Male Weapon Research Articles

Phylogeography of lethal male fighting in a social spider mite.

When males fight for access to females, such conflict rarely escalates into lethal fight because the risks and costs involved, that is, severe injury or death, are too high. The social spider mite, Stigmaeopsis miscanthi, does exhibit lethal male fights, and this male–male aggressiveness varies among populations. To understand the evolution of lethal fighting, we investigated aggressiveness in 42 populations and phylogenetic relationships in 47 populations along the Japanese archipelago. By analysis of the male weapon morph, a proxy for aggressiveness, we confirmed the existence of a mildly aggressive (ML) form, besides the low aggression (LW) and high aggression (HG) forms reported earlier. To evaluate demographic history of these three forms, we employed the approximate Bayesian computation approach using mtCOI sequences and taking into consideration the postlast glacial expansion history of the host plant, Miscanthus sinensis. As results, hierarchical split models are more likely to explain the observed genetic pattern than admixture models, and the ML form in the subtropical region was considered the ancestral group. The inferred demographic history was consistent with the one reconstructed for the host plant in a previous study. The LW form was split from the ML form during the last glacial period (20,000–40,000 years BP), and subsequently, the HG form was split from the ML form at the end of or after the last glacial period (5,494–10,988 years BP). The results also suggest that the mite invaded Japan more than once, resulting in the present parapatric distribution of LW and HG forms in eastern Japan.

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

AbstractExaggerated 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.

While males fight, females choose: male phenotypic quality informs female mate choice in mammals

Theoretical support exists for an exaggerated male structure to serve as both a weapon for intrasexual competition and as an ornament to signal quality and promote female choice. However, there is little, if any, evidence to support this theory in male–male competition breeding systems. Using white-tailed deer, Odocoileus virginianus, as a model species, we manipulated antler size on males while controlling for body size and age and allowed 25 oestrous females the opportunity to choose between pairs of segregated males with either large or small antlers. By segregating males, we were able to remove any intrasexual male competition and isolate the effects of female choice. Using various behavioural indications of female choice, we demonstrate that females prefer males with large antlers to those with small antlers. Because antler size is heritable in deer, this female preference for larger antlers may be adaptive by increasing the reproductive success of her male offspring. Our unique antler manipulation study supports the armament-ornament model where male weapons can simultaneously serve as ornaments to females and weapons in male–male competition breeding systems.

Novel host plant leads to the loss of sexual dimorphism in a sexually selected male weapon.

In this time of massive global change, species are now frequently interacting with novel players. Greater insight into the impact of these novel interactions on traits linked to fitness is essential, because effects on these traits can hinder population existence or promote rapid adaptation. Sexually selected weapons and ornaments frequently influence fitness and often have heightened condition-dependence in response to nutrition. Condition-dependence in response to different ecological conditions, a form of developmental plasticity, may be responsible for much of the intraspecific variation in sexually selected ornaments and weapons in wild populations. Here we examined the consequences of developing on a novel plant for the expression of size and shape in the leaf-footed cactus bug Narnia femorata (Hemiptera: Coreidae). The males of this species possess enlarged, sexually dimorphic femurs on their hind legs. These legs are used as weapons in male-male contests. Females are typically larger in overall body size. Our study revealed that developing upon a novel host can lead to pronounced phenotypically plastic change in sexually dimorphic traits. Male hind femurs were greatly impacted by the novel diet to the extent that the sexual dimorphism in hind femurs was lost. Further, dimorphism in body size increased, as males became tiny adults while females better maintained their body size. These patterns underscore the complex effects that novel species interactions may have on sexual phenotypes.

