Size-biased Mating in Both Sexes of the Black-horned Tree Cricket, <Emphasis Type="Italic">Oecanthus nigricornis</Emphasis> Walker (Orthoptera: Gryllidae: Oecanthinae)

Previous studies on tree crickets have demonstrated female choice of males based on size and courtship feeding but less is known about sexual selection under conditions of direct mating competition. I studied courtship, aggression and mating of the black-horned tree cricket Oecanthus nigricornis (Walker) to test size-related sexual selection under conditions of direct sexual competition. Results show that larger individuals of both sexes mated more frequently than their smaller counterparts, and this was due to the ability of large individuals to out compete rivals. Large males achieved the advantage by aggressively reducing courtship by small males, whereas large females responded to male courtship more quickly but with little aggression. Although there was no evidence here for mate choice, there were advantages for having larger mates; fecundity increased with female size and spermatophores (which females consume after mating) increased with male size. Size of the specialized metanotal courtship gift, however, was not related to male size.     

Pairing and insemination patterns in a giant weta (Deinacrida rugosa: Orthoptera; Anostostomatidae)

Positive size assortative mating can arise if either one or both sexes prefer bigger mates or if the success of larger males in contests for larger females leaves smaller males to mate with smaller females. Moreover, body size could not only influence pairing patterns before copulation but also the covariance between female size and size of ejaculate (number of spermatophores) transferred to a mate. In this field study, we examine the pre-copulatory mate choice, as well as insemination, patterns in the Cook Strait giant weta (Deinacrida rugosa). D. rugosa is a large orthopteran insect that exhibits strong female-biased sexual dimorphism, with females being nearly twice as heavy as males. Contrary to the general expectation of male preference for large females in insects with female-biased size dimorphism, we found only weak support for positive size assortative mating based on size (tibia length). Interestingly, although there was no correlation between male body size and the number of spermatophores transferred, we did find that males pass more spermatophores to lighter females. This pattern of sperm transfer does not appear to be a consequence of those males that mate heavier females being sperm depleted. Instead, males may provide lighter females with more spermatophores perhaps because these females pose less of a sperm competition risk to mates.     

Effects of female and male size on female mating and remating decisions in a bean beetle

The body sizes of individuals of the choosing and chosen sexes in a mate choice may affect sequential mating of females. We examined the effects of the body sizes of females and their mates on attributes of female first mating, and the effects of body sizes of females and their previous and potential future mates on female remating in the adzuki bean beetle, Callosobruchus chinensis. Large- and small-sized adults were derived from larvae reared under conditions of low and high density in a bean, respectively. The speed of first mating of large females was not affected by the size of courting males, whereas small females initiated mating more rapidly when courted by small males. The remating probability of large females was not affected by first male size, whereas small females that mated first with smaller males were more likely to remate. These data suggest that pre- and post-copulatory female choices for male size depend on the female’s size, and the small females might be more willing to copulate with smaller males but prefer larger males to sire their offspring after copulation. A possible explanation for this preference is that small females may suffer greater harm from copulating with larger males.     

SEXUAL SELECTION IN AMERICAN TOADS: A TEST OF A GOOD-GENES HYPOTHESIS

Adaptive mate choice in species lacking male resource control and/or paternal care might be maintained by selection because preferred males sire genetically superior offspring. For such a process to occur, some male phenotypic trait(s) must both reliably indicate male genetic quality and influence the pattern of mate choice by females. In American toads, Bufo americanus, male body length has been documented to influence female mating patterns: females usually mate with males that are larger than average. However, the relationship between male size and male genetic quality is unknown. We conducted a controlled breeding experiment using 48 sires and 19 dams to determine if larger males sire offspring with superior larval performance characteristics (greater survival to metamorphosis, larger mass at metamorphosis, and earlier metamorphosis). We also aged each sire to test the hypothesis that older males are, on average, genetically superior to younger males. We crossed each female with three sires representing three body size categories (mean and 1 SD ± mean snout-ischium length). Hatchlings (500 from each cross) were reared to metamorphosis in seminatural ponds in the field. Metamorph weight (log transformed) and age at metamorphosis showed significant heritability and were genetically correlated with each other. Hence, sires differed in genetic quality. However, none of the three measures of offspring performance was correlated with sire body size or age. Thus, we obtained no support for the prediction that sire body size or age is related to genetic quality.     

