Courtship raises male fertilization success through post-mating sexual selection in a spider

Neural mechanisms involved in female mate choice in invertebrates

Mate choice is a critical decision with direct implications for fitness. Although it has been recognized for over 150 years, our understanding of its underlying mechanisms is still limited. Most studies on mate choice focus on the evolutionary causes of behavior, with less attention given to the physiological and molecular mechanisms involved. This is especially true for invertebrates, where research on mate choice has largely focused on male behavior. This review summarizes the current state of knowledge on the neural, molecular and neurohormonal mechanisms of female choice in invertebrates, including behaviors before, during, and after copulation. We identify areas of research that have not been extensively explored in invertebrates, suggesting potential directions for future investigation. We hope that this review will stimulate further research in this area.

Resource-dependent investment in male sexual traits in a viviparous fish

Exaggerated and conspicuous sexually selected traits are often costly to produce and maintain. Costly traits are expected to show resource-dependent expression, since limited resources prevent animals from investing maximally in multiple traits simultaneously. However, there may be critical periods during an individual's life where the expression of traits is altered if resources are limited. Moreover, costly sexual traits may arise from sexual selection acting both before (pre-copulatory) and after mating (post-copulatory). Gaining a robust understanding of resource-dependent trait expression therefore requires an approach that examines both episodes of sexual selection after resource limitation during different times in an individual's life. Yet few studies have taken such an approach. Here, we examine how resource restriction influences a set of pre- and post-copulatory traits in male pygmy halfbeaks (Dermogenys collettei), which invest in sexual ornaments and routinely engage in male-male contests and sperm competition. Critically, we examined responses in males when resources were restricted during development and after reaching sexual maturity. Both pre- and post-copulatory traits are resource-dependent in male halfbeaks. Body size, beak size, courtship behavior, and testes size were reduced by diet restriction, while, unexpectedly, the restricted-diet group had a larger area of red color on the beak and fins after diet treatment. These patterns were generally consistent when resources were restricted during development and after reaching sexual maturity. The study reinforces the role of resource acquisition in maintaining variation among sexual traits.

Sperm competition intensity affects sperm precedence patterns in a polyandrous gift-giving spider

Sperm competition drives traits that enhance fertilization success. The amount of sperm transferred relative to competitors is key for attaining paternity. Female reproductive morphology and male mating order may also influence fertilization, however the outcome for sperm precedence under intense sperm competition remains poorly understood. In the polyandrous spider Pisaura mirabilis, males offer nuptial gifts which prolong copulation and increase sperm transfer, factors proposed to alter sperm precedence patterns under strong sperm competition. First, we assessed the degree of female polyandry by genotyping wild broods. A conservative analysis identified up to 4 sires, with a mean of 2 sires per brood, consistent with an optimal mating female rate. Then we asked whether intense sperm competition shifts sperm precedence patterns from first male priority, as expected from female morphology, to last male advantage. We varied sexual selection intensity experimentally and determined competitive fertilization outcome by genotyping broods. In double matings, one male monopolised paternity regardless of mating order. A mating order effect with first male priority was revealed when females were mated to 4 males, however this effect disappeared when females were mated to 6 males, likely due to increased sperm mixing. The proportion of males that successfully sired offspring drastically decreased with the number of competitors. Longer copulations translated into higher paternity shares independently of mating order, reinforcing the advantage of traits that prolong copulation duration under intense competition, such as the nuptial gift. Sperm competition intensity enhances the impact of competitive sexual traits and imposes multiple effects on paternity.

Strategic pheromone signalling by mate searching females of the sexually cannibalistic spider Argiope bruennichi

Reproduction often requires finding a mating partner. To this end, females of many arthropods advertise their presence to searching males via volatile chemical signals. Such pheromones are considered low-cost signals, although this notion is based on little evidence and has recently been challenged. Even when using comparatively low-cost signals, females should signal as little as possible to minimize costs while still ensuring mate attraction. Here, we test the strategic-signalling hypothesis using Argiope bruennichi. In this orb-weaving spider, egg maturation commences with adult moult, and females that do not attract a male in time will lay a large batch of unfertilized eggs approximately three weeks after maturation. Using GC-MS analyses, we show that virgin females enhance their signalling effort, i.e. pheromone quantity per unit body mass, with increasing age and approaching oviposition. We further demonstrate that pheromone release is condition dependent, suggesting the occurrence of physiological costs. Mate choice assays revealed that pheromone quantity is the only predictor of female attractiveness for males. In support of the strategic-signalling hypothesis, pheromone signals by female A. bruennichi become stronger with increased need as well as body condition, and might thus qualify as an honest signal of female quality.