Latitudinal cline in weapon allometry and phenology of the European stag beetle

Animal body size commonly exhibits a remarkable variation in response to environmental conditions. Latitude, when correlated with temperature, rainfall and seasonality, represents one of the main determinants of variation in body size, as well as in allometry. It has long been recognised that populations of larger body size are found in colder environments (Bergmann’s Rule), a cornerstone of evolutionary ecology. However, the way in which latitude might influence investment in exaggerated weapons of animals has received little attention. The European stag beetle Lucanus cervus (Linnaeus, 1758) is the focus of this study. Males of this species exhibit exaggerated mandibles, mainly used as weapons during intra-sexual conflicts. Five populations ranging from northern Italy to the southern limit of the distribution of L. cervus were analysed. Combining morphological and phenological data, latitudinal variation in body size, weapon investment and activity period of the adults were evaluated. The analysis of the allometry of mandibles strongly supported the presence of two male morphs. Large males (major morph) invest significantly more in weapons compared to males of the minor morph. Consistent with Bergmann’s Rule, these results confirmed that the stag beetle body size increased at higher latitudes (N) and that this increase in size triggers an arms race which leads to further exaggeration of male weapons which is particularly evident in major males. In this morph, the mandible allometric coefficient line was steeper for the northern populations. The activity period also varied with latitude, beginning later at lower latitudes. Characterisation and comparison of adult phenologies provide valuable data to be used in the design of monitoring programmes for this threatened species and are important for modelling the species responses to climate change.

It takes two to tango: functional roles, sexual selection and allometry of multiple male weapons in the flower beetle Dicronocephalus wallichii bourgoini

It takes two to tango: functional roles, sexual selection and allometry of multiple male weapons in the flower beetle Dicronocephalus wallichii bourgoini

Histone deacetylases control module-specific phenotypic plasticity in beetle weapons

Nutritional conditions during early development influence the plastic expression of adult phenotypes. Among several body modules of animals, the development of sexually selected exaggerated traits exhibits striking nutrition sensitivity, resulting in positive allometry and hypervariability distinct from other traits. Using de novo RNA sequencing and comprehensive RNA interference (RNAi) for epigenetic modifying factors, we found that histone deacetylases (HDACs) and polycomb group (PcG) proteins preferentially influence the size of mandibles (exaggerated male weapon) and demonstrate nutrition-dependent hypervariability in the broad-horned flour beetle, Gnatocerus cornutus RNAi-mediated HDAC1 knockdown (KD) in G. cornutus larvae caused specific curtailment of mandibles in adults, whereas HDAC3 KD led to hypertrophy. Notably, these KDs conferred opposite effects on wing size, but little effect on the size of the core body and genital modules. PcG RNAi also reduced adult mandible size. These results suggest that the plastic development of exaggerated traits is controlled in a module-specific manner by HDACs.

No evidence for a trade-off between sperm length and male premating weaponry.

Male ornaments and armaments that mediate success in mate acquisition and ejaculate traits influencing competitive fertilization success are under intense sexual selection. However, relative investment in these pre- and post-copulatory traits depends on the relative importance of either selection episode and on the energetic costs and fitness gains of investing in these traits. Theoretical and empirical work has improved our understanding of how precopulatory sexual traits and investments in sperm production covary in this context. It has recently also been suggested that male weapon size may trade off with sperm length as another post-copulatory sexual trait, but the theoretical framework for this suggestion remains unclear. We evaluated the relationship between precopulatory armaments and sperm length, previously reported in ungulates, in five taxa as well as meta-analytically. Within and between taxa, we found no evidence for a negative or positive relationship between sperm length and male traits that are important in male-male contest competition. It is important to consider pre- and post-copulatory sexual selection together to understand fitness, and to study investments in different reproductive traits jointly rather than separately. A trade-off between pre- and post-copulatory sexual traits may not manifest itself in sperm length but rather in sperm number or function. Particularly in large-bodied taxa such as ungulates, sperm number is more variable interspecifically and likely to be under more intense selection than sperm length. We discuss our and the previous results in this context.

Is diversification in male reproductive traits driven by evolutionary trade-offs between weapons and nuptial gifts?

