Sexual selection, phenotypic variation, and allometry in genitalic and non-genitalic traits in the sexually size-dimorphic stick insectMicrarchus hystriculeus

Plasticity of Life-History Traits and Adult Fitness of Fall Webworm in Relation to Climate Change

Temperature is an important environmental factor influencing the life-history traits of ectotherms. This study investigated the effects of larval-rearing temperature (21, 23, 25, and 27 °C) on the life-history traits and adult fitness of the fall webworm, Hyphantria cunea, an economically important invasive pest of China. With the increase in temperature during the larval stage, the larval developmental duration was significantly shortened, and the body mass was significantly increased, as was that of the body mass and size of pupae. The carbohydrate and lipid content of pupae significantly decreased with increasing larval-rearing temperature, whereas the protein content significantly increased. Adult body size and egg production increased significantly with increasing larval-rearing temperature, whereas there was no significant difference in egg diameter. These results indicate that H. cunea demonstrates life-history traits plasticity. In addition, the increase in fecundity would maintain a stable population size of H. cunea under higher temperatures. Such characteristics could enable H. cunea to spread to the more southern, warmer areas of China, posing an increased risk to the forestry industry in these regions.

Strongly sexually dimorphic forelegs are not more condition-dependent than less dimorphic traits in Drosophila prolongata

Directional sexual selection drives the evolution of traits that are most closely linked to reproductive success, giving rise to trait exaggeration and sexual dimorphism. Exaggerated structures are often costly and, therefore, thought to be expressed in a condition-dependent manner. Sexual selection theory thus predicts a direct link between directional sexual selection, sexual dimorphism, and sex-specific condition dependence. However, only a handful of studies investigate the relationship between sexual dimorphism and condition dependence. Using 21 genetic lines of Drosophila prolongata, we here compared the degree of sexual dimorphism and sex-specific condition dependence, measured as allometric slopes, in sexually selected and non-sexual traits. Our data revealed male-biased sexual dimorphism in all traits examined, most prominently in the sexually selected forelegs. However, there was no relationship between the degree of sex-specific condition dependence and sexual dimorphism across traits and genetic lines. Our results contradict theoretical predictions and highlight the importance of understanding the role of exaggerated traits in the context of both sexual and natural selection.

Sexual dimorphism divergence between sister species is associated with a switch in habitat use and mating system in thorny devil stick insects

The habitat and resource use of females critically affects their pattern of distribution and consequently their monoposibility by males and the mating system of a species. Shifts in habitat use are therefore likely to be associated with changes in mating system and sexual selection acting on males' phenotypes, consequently affecting patterns of sexual dimorphism. Although sexual dimorphism is often correlated with shifts in habitat use at the macroevolutionary scale, the underlying microevolutionary processes involved are typically unclear. Here, we used the New Guinean stick insect genus Eurycantha to investigate how changes in habitat use and mating system were associated with a change in sexual dimorphism seen specifically in the thorny devil stick insects (Eurycantha calcarata and Eurycantha horrida). Male thorny devils display sexually dimorphic and enlarged hindlegs endowed with a sharp spine. Sexual size dimorphism is also very reduced in these species relative to other phasmids. Using field observations, morphological measurements and radiotelemetry, we investigated changes in mating system associated with the reduction of sexual dimorphism and tested predictions from the hypothesis that sexual selection drove the evolution of this unusual male morphology. We found that thorny devils switched from solitary roosting in the canopy during the day to communal roosting inside cavities of a few host trees, shifting the distribution of females from scattered to clumped. Male thorny devils used their large hindlegs to fight with rivals for positions on the tree close to cavities containing females, and larger males were associated with cavities containing relatively more females. In contrast, the sister species, Eurycantha insularis, displays relatively small and unarmoured males (ancestral state). Adult female E. insularis were always scattered in the canopy, and this species displayed a scramble competition mating system typical of other phasmids, where mobility, rather than fighting ability, is probably critical to males' reproductive success. Overall, our study illustrates how a drastic change in sexual dimorphism can be associated with a switch from solitary to communal roosting and from a scramble competition to a defense-based polygyny mating system.