Family-specific chemical profiles provide potential kin recognition cues in the sexually cannibalistic spider Argiope bruennichi

Kin recognition, the ability to detect relatives, is important for cooperation, altruism and also inbreeding avoidance. A large body of research on kin recognition mechanisms exists for vertebrates and insects, while little is known for other arthropod taxa. In spiders, nepotism has been reported in social and solitary species. However, there are very few examples of kin discrimination in a mating context, one coming from the orb-weaver Argiope bruennichi. Owing to effective mating plugs and high rates of sexual cannibalism, both sexes of A. bruennichi are limited to a maximum of two copulations. Males surviving their first copulation can either re-mate with the current female (monopolizing paternity) or leave and search for another. Mating experiments have shown that males readily mate with sisters but are more likely to leave after one short copulation as compared with unrelated females, allowing them to search for another mate. Here, we ask whether the observed behaviour is based on chemical cues. We detected family-specific cuticular profiles that qualify as kin recognition cues. Moreover, correlations in the relative amounts of some of the detected substances between sexes within families indicate that kin recognition is likely based on subsets of cuticular substances, rather than entire profiles.

Sperm competition when transfer is dangerous

Aggressive and cannibalistic female spiders can impose strong selection on male mating and fertilization strategies. Furthermore, the distinctive reproductive morphology of spiders is predicted to influence the outcome of sperm competition. Polyandry is common in spiders, leading to defensive male strategies that include guarding, plugging and self-sacrifice. Paternity patterns are highly variable and unlikely to be determined solely by mating order, but rather by relative copulation duration, deployment of plugs and cryptic female choice. The ability to strategically allocate sperm is limited, either by the need to refill pedipalps periodically or owing to permanent sperm depletion after mating. Further insights now rely on unravelling several proximate mechanisms such as the process of sperm activation and the role of seminal fluids. This article is part of the theme issue 'Fifty years of sperm competition'.

Disentangling a shared trait: male control over mate guarding duration revealed by a mate exchange experiment

Abstract

Shared behavioural traits result from the interaction of two or more individuals, making it difficult to discern which individual is in control of the behaviour of interest. Especially in the case of shared reproductive traits such as mating duration or mate guarding duration is this an important issue to resolve, because these are potentially closely connected to fitness and are likely to exhibit sexual conflict. Here, we sought to disentangle which sex controls mate guarding duration in the tropical house cricket Gryllodes sigillatus, a species in which mate guarding and nuptial feeding by the male have been proposed to prevent premature removal of the transferred spermatophore by the female. To do so, we performed a series of mating experiments in a paired design, in which the first mating dyad was allowed to start mating some time before the second dyad. Once both dyads were in the mate guarding phase, we then interrupted them and exchanged partners, enabling us to determine whether the remaining guarding duration depended more on the duration of guarding already performed by the male in the new dyad (implying male control) or on the guarding already received by the female (implying female control). We found that the time a female was guarded overall was significantly affected by how long the exchanged male had already engaged in mate guarding with the previous female, but conversely, the total time males guarded both females was unaffected by the duration of guarding that the exchanged female had previously received. Our data thus clearly demonstrate that males rather than females control mate guarding duration and adjust the duration according to females’ weight.

Significance statement

It is not easy to determine which individual is in control of a shared behavioural trait (SBT). This information could provide insight into selection pressure on one sex and could help us understand differences in SBTs between related species or between different dyads of one species. We used a relative novel but simple method to disentangle a SBT in a cricket. We performed mating experiments and exchanged the mating partners after copulation within the mate guarding phase and measured the total guarding duration. Our analyses showed males were not influenced by the exchange and guarded as long as expected regardless how long the female were guarded before by another male. Our data suggest males are likely in control of mate guarding duration, and they have no ability to recognize post-copulatory mate exchange.