Many male animals have evolved exaggerated traits that they use in combat with rival males to gain access to females and secure their reproductive success. But some male animals invest in nuptial gifts that gains them access to females. Both these reproductive strategies are costly in that resources are needed to produce the weapon or nuptial gift. In closely related species where both weapons and nuptial gifts are present, little is known about the potential evolutionary trade-off faced by males that have these traits. In this study, we use dobsonflies (order Megaloptera, family Corydalidae, subfamily Corydalinae) to examine the presence and absence of enlarged male weapons versus nuptial gifts within and among species. Many dobsonfly species are sexually dimorphic, and males possess extremely enlarged mandibles that they use in battles, whereas in other species, males produce large nuptial gifts that increase female fecundity. In our study, we show that male accessory gland size strongly correlates with nuptial gift size and that when male weapons are large, nuptial gifts are small and vice versa. We mapped weapons and nuptial gifts onto a phylogeny we constructed of 57 species of dobsonflies. Our among-species comparison shows that large nuptial gift production evolved in many species of dobsonfly but is absent from those with exaggerated weapons. This pattern supports the potential explanation that the trade-off in resource allocation between weapons and nuptial gifts is important in driving the diversity of male mating strategies seen in the dobsonflies, whereas reduced male-male competition in the species producing large spermatophores could be an alternative explanation on their loss of male weapons. Our results shed new light on the evolutionary interplay of multiple sexually selected traits in animals.

Inter‐ and intrasexual genetic correlations of exaggerated traits and locomotor activity

Exaggerated traits in males can be costly and therefore can negatively affect fitness. Although these costs are thought to be male specific, traits that have a negative effect due to exaggeration are often shared between the sexes as life-history traits. When there are genetic intersexual correlations for these shared characters, the evolution of the exaggerated traits can impose these costs on nonadorned females through the intersexual correlation. Thus, the exaggerated traits can constrain optimum development of female characters, even if the females lack these exaggerations completely. However, investigation of this pattern has been largely ignored, and thus, it is necessary to investigate genetic architectures of these traits within and across the sexes. Male flour beetles, Gnatocerus cornutus, have enlarged mandibles that are used in male-male competition, but females lack this character completely. Using a traditional full-sib/half-sib breeding design, we detected a negative intrasexual genetic correlation between male weapon size and locomotor activity, but not an intersexual genetic correlation for locomotor activity. After subjecting this weapon to 17 generations of bidirectional selection, we found a correlated response to locomotor activity in the male, whereas there was no correlated response in the female. Our results suggest that the costs of exaggerated traits to locomotion are not imposed on females and would be male specific. This is partly explained by genetic decoupling of locomotor activities across the sexes.

Toxoplasma gondii: sexual transmission in mice.

This study was performed to evaluate sexual transmission of Toxoplasma gondii in mice. RH strain tachyzoites were intraperitoneally inoculated into 10 Balb/C male mice and after 48h, their semen were collected from epididymis and examined by giemsa staining and PCR. Twenty Balb/C female mice mated with four infected male mice four times and any mating time was 48h whilst 20 female control mice mated with four uninfected male mice for 8days. Female mate choice was assessed using a three-chambered cage. Four female mice were placed in a central chamber and in one side of it, two infected male mice were kept and in other side, two naïve male mice were placed. Due each quarter, every of the female movement was reported and then the female was replaced to middle chamber. Besides on the detection of DNA and whole parasite in semen, no abortion and death was seen in female mice. Pregnancy was seen only 4 out of 20 female mice which mated with infected males while 17 pregnancies were seen from 20 control female mice (P value=0.0001). No statistical significant was seen in female mate choice between naïve male (45 movement) and infected male (36 movement). This study showed that toxoplasmosis could not transmit to female mice and their offspring due to mating and the parasite had not effect on female mate choice. It seems that infected male mice cannot entirely mate with females due to reduction of male weapon and body size, physiological vigor and energy.

Sperm Traits Negatively Covary with Size and Asymmetry of a Secondary Sexual Trait in a Freshwater Crayfish

