Operational sex ratio and resource defence as predictors of the mating system in European bitterling

Operational sex ratio (OSR), the ratio of sexually active males to fertilizable females in a population, plays a central role in the theory of mating systems by predicting that the intensity of male–male competition and the degree of sexual selection increases as the OSR becomes increasingly male biased. At high values of OSR, however, resource defence theory predicts the breakdown of territoriality and a shift towards scramble competition with a decrease in sexual selection. The direction that correlations between OSR and resource competition and variance in mating success will take depends on the biology of the species of interest. We investigated the effects of male population density and male‐biased operational sex ratio on male mating tactics shown by a freshwater fish, the European bitterling, Rhodeus sericeus . This species spawns inside living unioneid mussels. Large males defended territories, were aggressive towards conspecifics under equal sex ratios and monopolized pair spawnings with females. The mating tactic, however, changed at high male density where large males ceased to be territorial and instead competed with groups of smaller males to release sperm when females spawned. This change in male behaviour from pair to group spawning has two ramifications for sexual selection. The intensity of sexual selection and variance in male mating success decrease, and the form of sexual competition changes from resource‐ to sperm competition. Thus, the use of alternative mating tactics renders the OSR unable to predict the direction of resource competition and variance in male mating success at high densities.     

Adaptive non-Fisherian sex ratios in a patchily distributed population with outbreeding

A model for sex allocation is presented where mothers have differing abilities to assess their arrival order at a patch in low-density populations. Mothers either oviposit with no knowledge, some knowledge, or they know their arrival order exactly. Furthermore they deposit single eggs in patches hence all matings within a patch are between unrelated individuals. The male offspring however, are also able to mate with females from male-less patches through dispersal. Whereas the first model predicted sex ratios of equality, the models incorporating knowledge both predicted female biased sex ratios. This is shown to be due to an asymmetry in mating opportunities that we have termed random asymmetrical mate competition (RAMC) that only knowledgeable females are able to use to their advantage. Data from the out-breeding non-pollinating fig wasp Otitesella pseudoserrata, suggests that these mothers do have knowledge of their arrival orders and supports the concept and predictions of RAMC.     

Evolutionary stable sex ratios with non‐facultative male‐eggs first sex allocation in fig wasps

Sex allocation theory has long generated insights into the nature of natural selection. Classical models have elucidated causal phenomena such as local mate competition and inbreeding on the degree of female bias exhibited by various invertebrates. Typically, these models assume mothers facultatively adjust sex allocation using predictive cues of future offspring mating conditions. Here we relax this assumption by developing a sex allocation model for haplodiploid mothers experiencing local mate competition that lay a fixed number of male eggs first. Female egg number is determined by remaining oviposition sites or remaining eggs of the mother, depending on which is exhausted first. Our model includes parameters for variation in foundress number, patch size, fecundity and offspring mortality that allow us to generate secondary sex ratio predictions based on specific parameterizations for natural populations. Simulations show that: 1) in line with classical models, factors that increase sib‐mating result in mothers laying relatively more female eggs; 2) high offspring mortality leads to relatively more males as fertilization insurance; 3) unlike classical model predictions, sub‐optimal predictions, such as more males than females are possible. In addition, our model provides the first quantitative predictions for the expected number of males and females in a patch where typically only one mother utilizes a given patch. We parameterized the model with data obtained from seven species of southern African fig wasps to predict expected means and variances for numbers of male and female offspring for typical numbers of mothers utilizing a patch. These predictions were compared to secondary sex ratio data from single foundress patches, the most commonly encountered situation for these species. Our predictions matched both the observed number and variance of male and female offspring with a high degree of accuracy suggesting that facultative adjustment is not required to produce evolutionary stable sex ratios.     

Callers and satellites in the natterjack toad: evolutionarily stable decision rules

Male natterjack toads, Bufo calamita, sometimes called to attract females and sometimes behaved as silent satellites, attempting to intercept females attracted by callers. Small males with low intensity calls most frequently adopted the satellite tactic and preferred to parasitize the loudest callers. Because body size and call intensity were highly correlated with male mating success, small males with weak calls were at a reproductive disadvantage, and probably achieved greater success by exploiting the displays of larger, louder males than by calling themselves. The behaviour adopted by individual males in a given situation was predicted by a game theory model which assumed that males used an evolutionarily stable decision rule when choosing between alternative mating tactics. However, the model provided close agreement with observed behaviour only when it assumed that males have imperfect information about the behaviour of competitors. It is concluded that male toads adopt a conditional strategy which depends on their attractiveness to females. By switching between mating tactics appropriately, males maximized expected mating success within the constraint of the information available to them. The decision rule of the model qualitatively corresponds to a proximate mechanism that males appeared to use when switching between tactics. The model offers insights into the behaviour of other species in which males display in groups to attract females.     