Mate availability does not influence mating strategies in males of the sexually cannibalistic spider Argiope bruennichi

Background

Sexual selection theory predicts that male investment in a current female should be a function of female density and male competition. While many studies have focused on male competition, the impact of female density on male mating investment has been widely neglected. Here, we aimed to close this gap and tested effects of mate density on male mating decisions in the orb-web spider Argiope bruennichi. Males of this species mutilate their genitalia during copulation, which reduces sperm competition and limits their mating rate to a maximum of two females (bigyny). The mating rate is frequently further reduced by female aggression and cannibalization. Males can reduce the risk of cannibalism if they jump off the female in time, but will then transfer fewer sperm. An alternative solution of this trade-off is to copulate longer, commit self-sacrifice and secure higher minimal paternity. The self-sacrificial strategy may be adaptive if prospective mating chances are uncertain. In A. bruennichi, this uncertainty may arise from quick changes in population dynamics. Therefore, we expected that males would immediately respond to information about low or high mate availability and opt for self-sacrifice after a single copulation under low mate availability. If male survival depends on information about prospective mating chances, we further predicted that under high mate availability, we would find a higher rate of males that leave the first mating partner to follow a bigynous mating strategy.

Method

We used naïve males and compared their mating decisions among two treatments that differed in the number of signalling females. In the high mate availability treatment, males perceived pheromone signals from four adult, virgin females, while in the low mate availability treatment only one of four females was adult and virgin and the other three were penultimate and unreceptive.

Results

Males took more time to start mate searching if mate availability was low. However, a self-sacrificial strategy was not more likely under low mate availability. We found no effects of treatment on the duration of copulation, the probability to survive the first copulation or the probability of bigyny. Interestingly, survival chances depended on male size and were higher in small males.

Discussion

Our results do not support the hypothesis that mate density variation affects male mating investment, although they clearly perceived mate density, which they presumably assessed by pheromone quantity. One reason for the absence of male adjustments to mating tactics could be that adaptations to survive female attacks veil adaptations that facilitate mating decisions.

Sexual cannibalism and population viability

Some behaviours that typically increase fitness at the individual level may reduce population persistence, particularly in the face of environmental changes. Sexual cannibalism is an extreme mating behaviour which typically involves a male being devoured by the female immediately before, during or after copulation, and is widespread amongst predatory invertebrates. Although the individual-level effects of sexual cannibalism are reasonably well understood, very little is known about the population-level effects. We constructed both a mathematical model and an individual-based model to predict how sexual cannibalism might affect population growth rate and extinction risk. We found that in the absence of any cannibalism-derived fecundity benefit, sexual cannibalism is always detrimental to population growth rate and leads to a higher population extinction risk. Increasing the fecundity benefits of sexual cannibalism leads to a consistently higher population growth rate and likely a lower extinction risk. However, even if cannibalism-derived fecundity benefits are large, very high rates of sexual cannibalism (>70%) can still drive the population to negative growth and potential extinction. Pre-copulatory cannibalism was particularly damaging for population growth rates and was the main predictor of growth declining below the replacement rate. Surprisingly, post-copulatory cannibalism had a largely positive effect on population growth rate when fecundity benefits were present. This study is the first to formally estimate the population-level effects of sexual cannibalism. We highlight the detrimental effect sexual cannibalism may have on population viability if (1) cannibalism rates become high, and/or (2) cannibalism-derived fecundity benefits become low. Decreased food availability could plausibly both increase the frequency of cannibalism, and reduce the fecundity benefit of cannibalism, suggesting that sexual cannibalism may increase the risk of population collapse in the face of environmental change.

