Availability Of Mates Research Articles

Novel ablation technique shows no sperm priming response by male eastern mosquitofish to cues of female availability

Changes in mate availability and sperm competition should generate selection to adjust investments into different pre- and post-copulatory traits so that the product of mating and fertilization success maximize net male reproductive success. Given costly sperm production and the risk of sperm depletion, males should invest strategically in ejaculates. Here, we use the eastern mosquitofish (Gambusia holbrooki), where males have a single coercive mating tactic, to test whether the number of cues indicative of female availability affects the rate of sperm available for mating (so-called sperm priming). We also tested whether plasticity in sperm production varies with male body size. We created four socio-sexual treatments that differ in the number of female-derived cues: none, chemical, chemical and visual, and full access to a female. We used ablation surgery, removing the tip of the male gonopodium (intromittent organ), to prevent males from mating with a female in the treatment where they interacted with females. We hypothesized that elevated sperm priming would be associated with more cues about female availability, and be more apparent in smaller, subordinate males due to their lower baseline sperm count (higher risk of sperm depletion) and their potential disadvantage during premating competition (leading to higher marginal benefits from sperm investment). There was, however, no evidence for sperm priming. The rate of sperm availability for mating and the baseline sperm reserves were, however, dependent on male body size. We discuss possible reasons for our findings. We also note that our study provides novel insights into the proximate mechanisms associated with sperm release in Poeciliids. Our confirmation of the fact that removing the gonopodium tip prevents a male from releasing sperm when housed with a female has many potential applications (e.g., in the study of effects of ejaculate investment and mating effort on male mating success and longevity). When ejaculates are energetically costly, males should strategically adjust sperm production in response to relevant social cues such as female availability. Using four socio-sexual scenarios, we demonstrate that male mosquitofish G. holbrooki do not produce less sperm when housed alone, compared to being exposed only to chemical cues from females, or to chemical and visual cues, or even when allowed full access to a female. The absence of plasticity in sperm production, when compared to that reported in other Poeciliid fishes, suggests that the relationship between sperm number and mating success might depend upon the mating system. If increased sperm number has a small effect on male reproductive success, there might not be selection for plastic shifts in sperm production. We also showed that ablation surgery on the male’s gonopodium allows us to let males interact with females without ejaculating.

Females of a cannibalistic spider control mutilation of their genitalia by males

AbstractWhen females can mate multiply, the interests of both sexes over female remating may not coincide, leading to selection for adaptations and counteradaptations in males and females. In several orb-weaving spiders, males damage external structures of the female genitalia during copulation, which hinders the female from remating. We investigated whether females have control over the mutilation of their genitalia in the orb-weaving spider Larinia jeskovi. We found that female sexual cannibalism during copulation reduced the number of insertions a male was able to perform and hence limited the probability of genital mutilation by the male. Genital mutilation did not differ between treatments in which females experienced different availabilities of other males before the mating trial: males absent, males near the female (“vicinity group”), and males in the female’s web (“web group”). However, traits of the mating male (size, condition) were significantly correlated with the occurrence of cannibalism during mating in “web” and “vicinity” treatments. These results suggest that females have control over mutilation by an early termination of mating, can respond to the availability of potential mates and can alter the probability of mutilation according to certain male traits. Female sexual cannibalism may represent a counteradaptation to genital mutilation allowing females to mate multiply.

Multiple mating is linked to social setting and benefits the males in a communally rearing mammal

