Interspecific relationship between the Japanese horned beetle and two Japanese stag beetle species

The Japanese horned beetle Trypoxylus dichotomus septentrionalis and the Japanese stag beetles Lucanus maculifemoratus maculifemoratus and Prosopocoilus inclinatus inclinatus generally occur syntopically and aggregate on oak tree surfaces that exude sap. Securement of these sap sites might be directly linked to male reproductive success. Among the three species, it is likely that males of T. d. septentrionalis are dominant in occupation of feeding sites because of their larger body size. However, there is no clear evidence of this superiority. Moreover, if T. d. septentrionalis is dominant, the mechanism by which the two stag beetle species secure the feeding sites remains unclear. In the present study, I used body mass to compare the body size among males of T. d. septentrionalis, L. m. maculifemoratus and P. i. inclinatus. Further, to clarify the interspecific relationship between the horned beetle and the two stag beetles, I investigated the seasonality of emergence pattern of males at the feeding sites in the field. Comparison of body mass and observation of fighting behavior revealed that males of T. d. septentrionalis have an apparent superiority over males of the two stag beetle species. The seasonal emergence patterns of the two stag beetle species at the feeding sites showed bimodal distributions, and avoided the peak of emergence in T. d. septentrionalis. My results suggest that the two stag beetle species exhibit mate‐securing tactics by emerging at the feeding sites early and late during the breeding season, in order to avoid encountering T. d. septentrionalis.     

Site‐specific profiles of biochemical properties in the larval digestive tract of Japanese rhinoceros beetle,Trypoxylus dichotomus (Coleoptera: Scarabaeidae)

The larvae of Japanese rhinoceros beetle, Trypoxylus dichotomus, feed on dead plant material in forest soils that are derived from fallen leaves broken down by basidiomycete fungi. Our previous work provided an understanding of the degradation of polysaccharides in dead plant material by T. dichotomus larvae and reported the complexity of the physicochemical and biochemical environment of the larval gut. Here, we examined ten divisions of the digestive tract of T. dichotomus larvae for physicochemical and biochemical conditions to elucidate site‐specifically functional properties along the tract. The distribution of potassium ions, pH, and acetic acid differed markedly along the length of the digestive tract with the potassium ion concentration profile closely reflecting that of pH along the length of the digestive tract. Distinct physicochemical environments were maintained in the digestive tract along with site‐specific polysaccharide degradation. Based on these findings, we suggest that there are metabolic relationships between the activities of the enzymes involved in polysaccharide degradation, the presence of intermediate metabolites and location along the digestive tract. Furthermore, we revealed that the anterior region of the gut plays an important role in the degradation of polysaccharides in the digestive tract of T. dichotomus larvae.     

Experimental evidence for the role of sexual selection in the evolution of cuticular hydrocarbons in the dung beetle,Onthophagus taurus

A role for sexual selection in the evolution of insect cuticular hydrocarbons (CHCs) is suggested by observations of selection acting on male CHCs during female mate choice. However, evidence that CHCs evolve in response to sexual selection is generally lacking, and there is a need to extend our understanding beyond well‐studied taxa. Experimental evolution offers a powerful approach to investigate the effect of sexual selection on the evolution of insect CHCs. We conducted such an experiment using the dung beetle, Onthophagus taurus. After six, 12 and 21 generations of experimental evolution, we measured the CHCs of beetles from three populations subject to sexual selection and three populations within which sexual selection had been removed via enforced monogamy. We found that the male CHC profile responded to the experimental removal of sexual selection. Conversely, the CHC profile of females responded to the presence of sexual selection but not to its removal. These results show that sexual selection can be an important mechanism affecting the evolution of insect CHCs and that male and female CHCs can evolve independently.     

Variation in body size in the giant rhinoceros beetle Trypoxylus dichotomus is mediated by maternal effects on egg size

1. The egg size of insects can vary depending on maternal body size or resource status, and it may influence offspring body size by determining initial resource level. 2. The giant rhinoceros beetle Trypoxylus dichotomus exhibits considerable variation in body size, some of which is attributed to the variation in larval food (humus) quality, although a substantial amount of variation in body size remains unexplained. In the present study, changes in the egg size and offspring body size in response to several maternal variables were examined (i.e. body size, age, and, nutritional status). 3. Nutritional intake of the females during the adult stage did not affect the egg size. Larvae hatched from small eggs partially recovered from the initial disadvantage during their ontogenetic processes by increasing growth rate (i.e. compensatory growth); however, there was still a positive relationship between egg size and pupal body size. 4. Older females produced small eggs, but because of compensatory growth, the pupae were no longer small. By contrast, due to a lack of compensatory growth, small females produced small eggs as well as small pupae. 5. These results suggest that maternal body size affects offspring body size through effects on egg size. This transgenerational effect may account for some of the variation in adult body size of T. dichotomus.     