Sexual selection on size and shape in Japanese beetles (Popillia japonica)

The mobility hypothesis argues that species in which males compete for mates in scrambles often exhibit female-biased size dimorphism because smaller male body size should increase male mobility and success in searching for mates. Sexual dimorphism can be further exaggerated if fecundity or sexual selection concurrently selects for larger female size. Scramble competition can select for trait characteristics that optimize locomotion; for example, long and slender wings should be favored if aerial speed is important to mating success. I tested these predictions in the scrambling Japanese beetle (Popillia japonica), a female-biased size dimorphic insect pest that is invasive to North America. Multivariate selection analyses support the prediction that smaller body size and larger wings in males benefit their mating success. My analyses also revealed significant selection for larger wings in females but, contrary to prediction, direct sexual selection favors smaller body size in females. These results support the mobility hypothesis and partially explain the evolution of female-biased size dimorphism in this species. Sexual selection favored rounder bodies in females and more tapered bodies in males, whereas, in both sexes, the effect of wing shape appears less important to fitness than wing size.

Parthenogenetic Females of the Stick Insect Clitarchus hookeri Maintain Sexual Traits

The New Zealand stick insect Clitarchus hookeri has both sexual and parthenogenetic (all-female) populations. Sexual populations exhibit a scramble competition mating system with distinctive sex roles, where females are signalers and males are searchers, which may lead to differences in the chemical and morphological traits between sexes. Evidence from a range of insect species has shown a decay of sexual traits is common in parthenogenetic lineages, especially those traits related to mate attraction and location, presumably due to their high cost. However, in some cases, sexual traits remain functional, either due to the recent evolution of the parthenogenetic lineage, low cost of maintenance, or because there might be an advantage in maintaining them. We measured morphological and chemical traits of C. hookeri to identify differences between males and females and between females from sexual and parthenogenetic populations. We also tested the ability of males to discriminate between sexual and parthenogenetic females in a laboratory bioassay. Our results show that male C. hookeri has morphological traits that facilitate mobility (smaller body with disproportionately longer legs) and mate detection (disproportionately longer antennae), and adult females release significantly higher amounts of volatile organic compounds than males when this species is sexually active, in accordance with their distinctive sex roles. Although some differences were detected between sexual and parthenogenetic females, the latter appear to maintain copulatory behaviors and chemical signaling. Males were unable to distinguish between sexual and parthenogenetic females, suggesting that there has been little decay in the sexual traits in the parthenogenetic lineage of C. hookeri.

Intrasexually selected weapons

We propose a practical concept that distinguishes the particular kind of weaponry that has evolved to be used in combat between individuals of the same species and sex, which we term intrasexually selected weapons (ISWs). We present a treatise of ISWs in nature, aiming to understand their distinction and evolution from other secondary sex traits, including from 'sexually selected weapons', and from sexually dimorphic and monomorphic weaponry. We focus on the subset of secondary sex traits that are the result of same-sex combat, defined here as ISWs, provide not previously reported evolutionary patterns, and offer hypotheses to answer questions such as: why have only some species evolved weapons to fight for the opposite sex or breeding resources? We examined traits that seem to have evolved as ISWs in the entire animal phylogeny, restricting the classification of ISW to traits that are only present or enlarged in adults of one of the sexes, and are used as weapons during intrasexual fights. Because of the absence of behavioural data and, in many cases, lack of sexually discriminated series from juveniles to adults, we exclude the fossil record from this review. We merge morphological, ontogenetic, and behavioural information, and for the first time thoroughly review the tree of life to identify separate evolution of ISWs. We found that ISWs are only found in bilateral animals, appearing independently in nematodes, various groups of arthropods, and vertebrates. Our review sets a reference point to explore other taxa that we identify with potential ISWs for which behavioural or morphological studies are warranted. We establish that most ISWs come in pairs, are located in or near the head, are endo- or exoskeletal modifications, are overdeveloped structures compared with those found in females, are modified feeding structures and/or locomotor appendages, are most common in terrestrial taxa, are frequently used to guard females, territories, or both, and are also used in signalling displays to deter rivals and/or attract females. We also found that most taxa lack ISWs, that females of only a few species possess better-developed weapons than males, that the cases of independent evolution of ISWs are not evenly distributed across the phylogeny, and that animals possessing the most developed ISWs have non-hunting habits (e.g. herbivores) or are faunivores that prey on very small prey relative to their body size (e.g. insectivores). Bringing together perspectives from studies on a variety of taxa, we conceptualize that there are five ways in which a sexually dimorphic trait, apart from the primary sex traits, can be fixed: sexual selection, fecundity selection, parental role division, differential niche occupation between the sexes, and interference competition. We discuss these trends and the factors involved in the evolution of intrasexually selected weaponry in nature.