Abstract Individuals in social species may mate with multiple opposite-sex individuals, including members of the same or different social groups. This variation may be linked to genetic benefits, where multiple mating decreases risk of inbreeding. Multiple mating may also be constrained by the sociospatial setting through its effect on availability of mates. Because multiple mating with individuals from same or different groups may determine sex-specific fitness effects, we also examined how multiple mating modulates social benefits of females and males. We used 7 years of data on demography, social organization, and genetics of a natural population of the group-living and colonial rodent, Octodon degus, to determine how kin and sex composition within social groups, and spatial relations between these groups (i.e., colonial habits) influence multiple mating and its fitness consequences. Males (81.3%) and females (64.9%) produced offspring with multiple opposite-sex individuals within groups and with individuals of neighboring groups. Thus, polygynandry was the dominant mating system in the degu population examined. Multiple mating in degus was high when compared with estimates reported in other social mammals. Variation in female and male multiple mating was better explained by social setting through its effect on availability of potential mates rather than by benefits derived from decreasing risk of inbreeding. Finally, our study revealed how multiple mating enhances male, but not female reproductive success.

Genetic rescue in a plant polyploid complex: Case study on the importance of genetic and trait data for conservation management.

Knowledge of the biology of rare plant species is indispensable to aid their survival and to inform efficient conservation actions, but in many cases relevant data are lacking. In addition, while studies of conservation genetics have provided a wealth of information on the considerations arising from inbreeding, mate limitation, or local adaptation, the impact of intraspecific polyploidy remains understudied. In this study, we examined the breeding system of the rare Australian daisy Rutidosis lanata (Asteraceae) and screened ten of its populations for their ploidy level to develop recommendations for management actions, in particular, with regard to seed sourcing and genetic rescue. We found R. lanata to represent a polyploid complex, with tetraploid, pentaploid and hexaploid individuals coexisting in the same species. Crossing experiments confirmed R. lanata to be self‐incompatible. Mate availability varied from c. 49% to c. 76% across populations. Most populations showed mate availability of c. 50%–70%, suggesting that mate limitation resulting from a lack of local genetic diversity may cause or at least contribute to reduced seed set. Crossing between populations resulted in significantly higher reproductive success for all populations except one, suggesting the possibility of genetic rescue through population mixing. However, the crossing experiments also showed that pentaploids suffer from a severely reduced paternal reproductive fitness. Any additional hybrids between tetraploids and pentaploids, as would be created by mixing populations with different genome copy numbers during conservation work, would consequently exacerbate mate limitation and thus reduce population viability. We conclude that seed set and thus population viability can be maximized by mixing populations with the same number of genome copies, but that populations with different numbers should be kept spatially separated. The case of Rutidosis lanata provides an example and a potential template for examining the conservation genetics of other species that may constitute polyploid complexes.

Harvesting changes mating behaviour in European lobster.

Removing individuals from a wild population can affect the availability of prospective mates and the outcome of competitive interactions, with subsequent effects on mating patterns and sexual selection. Consequently, the rate of harvest‐induced evolution is predicted to be strongly dependent on the strength and dynamics of sexual selection, yet there is limited empirical knowledge on the interplay between selective harvesting and the mating systems of exploited species. In this study, we used genetic parentage assignment to compare mating patterns of the highly valued and overexploited European lobster (Homarus gammarus) in a designated lobster reserve and nearby fished area in southern Norway. In the area open to fishing, the fishery is regulated by a closed season, a minimum legal size and a ban on the harvest of egg‐bearing females. Due to the differences in size and sex‐specific fishing mortality between the two areas, males and females are of approximately equal average size in the fished area, whereas males tend to be larger in the reserve. Our results show that females would mate with males larger than their own body size, but the relative size difference was significantly larger in the reserve. Sexual selection acted positively on both body size and claw size in males in the reserve, while it was nonsignificant in fished areas. This strongly suggests that size truncation of males by fishing reduces the variability of traits that sexual selection acts upon. If fisheries continue to target large individuals (particularly males) with higher relative reproductive success, the weakening of sexual selection will likely accelerate fisheries‐induced evolution towards smaller body size.

Chemical Cues that Guide Female Reproduction in Drosophila melanogaster.