In species where females mate promiscuously, the reproductive success of males depends both on their ability to acquire mates (pre-copulatory sexual selection) and ability of their ejaculates to outcompete those of other males (post-copulatory sexual selection). Sperm competition theory predicts a negative relationship between investment in body traits favouring mate acquisition (secondary sexual characters, SSCs) and investment in ejaculate size or quality, due to the inherent costs of sperm production. In contrast, the phenotype-linked fertility hypothesis posits that male fertilizing efficiency is reliably reflected by the phenotypic expression of male SSCs, allowing females to obtain direct benefits by selecting more ornamented males as copulation partners. In this study, we investigated the relationships between male SSCs and size and quality (viability and longevity) of ejaculates allocated to females in mating trials of the freshwater crayfish Austropotamobius italicus. We showed that the relative size of male weapons, the chelae, was negatively related to ejaculate size, and that chelae asymmetry, resulting from regeneration of lost chelipeds, negatively covaried with sperm longevity. Moreover, males allocated more viable sperm to mates from their own rather than different stream of origin. Our findings thus suggest that, according to sperm competition theory, pre-copulatory sexual selection for large weapons used in male fighting may counteract post-copulatory sperm competition in this crayfish species, and that investment in cheliped regeneration may impair ejaculate quality.

Across- and Within-Population Differences in the Size and Scaling Relationship of a Sexually Selected Trait in Leptoscelis tricolor(Hemiptera: Coreidae)

Abstract Sexually selected traits are often assumed to convey information to conspecifics about the condition and genetic quality of an individual. However, sexually selected traits also are hypothesized to be among the most rapidly evolving and phenotypically plastic traits in natural populations. When found, such variability can bring into question the reliability of these traits as sexually-selected signals. Here, we examine the expression of a male weapon over multiple spatial and temporal scales in the wild. Specifically, we measured the expression of enlarged hind femora across and within populations of the Neotropical bug Leptoscelis tricolor Westwood (Hemiptera: Coreidae). We found significant variation in the size of this trait across populations and even within a population according to the plant species where adult males were collected. We also found differences in the intercept of the scaling relationship with body size; for a given body size, males in some populations express larger sexually selected traits than males in other populations. These results support the hypothesis that the expression of sexually selected traits is dynamic over space and perhaps time. Such patterns suggest that environmental variation or small amounts of movement across genetically differentiated populations may confound the reliability of information contained in the expression of these traits.

Two Thresholds, Three Male Forms Result in Facultative Male Trimorphism in Beetles

Male animals of many species deploy conditional reproductive strategies that contain distinct alternative phenotypes. Such facultatively expressed male tactics are assumed to be due to a single developmental threshold mechanism switching between the expression of two alternative phenotypes. However, we discovered a clade of dung beetles that commonly expresses two threshold mechanisms, resulting in three alternative phenotypes (male trimorphism). Once recognized, we found trimorphism in other beetle families that involves different types of male weapons. Evidence that insects assumed to be dimorphic can express three facultative male forms suggests that we need to adjust how we think about animal mating systems and the evolution of conditional strategies.

Youths carrying a weapon or using a weapon in a fight: what makes the difference?

The objective of this study was to characterize weapon-carrying adolescents and to assess whether weapon carriers differ from weapon users. Data were drawn from a cross-sectional school-based survey of 7548 adolescents aged 16-20 years in Switzerland. Youths carrying a weapon were compared with those who do not. Subsequently, weapon carriers were divided into those who had used it in a fight and those who had not. Individual, family, school and social factors were analyzed using bivariate and stepwise multivariate analysis. For both genders, delinquent behavior and being victim of physical violence were associated with weapon carrying. For males, quarreling while intoxicated, being an apprentice, being sensation seekers, having a tattoo, having a poor relationship with parents and practicing unsafe sex were also related to weapon carrying. Compared with weapon carriers, female weapon users were more likely to be regular smokers. Male weapon users were foreign born, urban and apprentices; had poor school connectedness; practiced unsafe sex and quarreled while intoxicated. Carrying a weapon is a relatively frequent behavior among youths in Switzerland and a sizeable proportion of weapon carriers have used it in a fight. Weapon carrying should be part of the clinical assessment and preventive counseling of adolescents. Preventive programs specific for at-risk youth groups need to be developed.