Postmating Female Control: 20 Years of Cryptic Female Choice

Cryptic female choice (CFC) represents postmating intersexual selection arising from female-driven mechanisms at or after mating that bias sperm use and impact male paternity share. Although biologists began to study CFC relatively late, largely spurred by Eberhard's book published 20 years ago, the field has grown rapidly since then. Here, we review empirical progress to show that numerous female processes offer potential for CFC, from mating through to fertilization, although seldom has CFC been clearly demonstrated. We then evaluate functional implications, and argue that, under some conditions, CFC might have repercussions for female fitness, sexual conflict, and intersexual coevolution, with ramifications for related evolutionary phenomena, such as speciation. We conclude by identifying directions for future research in this rapidly growing field.

Male adaptations to minimize sexual cannibalism during reproduction in the funnel-web spider Hololena curta

Males of many spider species risk being attacked and cannibalized while searching for, courting, and mating with conspecific females. However, there are exceptions. We show that the funnel-web spider, Hololena curta, has 3 adaptations that minimize risk to males during courtship and mating, and enhance reproductive success. First, males detected chemical or tactile signals associated with webs of virgin females, and differentiated them from webs of mated females, enabling males to increase encounter rates with virgin females and avoid aggressive mated females. Second, males produced stereotyped vibrational signals during courting which induced female quiescence and suppressed female aggression. Third, when touched by males, sexually receptive females entered a cataleptic state, allowing males to safely approach and copulate. Because males can mate multiple times and the sex ratio in natural populations of H. curta is female biased, overall reproductive output is likely increased by males of this species avoiding sexual cannibalism.

Exploiting a moment of weakness: male spiders escape sexual cannibalism by copulating with moulting females

Sexual cannibalism is a particularly extreme example of conflict between the sexes, depriving the male of future reproduction. Theory predicts that sexual conflict should induce counter-adaptations in the victim. Observations of male spiders mating with moulting and hence largely immobile females suggest that this behaviour functions to circumvent female control and cannibalism. However, we lack quantitative estimates of natural frequencies and fitness consequences of these unconventional matings. To understand the importance of mating while moulting in cannibalistic mating systems, we combined mating experiments and paternity assessment in the laboratory with extensive field observations using the sexually cannibalistic orb-web spider Argiope bruennichi. Copulations with moulting females resulted in 97% male survival compared with only 20% in conventional matings. Mating while moulting provided similar paternity benefits compared with conventional matings. Our findings support the hypothesis that mating with moulting females evolved under sexual conflict and safely evades sexual cannibalism. Despite male benefits, natural frequencies were estimated around 44% and directly predicted by a male guarding a subadult female. Since only adult females signal their presence, the difficulty for males to locate subadult females might limit further spreading of mating with moulting females.

Worthless and Nutritive Nuptial Gifts: Mating Duration, Sperm Stored and Potential Female Decisions in Spiders

In nuptial gift-giving species females sometimes select their potential mates based on the presence and size of the gift. But in some species, such as the Neotropical polyandrous spider Paratrechalea ornate male gifts vary in quality, from nutritive to worthless, and this male strategy can be in conflict with female nutritional benefits. In this species, males without gifts experience a reduction in mating success and duration, while males that offer worthless or genuine nutritive gifts mate with similar frequencies and durations. The female apparently controls the duration of copulation. Thus, there is scope for females to favour males offering gifts and further if these are nutritious, via post-copulatory processes. We first tested whether females differentially store sperm from males that offer the highest nutritional benefits by experimentally presenting females with males that offer either nutritive or worthless gifts (uninterrupted matings). Second, we carried out another set of experiments to examine whether females can select sperm based only on gift presence. This time we interrupted matings after the first pedipalp insertion, thus matching number of insertions and mating duration for males that: offered and did not offer gift. Our results showed that the amount of sperm stored is positive related to mating duration in all groups, except in matings with worthless gifts. Gift presence itself did not affect the sperm stored by females, while they store similar number of sperm in matings with males offering either nutritive or worthless gifts. We discuss whether females prefer males with gifts regardless, if content, because it represents an attractive and/or reliable signal. Or alternatively, they prefer nutritive nuptial gifts, as they are an important source of food supply and/or signal of male donor ability.