Chemicals released into the environment by food, predators and conspecifics play critical roles in Drosophila reproduction. Females and males live in an environment full of smells, whose molecules communicate to them the availability of food, potential mates, competitors or predators. Volatile chemicals derived from fruit, yeast growing on the fruit, and flies already present on the fruit attract Drosophila, concentrating flies at food sites, where they will also mate. Species-specific cuticular hydrocarbons displayed on female Drosophila as they mature are sensed by males and act as pheromones to stimulate mating by conspecific males and inhibit heterospecific mating. The pheromonal profile of a female is also responsive to her nutritional environment, providing an honest signal of her fertility potential. After mating, cuticular and semen hydrocarbons transferred by the male change the female's chemical profile. These molecules make the female less attractive to other males, thus protecting her mate's sperm investment. Females have evolved the capacity to counteract this inhibition by ejecting the semen hydrocarbon (along with the rest of the remaining ejaculate) a few hours after mating. Although this ejection can temporarily restore the female's attractiveness, shortly thereafter another male pheromone, a seminal peptide, decreases the female's propensity to re-mate, thus continuing to protect the male's investment. Females use olfaction and taste sensing to select optimal egg-laying sites, integrating cues for the availability of food for her offspring, and the presence of other flies and of harmful species. We argue that taking into account evolutionary considerations such as sexual conflict, and the ecological conditions in which flies live, is helpful in understanding the role of highly species-specific pheromones and blends thereof, as well as an individual's response to the chemical cues in its environment.

Temporal patterns of extra-pair paternity in a population of Grasshopper Sparrows (Ammodramus savannarum) in Maryland

The incidence of extra-pair paternity (EPP) is highly variable across bird taxa. While EPP is known to affect reproductive variance, the causes of temporal variation in rates of EPP are poorly studied. Breeding density has often been proposed as an important factor influencing EPP variation, but it has received mixed support. Over a 5-year period we examined the rate of EPP in a socially monogamous Grasshopper Sparrow (Ammodramus savannarum) population on a restored grassland in Maryland. Of 124 broods, 62.1% had extra-pair young (EPY), and of 448 nestlings, 36.8% were the result of extra-pair fertilizations. Over the 5-year study, percent EPY ranged from 26% to 60%. EPP rates were positively correlated with the local density of territorial males, supporting the density hypothesis, and 81% of the EPY were from neighboring males. Male within-pair mating success was related to extra-pair mating, suggesting an active behavioral tradeoff between within- and extra-pair matings, especially at high densities. By examining males at the individual level, we found males that were not cuckolded also had the most genetic offspring (within-pair and extra-pair), whereas males that were cuckolded also failed to sire many extra-pair offspring. Here we show that density plays an important role in the availability of mates, EPP, and the opportunity for sexual selection. Received 9 April 2016. Accepted 3 August 2017.

Social structure affects mating competition in a damselfish

The strength of mating competition and sexual selection varies over space and time in many animals. Such variation is typically driven by ecological and demographic factors, including adult sex ratio and consequent availability of mates. The spatial scale at which demographic factors affect mating competition and sexual selection may vary but is not often investigated. Here, we analyse variation in size and sex ratio of social groups, and how group structure affects mating competition, in the site-attached damselfish Chrysiptera cyanea. Site-attached reef fishes are known to show extensive intraspecific variation in social structure. Previous work has focused on species for which the size and dynamics of social groups are constrained by habitat, whereas species with group structure unconstrained by habitat have received little attention. Chrysiptera cyanea is such a species, with individuals occurring in spatial clusters that varied widely in size and sex ratio. Typically, only one male defended a nest in multi-male groups. Nest-holding males were frequently visited by mate-searching females, with more visits in groups with more females, suggesting that courtship and mating mostly occur within groups and that male mating success depends on the number of females in the group. Male–male aggression was frequent in multi-male groups but absent in single-male groups. These findings demonstrate that groups are distinct social units. In consequence, the dynamics of mating and reproduction are mainly a result of group structure, largely unaffected short term by overall population demography which would be important in open social systems. Future studies of the C. cyanea model system should analyse longer-term dynamics, including how groups are formed, how they vary in relation to density and time of season and how social structure affects sexual selection.