Female mate choice tactics in a resource-based mating system: field tests of alternative models

Abstract In this study we test theoretical models of female mate choice tactics in natural populations of pine engravers, Ips pini (Say) (Coleoptera: Scolytidae), a species with a resource-based mating system and high search costs. We first develop distinguishing predictions for each of four models of mate choice: random, comparison tactics, and fixed and adjustable thresholds. These predictions relate to commonly collected field data that include the visiting behavior of females and the quality of accepted and rejected mates. Using these types of data, we conclude that pine engravers use an adjustable threshold mate choice tactic because females often accepted the first male encountered, rarely revisited males, visited similar numbers of males in patches of different quality, accepted higher-quality males than those they rejected even on their first encounter with a male in a patch, and had higher acceptance thresholds in high-quality patches than in low-quality patches. This adjustable threshold tactic is consistent with a one-step decision rule and is predicted to occur in species such as pine engravers in which search costs are high and females have information about patch quality before beginning a search in a patch.     

Male mate choice and the potential for complex mating dynamics in the tree lizard (Urosaurus ornatus)

A growing body of literature is recognizing that males may also play a role in the mating process by behaving non‐randomly toward potential female mates during courtship. In numerous species, discrete color polymorphisms in males are inferred to represent alternative mating tactics, which often correspond with concomitant asymmetries in ecology and behavior. In terms of their mating behavior, these ecological outcomes of a color polymorphism should affect a morph's likelihood and frequency of encountering females in a population, possibly favoring the evolution of morph‐specific mating preferences. Knowledge of how male morphs contribute to a species’ overall mating dynamics will improve our understanding of how sexual selection shapes phenotypic diversity in color polymorphic systems. We conducted a mate choice experiment to evaluate the extent and morph specificity of non‐random mating preferences by male ornate tree lizards, Urosaurus ornatus. We observed the behavior of blue and yellow males in an experimental arena in response to a choice between an orange or yellow female. We found that blue males preferred yellow females over orange females, and although yellow males visited females more often than blue males overall, their attention was not morph‐specific. Given male morph differences in choosiness, and their differences in social dominance, we conclude that female throat color may be partly under sexual selection in U. ornatus. However, a lack of concordance between male and female mating preferences (drawn from an earlier study) suggests that overall mating dynamics may serve to maintain, rather than enhance, color morph differences in this species.     

Impact of male alternative reproductive tactics on female costs of sexual conflict under variation in operational sex ratio and population density

Sexual conflict over mating rate is both pervasive and evolutionarily costly. For females, the lifetime reproductive fitness costs that arise through interactions with potential mates will be influenced by the frequency of such interactions, and the fitness cost of each interaction. Both of these factors are likely to be influenced by variation in operational sex ratio (OSR) and population density. Variation in OSR‐ and density‐dependent male alternative reproductive tactics (ARTs) may be particularly important if the fitness costs that females experience vary with the reproductive tactics that males express. Using a simple model, we consider several examples of OSR‐ and/or density‐dependent variation in male ARTs and the frequency of male–female interactions, and find that variation in the expression of male ARTs has the potential to augment or diminish the costs of frequent male interactions for females. Accurately documenting variation in the expression of male ARTs and associated female fitness costs will benefit future work in this area.     

Consequence of emergence pattern on inbreeding risk in the aphid parasitoid Aphidius matricariae (Hymenoptera: Braconidae)

In insect parasitoids, mating strategy depends on mate availability and is influenced by the spatial and temporal emergence patterns of adults. For quasi-gregarious species, simultaneous emergence favors local mating and reduces search costs for partners while increasing the risk of inbreeding, particularly when only one female parasitizes the initial host patch. Consequently, in inbreeding sensitive species, mating on the place of adult emergence (patch mating) between siblings should be counter selected. In practice, the timing of male and female emergence and of their dispersal influences mate availability and can limit on patch mating. To test the role of these two factors, we analyzed the daily distribution of emergence and patch residence time of a cohort in the aphid parasitoid Aphidius matricariae (Hymenoptera: Braconidae). We observed that adult emergence is concentrated on the morning with males emerging on average before females with some overlaps. A more precise evaluation of emergence pattern within a brood suggests that brothers and sisters rarely emerge at the same time and rapid dispersal of males and females favors off-patch mating. The relationships between timing of emergence including differences between sex and consequences on inbreeding probability in these species are thus discussed.     