Molecular information on bowerbird phylogeny and the evolution of exaggerated male characteristics

Mitochondrial DNA cytochrome b sequences of 849 base pairs are reported from eight species of Australian bowerbirds. These sequences are used with three from the literature (Edwards et al., 1991) to investigate bowerbird phylogeny using maximum parsimony and maximum likelihood methods. With respect to the three outgroup species, bowerbirds are shown to be monophyletic with high confidence using the bootstrap. The monogamous Ailuroedus crassirostris (which does not clear display courts) is indicated as the sister group to other bowerbirds. The maypole-builders (Amblyornis macgregoriae and Prionodura newtoniana) are significantly supported as a clade indicating a common origin for maypole type bowers, despite large differences in the design of these species' bowers. The avenue-builders (Sericulus chrysocephalus, Ptilonorhynchus violaceus, Chlamydera maculata and C. nuchalis) are also monophyletic. The pattern of divergence in avenue builders accords with the predictions of Gilliard's (1956, 1963) “transferral effect”. The transference hypothesis is not supported by evidence suggesting that the dull plumage of Scenopoeetes is an ancestral condition in bowerbirds. The use of sticks to build bowers could have had a single evolutionary origin and been secondarily lost in Scenopoeetes, or evolved independently in the avenue and maypole builders.     

The oestrogen pathway underlies the evolution of exaggerated male cranial shapes in Anolis lizards

Sexual dimorphisms vary widely among species. This variation must arise through sex-specific evolutionary modifications to developmental processes. Anolis lizards vary extensively in their expression of cranial dimorphism. Compared with other Anolis species, members of the carolinensis clade have evolved relatively high levels of cranial dimorphism; males of this clade have exceptionally long faces relative to conspecific females. Developmentally, this facial length dimorphism arises through an evolutionarily novel, clade-specific strategy. Our analyses herein reveal that sex-specific regulation of the oestrogen pathway underlies evolution of this exaggerated male phenotype, rather than the androgen or insulin growth factor pathways that have long been considered the primary regulators of male-biased dimorphism among vertebrates. Our results suggest greater intricacy in the genetic mechanisms that underlie sexual dimorphisms than previously appreciated.     

A comparative study on the evolution of reversed size dimorphism in monogamous waders

Sexual dimorphism in size is common in birds. Males are usually larger than females, although in some taxa reversed size dimorphism (RSD) predominates. Whilst direct dimorphism is attributed to sexual selection in males giving greater reproductive access to females, the evolutionary causes of RSD are still unclear. Four different hypotheses could explain the evolution of RSD in monogamous birds: (1) The ‘energy storing’ hypothesis suggests that larger females could accumulate more reserves at wintering or refuelling areas to enable an earlier start to egg laying. (2) According to the ‘incubation ability’ hypothesis, RSD has evolved because large females can incubate more efficiently than small ones. (3) The ‘parental role division’ hypothesis suggests that RSD in monogamous waders has evolved in species with parental role division and uniparental male care of the chicks. It is based on the assumption that small male size facilitates food acquisition in terrestrial habitats where chick rearing takes place and that larger females can accumulate more reserves for egg laying in coastal sites. (3) The ‘display agility’ hypothesis suggests that small males perform better in acrobatic displays presumably involved in mate choice and so RSD may have evolved due to female preference for agile males. I tested these hypotheses in monogamous waders using several comparative methods. Given the current knowledge of the phylogeny of this group, the evolutionary history of waders seems only compatible with the hypothesis that RSD has evolved as an adaptation for increasing display performance in males. In addition, the analysis of wing shape showed that males of species with acrobatic flight displays had wings with higher aspect ratio (wing span/²wing area) than non-acrobatic species, which probably increases flight manoeuvrability during acrobatic displays. In species with acrobatic displays males also had a higher aspect ratio than females although no sexual difference was found in non-acrobatic species. These results suggest that acrobatic flight displays could have produced changes in the morphology of some species and suggest the existence of selection favouring higher manoeuvrability in species with acrobatic flight displays. This supports the validity of the mechanisms proposed by the ‘display agility’ hypothesis to explain the evolution of RSD in waders.     