Predation on reproducing wolf spiders: access to information has differential effects on male and female survival

Predation has widespread influences on animal behaviour, and reproductive activities can be particularly dangerous. Males and females differ in their reactions to sensory stimuli from predators and potential mates, which affects the risk experienced by each sex. Thus, the information available can cause differential survival and have profound implications for mating opportunities and population structure. The wolf spider, Pardosa milvina, detects and responds in a risk-sensitive manner to chemotactile information from a larger predator, the wolf spider Tigrosa helluo. Male P. milvina use similar chemotactile cues to find females whereas female P. milvina focus on the visual, and perhaps vibratory, aspects of the male display. Our aim was to document the risk posed by T. helluo predators on P. milvina during reproduction and to determine whether augmenting chemotactile information would affect that outcome. In the laboratory, we explored the effects of adding predator and/or female cues on the predatory success of T. helluo on P. milvina males or observing females. Additional cues from prospective mates or from predators enhanced male survival. The addition of female cues increased predation on females whereas predator cues augmented female survival. In field enclosures, we documented the impact of T. helluo, with and without additional predator cues, on the sex ratio of survivors and the reproductive success of females. Additional predator cues shifted the sex ratio towards males, however, 90% of the remaining females in that treatment produced eggsacs whereas less than 60% reproduced in female-biased populations. Thus, augmenting the available predator information shifted the risk from males to females, presumably due to differences in their sensory priorities. By altering the availability of potential mates, this shift appears to have influenced the intensity of sexual selection for this spider.

A Pheromone Antagonist Regulates Optimal Mating Time in the Moth Helicoverpa armigera

Many insect species use multi-component sex pheromones to discriminate among potential mating partners [1-5]. In moths, pheromone blends tend to be dominated by one or two major components, but behavioral responses are frequently optimized by the inclusion of less abundant minor components [6]. An increasing number of studies have shown that female insects use these chemicals to convey their mating availability to males, who can assess the maturity of females and thus decide when to mate [7, 8]. However, little is known about the biological mechanisms that enable males to assess female reproductive status. In this study, we found that females of Helicoverpa armigera avoid nonoptimal mating by inhibiting males with pheromone antagonist cis-11-Hexadecenol (Z11-16:OH). We also show that this antagonist-mediated optimization of mating time ensures maximum fecundity. To further investigate molecular aspects of this phenomenon, we used the CRISPR/Cas9 system to knock out odorant receptor 16 (OR16), the only pheromone receptor tuned to Z11-16:OH. In mutant males, electrophysiological and behavioral responses to Z11-16:OH were abolished. Inability to detect Z11-16:OH prompted the males to mate with immature females, which resulted in significantly reduced viability of eggs. In conclusion, our study demonstrates that the sensitivity of OR16 to Z11-16:OH regulates optimal mating time and thus ensures maximum fecundity. These results may suggest novel strategies to disrupt pest insect mating.

Genetic structure among greater white-fronted goose populations of the Pacific Flyway.

An understanding of the genetic structure of populations in the wild is essential for long‐term conservation and stewardship in the face of environmental change. Knowledge of the present‐day distribution of genetic lineages (phylogeography) of a species is especially important for organisms that are exploited or utilize habitats that may be jeopardized by human intervention, including climate change. Here, we describe mitochondrial (mtDNA) and nuclear genetic (microsatellite) diversity among three populations of a migratory bird, the greater white‐fronted goose (Anser albifrons), which breeds discontinuously in western and southwestern Alaska and winters in the Pacific Flyway of North America. Significant genetic structure was evident at both marker types. All three populations were differentiated for mtDNA, whereas microsatellite analysis only differentiated geese from the Cook Inlet Basin. In sexual reproducing species, nonrandom mate selection, when occurring in concert with fine‐scale resource partitioning, can lead to phenotypic and genetic divergence as we observed in our study. If mate selection does not occur at the time of reproduction, which is not uncommon in long‐lived organisms, then mechanisms influencing the true availability of potential mates may be obscured, and the degree of genetic and phenotypic diversity may appear incongruous with presumed patterns of gene flow. Previous investigations revealed population‐specific behavioral, temporal, and spatial mechanisms that likely influence the amount of gene flow measured among greater white‐fronted goose populations. The degree of observed genetic structuring aligns well with our current understanding of population differences pertaining to seasonal movements, social structure, pairing behavior, and resource partitioning.

