甘南草原高原鼢鼠年龄划分及其组成分析

建立高原鼢鼠(Eospalax baileyi)种群年龄鉴定划分标准,可为对其的监测及防控提供科学依据。研究于2015年5月、10月分别在甘南草原碌曲县采用夹捕法捕捉高原鼢鼠469只(雄鼠209只,雌鼠260只),在对样本进行逐个解剖和数据记录之后,参考其体重、体长、胴体重及毛色等,对高原鼢鼠的年龄划分标准进行研究。依据胴体重将高原鼢鼠分为6组:亚成年组(雄144 g,雌106 g),成年Ⅰ组(雄144~210 g,雌106~148 g)、成年Ⅱ组(雄211~276 g,雌149~190 g)、成年Ⅲ组(雄277~342 g,雌191~232 g)、成年Ⅳ组(雄343~408 g,雌233~274 g)和老年组(雄409 g,雌275 g)。年龄结构表显示,不同的季节和不同地理区域高原鼢鼠的种群年龄结构类型均为增长型。     

The Evolution of Reversed Sexual Size Dimorphism in Hawks,Falcons and Owls: A Comparative Study

Many hypotheses have been proposed to account for the origin and maintenance of reversed size dimorphism (RSD, females being larger than males) in hawks, falcons and owls, but no consensus has been reached. I performed comparative analyses, using both cross-taxa data and phylogenetically independent contrasts, to investigate potential correlates of reversed size dimorphism. Using a similar set of explanatory variables, covering morphology, life history and ecology, I tested whether any trait coevolved with size dimorphism in all three groups and hence provided a general explanation for the evolution of RSD. For hawks, strong correlates were found in the foraging-variable complex, so RSD might have evolved in species hunting large and agile prey. This is consistent with the intersexual-competition hypothesis (sexes have evolved different sizes to lessen intersexual competition for food), but especially the small-male hypothesis (males have evolved to be smaller to be more efficient foragers). Evolutionary pathway analyses suggest that RSD evolved most likely as a precursor of changes in hunting strategy but as a consequence of high reproduction. The falcons showed a similar pattern: species with strong RSD hunted larger and more agile prey. The evolutionary pathway analysis supported the idea that RSD evolved before the specialisation on more agile and/or larger prey. Finally for owls, the results showed clear parallels. RSD increased with prey size, consistent with the small-male hypothesis. Evolutionary pathway analysis suggests that RSD in owls has most likely evolved before specialisation on large prey, so a small and more agile male might be advantageous even when hunting small prey. These results suggest that RSD in hawks, falcons and owls evolved due to natural-selection pressures rather than sexual-selection pressures. Co-ordinating editor: J. Tuomi     

Sexual selection explains Rensch's rule of allometry for sexual size dimorphism

In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as 'Rensch's rule'. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.     

Scaling of Size and Dimorphism in Primates II: Macroevolution

Previous researchers found positive scaling of body size and sexual size dimorphism (SSD) in primates, known as Rensch's rule. The pattern is present in Haplorhini, but absent in Strepsirhini. I found that positive evolutionary correlations between size and SSD drive positive scaling relationships within Haplorhini as a whole and Platyrrhini, Cercopithecinae, Colobinae, and Hominoidea individually at the generic level and higher, but that evolutionary correlations within genera in these clades are often nonsignificant or negative. I suggest that positive evolutionary correlations result from greater change in male than in female size, usually because of sexual selection acting on polygynous populations. I suggest that negative evolutionary correlations result from greater change in female size, owing to either natural selection or, in Callitrichidae, sexual selection acting on polyandrous populations. The high incidence of negative evolutionary correlations within Haplorhini suggests a relatively large influence of natural selection on SSD, at least with regard to differences in SSD between congeners. I propose two possible explanations for the difference in intrageneric and supergeneric evolutionary patterns: 1) natural selection is a relatively weak force for modifying SSD and has a noticeable effect only when one compares related species experiencing similar levels of sexual selection, and 2) natural selection is a relatively strong force for modifying SSD but is less likely than sexual selection to affect higher level taxonomic comparisons noticeably because of the cumulative effect over time of marginal differences in mortality rates of these two types of selection. I discuss types of data required to test these explanations and implications for reconstructing fossil behavior.     

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.     

