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Sexual conflict is a strong driver of evolution. The evolutionary outcomes of sexual conflict can, in turn, influence ecological processes within populations, for example, demography. However, evidence for the latter hypothesis is scarce, especially in the wild. Here, we show that sexual conflict is associated with demographic processes determining population size in the ground beetle Carabus insulicola with elaborate male and female genitalia based on individual-and population-level analyses. We found that sexually antagonistic selection can operate on the genitalia: longer male genitalia can be beneficial in sperm competition but decrease female reproductive success with increased egg dumping, whereas longer female genitalia are resistant to this male harassment via decreased egg dumping and increased fertilization rate. As expected from sexually antagonistic coevolution due to sexual conflict, we detected coevolutionary divergence between male and female genital sizes among populations. In parallel with decrease in female reproductive success, more harmful males with longer genitalia and less resistant females with shorter genitalia were related to small effective population sizes. Thus, sexual conflict may promote coevolutionary diversification between sexual traits, and this was associated with a demographic process. Our findings provide an insight into sex-driven eco-evolutionary dynamics in the wild.

Predator–prey interactions may be responsible for enormous morphological diversity in prey species. We performed predation experiments with morphological manipulations (ablation) to investigate the defensive function of dorsal spines and explanate margins in Cassidinae leaf beetles against three types of predators: assassin bugs (stinger), crab spiders (biter), and tree frogs (swallower). There was mixed support for the importance of primary defense mechanisms (i.e., preventing detection or identification). Intact spined prey possessing dorsal spines were more likely to be attacked by assassin bugs and tree frogs, while intact armored prey possessing explanate margins were likely to avoid attack by assassin bugs. In support of the secondary defense mechanisms (i.e., preventing subjugation), dorsal spines had a significant physical defensive function against tree frogs, and explanate margins protected against assassin bugs and crab spiders. Our results suggest a trade‐off between primary and secondary defenses. Dorsal spines improved the secondary defense but weakened the primary defense against tree frogs. We also detected a trade‐off in which dorsal spines and explanate margins improved secondary defenses against mutually exclusive predator types. Adaptation to different predatory regimes and functional trade‐offs may mediate the diversification of external morphological defenses in Cassidinae leaf beetles. We revealed defensive function of dorsal spines and explanate margins in Cassidinae leaf beetles and detected functional trade‐offs. These results suggest that adaptation to different predatory regimes and functional trade‐offs contribute to the diversification of external morphologies in herbivorous insects.

1. Body size differences among coexisting related species are common, but the actual effect of these differences in mitigating interspecific interactions, such as resource competition and reproductive interference, is poorly understood. 2. Local assemblages of the ground beetle genus Carabus (subgenus Ohomopterus) typically consist of two or more species of varying sizes. Through foraging and mating experiments using four Ohomopterus species in parapatry and sympatry, we examined whether interspecific body size differences are effective in partitioning food resources or reducing reproductive interference. 3. Because larval Ohomopterus feed exclusively on earthworms, body size differences may be related to partitioning earthworms of different sizes. However, larvae did not exhibit differences in selectivity or attack success on earthworms of different sizes based on larval body size, indicating little possibility of partitioning food by body size. 4. In contrast, interspecific mating behaviours, such as mate recognition, mounting, and copulation, were hindered when body size differences were large; copulation was frequently accomplished between parapatric species with smaller body size differences. 5. These results suggest that body size differences between species effectively reduce reproductive interference, rather than resource competition. Although body size differences in coexisting closely related species have been considered to function in resource partitioning, they may function primarily in reproductive isolation and thereby facilitate coexistence of species.