The sterile male technique: irradiation negatively affects male fertility but not male courtship

The sterile male technique is a common method to assign paternity, widely adopted due to its relative simplicity and low cost. Male sterility is induced by exposure to sub lethal doses of chemosterilants or irradiation, the dosage of which has to be calibrated for every species to provide successful male sterilisation, without affecting male physiology and behaviour. While the physiological effects of sterilisation are usually assessed for each study, the behavioural ones are rarely analysed in detail. Using the orb web spider Argiope keyserlingi as a model we first tested (1) the validity of the thread assay, which simulates male courtship behaviour in a standardised context, as a proxy representing courtship on a female web. We then investigated (2) the effectiveness of male sterilisation via irradiation and (3) its consequences on male courtship behaviour. Our results validate the thread assay and the sterile male technique as legitimate tools for the study of male courtship behaviour and fertilisation success. We show that these techniques are time and cost effective and reduce undesirable variation, thereby creating opportunities to study and understand the mechanisms underlying sexual selection.

Sexual cannibalism as a manifestation of sexual conflict

Sexual cannibalism is a well-known example for sexual conflict and has many facets that determine the costs and benefits for the cannibal and the victim. Here, I focus on species in which sexual cannibalism is a general component of a mating system in which males invest maximally in mating with a single (monogyny) or two (bigyny) females. Sexual cannibalism can be a male strategy to maximize paternity and a female strategy to prevent paternity monopolization by any or a particular male. Considerable variation exists between species (1) in the potential of males to monopolize females, and (2) in the success of females in preventing monopolization by males. This opens up exciting future possibilities to investigate sexually antagonistic coevolution in a largely unstudied mating system.

Sperm storage mediated by cryptic female choice for nuptial gifts

Polyandrous females are expected to discriminate among males through postcopulatory cryptic mate choice. Yet, there is surprisingly little unequivocal evidence for female-mediated cryptic sperm choice. In species in which nuptial gifts facilitate mating, females may gain indirect benefits through preferential storage of sperm from gift-giving males if the gift signals male quality. We tested this hypothesis in the spider Pisaura mirabilis by quantifying the number of sperm stored in response to copulation with males with or without a nuptial gift, while experimentally controlling copulation duration. We further assessed the effect of gift presence and copulation duration on egg-hatching success in matings with uninterrupted copulations with gift-giving males. We show that females mated to gift-giving males stored more sperm and experienced 17% higher egg-hatching success, compared with those mated to no-gift males, despite matched copulation durations. Uninterrupted copulations resulted in both increased sperm storage and egg-hatching success. Our study confirms the prediction that the nuptial gift as a male signal is under positive sexual selection by females through cryptic sperm storage. In addition, the gift facilitates longer copulations and increased sperm transfer providing two different types of advantage to gift-giving in males.

Rapid range expansion is not restricted by inbreeding in a sexually cannibalistic spider

Few studies investigated whether rapid range expansion is associated with an individual's short-term fitness costs due to an increased risk of inbred mating at the front of expansion. In mating systems with low male mating rates both sexes share potential inbreeding costs and general mechanisms to avoid or reduce these costs are expected. The spider Argiope bruennichi expanded its range recently and we asked whether rapid settlement of new sites exposes individuals to a risk of inbreeding. We sampled four geographically separated subpopulations, genotyped individuals, arranged matings and monitored hatching success. Hatching success was lowest in egg-sacs derived from sibling pairs and highest in egg-sacs derived from among-population crosses, while within-population crosses were intermediate. This indicates that inbreeding might affect hatching success in the wild. Unlike expected, differential hatching success of within- and among-population crosses did not correlate with genetic distance of mating pairs. In contrast, we found high genetic diversity based on 16 microsatellite markers and a fragment of the mitochondrial COI gene in all populations. Our results suggest that even a very recent settlement secures the presence of genetically different mating partners. This leads to costs of inbreeding since the population is not inbred.