Scaling of Size and Dimorphism in Primates I: Microevolution

I used a new quantitative genetics model to predict relationships between sex-specific body size and sex-specific relative variability when populations experience differences in relative intensity of sex-specific selection pressures—stronger selection on males or females—and direction of selection: increase or decrease in size. I combined Lande's (Evolution 34: 292–305) model for the response of sex-specific means to selection with a newly derived generalization of Bulmer's (Am. Nat. 105: 201–211) model for the response of relative variability to selection. I used this combined response model to predict correlations of sex-specific size and relative variability under various starting conditions, which one can compare to correlations between closely related primate populations. One can then compare predicted patterns of sex-specific selection pressures to social and ecological variables pertaining to those populations to identify likely forces producing microevolutionary change in sexual size dimorphism (SSD). I provide examples of this approach for populations representing three taxa: Papio anubis, Saguinus mystax, and Cercopithecus aethiops pygerythrus. Model results suggest that microevolutionary changes in SSD can result from greater selection acting on males or females, and that natural selection or natural and sexual selection combined, rather than sexual selection alone, may sometimes explain sex-specific selection differentials.     

Ecological drivers of body size evolution and sexual size dimorphism in short‐horned grasshoppers (Orthoptera: Acrididae)

Sexual size dimorphism (SSD) is widespread and variable in nature. Although female‐biased SSD predominates among insects, the proximate ecological and evolutionary factors promoting this phenomenon remain largely unstudied. Here, we employ modern phylogenetic comparative methods on eight subfamilies of Iberian grasshoppers (85 species) to examine the validity of different models of evolution of body size and SSD and explore how they are shaped by a suite of ecological variables (habitat specialization, substrate use, altitude) and/or constrained by different evolutionary pressures (female fecundity, strength of sexual selection, length of the breeding season). Body size disparity primarily accumulated late in the history of the group and did not follow a Brownian motion pattern, indicating the existence of directional evolution for this trait. We found support for the converse of Rensch's rule (i.e. females are proportionally bigger than males in large species) across all taxa but not within the two most speciose subfamilies (Gomphocerinae and Oedipodinae), which showed an isometric pattern. Our results do not provide support for the fecundity or sexual selection hypotheses, and we did not find evidence for significant effects of habitat use. Contrary to that expected, we found that species with narrower reproductive window are less dimorphic in size than those that exhibit a longer breeding cycle, suggesting that male protandry cannot solely account for the evolution of female‐biased SSD in Orthoptera. Our study highlights the need to consider alternatives to the classical evolutionary hypotheses when trying to explain why in certain insect groups males remain small.     

Allopatric divergence and phylogeographic structure of the plateau zokor (Eospalax baileyi), a fossorial rodent endemic to the Qinghai–Tibetan Plateau

Aim Most species of temperate regions are believed to have shifted to lower latitudes or elevations during the glacial periods of the Quaternary. In this study we test whether this phylogeographic assumption is also true for the plateau zokor (Eospalax baileyi), a fossorial rodent endemic to the climate-sensitive Qinghai–Tibetan Plateau (QTP), which ranges in elevation from 2600 to 4600 m. Location The QTP of western China. Methods Phylogeographic analyses were conducted based on the mitochondrial cytochrome b gene sequences of 193 individuals from 20 populations over the entire range of the species. Results A total of 54 haplotypes identified in the present study clustered into four geographically correlated clades located in the interior of the QTP (clade A) and at the plateau edge (B, C and D). Molecular calibrations suggest that the interior plateau (A) and plateau-edge (B–D) clades diverged at 1.2 Ma and that the three plateau-edge clades diverged between 0.85 and 0.80 Ma. These estimates are concordant with diastrophism and glaciation events in the QTP. Coalescent tests rejected both the hypothesis that all current populations originated from a single refugium at a low elevation during the Last Glacial Maximum (LGM) and the hypothesis that the two lineages diverged during the LGM. The tests instead supported the hypothesis that there were four refugia during the LGM, and that the four clades diverged prior to the late Pleistocene. Main conclusions Our results suggest that Quaternary diastrophisms and glaciations repeatedly promoted allopatric divergence of the plateau zokor into geographical clades, and that these regional clades subsequently persisted at high elevations, rather than migrating to the low-elevation plateau edge during subsequent glacial ages.     

