual selection is indeed the main mechanism underlying Rensch's rule, we predict that other sexually selected traits, including coloration‐based ornaments, will also exhibit interspecific allometric scaling consistent with Rensch's rule. We tested this prediction using wing pigmentation of 89 species of dragonflies. Studies show that male wing pigmentation is generally under strong intra‐ and intersexual selection, so that sexual dichromatism in this trait should follow Rensch's rule. Conversely, the available evidence suggests that male body size is usually not sexually selected in dragonflies, so we do not expect SSD to follow Rensch's rule. First, we found that sexual dichromatism in wing pigmentation was consistent with Rensch's rule. The phylogenetic major axis regression slope was significantly greater than one. We also showed that the allometric slope for SSD was not different from unity, providing no support for Rensch's rule. Our results provide the first evidence that a trait which appears to be under strong sexual selection exhibits a pattern consistent with Rensch's rule.     

A test of Rensch’s rule in dwarf chameleons (Bradypodion spp.), a group with female-biased sexual size dimorphism

Rensch’s rule describes a pattern of allometry in sexual size dimorphism (SSD): when males are the larger sex (male-biased SSD), SSD increases with increasing body size, and when females are the larger sex (female-biased SSD), SSD decreases with increasing body size. While this expectation generally holds for taxa with male-biased or mixed SSD, examples of allometry for SSD consistent with Rensch’s rule in groups with primarily female-biased SSD are remarkably rare. Here, I show that the majority of dwarf chameleons (Bradypodion spp.) have female-biased SSD. In accordance with Rensch’s rule, the group exhibits an allometric slope of log(female size) on log(male size) less than one, although statistical significance is dependent on the phylogenetic comparative method used. In this system, this pattern is likely due to natural selection on both male and female body size, combined with fecundity selection on female body size. In addition to quantifying SSD and testing Rensch’s rule in dwarf chameleons, I discuss reasons why Rensch’s rule may only rarely apply to taxa with female-biased SSD.     

Patterns of sexual size dimorphism in stingless bees: Testing Rensch’s rule and potential causes in highly eusocial bees (Hymenoptera: Apidae,Meliponini)

Eusocial insects offer a unique opportunity to analyze the evolution of body size differences between sexes in relation to social environment. The workers, being sterile females, are not subject to selection for reproductive function providing a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other kinds of natural selection. Patterns of sexual size dimorphism (SSD) and testing of Rensch's rule controlling for phylogenetic effects were analyzed in the Meliponini or stingless bees. Theory predicts that queens may exhibit higher selection for fecundity in eusocial taxa, but contrary to this, we found mixed patterns of SSD in Meliponini. Non‐Melipona species generally have a female‐biased SSD, while all analyzed species of Melipona showed a male‐biased SSD, indicating that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. The phylogenetic regressions revealed that the rate of divergence has not differed between the two castes of females and the males, that is, stingless bees do not seem to follow Rensch's rule (a slope >1), adding this highly eusocial taxon to the various solitary insect taxa not conforming with it. Noteworthy, when Melipona was removed from the analysis, the phylogenetic regressions for the thorax width of males on queens had a slope significantly smaller than 1, suggesting that the evolutionary divergence has been larger in queens than males, and could be explained by stronger selection on female fecundity only in non‐Melipona species. Our results in the stingless bees question the classical explanation of female‐biased SSD via fecundity and provide a first evidence of a more complex determination of SSD in highly eusocial species. We suggest that in highly eusocial taxa, additional selection mechanisms, possibly related to individual and colonial interests, could influence the evolution of environmentally determined traits such as body size.     

Variation of life‐history traits of the Asian corn borer,Ostrinia furnacalis in relation to temperature and geographical latitude

Life‐history traits from four geographical populations (tropical Ledong population [LD], subtropical Guangzhou [GZ] and Yongxiu populations, and temperate Langfang population [LF]) of the Asian corn borer, Ostrinia furnacalis were investigated at a wide range of temperatures (20–32°C). The larval and pupal times were significantly decreased with increasing rearing temperature, and growth rate was positively correlated with temperature. The relationship between body weight and rearing temperature in O. furnacalis did not follow the temperature–size rule (TSR); all populations exhibited the highest pupal and adult weights at high temperatures or intermediate temperatures. However, development time, growth rate, and body weight did not show a constant latitudinal gradient. Across all populations at each temperature, female were significantly bigger than males, showing a female‐biased sexual size dimorphism (SSD). Contrary to Rensch's rule, the SSD tended to increase with rising temperature. The subtropical GZ population exhibited the largest degree of dimorphism while the temperate LF exhibited the smallest. Male pupae lose significantly more weight at metamorphosis compared to females. The proportionate weight losses of different populations were significantly different. Adult longevity was significantly decreased with increasing temperature. Between sexes, all populations exhibit a rather female‐biased adult longevity. Finally, we discuss the adaptive significance of higher temperature‐inducing high body weight in the moth's life history and why the moth exhibits the reverse TSR.     