Identifying a causal agent of sexual selection on weaponry in an insect

In many animal species, males do not seek females directly but instead locate and defend sites that contain spatially or temporally limited resources essential to female survival and reproduction. Resident males that successfully repel conspecific rivals can mate with females attracted to these resources. In theory, increasing resource value increases harem size and thus increases the opportunity (Imates) for and strength of sexual selection on traits crucial to male resource-holding potential and mating success. I experimentally tested this hypothesis in the field using the Wellington tree weta, Hemideina crassidens (Orthoptera: Tettigonioidea: Anostostomatidae), a sexually dimorphic insect in which males use their enlarged mandibles as weapons in male‐male contests over access to females sheltering in tree cavities (galleries). By manipulating gallery size, I showed that, compared with smaller galleries, larger galleries housed larger harems. Variation in gallery size was an important determinant of Imates, but contrary to expectation, greater opportunity existed in small galleries compared with large galleries. As predicted, male weapon size was under stronger directional selection in large galleries because the fitness benefits were greater under these conditions compared with small galleries. My results help explain the positive association between average weapon size and average gallery size observed within and among tree weta populations in New Zealand. Key words: conservation, Hemideina, longitudinal study, resource defense, selection gradient, sexual dimorphism. [Behav Ecol]

The Relationship Between Resource Control, Association with Females and Male Weapon Size in a Male Dominance Insect

AbstractIn species with a resource‐defence (male dominance) mating system, males are expected to maximize fitness by controlling resources deemed more valuable by sexually receptive females because these sites attract more mates. Furthermore, males, which control more valuable resources should themselves be of high quality. I experimentally tested these predictions in the laboratory using the sexually dimorphic Wellington tree weta, Hemideina crassidens (Blanchard) (Orthoptera: Tettigonioidea: Anostostomatidae). Male H. crassidens use their mandibular weaponry to fight for control of harems (groups of adult females) that seek shelter in trees cavities (galleries). As predicted, larger galleries housed significantly larger groups of females and males with larger weaponry controlled large galleries significantly more often. Therefore, galleries with a larger volume are likely considered more valuable by males because they house larger harems. However, contrary to prediction, males with larger weaponry did not reside with significantly more females overall because females did not always form the largest possible groups in galleries and males with smaller weaponry were able to reside with single females in small galleries. The latter observation suggests a possible alternative mating strategy by disadvantaged males.

Vertebrate Secondary Sexual Characteristics-Physiological Mechanisms and Evolutionary Patterns.

As most commonly presented, the organization-activation theory of sexual differentiation emphasizes the importance of the relative age of the organism for understanding steroid hormone effects. However, considering the actual physiological mechanisms of tissue enlargement (hyperplasia/hypertrophy) provides an additional perspective for interpreting patterns of evolutionary change in sexual dimorphism. Using that focus, it is possible to suggest mechanistic explanations for patterns of allometry and the size of some secondary sexual characteristics produced by "runaway" selection. It can also lead to the formulation of testable hypotheses regarding the type of sexually dimorphic characters that might evolve through "good genes" models; the relationship between intrasexual competition, polygyny, and the development and size of male weapons; and the control and type of secondary sexual characteristics that will be present in males with associated and disassociated reproductive patterns.

Exaggerated male eye span influences contest outcome in stalk-eyed flies (Diopsidae)

Evolution of male weapons or status signals has been hypothesized to precede evolution of female mating preferences for those traits. We used staged male fights among three species of Malaysian stalk-eyed flies (Diptera: Diopsidae) to determine if elongated eye span, which is preferred by females in two sexually dimorphic species, influences contest outcome. Extreme sexual di- morphism, with large males possessing longer eye span than females, is shared by Cyrtodiopsis whitei and C. dalmanni. In contrast, C. quinqueguttata exhibits a more ancestral condition - short, sexually monomorphic eye stalks. Videotape analysis of 20-min paired contests revealed that males with larger eye span and body size won more fights in the dimorphic, but not monomor- phic, species. To determine if males from the dimorphic species use eye span directly to resolve contests, we competed male C. dalmanni from lines that had under- gone artificial selection for 30 generations to increase or decrease eye span. We found that eye span, indepen- dently of body size, determines contest outcome in selected-line males. Furthermore, in both dimorphic species, the average encounter duration declined as the eye span diAerence between contestants increased, as expected if males use eye span to assess opponent size. The number of encounters also increased with age in dimorphic, but not monomorphic, species. Selected-line males did not diAer from outbred males in either fight duration or number of encounters. We conclude that exaggerated male eye stalks evolved to influence both competitive interactions and female mating preferences in these spectacular flies.