Mate choice for major histocompatibility complex complementarity in a strictly monogamous bird, the grey partridge (Perdix perdix)

BackgroundSexual selection has been hypothesised as favouring mate choice resulting in production of viable offspring with genotypes providing high pathogen resistance. Specific pathogen recognition is mediated by genes of the major histocompatibility complex (MHC) encoding proteins fundamental for adaptive immune response in jawed vertebrates. MHC genes may also play a role in odour-based individual recognition and mate choice, aimed at avoiding inbreeding. MHC genes are known to be involved in mate choice in a number of species, with ‘good genes’ (absolute criteria) and ‘complementary genes’ (self-referential criteria) being used to explain MHC-based mating. Here, we focus on the effect of morphological traits and variation and genetic similarity between individuals in MHC class IIB (MHCIIB) exon 2 on mating in a free-living population of a monogamous bird, the grey partridge.ResultsWe found no evidence for absolute mate choice criteria as regards grey partridge MHCIIB genotypes, i.e., number and occurrence of amino acid variants, though red chroma of the spot behind eyes was positively associated with male pairing success. On the other hand, mate choice at MHCIIB was based on relative criteria as females preferentially paired with more dissimilar males having a lower number of shared amino acid variants. This observation supports the ‘inbreeding avoidance’ and ‘complementary genes’ hypotheses.ConclusionsOur study provides one of the first pieces of evidence for MHC-based mate choice for genetic complementarity in a strictly monogamous bird. The statistical approach employed can be recommended for testing mating preferences in cases where availability of potential mates (recorded with an appropriate method such as radio-tracking) shows considerable temporal variation. Additional genetic analyses using neutral markers may detect whether MHC-based mate choice for complementarity emerges as a by-product of general inbreeding avoidance in grey partridges.

Mate availability affects the trade-off between producing one or multiple annual clutches

Females of many iteroparous species face trade-offs between producing one or multiple broods per reproductive season, and over fertilizing broods with sperm from the same or different mates. Both trade-offs might be affected by the availability of males (i.e. absence/presence of males) and the timing and duration of male encounters. Here, we experimentally manipulated the duration of mate availability at the first brood and mate availability per se (i.e. absence/presence of mates) at the second brood, and tested their effects on female and male reproductive success, using the common lizard, Zootoca vivipara, as a model species. Females were either exposed to males for a long period before their first annual reproduction and they could remate before their second annual reproduction (unrestricted treatment), or they were exposed to males for a short period before their first annual reproduction and were not allowed to remate (restricted treatment). Reproductive success of first clutches was not directly affected by the duration of access to males. Remating positively affected the probability of producing a second clutch, and the proportion of viable offspring. Remating by females also affected the reproductive success of males: fewer second clutch eggs were fertilized with stored sperm in unrestricted than restricted females. Sperm presence in males was high until the end of the remating period. Our results suggest a close coevolution between male and female reproductive strategies and point to facultative skipping of second broods when fitness benefits are small. This shows that behavioural strategies are at least partially responsible for multiple annual broods. These behavioural strategies are likely to be widespread, given the multitude of taxa raising multiple broods in some but not all years, and given that in most taxa some but not all individuals produce multiple annual broods.

