The sterile insect technique is protected from evolution of mate discrimination

Background

The sterile insect technique (SIT) has been used to suppress and even extinguish pest insect populations. The method involves releasing artificially reared insects (usually males) that, when mating with wild individuals, sterilize the broods. If administered on a large enough scale, the sterility can collapse the population. Precedents from other forms of population suppression, especially chemicals, raise the possibility of resistance evolving against the SIT. Here, we consider resistance in the form of evolution of female discrimination to avoid mating with sterile males. Is resistance evolution expected?

Methods

We offer mathematical models to consider the dynamics of this process. Most of our models assume a constant-release protocol, in which the same density of males is released every generation, regardless of wild male density. A few models instead assume proportional release, in which sterile releases are adjusted to be a constant proportion of wild males.

Results

We generally find that the evolution of female discrimination, although favored by selection, will often be too slow to halt population collapse when a constant-release implementation of the SIT is applied appropriately and continually. The accelerating efficacy of sterile males in dominating matings as the population collapses works equally against discriminating females as against non-discriminating females, and rare genes for discrimination are too slow to ascend to prevent the loss of females that discriminate. Even when migration from source populations sustains the treated population, continued application of the SIT can prevent evolution of discrimination. However, periodic premature cessation of the SIT does allow discrimination to evolve. Likewise, use of a 'proportional-release' protocol is also prone to escape from extinction if discriminating genotypes exist in the population, even if those genotypes are initially rare. Overall, the SIT is robust against the evolution of mate discrimination provided care is taken to avoid some basic pitfalls. The models here provide insight for designing programs to avoid those pitfalls.

The Effects of a Male Audience on Male and Female Mating Behaviour in the Blood-Sucking Bug Rhodnius prolixus

Social context is a key factor affecting sexual behaviour and cannot be neglected in gregarious species, such as triatomine blood-sucking bugs. Here we study the influence of the social context on the sexual reproductive behaviour of males and females of Rhodnius prolixus Stål. Specifically, we identify and compare the frequencies and sequence of sexual behaviours exhibited by a focal pair in presence or absence of a male audience. We expect that in presence of a male audience females increase their selectiveness level since the risk of losing infertile eggs decreases in presence of more candidates and females can benefit from copulating with a better quality male. Besides, in presence of potential rivals, we expect changes in the sexual behaviour of focal males, associated to a reduction in the risk of sperm competition. As expected, in presence of a male audience, females significantly increased the exhibition of rejection behaviour. Moreover, focal males exhibited shorter latencies to mount the female, longer duration of copula, and differences in their stereotyped behaviour exhibited during copula. We discuss the influence of the social context on the reproductive behaviour of females and males of R. prolixus.

Lack of sibling avoidance during mate selection in the butterfly Bicyclus anynana

Species susceptible to inbreeding depression are hypothesized to combat this problem through a number of different mechanisms, including kin recognition. For species with kin recognition, it is unknown if filial recognition is innate or due to prior juvenile experience with siblings. Here, we first test for the presence of kin recognition, and then test these two hypotheses for the development of filial recognition, in the butterfly Bicyclus anynana, a species that suffers from inbreeding depression when forcibly inbred but recovers within a few generations when allowed to breed freely. We evaluate whether the rapid recovery from inbreeding depression is associated with either innate or learned filial recognition. First, we determined whether females innately prefer unrelated males over sibling males using females reared in isolation and then given a choice between an unrelated and a sibling male. Then, we determined if females raised with siblings learned to detect and avoid mating with siblings as adults when provided a choice between an unrelated male and a sibling male. Finally, we determined if females raised with siblings could learn to detect and avoid mating with familiar siblings when given a choice between familiar and unfamiliar siblings. We found that females mated randomly in all three choice combinations. Observed male behavior also did not influence female mating outcome. Our results suggest that adult females do not innately avoid or learn to avoid siblings during mate selection, and that filial detection may not be as critical to reproductive fitness in B. anynana as previously thought.