Variation in body size and sexual size dimorphism in the most widely ranging lizard: testing the effects of reproductive mode and climate

Reproductive mode, ancestry, and climate are hypothesized to determine body size variation in reptiles but their effects have rarely been estimated simultaneously, especially at the intraspecific level. The common lizard (Zootoca vivipara) occupies almost the entire Northern Eurasia and includes viviparous and oviparous lineages, thus representing an excellent model for such studies. Using body length data for >10,000 individuals from 72 geographically distinct populations over the species' range, we analyzed how sex‐specific adult body size and sexual size dimorphism (SSD) is associated with reproductive mode, lineage identity, and several climatic variables. Variation in male size was low and poorly explained by our predictors. In contrast, female size and SSD varied considerably, demonstrating significant effects of reproductive mode and particularly seasonality. Populations of the western oviparous lineage (northern Spain, south‐western France) exhibited a smaller female size and less female‐biased SSD than those of the western viviparous (France to Eastern Europe) and the eastern viviparous (Eastern Europe to Far East) lineages; this pattern persisted even after controlling for climatic effects. The phenotypic response to seasonality was complex: across the lineages, as well as within the eastern viviparous lineage, female size and SSD increase with increasing seasonality, whereas the western viviparous lineage followed the opposing trends. Altogether, viviparous populations seem to follow a saw‐tooth geographic cline, which might reflect the nonmonotonic relationship of body size at maturity in females with the length of activity season. This relationship is predicted to arise in perennial ectotherms as a response to environmental constraints caused by seasonality of growth and reproduction. The SSD allometry followed the converse of Rensch's rule, a rare pattern for amniotes. Our results provide the first evidence of opposing body size—climate relationships in intraspecific units.     

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

In 1950, Rensch noted that in clades where males are the larger sex, sexual size dimorphism (SSD) tends to be more pronounced in larger species. This fundamental allometric relationship is now known as ‘Rensch''s rule’. While most researchers attribute Rensch''s rule to sexual selection for male size, experimental evidence is lacking. Here, we suggest that ultimate hypotheses for Rensch''s rule should also apply to groups of individuals and that individual trait plasticity can be used to test those hypotheses experimentally. Specifically, we show that in the sex-changing fish Parapercis cylindrica, larger males have larger harems with larger females, and that SSD increases with harem size. Thus, sexual selection for male body size is the ultimate cause of sexual size allometry. In addition, we experimentally illustrate a positive relationship between polygyny potential and individual growth rate during sex change from female to male. Thus, sexual selection is the ultimate cause of variation in growth rate, and variation in growth rate is the proximate cause of sexual size allometry. Taken together, our results provide compelling evidence in support of the sexual selection hypothesis for Rensch''s rule and highlight the potential importance of individual growth modification in the shaping of morphological patterns in Nature.     

Phylogeny suggests nondirectional and isometric evolution of sexual size dimorphism in argiopine spiders

Sexual dimorphism describes substantial differences between male and female phenotypes. In spiders, sexual dimorphism research almost exclusively focuses on size, and recent studies have recovered steady evolutionary size increases in females, and independent evolutionary size changes in males. Their discordance is due to negative allometric size patterns caused by different selection pressures on male and female sizes (converse Rensch's rule). Here, we investigated macroevolutionary patterns of sexual size dimorphism (SSD) in Argiopinae, a global lineage of orb‐weaving spiders with varying degrees of SSD. We devised a Bayesian and maximum‐likelihood molecular species‐level phylogeny, and then used it to reconstruct sex‐specific size evolution, to examine general hypotheses and different models of size evolution, to test for sexual size coevolution, and to examine allometric patterns of SSD. Our results, revealing ancestral moderate sizes and SSD, failed to reject the Brownian motion model, which suggests a nondirectional size evolution. Contrary to predictions, male and female sizes were phylogenetically correlated, and SSD evolution was isometric. We interpret these results to question the classical explanations of female‐biased SSD via fecundity, gravity, and differential mortality. In argiopines, SSD evolution may be driven by these or additional selection mechanisms, but perhaps at different phylogenetic scales.     

Bergmann's and Rensch's rules and the spur‐thighed tortoise (Testudo graeca)

Body size is an ecologically important variable in animals. The geographical size variation of most snakes and some lizards counters Bergmann's rule in that, among related taxa, the larger ones live at warmer latitudes. However, exceptions notwithstanding, and despite being ectothermic, turtles as a group tend to obey Bergmann's rule. We examined this idea in Testudo graeca, ranging from Morocco to Romania and to Iran with disputed systematics, both at the global scale (using literature) and within the focal area of Israel (using museum specimens). Both globally and locally, carapace length correlated with latitude, in accordance with Bergmann's rule. The scant data on reproduction fully support the hypothesis that Bergmann's rule enables larger clutches where the climate would limit repeated clutches. The sexual size dimorphism (SSD) was approached using two methodologies: (1) ‘conventional’, using globally literature data and locally museum samples and (2) ‘innovated’, using photographs of copulating tortoises from Israel and Turkey. By each methodology, SSD emerged as being male biased in the larger‐bodied populations and female biased in the smaller‐bodied populations, obeying Rensch's rule. Some observations support the hypothesis that the evolution of large males serves intermale combating. Finally, Rensch's rule was found to apply separately within Anatolia and within the Levant, possibly indicating that these populations are separate.