Variability of GnRH secretion in two goby species with socially controlled alternative male mating tactics

Male reproductive phenotypic plasticity related to environmental-social conditions is common among teleost fish. In several species, males adopt different mating tactics depending on their size, monopolizing mates when larger, while parasitizing dominant male spawns when smaller. Males performing alternative mating tactics are often characterized by a strong dimorphism in both primary and secondary reproductive traits. According to studies on sex-changing species and on species where only one male morph is reproductively active, male alternative phenotypes are expected to vary also in gonadotropin-releasing hormone (GnRH) neurons in forebrain preoptic area (POA). Here, we compared the intra- and inter-sexual variations in number and size of GnRH neurons, along with gonads and male accessory structure investment, in two goby species, the grass goby, Zosterisessor ophiocephalus, and the black goby, Gobius niger, characterized by male alternative mating phenotypes. In both species, older and larger males defend nests, court and perform parental care, while younger and smaller ones try to sneak territorial male spawning. We found that grass goby and black goby have different patterns of GnRH expression. Grass goby presents a clear intra-sexual dimorphism in GnRH expression, related to the occurrence of alternative mating tactics, while in the black goby, only inter-sexual differences are observed. The inter- and intra-specific variability in the GnRH neurons in these two goby species is discussed in light of the differences in migratory behavior, nest type, and mating system.     

Mothers matter! Maternal support,dominance status and mating success in male bonobos (Pan paniscus)

Variation in male mating success is often related to rank differences. Males who are unable to monopolize oestrous females alone may engage in coalitions, thus enhancing their mating success. While studies on chimpanzees and dolphins suggest that coalitions are independent of kinship, information from female philopatric species shows the importance of kin support, especially from mothers, on the reproductive success of females. Therefore, one might expect a similar effect on sons in male philopatric species. We evaluate mating success determinants in male bonobos using data from nine male individuals from a wild population. Results reveal a steep, linear male dominance hierarchy and a positive correlation between dominance status and mating success. In addition to rank, the presence of mothers enhances the mating success of sons and reduces the proportion of matings by the highest ranking male. Mothers and sons have high association rates and mothers provide agonistic aid to sons in conflicts with other males. As bonobos are male-philopatric and adult females occupy high dominance status, maternal support extends into adulthood and females have the leverage to intervene in male conflicts. The absence of female support to unrelated males suggests that mothers gain indirect fitness benefits by supporting their sons.     

Mating system and sex allocation in the gregarious parasitoid Cotesia glomerata

The gregarious parasitoid Cotesia glomerata (L.) is often presumed to possess the characteristic attributes of a species that manifests local mate competition (LMC), as it commonly produces female-biased broods. However, our field surveys of sex ratio and laboratory observations of adult behaviour showed that this species is subject to partial local mate competition caused by natal dispersal. On average, 30% of males left their natal patch before mating, with the proportion of dispersing males increasing with an increase in the patch's sex ratio (i.e. proportion of males). Over 50% of females left their natal patch before mating, and only 27.5% of females mated with males emerging from the same natal patch. Although females showed no preference between males that were and were not their siblings, broods from females that mated with siblings had a significantly higher mean brood sex ratio (0.56) than broods from females that mated with nonsiblings (0.39). Furthermore, brood sex ratios increased as inbreeding was intensified over four generations. A field population of this wasp had a mean brood sex ratio of 0.35 over 3 years, which conformed well to the evolutionarily stable strategy sex ratio (r=0.34) predicted by Taylor's partial sibmating model for haplodiploid species. These results suggest that the sex allocation strategy of C. glomerata is based on both partial local mate competition in males and inbreeding avoidance in females. In turn, this mating system plays a role in the evolution of natal dispersal behaviour in this species.Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Partial local mate competition in the wasp Trichogramma euproctidis: the role of emergence sex ratio on female mating behaviour

1. Parasitic wasps with structured populations are generally assumed to follow the local mate competition (LMC) model: females lay only the minimal number of sons necessary to inseminate all daughters in the emergence patch, and increase this number when faced with additional broods from unrelated females. After emergence, daughters mate with local males before dispersing for host location and oviposition. The main predictions from the model have been verified for many species. 2. Conflicting evidence exists on the status of the egg parasitoids Trichogramma regarding their on‐patch versus off‐patch mating. Although the life‐history traits of several species indicate that mating must occur on the emergence patch, recent data suggest that mating could occur outside the natal patch. 3. In this study, we measured the level of off‐patch mating in the egg parasitoid Trichogramma euproctidis using two isofemale lines in a greenhouse experiment. The impact of the sex ratio on the level of off‐patch mating was also tested. 4. The overall off‐patch mating proportion was 40.5% with a range between 0 and 85.7%, and was influenced by the sex ratio on the emergence patch: the more males available at emergence, the less off‐patch mating occurring. 5. The mating structure of this species can be described as partial LMC.     