Female cognitive performance and mass are correlated with different aspects of mate choice in the zebra finch (Taeniopygia guttata)

A female's cognitive ability may influence her mate preferences through various mechanisms. These mechanisms include the direct effect of cognitive ability on the information-processing skills used during mate choice, and the indirect effect of cognitive ability on quality when females mate assortatively. Here, we examined whether the ability to learn a novel foraging task, a cognitive skill which has been associated with reproductive success in other capacities, was correlated with song preferences in female zebra finches (Taeniopygia guttata). Female preferences were measured in an operant testing chamber where hops on a perch triggered song playback. Females were given the choice of (1) conspecific vs. heterospecific song and (2) high-quality male vs. low-quality male conspecific song. We found that female performance on the novel foraging task was positively correlated with preference for conspecific song, but not with preference for high-quality male song. Instead, female mass was positively correlated with preference for high-quality male song, potentially signifying that female mass is a stronger predictor of female quality in assortative mating than female cognitive performance. Female mass and cognitive performance were unrelated. Our results suggest that the particular traits of a female that affect conspecific preference do not necessarily affect preference for high-quality males.

Causes of variability in male vibratory signals and the role of female choice in Mantophasmatodea

Communication systems that involve substrate vibrations are increasingly a focus of research since this communication mode - recently termed biotremology - has been found to be remarkably widespread in the animal kingdom. Vibrational signals are often used during courtship and therefore underlie both natural and sexual selection. Mantophasmatodea use species- and sex-specific substrate vibrational signals during courtship. We explored whether male vibrational signals of the South African heelwalker Karoophasma biedouwense vary with temperature, body condition and age, and tested female preference towards various signal pattern combinations. We recorded male signals under varying temperatures and over 3.5 weeks after onset of signaling. Our results show that the temporal structure of male signals is modified by changes in temperature, and changes with male age. Other characteristics, especially duty cycles, are less affected, but correlate with body condition. Females responded along a broad spectrum of signaling patterns, indicating that they do not favor signals of males of a certain age or condition. They were selective towards the fine structure of vibratory signals, suggesting that pulse repetition times carry species-specific information. Mantophasmatodea thus use vibrational signals to identify and localize a mating partner, but presumably not for precopulatory mate selection.

Corticosterone mediated mate choice affects female mating reluctance and reproductive success

The study of stress-related hormones as mediators of sexual selection has traditionally focused on the effect of glucocorticoids on male quality and competing ability. However, environmental stressors are expected to affect both males and females, and the strength of sexual selection might be affected by changes in female mating decisions, a hypothesis that has rarely been tested. Here, we investigated whether female common lizard (Zootoca vivipara) mating behaviour and mating preferences are affected by different levels of administered corticosterone and conditioned by the familiarity of their partners, which is known to influence Z. vivipara social behaviour. To this end, two females, one corticosterone-treated and one control female, were simultaneously presented with an unfamiliar male and the following day with either a familiar or an unfamiliar male. Females treated with corticosterone (Cort) were more aggressive towards males and mated less. Furthermore, copulation probability in Cort females, but not in control females, increased with body size. On the second day, Cort females only mated with familiar partners. In contrast, male behaviour towards females was not affected by treatment and only bigger males successfully copulated with Cort females. This shows that corticosterone directly affected female mating behaviour and mating preferences, while male mating behaviour was unaffected by the female's level of corticosterone. Environmental and social stressors may affect reproductive strategies of females, the strength of sexual selection, and sexual conflict through their effects on female glucocorticoid levels, potentially in a wide range of species.

The impact of age and egg-laying cycle on female grasshoppers' preference functions for acoustic signals

Female responsiveness and the shape of preference functions for male signal traits are important determinants for male mating success. We observed the responsiveness and the selectivity of virgin grasshopper females (Chorthippus biguttulus L.) for different features of males' acoustic signals throughout their life span to detect possible influences of age on the females' preference functions. In particular, we explored the hypothesis that the females may become less selective with increasing age and, therefore, would start to accept songs that are normally rejected. Such an age effect could relieve the selection pressure on male signal traits. In the majority of animals tested, the general responsiveness decreased with age although a few individuals exhibited an opposite trend. Contrary to the above expectation, there was no indication of a loss of selectivity in older females or an increased acceptance of normally unattractive song models. The timing within the oviposition cycle had a strong effect on responsiveness: near oviposition the general responsiveness increased and with it also the half width of the preference functions. However, highly unattractive song models remained unattractive also near oviposition.