New perspectives on the evolution of exaggerated traits

The scaling of body parts is central to the evolution of morphology and shape. Most traits scale proportionally with each other and body size such that larger adults are essentially magnified versions of smaller ones. This pattern is so ubiquitous that departures from it – disproportionate scaling between trait and body size – pique interest because it can generate dramatically exaggerated traits. These extreme morphologies are frequently hypothesized to result from sexual selection and their study has a long history, with several hypotheses seeking to explain their evolution. Despite this effort, surprisingly little progress has been made in demonstrating the forms of selection that produce different scaling patterns or in identifying the mechanisms that underlie the expression and evolution of scaling relationships. Here we review recent insights regarding the proximate mechanisms that regulate and integrate trait growth and that offer a new framework for studying the evolution of morphological scaling.     

Mechanical limits to maximum weapon size in a giant rhinoceros beetle

The horns of giant rhinoceros beetles are a classic example of the elaborate morphologies that can result from sexual selection. Theory predicts that sexual traits will evolve to be increasingly exaggerated until survival costs balance the reproductive benefits of further trait elaboration. In Trypoxylus dichotomus, long horns confer a competitive advantage to males, yet previous studies have found that they do not incur survival costs. It is therefore unlikely that horn size is limited by the theoretical cost–benefit equilibrium. However, males sometimes fight vigorously enough to break their horns, so mechanical limits may set an upper bound on horn size. Here, I tested this mechanical limit hypothesis by measuring safety factors across the full range of horn sizes. Safety factors were calculated as the ratio between the force required to break a horn and the maximum force exerted on a horn during a typical fight. I found that safety factors decrease with increasing horn length, indicating that the risk of breakage is indeed highest for the longest horns. Structural failure of oversized horns may therefore oppose the continued exaggeration of horn length driven by male–male competition and set a mechanical limit on the maximum size of rhinoceros beetle horns.     

The evolution of static allometry in sexually selected traits

Although it has been the subject of verbal theory since Darwin, the evolution of morphological trait allometries remains poorly understood, especially in the context of sexual selection. Here we present an allocation trade-off model that predicts the optimal pattern of allometry under different selective regimes. We derive a general solution that has a simple and intuitive interpretation and use it to investigate several examples of fitness functions. Verbal arguments have suggested cost or benefit scenarios under which sexual selection on signal or weapon traits may favor larger individuals with disproportionately larger traits (i.e., positive allometry). However, our results suggest that this is necessarily true only under a precisely specified set of conditions: positive allometry will evolve when the marginal fitness gains from an increase in relative trait size are greater for large individuals than for small ones. Thus, the optimal allometric pattern depends on the precise nature of net selection, and simple examples readily yield isometry, positive or negative allometry, or polymorphisms corresponding to sigmoidal scaling. The variety of allometric patterns predicted by our model is consistent with the diversity of patterns observed in empirical studies on the allometries of sexually selected traits. More generally, our findings highlight the difficulty of inferring complex underlying processes from simple emergent patterns.     

The phylogeny of diving beetles (Coleoptera: Dytiscidae) and the evolution of sexual conflict

A model of evolution based on conflicts of interest between the sexes over mating decisions is examined in relation to diving beetles (Dytiscidae). The model predicts the following evolutionary sequence: (1) cost to females of mating increases, (2) females evolve behavioural resistance to male mating attempts, (3) males evolve devices to overcome female resistance, and (4) females evolve morphological counter-adaptations to the male devices. This model is tested using species of Dytiscidae, in which (1) some species have a very long mating duration while others mate quickly, (2) females of some species resist male mating attempts by swift and erratic swimming when seized by a male, (3) males of some species possess a grasping device in the form of sucker-shaped setae on the legs used to adhere to the pronota or elytra of females prior to mating, and (4) females of some species have a modified dorsal cuticle with irregular sculpturing which appears to interfere with the male adhesive setae. The predicted order of evolution of some of these features was tested in a cladistic analysis of 52 taxa in Dytiscidae and Hygrobiidae using characters from adult and larval morphology and a portion of the gene wingless . The combined analysis resulted in nine most parsimonious cladograms. The consensus cladogram of these indicates that male sucker setae arose a single time in a clade of Dytiscinae. Nested within this clade are five groups with an independently derived, modified dorsal cuticle in females. This pattern of characters in Dytiscinae is consistent with the prediction implied by the model of sexual selection. The utility of wingless as a marker for phylogenetic analysis of diving beetles is discussed, and the resulting phylogeny is compared with previous analyses and current classification.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79 , 359–388.     