The interplay between natural and sexual selection in the evolution of sexual size dimorphism in Sceloporus lizards (Squamata: Phrynosomatidae)

Sexual size dimorphism (SSD) evolves because body size is usually related to reproductive success through different pathways in females and males. Female body size is strongly correlated with fecundity, while in males, body size is correlated with mating success. In many lizard species, males are larger than females, whereas in others, females are the larger sex, suggesting that selection on fecundity has been stronger than sexual selection on males. As placental development or egg retention requires more space within the abdominal cavity, it has been suggested that females of viviparous lizards have larger abdomens or body size than their oviparous relatives. Thus, it would be expected that females of viviparous species attain larger sizes than their oviparous relatives, generating more biased patterns of SSD. We test these predictions using lizards of the genus Sceloporus. After controlling for phylogenetic effects, our results confirm a strong relationship between female body size and fecundity, suggesting that selection for higher fecundity has had a main role in the evolution of female body size. However, oviparous and viviparous females exhibit similar sizes and allometric relationships. Even though there is a strong effect of body size on female fecundity, once phylogenetic effects are considered, we find that the slope of male on female body size is significantly larger than one, providing evidence of greater evolutionary divergence of male body size. These results suggest that the relative impact of sexual selection acting on males has been stronger than fecundity selection acting on females within Sceloporus lizards.     

Sexual selection, sexual size dimorphism and Rensch's rule in Odonata

Odonata (dragonflies and damselflies) exhibit a range of sexual size dimorphism (SSD) that includes species with male-biased (males > females) or female-biased SSD (males < females) and species exhibiting nonterritorial or territorial mating strategies. Here, we use phylogenetic comparative analyses to investigate the influence of sexual selection on SSD in both suborders: dragonflies (Anisoptera) and damselflies (Zygoptera). First, we show that damselflies have male-biased SSD, and exhibit an allometric relationship between body size and SSD, that is consistent with Rensch's rule. Second, SSD of dragonflies is not different from unit, and this suborder does not exhibit Rensch's rule. Third, we test the influence of sexual selection on SSD using proxy variables of territorial mating strategy and male agility. Using generalized least squares to account for phylogenetic relationships between species, we show that male-biased SSD increases with territoriality in damselflies, but not in dragonflies. Finally, we show that nonagile territorial odonates exhibit male-biased SSD, whereas male agility is not related to SSD in nonterritorial odonates. These results suggest that sexual selection acting on male sizes influences SSD in Odonata. Taken together, our results, along with avian studies (bustards and shorebirds), suggest that male agility influences SSD, although this influence is modulated by territorial mating strategy and thus the likely advantage of being large. Other evolutionary processes, such as fecundity selection and viability selection, however, need further investigation.     

Is body size a good indicator of fecundity in the genus Thaumetopoea? A story told by two intrageneric Mediterranean forest defoliators

1. Body size correlates with several factors such as reproductive fitness, environmental changes, the quality and quantity of food during critical development stages, and the feeding season. For the Palearctic moths of the genus Thaumetopoea (Lepidoptera; Notodontidae), the larval development is crucial and differs between species according to their feeding season; larvae of the pine processionary moth Thaumetopoea pityocampa (Denis & Schiffermüller 1775) feed during winter while larvae of its congeneric cedar processionary moth Thaumetopoea bonjeani (Powell 1922) develop during summer in North Africa.
2. This discrepancy in lifecycles leads to different reproductive traits such as egg batch length, number of eggs per batch, eggs protection mechanisms and female body size.
3. According to Darwin's fecundity advantage hypothesis (1871), female body size should have a positive influence on reproductive fitness, since larger females supposedly have higher fecundity. The universal allometric scaling phenomenon rule proposed by Rensch (1950) predicts that the degree of sexual size dimorphism tends to decrease with the increase of female body size.
4. Here, two morphometrical parameters that is, body size and scale size, estimated from body measurements of individuals of both species, feeding on the same host Atlantic cedar Cedrus atlantica (Manetti & Carrière 1855) (Pinales; Pinaceae) in Algeria were proposed. The aim was to find out traits that might rule the competition for food and space, in particularly fecundity and body size.
5. Results of the present study highlight a female-biased sexual size dimorphism in both species. The positive correlation between female body size and fecundity shown in this study weakly supports Darwin's hypothesis. Finally, the intrageneric test performed leads to conclude that Rensch's rule does not hold in the considered species.