Assessing the use of wing ornamentation and visual display in female choice sexual selection

Conspicuous sexual dimorphism is often ascribed to sexual selection. When the differences between the sexes are ornamental, this is thought to indicate a role for female choice. In spotted winged Drosophila species courtship, a male positioned in front of a female waves his wings, which have a patch of melanization on the exterior margin. In this study, we examine both female preference for wing spots and the role of vision in mating success in three species of the suzukii group: Drosophila biarmipes, D. suzukii, and D. subpulchrella. To assess female preference for wing spot, we removed the spot with a novel, non-invasive method, and competed spotless males with males with two spots. Phenotype did not affect mating success in any species. To eliminate the potential effect of competitive behavior on male mating success, we also ran a no-choice analysis. Mating frequency and timing was not different between phenotypes within these species. The effect of vision on mating success was assessed by comparing mating success of spotted males between light and dark conditions, both for frequency of mating, as well as timing of multiple courtship parameters. Species varied in the extent that lack of vision negatively affected mating success. Though vision is important for mating success, the spot itself may not be providing the signal that females use to make mating decisions.

Sexual ornaments reveal the strength of melanization immune response and longevity of male paper wasps

It has been recently suggested that female mate choice, based on sexually selected ornaments, is an important component of social wasps' reproductive biology. The correlates of male ornaments that could be of a female's interest, however, remain to be investigated. Males of the Neotropical paper wasp Polistes simillimus have sexually dimorphic melanin-based black spots on their faces. In this species, male spots work like sexual ornaments, as it has been experimentally demonstrated that females prefer sexual partners with a higher proportion of black pigment on their faces. We have shown that, under laboratory conditions, male sexual ornamentation positively predicts the strength of the melanization immune response and longevity. Therefore, in P. simillimus, melanin-based facial patterns (ornaments) seem to be honest indicators of male quality.

Male preference for conspecific mates is stronger than females' in Betta splendens

The higher energetic cost related to female gamete investment is classically considered the driving force behind sexual selection. This asymmetric cost of reproduction is thought to cause female preference for elaborate male ornamentation. Subsequent co-evolution of female preferences and male ornaments is thought then to lead to a greater preference for conspecific mates in females as compared to males. Thus, female choice is classically assumed to contribute more than male choice to behavioral isolation between sexually dimorphic species. However, this hypothesis fails to account for the cost of maintaining a territory, building a nest, courtship displays, ornament investment, and parental care, as seen in males of the Siamese fighting fish, Betta splendens. Here we show that male B. splendens have a greater preference for female conspecifics than females have for male conspecifics, when given a choice between conspecifics and the allopatric Betta imbellis. We hypothesize that in B. splendens, the cost of mating may be higher for males than females, and predict that male choice would contribute to behavioral isolation upon secondary contact of wild populations.

Male-male competition and female choice are differentially affected by male call acoustics in the serrate-legged small treefrog, Kurixalus odontotarsus

Background

The evolution of exaggerated vocal signals in anuran species is an important topic. Males and females have both evolved the ability to discriminate communication sounds. However, the nature of sexual dimorphism in cognition and sensory discrimination and in the evolution and limitation of sexual signal exaggeration remain relatively unexplored.

Methods

In the present study, we used male calls of varied complexity in the serrate-legged small treefrog, Kurixalus odontotarsus, as probes to investigate how both sexes respond to variations in call complexity and how sex differences in signal discrimination play a role in the evolution of sexual signal exaggeration. The compound calls of male K. odontotarsus consist of a series of one or more harmonic notes (A notes) which may be followed by one or more short broadband notes (B notes).

Results

Male playback experiments and female phonotaxis tests showed that increasing the number of A notes in stimulus calls elicits increased numbers of response calls by males and increases the attractiveness of the stimulus calls to females. The addition of B notes, however, reduces male calling responses. Moreover, call stimuli which contain only B notes suppress spontaneous male calling responses. Phonotaxis experiments show that females prefer calls with greater numbers of A notes and calls containing both A notes and B notes, but do not prefer calls with only B notes.