Age‐dependent male mating tactics in a spider mite—A life‐history perspective

Males often fight with rival males for access to females. However, some males display nonfighting tactics such as sneaking, satellite behavior, or female mimicking. When these mating tactics comprise a conditional strategy, they are often thought to be explained by resource holding potential (RHP), that is, nonfighting tactics are displayed by less competitive males who are more likely to lose a fight. The alternative mating tactics, however, can also be explained by life‐history theory, which predicts that young males avoid fighting, regardless of their RHP, if it pays off to wait for future reproduction. Here, we test whether the sneaking tactic displayed by young males of the two‐spotted spider mite can be explained by life‐history theory. We tested whether young sneaker males survive longer than young fighter males after a bout of mild or strong competition with old fighter males. We also investigated whether old males have a more protective outer skin—a possible proxy for RHP—by measuring cuticle hardness and elasticity using nanoindentation. We found that young sneaker males survived longer than young fighter males after mild male competition. This difference was not found after strong male competition, which suggests that induction of sneaking tactic is affected by male density. Hardness and elasticity of the skin did not vary with male age. Given that earlier work could also not detect morphometric differences between fighter and sneaker males, we conclude that there is no apparent increase in RHP with age in the mite and age‐dependent male mating tactics in the mite can be explained only by life‐history theory. Because it is likely that fighting incurs a survival cost, age‐dependent alternative mating tactics may be explained by life‐history theory in many species when reproduction of old males is a significant factor in fitness.     

Predation risk and alternative mating tactics in male Trinidadian guppies (Poecilia reticulata)

In the guppy (Poecilia reticulata), males have two alternative mating tactics. Individual males may either display to a receptive female prior to attempting to copulate with her or attempt to quickly sneakcopulate with a female without first displaying to her or without a prior receptive response from her. In this study, I experimentally investigated the effects of simulated local increases in the risk of predation (in the form of a cichlid fish predator model in situ) on the mating tactics used by free-ranging male guppies in two typical macrohabitats (riffle and pool) of a Trinidadian river. Focal male guppies displayed to females significantly less often on average, and conversely attempted sneak copulations more often, in the presence of the predator model than in its absence; this pattern was similar for both habitats. These fish therefore performed a lower proportion of sigmoid displays and increased their sneaky mating attempts when the apparent risk of predation had increased locally. This predator-mediated response is consistent with a trade-off between mating success and risk of mortality due to predation. The results are the first to confirm risk-sensitive mating behaviour in free-ranging male guppies within a population, and demonstrate the potential importance of predators in influencing the relative use of alternative mating tactics in this species on a microgeographical scale in the wild.     

Persistent courtship reduces male and female longevity in captive tsetse flies Glossina morsitans morsitans Westwood (Diptera: Glossinidae)

Where males can increase their mating success by harassing femalesuntil they accept copulation, harassing tactics can be expectedto evolve to a point where they have costs to the longevityof both sexes. By experimentally manipulating the sex ratioin captive groups of tsetse flies Glossina morsitans morsitans,we demonstrated that the longevity of females declines wheresex ratios are biased toward males, while the longevity of malesdeclines where the sex ratio is biased toward females. Neitherirradiation of males nor prevention of copulation by blockingor damaging the external male genitalia increased the longevityof females caged with them, suggesting that female longevitywas reduced by the physical aspects of male harassment ratherthan by components of the ejaculate     

Experimental field studies of Asian ape social systems

The Asian apes, orangutans and gibbons, possess unusual social systems among anthropoid primates. Social groups of gibbons consist of mated adult pairs and their offspring; mature orangutans are primarily solitary. Recent experimental field research has begun to yield insights into the behavioral mechanisms employed by these animals to maintain their characteristic patterns of social dispersion. While spatial separation between female orangutans appears to be maintained passively, aggression, which is manifest during direct encounters and long-distance vocal interactions, mediates male asociality. Male-male aggression is the result of intense intrasexual competition occurring between animals for mating access to females. To reduce intrasexual competition, male orangutans have adopted alternative mating tactics. In contrast to female orangutans, female gibbons show marked agonistic tendencies toward conspecifics. Female territoriality contributes to preventing males from becoming polygynous. Male gibbons, restricted to monogamous relationships, attempt to ensure their paternity through intrasexual aggression. These observations suggest that the spatial dispersion of females constrains male mating options in both species. However, variations between orangutan and gibbon social systems can be understood as consequences of the temporal dispersion of sexually receptive females. The temporal clumping of females, due to relatively high operational sex ratios, limits the ability of male gibbons to acquire multiple mates. Conversely, an extremely low operational sex ratio in orangutans creates a strong selection pressure for intrasexual competition and polygamous mating. These considerations provide a novel framework for interpreting the social systems of the African apes.     