Female mate preferences on high-dimensional shape variation for male species recognition traits

Females in many animal species must discriminate between conspecific and heterospecific males when choosing mates. Such mating preferences that discriminate against heterospecifics may inadvertently also affect the mating success of conspecific males, particularly those with more extreme phenotypes. From this expectation, we hypothesized that female mate choice should cause Enallagma females (Odonata: Coenagrionidae) to discriminate against conspecific males with more extreme phenotypes of the claspers males use to grasp females while mating - the main feature of species mate recognition in these species. To test this, we compared cerci sizes and shapes between males that were captured while mating with females to males that were captured at the same time but not mating in three Enallagma species. In contrast to our hypothesis, we found only one of forty comparisons of shape variation that was consistent with females discriminating against males with more extreme cerci shapes. Instead, differences in cerci shape between mating and single males suggested that females displayed directional preferences on 1-4 aspects of cerci shape in two of the species in our samples. These results suggest that whereas some directional biases in mating based on cerci shape occur, the intraspecific phenotypic variation in male cerci size and shape is likely not large enough for females to express any significant incidental discrimination among conspecifics with more extreme shapes.

Using geometric morphometric visualizations of directional selection gradients to investigate morphological differentiation

Researchers studying extant and extinct taxa are often interested in identifying the evolutionary processes that have lead to the morphological differences among the taxa. Ideally, one could distinguish the influences of neutral evolutionary processes (genetic drift, mutation) from natural selection, and in situations for which selection is implicated, identify the targets of selection. The directional selection gradient is an effective tool for investigating evolutionary process, because it can relate form (size and shape) differences between taxa to the variation and covariation found within taxa. However, although most modern morphometric analyses use the tools of geometric morphometrics (GM) to analyze landmark data, to date, selection gradients have mainly been calculated from linear measurements. To address this methodological gap, here we present a GM approach for visualizing and comparing between-taxon selection gradients with each other, associated difference vectors, and "selection" gradients from neutral simulations. To exemplify our approach, we use a dataset of 347 three-dimensional landmarks and semilandmarks recorded on the crania of 260 primate specimens (112 humans, 67 common chimpanzees, 36 bonobos, 45 gorillas). Results on this example dataset show how incorporating geometric information can provide important insights into the evolution of the human braincase, and serve to demonstrate the utility of our approach for understanding morphological evolution.

Sexual dimorphism and allometry in the sphecophilous rove beetle Triacrus dilatus

The rove beetle Triacrus dilatus is found in the Atlantic forest of South America and lives in the refuse piles of the paper wasp Agelaia vicina. Adults of T. dilatus are among the largest rove beetles, frequently measuring over 3 cm, and exhibit remarkable variation in body size. To examine sexual dimorphism and allometric relationships we measured the length of the left mandible, ocular distance and elytra. We were interested in determining if there are quantifiable differences between sexes, if there are major and minor forms within each sex and if males exhibit mandibular allometry. For all variables, a t-test was run to determine if there were significant differences between the sexes. Linear regressions were run to examine if there were significant relationships between the different measurements. A heterogeneity of slopes test was used to determine if there were significant differences between males and females. Our results indicated that males had significantly larger mandibles and ocular distances than females, but the overall body length was not significantly different between the sexes. Unlike most insects, both sexes showed positive linear allometric relationships for mandible length and head size (as measured by the ocular distance). We found no evidence of major and minor forms in either sex.

Mate detection and seasonal variation in stick insect mating behaviour (Phamatodea: Clitarchus hookeri)

For animals that exhibit a scramble competition mating system, sexual selection pressures on mate searching ability are expected to be strong. Scramble competition mating systems evolve when populations provide females with equal accessibility to all male competitors, yet sex ratio and population density influences mating systems and varies seasonally. The stick insect species,Clitarchus hookeri, is frequently found in copula, yet very little is known about it’s mating behaviour. We preformed behavioural tests and assayed antennal sensory morphology to determine whether males used chemosensory cues to detect females. Through natural field observations we found populations to be significantly male-biased earlier in the season, while later, populations began to display equal sex ratios. With increasing female availability mating pair proportions steadily increased, while copulation duration declined. These results supportC. hookerias a scramble competitor, and demonstrate males may alter their behaviour in response to the seasonal variation in female density.