Discussion

Male-male competition and female choice appear to have played different roles in the evolution and limitation of signal complexity in K. odontotarsus. These results provide new insights into how exaggerated compound signals evolve and how signal complexity may be limited in anurans.

Plasticity of signaling and mate choice in a trilling species of the Mecopoda complex (Orthoptera: Tettigoniidae)

Abstract

Males of a trilling species in the Mecopoda complex produce conspicuous calling songs that consist of two motifs: an amplitude-modulated motif with alternating loud and soft segments (AM-motif) and a continuous, high-intensity trill. The function of these song motifs for female attraction and competition between males was investigated. We tested the hypothesis that males modify their signaling behavior depending on the social environment (presence/absence of females or rival males) when they compete for mates. Therefore, we analyzed acoustic signaling of males in three different situations: (1) solo singing, (2) acoustic interaction with another male, and (3) singing in the presence of a female. In addition, the preference of females for these song motifs and further song parameters was studied in two-choice experiments. As expected, females showed a preference for conspicuous and loud song elements, but nevertheless, males increased the proportion of the AM-motif in the presence of a female. In acoustic interactions, males reduced bout duration significantly compared to both other situations. However, song bouts in this situation still overlapped more than expected by chance, which indicates intentionally simultaneous singing. A multivariate statistical analysis revealed that the proportion of the AM-motif and the duration of loud segments within the AM-motif allow a reliable prediction of whether males sing in isolation, compete with another male, or sing in the presence of a female. These results indicate that the AM-motif plays a dominant role especially in close-range courtship and that males are challenged in finding a balance between attracting females and saving energy during repeated acoustic interactions.

Significance statement

Males of acoustic insects often produce conspicuous calling songs that have a dual function in male-male competition and mate attraction. High signal amplitudes and signal rates are associated with high energetic costs for signal production. We would therefore predict that males adjust their signaling behavior according to their perception of the social context. Here we studied signal production and mate choice in a katydid, where males switch between loud and soft song segments in a dynamic way. Additionally, we examined the attractiveness of different song elements in female choice tests. Our results show how males of this katydid deal with the conflict of remaining attractive for females and competing with a costly signal with rivals.

Implications of nutritional stress as nestling or fledgling on subsequent attractiveness and fecundity in zebra finches (Taeniopygia guttata)

The conditions an organism experiences during early development can have profound and long lasting effects on its subsequent behavior, attractiveness, and life history decisions. Most previous studies have exposed individuals to different conditions throughout development until nutritional independence. Yet under natural conditions, individuals may experience limitations for much shorter periods due to transient environmental fluctuations. Here, we used zebra finches (Taeniopygia guttata) in captivity to determine if conditions experienced during distinctly different early developmental phases contribute differently to male and female attractiveness and subsequent reproduction. We conducted a breeding experiment in which offspring were exposed to food regimes with (a) low quality food provided only during the nestling period, (b) low quality food provided only during the fledgling period, or (c) high quality food throughout early development. We show that despite short-term effects on biometry and physiology, there were no effects on either male or female attractiveness, as tested in two-way mate choice free-flight aviary experiments. In a subsequent breeding experiment, the offspring from the initial experiment were allowed to breed themselves. The next generation offspring from mothers raised under lower quality nutrition as either nestling or fledging were lighter at hatching compared to offspring from mothers raised under higher quality nutrition whereas paternal early nutrition had no such effects. The lack of early developmental limitations on attractiveness suggests that attractiveness traits were not affected or that birds compensated for any such effects. Furthermore, maternal trans-generational effects of dietary restrictions emphasize the importance of role of limited periods of early developmental stress in the expression of environmentally determined fitness components.