Polyandry and alternative mating tactics

Many species in the animal kingdom are characterized by alternative mating tactics (AMTs) within a sex. In males, such tactics include mate guarding versus sneaking behaviours, or territorial versus female mimicry. Although AMTs can occur in either sex, they have been most commonly described in males. This sex bias may, in part, reflect the increased opportunity for sexual selection that typically exists in males, which can result in a higher probability that AMTs evolve in that sex. Consequently, females and polyandry can play a pivotal role in governing the reproductive success associated with male AMTs and in the evolutionary dynamics of the tactics. In this review, we discuss polyandry and the evolution of AMTs. First, we define AMTs and review game theoretical and quantitative genetic approaches used to model their evolution. Second, we review several examples of AMTs, highlighting the roles that genes and environment play in phenotype expression and development of the tactics, as well as empirical approaches to differentiating among the mechanisms. Third, ecological and genetic constraints to the evolution of AMTs are discussed. Fourth, we speculate on why female AMTs are less reported on in the literature than male tactics. Fifth, we examine the effects of AMTs on breeding outcomes and female fitness, and as a source, and possibly also a consequence, of sexual conflict. We conclude by suggesting a new model for the evolution of AMTs that incorporates both environmental and genetic effects, and discuss some future avenues of research.     

Partial local mate competition and the sex ratio: A study on non-pollinating fig wasps

In many species, mating takes place in temporary patches where only a small number of females produce offspring. In this situation Local Mate Competition (LMC) theory predicts that the optimal sex ratio (defined as proportion males) should become increasingly female biased as the number of females contributing offspring to a patch decreases. However, in a large number of these species, some mating is also likely to occur away from the natal patch (termed partial LMC). In this case the degree of LMC is reduced, and theory predicts a relatively less female biased sex ratio. We tested these two predictions with field data from 17 species of New World non-pollinating fig wasps representing three genera. We present a model which suggests that the average number of females ovipositing in a fruit (i.e. patch) is positively correlated with the proportion of fruit of a given tree species in which that species of wasp occurs. Across species, the overall sex ratio was positively correlated with the proportion of fruit in which that species occurs. Furthermore, the males of some species are wingless, and in these species all mating must take place before females disperse from their natal fruit. In contrast, the males of other species are winged, and in these species mating may also take place away from the natal fruit. Species with winged males had less female biased sex ratios than species with wingless males that occurred in a similar proportion of fruit. Finally, the correlation between sex ratio and the proportion of fruit in which a species occurs was also observed within species when comparing between the fruit crops of different trees. This suggests that individual females facultatively adjust the sex ratio of their offspring in response to variable LMC.     

Female mating success and risk of pre-reproductive death in a protandrous grasshopper

Numerous studies have assessed the adaptive value of protandry for males in several insect species, considering that male emergence is determined by female availability. However, the possible advantage of the time of emergence for females on their mating success in protandrous insect species has only been explored theoretically. By studying the grasshopper Sphenarium purpurascens we evaluated the hypothesis that late emergence could be adaptive for females. If female maturation occurs when the population density is higher and the sex ratio (males/females) is biased to males, their probability of mating increases. Thus, in this study we estimated (1) the opportunity for mating in females as a function of their sexual maturation time, population density, and sex ratio at the moment they reached sexual maturity. In addition, (2) an analysis incorporating female body size and the total number of female matings was performed. Both analyses support the occurrence of protandry in the studied population. Under the first approach, females with intermediate maturation time had a higher probability of being mated than earlier and late matured females. Thus, it suggests that stabilising selection is acting on female maturation time and this may affect selection on male maturation time. Furthermore, the proportion of mated females increased when the sex ratio was biased to males, and stabilising selection on maturation time was detected also. However, the number of matings of a female depended on her body size. Females with larger body size had more matings than smaller ones at the beginning of the reproductive season. Because selection acts differently on maturation time in males and females of S. purpurascens this result is consistent with a condition for the maintenance of protandry in the population. The present results are discussed in the light of the models for the evolution of protandry.