Mate sampling and the sexual conflict over mating in seaweed flies

The order in which females encounter, or sample, males in apopulation may have important consequences for mate choice,with the information gathered about males influencing boththe preference function and degree of choosiness of females.Sexual selection may be affected as a result. Sampling of particularsubsets of males may be a crucial component of individual variation in mate preferences within populations. However, the sequencein which males are sampled may also be important in specieswithout traditional, active mate choice, such as when sexualselection involves sexual conflict over mating. This wouldoccur if the likelihood of a female mating with a male of acertain phenotype changes as a result of previous encounters.We examined the effects of encountering males differing inbody size, a sexually selected phenotype, in the seaweed flyCoelopa frigida. Sexual selection occurs in this species asa result of a sexual conflict over mating. We show that theoutcome of the sexual conflict is independent of the orderin which males are encountered by female seaweed flies, withthe overall mating advantage to large males being unaffected.In addition, we explored female preference functions and evaluatethe heterogeneity in female willingness to mate. We suggestthat consideration of mate sampling theory is valuable whenexamining mate choice in species in which sexual selectionis driven by sexual conflict.     

Secondary sexual ornamentation and non-additive genetic benefits of female mate choice

Ornamental secondary sexual traits are hypothesized to evolve in response to directional mating preferences for more ornamented mates. Such mating preferences may themselves evolve partly because ornamentation indicates an individual's additive genetic quality (good genes). While mate choice can also confer non-additive genetic benefits (compatible genes), the identity of the most 'compatible' mate is assumed to depend on the choosy individual's own genotype. It is therefore unclear how choice for non-additive genetic benefits could contribute to directional mating preferences and consequently the evolution of ornamentation. In free-living song sparrows (Melospiza melodia), individual males varied in their kinship with the female population. Furthermore, a male's song repertoire size, a secondary sexual trait, was negatively correlated with kinship such that males with larger repertoires were less closely related to the female population. After excluding close relatives as potential mates, individual females were on average less closely related to males with larger repertoires. Therefore, female song sparrows expressing directional preferences for males with larger repertoires would on average acquire relatively unrelated mates and produce relatively outbred offspring. Such non-additive genetic fitness benefits of directional mating preferences, which may reflect genetic dominance variance expressed in structured populations, should be incorporated into genetic models of sexual selection.     

Female Choice or Male Sex Drive? The Advantages of Male Body Size during Mating in Drosophila Melanogaster

The mating success of larger male Drosophila melanogaster in the laboratory and the wild has been traditionally been explained by female choice, even though the reasons are generally hard to reconcile. Female choice can explain this success by virtue of females taking less time to mate with preferred males, but so can the more aggressive or persistent courtships efforts of large males. Since mating is a negotiation between the two sexes, the behaviors of both are likely to interact and influence mating outcomes. Using a series of assays, we explored these negotiations by testing for the relative influence of male behaviors and its effect on influencing female courtship arousal threshold, which is the time taken for females to accept copulation. Our results show that large males indeed have higher copulation success compared to smaller males. Competition between two males or an increasing number of males had no influence on female sexual arousal threshold;—females therefore may have a relatively fixed ‘arousal threshold’ that must be reached before they are ready to mate, and larger males appear to be able to manipulate this threshold sooner. On the other hand, the females’ physiological and behavioral state drastically influences mating; once females have crossed the courtship arousal threshold they take less time to mate and mate indiscriminately with large and small males. Mating quicker with larger males may be misconstrued to be due to female choice; our results suggest that the mating advantage of larger males may be more a result of heightened male activity and relatively less of female choice. Body size per se may not be a trait under selection by female choice, but size likely amplifies male activity and signal outputs in courtship, allowing them to influence female arousal threshold faster.     

Indirect mate choice, direct mate choice and species recognition in a bower-building cichlid fish lek

Sexual selection arising through female mate choice typically favours males with larger, brighter and louder signals. A critical challenge in sexual selection research is to determine the degree to which this pattern results from direct mate choice, where females select individual males based on variation in signalling traits, or indirect mate choice, where male competition governs access to reproductively active females. We investigated female mate choice in a lekking Lake Malawi cichlid fish, Hemitilapia oxyrhynchus, in which males build and aggressively defend sand 'bowers'. Similar to previous studies, we found that male reproductive success was positively associated with bower height and centrality on the lek. However, this pattern resulted from males holding these territories encountering more females, and thus their greater success was due to indirect mate choice. Following initial male courtship, an increase in the relative mating success of some males was observed, but this relative increase was unrelated to bower size or position. Crucially, experimentally manipulating bowers to resemble those of a co-occurring species had no appreciable effect on direct choice by females or male spawning success. Together, these results suggest indirect mate choice is the dominant force determining male-mating success in this species, and that bowers are not signals used in direct mate choice by females. We propose that, in this species, bowers have a primary function in intraspecific male competition, with the most competitive males maintaining larger and more central bowers that are favoured by sexual selection due to higher female encounter rates.     