     

Latitudinal variation in sexual dimorphism in life‐history traits of a freshwater fish

Sexual dimorphism is common across the animal kingdom, but the contribution of environmental factors shaping differences between the sexes remains controversial. In ectotherms, life‐history traits are known to correlate with latitude, but sex‐specific responses are not well understood. We analyzed life‐history trait variation between the sexes of European perch (Perca fluviatilis L.), a common freshwater fish displaying larger female size, by employing a wide latitudinal gradient. We expected to find sex‐dependent latitudinal variation in life‐history variables: length at age, length increment, and size at maturity, with females showing consistently higher values than males at all latitudes. We further anticipated that this gender difference would progressively decrease with the increasingly harsh environmental conditions toward higher latitude. We hypothesized that growth and length increment would decrease and size/age at maturity would increase at higher latitudes. Our results confirmed female‐biased sexual size dimorphism at all latitudes and the magnitude of sexual dimorphism diminished with increase in latitude. Growth of both sexes decreased with increase in latitude, and the female latitudinal clines were steeper than those of males. Hence, we challenge two predominant ecological rules (Rensch's and Bergmann's rules) that describe common large‐scale patterns of body size variation. Our data demonstrate that these two rules are not universally applicable in ectotherms or female‐biased species. Our study highlights the importance of sex‐specific differences in life‐history traits along a latitudinal gradient, with evident implications for a wide range of studies from individual to ecosystems level.     

Behavioural Tests Reveal Severe Visual Deficits in the Strictly Subterranean African Mole‐Rats (Bathyergidae) but Efficient Vision in the Fossorial Rodent Coruro (Spalacopus cyanus,Octodontidae)

Vision has long been considered purposeless in the dark underground ecotope. However, recent anatomical studies revealed an unexpected diversity of ocular and retinal features and various degrees of development of the visual system in mammals with predominantly subterranean activity, and have suggested retention of basic visual capabilities even in some strictly subterranean mammals such as the African mole‐rats. Behavioural tests assessing image‐forming vision have not yet been conducted in subterranean mammals. Here, we investigated the visual capacities in three species of the African mole‐rats, namely the giant mole‐rat Fukomys mechowii, the Mashona mole‐rat Fukomys darlingi and the silvery mole‐rat Heliophobius argenteocinereus, in the fossorial coruro Spalacopus cyanus and the inbred C57L/J mouse. The behavioural assays performed in this study revealed severe visual deficits in all three species of mole‐rats. The absence of the visual placing reflex suggested impairment of either image‐forming vision or visuomotor integration. The random choice between the shallow and the deep side of a visual cliff clearly demonstrated inability of mole‐rats to perceive depth. The nesting assay did not yield conclusive evidence regarding the capacity for visually guided spatial orientation in the only tested species, the giant mole‐rat. In contrast, both the coruro and the mouse exhibited a clear placing reaction and preferred the shallow side of the visual cliff, implying functional image‐forming vision. Thus, the behavioural data gathered in this study show that vision is seriously compromised in the strictly subterranean, congenitally microphthalmic African mole‐rats but efficient (i.e. comparable to that of surface‐dwelling rodents) in a species with regular surface activity, the coruro.     

Sexual size dimorphism in greater mouse‐eared bat Myotis myotis (Chiroptera: Vespertilionidae) from a Mediterranean region

Although sexual size dimorphism (SSD) is common among mammals, there is no clear explanation for its maintenance in nature. Bats are one of the few groups of mammals where reverse SSD appears. In this group, the size of individuals may have very important ecological consequences related with flight. In this study, we examine sexual dimorphism in the wing measurements of 195 adult individuals (141 males and 54 females) of the greater mouse‐eared bat Myotis myotis in the south‐east of the Iberian Peninsula. We also investigated size differences between paired and single males in a swarming roost. The results indicate that there are significant differences in the wing measurements between sexes, females being bigger than males in this respect. While no significant differences in the wing measurements of paired and single males were observed, significant differences were found in their relative weight and fitness, single males being significantly heavier and having a better physical condition. We discuss the implications of SSD for the female of M. myotis in terms of reproductive advantages, trophic niche segregation and a greater ability to move, which may favour gene flow between populations.     

Role of sexual and natural selection in evolution of body size and shape: a phylogenetic study of morphological radiation in grouse

We use standardized independent contrasts (SICs) to elucidate the effect of ecology and mating systems on morphological radiation in grouse. The analysis of SICs for 38 skeletal measurements from 20 taxa, showed that changes in mating system had a significant effect on body size of both sexes. Sexual size dimorphism in grouse is consistent with Rensch's rule; the slope of the regression of male vs. female size SICs was 1.4, significantly >1. Changes in habitat were associated with accelerated rates of evolution of body proportions. SICs for male and female scores of size independent factors were directly proportional to each other (slope = 1), indicating extreme similarities between male and female ecology. Females, however, were better adapted to longer, more energy efficient flight than males. Size independent morphological differences among grouse are adaptive and are related to the differences in habitat and foraging behaviour among the species.     