The influence of vibratory courtship on female mating behaviour in orb-web spiders (Argiope keyserlingi, Karsch 1878)

Web-building spiders are important models for sexual selection. While our understanding of post-copulatory mechanisms including sperm competition and cryptic female choice is considerable, our knowledge of courtship and how it influences male and female mating decisions is still extremely poor. Here, we provide the first comprehensive description of male courtship behaviour and vibrations generated in the web by the orb-web spider, Argiope keyserlingi – a recognised model species. We identified three main elements of male courtship: shudders, abdominal wags and mating thread dances (including both plucks and bounces). The vibrations generated by these behaviours are described in detail. Male shuddering behaviour appears to have a strong influence on female latency to mate acceptance, with males that shudder at high rates without compromising shudder duration being preferred. Shuddering behaviour may also mediate female aggressive behaviour, with males that generate long shudders less likely to be cannibalised after copulation. Male abdominal wagging behaviour, however, appears to have only limited influence on female mating decisions. This study provides avenues for future work that synthesises pre- and post-copulatory mechanisms in web-building spiders to generate an all-encompassing model of how sexual selection operates.

Conditional monogyny: female quality predicts male faithfulness

INTRODUCTION

Male monogyny in the absence of paternal investment is arguably one of the most puzzling mating systems. Recent evidence suggests that males of monogynous species adjust their life-history and their mating decision to shifting spatial and temporal selection regimes. In the cannibalistic wasp spider Argiope bruennichi males can be either monogynous or mate with a maximum of two females. We studied factors underlying male mating decisions in a natural population over a whole mating season. We documented all matings and categorized the males into single-mated and double-mated monogynous as well as bigynous males.

RESULTS

We found that all categories were continuously present with relatively stable frequencies despite changes in the operational sex ratio. Males were more likely monogynous when copulating with relatively heavy and old females and otherwise bigynous.

CONCLUSION

Our results imply that males make conditional mating decisions based on the quality of the first female they encounter but do not adjust their mating tactic to the local selection regime.

Determinants of natural mating success in the cannibalistic orb-web spider Argiope bruennichi

Monogynous mating systems (low male mating rates) occur in various taxa and have evolved several times independently in spiders. Monogyny is associated with remarkable male mating strategies and predicted to evolve under a male-biased sex ratio. While male reproductive strategies are well documented and male mating rates are easy to quantify, especially in sexually cannibalistic species, female reproductive strategies, the optimal female mating rate, and the factors that affect the evolution of female mating rates are still unclear. In this study, we examined natural female mating rates and tested the assumption of a male-biased sex ratio and female polyandry in a natural population of Argiope bruennichi in which we controlled female mating status prior to observations. We predicted variation in female mating frequencies as a result of spatial and temporal heterogeneity in the distribution of mature females and males. Females had a low average mating rate of 1.3 and the majority copulated only once. Polyandry did not entirely result from a male-biased sex-ratio but closely matched the rate of male bigamy. Male activity and the probability of polyandry correlated with factors affecting pheromone presence such as virgin females' density. We conclude that a strong sex ratio bias and high female mating rates are not necessary components of monogynous mating systems as long as males protect their paternity effectively and certain frequencies of bigyny stabilise the mating system.

Males of the orb-web spider Argiope bruennichi sacrifice themselves to unrelated females

Costs of inbreeding can lead to total reproductive failure and inbreeding avoidance is, therefore, common. In classical sex roles with no paternal care, the selective pressure to avoid inbreeding is mostly on the female, which carries the higher costs. In some orb-web spiders, this situation is very different because females are polyandrous and males are monogynous or at most bigynous. Additionally, females of many entelegyne orb weavers are thought to bias paternity post-copulatorily towards a desired mate. This increases the selective pressure on males to adjust their investment in a mating with regard to the compatibility to a female. Here, we examine whether genetic relatedness influences mating behaviour in the orb-web spider Argiope bruennichi. We mated either a sibling or a non-sibling male to a female in single copulation trials and compared copulation duration, cannibalism rate and female fecundity. Our experiment revealed that males prolonged their copulation duration and were cannibalized more frequently when mating with a non-sibling female. Males mating with a sibling female were more likely to escape cannibalism by copulating briefly, thus presumably increasing their chances of re-mating with a more compatible female. This suggests that males can adaptively adjust their investment relating to the compatibility of a female.