Sexual traits are subject to evolutionary forces that maximize reproductive benefits and minimize survival costs, both of which can depend on environmental conditions. Latitude explains substantial variation in environmental conditions. However, little is known about the relationship between sexual trait variation and latitude, although body size often correlates with latitude. We examined latitudinal variation in male and female sexual traits in 22 populations of the false blister beetle Oedemera sexualis in the Japanese Archipelago. Males possess massive hind legs that function as a female‐grasping apparatus, while females possess slender hind legs that are used to dislodge mounting males. Morphometric analyses revealed that male and female body size (elytron length), length and width of the hind femur and tibia, and allometric slopes of these four hind leg dimensions differed significantly among populations. Of these, three traits showed latitudinal variation, namely, male hind femur was stouter; female hind tibia was slenderer, and female body was smaller at lower latitudes than at higher latitudes. Hind leg sizes and shapes, as measured by principal component analysis of these four hind leg dimensions in each sex, covaried significantly between sexes, suggesting coevolutionary diversification in sexual traits. Covariation between sexes was weaker when variation in these traits with latitude was removed. These results suggest that coevolutionary diversification between male and female sexual traits is mediated by environmental conditions that vary with latitude. Male Oedemera sexualis possess massive hind legs that function as a female‐grasping apparatus, while females possess slender hind legs that are used to dislodge mounting males. These sexually dimorphic traits show coevolutionary diversification along latitude in the Japanese Archipelago.

The lacustrine shrimp Palaemon paucidens undertakes seasonal migration between shallow waters in spring and summer to deep waters in autumn and winter in Lake Biwa in Japan. Previous studies hypothesized that the migration to deep waters was for hibernation. The findings of this study oppose this hypothesis. We showed that P. paucidens is physiologically active in deep waters during winter; that the carbon stable isotope ratio indicated that the species forages in winter and that there was a difference in food sources between summer and winter; that lipid content was highest in January; and that gene expression activity (as measured by the RNA/DNA ratio) remained constant throughout the year. Thus, P. paucidens individuals that migrate to the bottom are likely to forage in winter, but do not hibernate. Nutrients gained in winter were not reflected in individual growth but may have been allocated to gonadal growth and the production of gametes, suggesting that winter foraging by this species in deep sites contributes to obtain resources for reproductive investment. In addition, we found that both small individuals with no reproductive experience and relatively large females with reproductive experience overwintered, indicating that P. paucidens has a life‐cycle that is longer than 1 yr in Lake Biwa.

Spatial and temporal variation in ecological environments may result in spatial and temporal variation in communities. Temporal studies of biodiversity are essential for forecasting future changes in community structure and ecosystem function. Therefore, determining the mechanisms that drive temporal change in communities remains an important and interesting challenge in ecology. We quantified spatial and temporal variations in carabid beetle communities and site-specific environmental factors for 5 years at nine study sites on three mountains in the Baekdudaegan Mountain Range, Korea. Carabid beetle communities exhibited significant temporal variation, which was larger than spatial variations between and within mountains. Environmental factors mostly varied between sites within mountains. Community variation was only weakly associated with environmental factors at wide scales, i.e., between sites on three mountains, but was strongly associated at narrow spatial scales, i.e., between sites within one mountain. Our results indicate that temporal variation in communities occurs in response to variations in the local climate, and that the patterns of temporal variation differ between mountains. Thus, temporal surveys of insect communities and climates at local scales are important for predicting temporal changes in the communities.

Exogenous selection via interactions between organisms and environments may influence the dynamics of hybrid zones between species in multiple ways. Two major models of a hybrid zone allowed us to hypothesize that environmental conditions influence hybrid zone dynamics in two ways. In the first model, an environmental gradient determines the mosaic distribution at the boundary between ecologically differentiated species (mosaic hybrid zone model). In the second model, a patch of unsuitable habitat traps a hybrid zone between species whose hybrids are unfit (tension zone model). To test these, we examined the environmental factors influencing the spatial structure of a hybrid zone between the ground beetles Carabus maiyasanus and C. iwawakianus using GIS‐based quantification of environmental factors and a statistical comparison of species distribution models (SDMs). We determined that both of the hypothetical processes can be important in the hybrid zone. We detected interspecific differences in the environmental factors in presence localities and their relative contribution in SDMs. SDMs were not identical between species even within contact areas, but tended to be similar within the range of each species. These results suggest an association between environments and species, and provide evidence that ecological differentiation between species plays a role in the maintenance of the hybrid zone. Contact areas were characterized by a relatively high temperature, low precipitation, and high topological wetness. Thus, the contact areas were regarded as being located in an unsuitable habitat with a drier climate, where those populations are likely to occur in patches with limited precipitation concentrated. A comparison of spatial scales suggests that exogenous selection via environmental factors may be weaker than endogenous selection via genitalic incompatibility. GIS‐based environmental quantification and species distribution modeling revealed ecological differentiation between parapatric ground beetle species Carabus maiyasanus and C. iwawakianus and relatively drier environmental condition in their contact area. These results suggest that ecological differentiation and unsuitable habitat mediate the dynamics of the hybrid zone.