BENEFITS OF MULTIPLE MATES IN THE CRICKET GRYLLUS BIMACULATUS

Despite the importance of polyandry for sexual selection, the reasons why females frequently mate with several males remain poorly understood. A number of genetic benefits have been proposed, based on the idea that by taking multiple mates, females increase the likelihood that their offspring will be sired by genetically more compatible or superior males. If certain males have intrinsically "good genes," any female mating with them will produce superior offspring. Alternatively, if some males have genetic elements that are incompatible with a particular female, then she may benefit from polyandry if the sperm of such males are less likely to fertilize her eggs. We examined these hypotheses in the field cricket Gryllus bimaculatus (Orthoptera: Gryllidae). By allocating females identical numbers of matings but different numbers of mates we investigated the influence of number of mates on female fecundity, and both short- and long-term offspring fitness. This revealed no effect of number of mates on number of eggs laid. However, hatching success of eggs increased with number of mates. This effect could not be attributed to nongenetic effects such as the possibility that polyandry reduces variance in the quantity or fertilizing ability of sperm females receive, because a control group receiving half the number of copulations showed no drop in hatching success. Offspring did not differ in survival, adult mass, size, or development time with treatment. When males were mated to several different females there were no repeatable differences between individual males in the hatching success of their mate's eggs. This suggests that improved hatching success of polyandrous females is not due to certain males having genes that improve egg viability regardless of their mate. Instead, our results support the hypothesis that certain males are genetically more compatible with certain females, and that this drives polyandry through differential fertilization success of sperm from more compatible males.

Larger females are choosier in the gray treefrog (Hyla versicolor)

Individual variation in female mate choice has important implications for sexual trait evolution and the maintenance of phenotypic diversity. In this study we examined several potential drivers of individual variation in female choosiness for the well-studied, energetically expensive courtship signal of male gray treefrogs, Hyla versicolor. Specifically, we investigated the relationship between female choosiness and other female traits (female body size, physical condition, and age) using a costly choice playback experiment where females traveled different simulated distances to reach attractive mates. We found that larger females maintained their preferences for attractive male calls over greater simulated distances (i.e. were choosier) than smaller females. We discuss possible explanations for why larger females may be choosier and suggest several potential avenues of future research.

Experiments with claw models explain the function of the waving display of Ilyoplax pusilla (Brachyura: Dotillidae)

Males of the dotillid crab, Ilyoplax pusilla, perform waving displays during the reproductive season. Unlike many fiddler crabs (genus Uca), however, the function of the waving display is unclear for this species. Experiments using waving and static claw models over short (10 cm) and long (25 cm) distances were conducted to investigate responses by wandering females to artificial waving signals. In long-distance choice experiments, presuming undirected waving to unseen females as broadcast waving, females were equally likely to approach the waving and static claw sections during the non-reproductive season, but significantly more females (65 %) chose the waving claw section during the reproductive season. In short-distance choice experiments, presuming directed courtship waving toward a particular female, there was no significant difference between the waving and static claw models during the non-reproductive season, but significantly more females (88 %) chose the waving claw during the reproductive season. These results suggest that one function of the waving display of I. pusilla is mate attraction and that waving from a short distance is more effective.

How females of chirping and trilling field crickets integrate the 'what' and 'where' of male acoustic signals during decision making

In crickets acoustic communication serves mate selection. Female crickets have to perceive and integrate male cues relevant for mate choice while confronted with several different signals in an acoustically diverse background. Overall female decisions are based on the attractiveness of the temporal pattern (informative about the 'what') and on signal intensity (informative about the 'where') of male calling songs. Here, we investigated how the relevant cues for mate choice are integrated during the decision process by females of five different species of chirping and trilling field crickets. Using a behavioral design, female preferences in no-choice and choice situations for male calling songs differing in pulse rate, modulation depth, intensities, chirp/trill arrangements and temporal shifts were examined. Sensory processing underlying decisions in female field crickets is rather similar as combined evidence suggested that incoming song patterns were analyzed separately by bilaterally paired networks for pattern attractiveness and pattern intensity. A downstream gain control mechanism leads to a weighting of the intensity cue by pattern attractiveness. While remarkable differences between species were observed with respect to specific processing steps, closely related species exhibited more similar preferences than did more distantly related species.

Is Juvenile Hormone a potential mechanism that underlay the "branched Y-model"?