Assortative mating in the online market for sperm donation

Previous studies on assortative mating have struggled to isolate preferences from actual constraints faced throughout the matching process, including the geographic and social propinquity that limit the availability of possible mates. Because such passive factors restrict the possibility set of potential partners, they may either restrict the chance of fulfilling mating preferences or lead to a high level of positive assortative mating. The possibility set may be further reduced by competition in the mating market. It is also unclear from couple’s data how much assortative mating is driven by partner selection to reduce anticipated child rearing problem and how much by a desire for parental assistance and altruistic preferences for offspring. Adopting the online market for sperm donation as the research setting reduces such problems: the more controlled setting ensures isolation of a male’s genetic impact on his offspring from other factors. By identifying the factors that influence the symmetry of characteristics between recipients and partners and recipients and donors chosen, we provide empirical evidence that even with limited constraints on available choice, women still exhibit homogamous donor preferences. Likewise, by exploring how potential donors’ characteristics match partner characteristics, we offer insights into what drives recipients’ desires to find donors who surpass both their own and their partners’ characteristics.

Sibship effects on dispersal behaviour in a pre-industrial human population.

Understanding dispersal behaviour and its determinants is critical for studies on life-history maximizing strategies. Although many studies have investigated the causes of dispersal, few have focused on the importance of sibship, despite that sibling interactions are predicted to lead to intrafamilial differences in dispersal patterns. Using a large demographic data set from pre-industrial Finland (n=9000), we tested whether the sex-specific probability of dispersal depended on the presence of same-sex or opposite-sex elder siblings who can both compete and cooperate in the family. Overall, following our predictions, the presence of same-sex elder siblings increased the probability of dispersal from natal population for both sexes, whereas the number of opposite-sex siblings had less influence. Among males, dispersal was strongly linked to access to land resources. Female dispersal was mainly associated with competition over availability of mates but likely mediated by competition over access to wealthy mates rather mate availability per se. Besides ecological constraints, sibling interactions are strongly linked with dispersal decisions and need to be better considered in the studies on the evolution of family dynamics and fitness maximizing strategies in humans and other species.

COHORT MARRIAGE KINETICS IN THE CONTEXT OF MIGRATION, WITH A CASE STUDY OF JAPAN, 1920-1940.

The concept of marriage squeeze expects a positive association between marriage formation and the availability of preferred mates. Previous research to test the hypothesis has had mixed results owing to inconsistent marriage measures, inconsistent age focuses and insufficient attention to migration. This study derives kinetics equations of marriage formation to link cohort age-specific mate availability to migration-adjusted marriage rate/incidence, a measure in contrast to nominal marriage rate. On testing the equations with Japanese census data for 1920-1940, it is found that, in female cohorts, mate availability impacts first marriage rate at the life-course stage from 15-19 to 20-24 years, but not at later stages. Among young females, the decline in mate availability accounted for about 21% of the decline in first marriage rate over the period 1920-1940, when there was a trend towards later but not less marriage in Japan. The study suggests that the flexibility of mate/spouse choice in females varies along the marriageable life course and is more manifest at older ages. At young ages, however, the marriage squeeze hypothesis could hold, presumably because young women are evolutionarily shaped to be choosier, perhaps postponing their marriages when preferred mates are in short supply.

Females sample more males at high nesting densities, but ultimately obtain less attractive mates.

BackgroundSexual selection is largely driven by the availability of mates. Theory predicts that male competition and female choice should be density-dependent, with males competing more intensely at relatively high density, and females becoming increasingly discriminating when there are more males from whom to choose. Evidence for flexible mating decisions is growing, but we do not understand how environmental variation is incorporated into mate sampling strategies. We mimicked threespine stickleback (Gasterosteus aculeatus) breeding conditions in pools with high and low densities of nesting males and allowed females to search for mates to determine whether 1) mate search strategies change with the density of breeding males and 2) pre-copulatory components of mate choice (signalling, competition, search patterns, and mating decisions) are modified in parallel.ResultsWhile females sampled more males at high male density, suggesting greater opportunity for sexual selection, the expanded search did not result in females choosing males with more attractive sexual signals. This is likely because red throat colouration was twice as great when half as many males competed. Instead, females chose similarly at high and low male density, using a relative strategy to compare male traits amongst potential suitors. Reduced throat colour could reflect a trade-off with costly male competition. However, we did not observe more intense competition at higher relative density. Density-dependent signalling appears largely responsible for females associating with males who have more attractive signals at low density. If we lacked knowledge of plasticity in signalling, we might have concluded that females are more discriminating at low male density.ConclusionsTo understand interactions between mate choice and population dynamics, we should consider how components of mate choice that precede the mating decision interact.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0481-3) contains supplementary material, which is available to authorized users.