Speciation studies seek to clarify the origin of reproductive isolation, the various mechanisms working from mate recognition through postzygotic stages. Asymmetric effects of isolating barriers can result in asymmetrical gene introgression during interspecific hybridization. The flightless ground beetles Carabus yamato and C. albrechti are distributed parapatrically in Japan, showing repeated asymmetrical introgression of mitochondria from C. albrechti to C. yamato. This pattern suggests that reproductive isolation between these species is strong, but incomplete and asymmetric (i.e., weaker for the cross between a C. albrechti female and a C. yamato male). To test this hypothesis, we conducted interspecific mating experiments in the laboratory. The estimates of total reproductive isolation, which occurred mainly at the premating and postmating/prezygotic stages, were high (isolation index = 0.964 for C. yamato female x C. albrechti male and 0.886 for the reciprocal cross), supporting the hypothesis of strong, but incomplete isolation. However, the observed difference between the reciprocal crosses was not sufficiently large to conclude that it caused directional introgression of mitochondria. Instead, we found asymmetry in individual isolating barriers in the postmating/prezygotic stages that coincided with the prediction, perhaps resulting from morphological mismatch of heterospecific genitalia. Although this asymmetry was compensated for by an inverse asymmetry of isolation in the postzygotic stage, the contribution of these individual barriers to total isolation may change for our expectation when considering females mating with multiple heterospecific males.

Sexually cannibalized males incur a significant fitness cost due to the loss of future mating opportunities and are expected to evolve behaviors to avoid or compensate for such costs. For example, partially cannibalized males may exhibit mate guarding, in which they accompany the female to prevent her from mating with another male. In some species, cannibalized males prolong the duration of copulation. However, little is known about the adaptive significance of the mating behavior of sexually cannibalized males. We hypothesized that mating itself serves a mate guarding function, and that behavioral change caused by cannibalism enhances the mate guarding function. We tested these hypotheses using the polyandrous and sexually cannibalistic praying mantid Tenodera angustipennis, with decapitation as a model of sexual cannibalism. We compared latencies to female mating with a rival male among three experimental treatments: unmated treatment, noncannibalistic mating treatment, and cannibalistic mating treatment. Mating itself delayed female remating, revealing its function in mate guarding. Decapitated males exhibited a higher guarding efficiency against rival males via firmer genital coupling. In addition, spermatophore attached to the female genital opening also delayed female remating, revealing an additional function in postmating mate guarding. Although copulation was prolonged due to decapitation, mating by a rival male was not delayed compared to noncannibalistic mating, probably because of weaker postcopulatory guarding. These findings suggest that greater mate guarding by decapitated males during copulation was offset by processes after copulation.Significance statementSexually cannibalized males die and lose the chance for future mating. This means that males that can avoid or compensate for this fitness loss may be favored. We examined this possibility by focusing on the postmating behavior of sexually cannibalized male mantises. Experimental analysis revealed that cannibalized males grasped the female more firmly during copulation to avoid disruption by other males, and prolonged copulation duration compared with noncannibalized males. These behavioral changes by cannibalized males contributed to delaying female remating with other males to the same extent as noncannibalized males. This suggests that sexually cannibalized males did not fully compensate for the loss of future mating opportunities. Stronger mate guarding via firm genital coupling and prolonged copulation duration in cannibalized males may be offset by weaker postcopulatory guarding such as shorter duration of copulatory plug attachment.