Trade-offs are a central tenet in the life-history evolution and the simplest model to understand it is the "Y" model: the investment of one arm will affect the investment of the other arm. However, this model is by far more complex, and a "branched Y-model" is proposed: trade-offs could exist within each arm of the Y, but the mechanistic link is unknown. Here we used Tenebrio molitor to test if Juvenile Hormone (JH) could be a mechanistic link behind the "branched Y-model". Larvae were assigned to one of the following experimental groups: (1) low, (2) medium and (3) high doses of methoprene (a Juvenile Hormone analogue, JHa), (4) acetone (methoprene diluents; control one) or (5) näive (handled in the same way as other groups; control two). The JHa lengthened the time of development from larvae to pupae and larvae to adults, resulting in adults with a larger size. Males with medium and long JHa treatment doses were favored with female choice, but had smaller testes and fewer viable sperm. There were no differences between groups in regard to the number of spermatozoa of males, or the number of ovarioles or eggs of females. This results suggest that JH: (i) is a mechanistic link of insects "branched Y model", (ii) is a double ended-sword because it may not only provide benefits on reproduction but could also impose costs, and (iii) has a differential effect on each sex, being males more affected than females.

Red Queen dancing in the lek: effects of mating skew on host-parasite interactions

The RQH (Red Queen hypothesis), which argues that hosts need to be continuously finding new ways to avoid parasites that are able to infect common host genotypes, has been at the center of discussions on the maintenance of sex. This is because diversity is favored under the host–parasite coevolution based on negative frequency‐dependent selection, and sexual reproduction is a mechanism that generates genetic diversity in the host population. Together with parasite infections, sexual organisms are usually under sexual selection, which leads to mating skew or mating success biased toward males with a particular phenotype. Thus, strong mating skew would affect genetic variance in a population and should affect the benefit of the RQH. However, most models have investigated the RQH under a random mating system and not under mating skew. In this study, I show that sexual selection and the resultant mating skew may increase parasite load in the hosts. An IBM (individual‐based model), which included host–parasite interactions and sexual selection among hosts, demonstrates that mating skew influenced parasite infection in the hosts under various conditions. Moreover, the IBM showed that the mating skew evolves easily in cases of male–male competition and female mate choice, even though it imposes an increased risk of parasite infection on the hosts. These findings indicated that whether the RQH favored sexual reproduction depended on the condition of mating skew. That is, consideration of the host mating system would provide further understanding of conditions in which the RQH favors sexual reproduction in real organisms.

Progesterone and prostaglandin F2α induce species-typical female preferences for male sexual displays in Cope's gray treefrog (Hyla chrysoscelis)

Endocrine systems play critical roles in facilitating sexual behavior in seasonally breeding vertebrates. Much of the research exploring this topic has focused on the endocrine correlates of signaling behavior in males and sexual proceptivity in females. What is less understood is how hormones promote the expression of the often complex and highly selective set of stimulus-response behaviors that are observed in naturally breeding animals. In female frogs, phonotaxis is a robust and sensitive bioassay of mate choice and is exhibited by gravid females during the breeding season. In stark contrast, females exhibit low phonotactic responsiveness outside the breeding season, but the administration of hormones can induce sexual proceptivity. Here we test the hypothesis that manipulation of a minimal set of reproductive hormones-progesterone and prostaglandin F2α-are capable of evoking not only proceptive behavior in non-breeding females, but also the patterns of intraspecific selectivity for male sexual displays observed in gravid females tested during the breeding season. Specifically, we investigated whether preferences for faster call rates, longer call durations, and higher call efforts were similar between breeding and hormone-treated females of Cope's gray treefrog (Hyla chrysoscelis). Hormone injections induced patterns of selective phonotaxis in non-breeding females that were remarkably similar to those observed in breeding females. These results suggest that there may be an important contribution of hormonal pleiotropy in regulating this complex, acoustically-guided sexual behavior. Our findings also support the idea that hormonal induction could be used to evaluate hypotheses about selective mate choice, and its underlying mechanisms, using non-breeding females.