Investment choice and perceived mating intentions regulated by external resource cues and internal fluctuation in blood glucose levels.

We examined resource allocation priorities in the framework of an updated Maslow hierarchy of fundamental human needs. In Experiment 1, the participants in the food abundance priming condition viewing photos of high-calorie food allocated more money to savings than to spending. However, the participants preferred spending to savings under the condition of mating availability priming with romantic photographs. In Experiment 2, before and after drinking either water or a sugary beverage, fasting participants rated photos of a conversation between a man and a woman. Water drinking lowered the rating scores of mating intentions as well as blood glucose (BG) levels. The sugary drink buffered this decline in sexual perceptivity. Overall, the change in BG levels was positively associated with changes in the ratings of mating intentions but was not associated with other likelihood ratings. These results suggest that both external cues of food and mating resources and internal BG fluctuation regulate the cognitive priority of physiological needs vs. mate acquisition and retention.

Multiple paternity and mating group size in the European earwig, Forficula auricularia

1. The patterns of multiple paternity among the progeny of females are key properties of genetic mating systems. Female multiple mating should evolve due to direct or indirect benefits, but it may also partly be driven by the encounter rate with different potential mates.2. In this study this hypothesis was experimentally tested in the European earwig (Forficula auricularia L.) by establishing experimental mating groups that differed in the number of males and females (i.e. density). The number of sires and mean sibling relatedness in each clutch were estimated using microsatellite‐based paternity analysis.3. As predicted, the mean number of sires per clutch was significantly increased, and sibling relatedness decreased, in the higher density treatment where more potential male mates were available. This change was less than proportional to the number of males in the mating groups, indicating that mechanisms limiting multiple paternity in large mating groups were involved. There were no significant relationships between female reproductive success or male siring success with morphology (body size, weight, and forceps size).4. The present results show that multiple paternity in F. auricularia clutches is partly determined by the availability of male mates and suggest that this effect is modulated by mechanisms in males and/or females that limit multiple paternity.

Frequency-dependent population dynamics: Effect of sex ratio and mating system on the elasticity of population growth rate

When vital rates depend on population structure (e.g., relative frequencies of males or females), an important question is how the long-term population growth rate λ responds to changes in rates. For instance, availability of mates may depend on the sex ratio of the population and hence reproductive rates could be frequency-dependent. In such cases change in any vital rate alters the structure, which in turn, affect frequency-dependent rates. We show that the elasticity of λ to a rate is the sum of (i) the effect of the linear change in the rate and (ii) the effect of nonlinear changes in frequency-dependent rates. The first component is always positive and is the classical elasticity in density-independent models obtained directly from the population projection matrix. The second component can be positive or negative and is absent in density-independent models. We explicitly express each component of the elasticity as a function of vital rates, eigenvalues and eigenvectors of the population projection matrix. We apply this result to a two-sex model, where male and female fertilities depend on adult sex ratio α (ratio of females to males) and the mating system (e.g., polygyny) through a harmonic mating function. We show that the nonlinear component of elasticity to a survival rate is negligible only when the average number of mates (per male) is close to α. In a strictly monogamous species, elasticity to female survival is larger than elasticity to male survival when α<1 (less females). In a polygynous species, elasticity to female survival can be larger than that of male survival even when sex ratio is female biased. Our results show how demography and mating system together determine the response to selection on sex-specific vital rates.