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.     

Sexual size dimorphism in domestic goats,sheep, and their wild relatives

Sexual size dimorphism (SSD) is a widespread phenomenon in different animal taxa, including the subfamily of goats and sheep (Caprinae), which belongs to the most dimorphic mammalian groups. Rensch's rule describes the pattern of SSD, claiming that larger species generally exhibits higher male to female body size ratio. Agreement with Rensch's rule is manifested by slope of the allometric relationship between male and female body size exceeding one. To test this rule, we analysed the data available in the literature on adult body mass of males and females in domestic goat and sheep breeds (169 and 303, respectively) and 37 wild species/subspecies of the subfamily Caprinae. According to the current phylogenetical hypotheses, there are six distinct monophyletic groups with different levels of SSD (expressed as M/F): (1) wild goats (1.83); (2) wild sheep (1.67); (3) non‐European chamoises, including Ovibos moschatus (1.18); (4) European chamoises (1.27); (5) Budorcas taxicolor (1.01); and (6) Pantholops hodgsonii (1.65). Domestication has led to a remarkable decline in SSD of both domestic goats (1.36) and sheep (1.41). The highest regression slope of the relationship between male and female body size is that estimated for wild goats (1.32), followed by wild sheep (1.24), non‐European chamoises (1.14), domestic sheep (1.13), and domestic goats (1.10). Nevertheless, only the last two values are statistically different from one and thus corroborate Rensch's rule. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 872–883.     

Insularity and the evolution of melanism,sexual dichromatism and body size in the worldwide‐distributed barn owl

Island biogeography has provided fundamental hypotheses in population genetics, ecology and evolutionary biology. Insular populations usually face different feeding conditions, predation pressure, intraspecific and interspecific competition than continental populations. This so‐called island syndrome can promote the evolution of specific phenotypes like a small (or large) body size and a light (or dark) colouration as well as influence the evolution of sexual dimorphism. To examine whether insularity leads to phenotypic differentiation in a consistent way in a worldwide‐distributed nonmigratory species, we compared body size, body shape and colouration between insular and continental barn owl (Tyto alba) populations by controlling indirectly for phylogeny. This species is suitable because it varies in pheomelanin‐based colouration from reddish‐brown to white, and it displays eumelanic black spots for which the number and size vary between individuals, populations and species. Females are on average darker pheomelanic and display more and larger eumelanic spots than males. Our results show that on islands barn owls exhibited smaller and fewer eumelanic spots and lighter pheomelanic colouration, and shorter wings than on continents. Sexual dimorphism in pheomelanin‐based colouration was less pronounced on islands than continents (i.e. on islands males tended to be as pheomelanic as females), and on small islands owls were redder pheomelanic and smaller in size than owls living on larger islands. Sexual dimorphism in the size of eumelanic spots was more pronounced (i.e. females displayed much larger spots than males) in barn owls living on islands located further away from a continent. Our study indicates that insular conditions drive the evolution towards a lower degree of eumelanism, smaller body size and affects the evolution of sexual dichromatism in melanin‐based colour traits. The effect of insularity was more pronounced on body size and shape than on melanic traits.     

Sex‐specific phenotypic plasticity in response to the trade‐off between developmental time and body size supports the dimorphic niche hypothesis

Female‐biased sexual size dimorphism (SSD) is widespread in many invertebrate taxa. One hypothesis for the evolution of SSD is the dimorphic niche hypothesis, which states that SSD evolved in response to the different sexual reproductive roles. While females benefit from a larger body size by producing more or larger eggs, males benefit from a faster development, which allows them to fertilize virgin females (protandry). To test this hypothesis, we studied the influence of temperature and intraspecific density on the development of Chorthippus montanus. We reared them in monosexual groups under different conditions and measured adult body size, wing length, nymphal mortality, and development time. The present study revealed an inverse temperature–size relationship: body size increased with increasing temperature in both sexes. Furthermore, we found intersexual differences in the phenotypic response to population density, supporting the dimorphic niches hypothesis. At a lower temperature, female development time increased and male body size decreased with increasing density. Because there was no food limitation, we conclude that interference competition hampered development. By contrast to expectations, mortality decreased with increasing density, suggesting that interference did not negatively affect survival. The present study shows that sex‐specific niche optima may be a major trigger of sexual dimorphisms. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 48–57.