Sprint speed is not reduced by exaggerated male weapons in a flower beetle Dicronocephalus wallichii

Exaggerated sexually selected traits are assumed to decrease the mobility of bearers. However, previous empirical studies have often failed to support this assumption, possibly because locomotor performance represents the integration of numerous morphological, physiological and behavioural traits. Males of a flower beetle Dicronocephalus wallichii Pouillaude 1914 (Coleoptera: Scarabaeidae: Cetoniinae) possess elongated forelegs and a pair of exaggerated horns, which are used as dual weapons in male–male competition for mates. We investigated whether these two sexual traits impede the maximum sprint speed on bamboo branches with different angles and thicknesses under laboratory conditions. Our results suggested that no negative relationship exists between relative foreleg length or horn length and sprint speed. Elongated forelegs and horns may entail negligible locomotor costs. Males with longer horns and forelegs were found to have longer midlegs and hindlegs independent of body size. Thus, elongated midlegs and hindlegs in males may enhance balance, stabilize running on bamboo branches and compensate for the locomotor costs of bearing exaggerated weapons. Furthermore, a positive relationship was found between horn length and sprint speed on a horizontal branch. Males with longer horns probably have more energy and/or invest more heavily in appendage musculature. As is known in other animals, male horns of D. wallichii may act as honest indicators of body condition.     

Predicting the evolution of sexual size dimorphism

11 , Evolution 34 : 292–305) equations for predicting the evolution of sexual size dimorphism (SSD) through frequency‐dependent sexual selection, and frequency‐independent natural selection, were tested against results obtained from a stochastic genetic simulation model. The SSD evolved faster than predicted, due to temporary increases in the genetic variance brought about by directional selection. Predictions for the magnitude of SSD at equilibrium were very accurate for weak sexual selection. With stronger sexual selection the total response was greater than predicted. Large changes in SSD can occur without significant long‐term change in the genetic correlation between the sexes. Our results suggest that genetic correlations constrain both the short‐term and long‐term evolution of SSD less than predicted by the Lande model.     

Variation in cross‐sectional horn shape within and among rhinoceros beetle species

Sexual selection has equipped male rhinoceros beetles with large horns on their head and prothorax to aid in battle over access to females. Horns are used to pry and dislodge opponents from resource sites that attract females, so an optimal horn should be able both to withstand the high stresses imposed during fights, and to resist deflection in response to these loads. We examined the cross‐sectional morphology of horns using micro‐computed tomography scanning to determine how horn structure changes with horn length to withstand the different fighting loads. Specifically, we measured the second moment of area of horns within and among rhinoceros beetle species to assess whether changes in cross‐sectional morphology accompany changes in body size in order to maintain high strength and stiffness during fights. We find that the second moment of area of horns increases with body size both intra‐specifically and inter‐specifically, and that these relationships closely fit those predicted if horns have been selected to be strong and stiff fighting structures. Our results therefore support the hypothesis that rhinoceros beetle horns are structurally adapted for combat.     

On the threshold of dispersal: hitchhiking on a giant fly favours exaggerated male traits in a male‐dimorphic pseudoscorpion

The evolution of exaggerated male traits is frequently driven by competition between males to control resources critical for female survival and/or reproductive success. For flightless arthropods specializing on patchy habitats, dispersal agents may represent one such critical resource. The Neotropical pseudoscorpion, Semeiochernes armiger, disperses to new habitats by attaching to the giant timber fly, Pantophthalmus tabaninus, as it ecloses from pupal boreholes within decaying Ficus trees. In a study that combined field observations of mating with experimental removal of individuals from a large, pre‐dispersal population, our morphometric analyses revealed that S. armiger is among the most highly sexually dimorphic pseudoscorpions known, with males possessing unusual, triangular‐shaped pedipalpal chelae (hands) and a male‐specific, dimorphic chela peg that exhibits threshold trait expression. Several lines of evidence indicate that extreme sexual dimorphism in S. armiger results from male competition to monopolize pantophthalmid bores as strategic sites for inseminating females on the verge of dispersal. Sexually dimorphic pedipalpal characters were significantly larger in males located in and around pantophthalmid boreholes, compared with males collected at the periphery of the pantophthalmid emergence zone. Removal of pseudoscorpions resulted in a significant decline in pedipalpal size of males associated with pantophthalmid bores, followed by a rebound in size after collected individuals were returned to the tree. Most significantly, field observations of mating indicate that this competition translates into intense selection for exaggerated male traits, with all traits of the sexually dimorphic chelae exhibiting highly significant sexual selection differentials in males. © 2013 The Linnean Society of London