Copulatory courtship by bushcricket genital titillators revealed by functional morphology, μCT scanning for 3D reconstruction and female sense structures

Genitalia are rapidly evolving morphological structures most likely under sexual selection. Due to their internal nature they are often hidden inside the body, thus morpho-functional studies of animal genitalia are broadly lacking. Males of some bushcricket taxa bear paired genital appendices called titillators, the exact function of which is unknown since they are obscured inside the female body during pairing. To investigate titillator morphology and possible function during copulation, we studied the bushcricket Metrioptera roeselii (Hagenbach, 1822) using a novel combination of independent, yet complementary, techniques. Copulating pairs were snap-frozen and scanned by X-ray micro-computed tomography (μCT) to visualize the coupling of male and female genitalia in situ. Video recordings of copulating pairs also showed rhythmical insertion of male titillators into the female's genital chamber, where they percuss a softened structure on the female's subgenital plate. Movements did not induce damage to the female's structure, which lacks any sclerotized genital counterparts. Instead, scanning electron microscopy and histological sections show the female subgenital plate to be covered with two different types of sensory receptors at the contact zone between the male's titillator and the female genital chamber. We interpret the non-harmful function of the titillator processes, the lack of a genital counter-structure and the presence of sensory cells on the female's subgenital plate as indicators of a copulatory courtship function of titillators, subject to sexual selection by female choice.

What is the password? Female bark beetles (Scolytinae) grant males access to their galleries based on courtship song

Acoustic signals are commonly used by insects in the context of mating, and signals can vary depending on the stage of interaction between a male and female. While calling songs have been studied extensively, particularly in the Orthoptera, much less is known about courtship songs. One outstanding question is how potential mates are differentiated by their courtship signal characteristics. We examined acoustic courtship signals in a new system, bark beetles (Scolytinae). In the red turpentine beetle (Dendroctonus valens) males produce chirp trains upon approaching the entrance of a female's gallery. We tested the hypotheses that acoustic signals are honest indicators of male condition and that females choose males based on signal characteristics. Males generated two distinct chirp types (simple and interrupted), and variability in their prevalence correlated with an indicator of male quality, body size, with larger males producing significantly more interrupted chirps. Females showed a significant preference for males who produced interrupted chirps, suggesting that females distinguish between males on the basis of their chirp performances. We suggest that interrupted chirps during courtship advertise a male's size and/or motor skills, and function as the proverbial 'passwords' that allow him entry to a female's gallery.

A male spider's ornamentation polymorphism maintained by opposing selection with two niches

The Levene mechanism to maintain genotypic polymorphism by opposing selection on genotypes in multiple niches was proposed 60 years ago, and yet no systems were found to satisfy the mechanism's rather restrictive conditions. Reported here is such an example that a wolf spider population lives in a habitat of mixed rocks and leafy litter for which the females are phenotypically indistinguishable and the males have two distinct phenotypes subject to opposing selection with respect to the substrates. Census data is best-fitted to a population genetics model of the Levene type. A majority of the best fit support polymorphism, with many fitted parameter values quantitatively consistent with various laboratory studies on two closely related species.

Male dimorphism and alternative reproductive tactics in harvestmen (Arachnida: Opiliones)

Strong sexual selection may lead small males or males in poor condition to adopt alternative reproductive tactics (ARTs) as a way to avoid the risk of being completely excluded from the mating pool. ARTs, sometimes accompanying morphological dimorphism among males, are taxonomically widespread, especially common in arthropods. Here we review the current knowledge on ARTs and male dimorphism in a diverse but relatively overlooked group of arachnids, the order Opiliones, popularly known as harvestmen or daddy long-legs. We begin with a summary of harvestman mating systems, followed by a review of the two lines of evidence for the presence of ARTs in the group: (1) morphological data from natural populations and museum collections; and (2) behavioral information from field studies. Despite receiving less attention than spiders, scorpions and insects, our review shows that harvestmen are an exciting group of organisms that are potentially great models for sexual selection studies focused on ARTs. We also suggest that investigating the proximate mechanisms underlying male dimorphism in the order would be especially important. New research on ARTs and male dimorphism will have implications for our understanding of the evolution of mating systems, sperm competition, and polyandry. This article is part of a Special Issue entitled: Neotropical Behaviour.