Cryptic female choice: frogs reduce clutch size when amplexed by undesired males

In species with internal fertilization, females can 'cryptically' choose (e.g. through sperm selection) which individuals sire their offspring, even when their overt preferences for copulatory partners are overrun by male-male competition and sexual coercion. The experiment presented here reveals that control of paternity after copulation has begun is also possible in species with external fertilization. Females of the hybridogenetic Rana essonae-Rana esculenta (LL-LR) waterfrog complex adjust their clutch size in response to mate type: they release fewer eggs when amplexed by hybrid LR males who--jeopardize successful reproduction--than when amplexed by parental LL males. This reduction in the number of eggs laid can increase a female's residual reproductive value through a second mating in the same breeding season or a larger clutch size in the next year. We argue that cryptic female choice through clutch size adjustment (i) may have evolved more often than previously assumed, and (ii) can arise even where females mate only once during a reproductive period.     

Polyandry and postcopulatory sexual selection in a wild population

When females mate multiply, postcopulatory sexual selection can occur via sperm competition and cryptic female choice. Although postcopulatory selection has the potential to be a major force in driving evolution, few studies have estimated its strength in natural populations. Likewise, although polyandry is widespread across taxa and is the focus of a growing body of research, estimates of natural female mating rates are still limited in number. Microsatellites can be used to estimate the number of mates represented in females' sperm stores and the number of sires contributing to their offspring, enabling comparisons both of polyandry and of two components of postcopulatory selection: the proportion of males that mate but fail to sire offspring, and the degree of paternity skew among the males that do sire offspring. Here, we estimate the number of mates and sires among wild females in the Hawaiian swordtail cricket Laupala cerasina. We compare these estimates to the actual mating rates and paternity shares we observed in a semi‐natural population. Our results show that postcopulatory sexual selection operates strongly in this species: wild females mated with an average minimum of 3.6 males but used the sperm from only 58% of them. Furthermore, among the males that did sire offspring, paternity was significantly skewed. These patterns were similar to those observed in the field enclosure, where females mated with an average of 5.7 males and used the sperm from 62% of their mates, with paternity significantly skewed among the sires.     

Male age, mating status and nuptial gift quality in a bushcricket

Female mate choice occurs in many animals, and in some species females prefer older males. Because older males have demonstrated their survival ability, they may be of higher genetic quality, providing genetic benefits to the offspring of their mates. However, in species where females receive direct benefits of matings, younger males may be more likely to provide more fertile or more nutritious ejaculates, so females may discriminate against older males. Males of the bushcricket Ephippiger ephippiger (Orthoptera: Tettigoniidae) produce large spermatophores at mating (>30% of body weight, circa 10% protein content). Female E. ephippiger discriminate against the song of older males. We examined the effects of male age and mating history on male reproductive investment (spermatophore size, sperm number, nitrogen content). Males produced spermatophores with significantly fewer sperm and of lower nitrogen content on their fourth mating, despite free access to food and a 1-week interval between matings, indicating that there is a cost of mating to males. There was no indication that older virgin males produced lower-quality spermatophores. Rather, older males produced bigger spermatophores of higher nutritional value and containing more sperm. Male age and mating history seem likely to be strongly correlated in the field. We conclude that female E. ephippiger probably prefer the songs of younger males, because in the field, this preference correlates with male mating history and therefore resources provided at mating. Thus, female preference for younger males could reflect discrimination against low-quality nuptial gifts.     

When are good genes good? Variable outcomes of female choice in wax moths

Female lesser wax moths (Achroia grisella) choose males based on characters of their ultrasonic advertisement signals. Because a female''s opportunity to obtain increased somatic benefits by mating with a particular male is limited, we investigated whether females obtain genetic benefits for their offspring via mate choice. Controlled breeding experiments conducted under favourable food and temperature conditions showed that developmental characters are heritable, that sire attractiveness and offspring survivorship are unrelated, but that females mating with attractive signallers produce offspring who mature faster than the offspring of females mating with non-attractive signallers. However, under some unfavourable food or temperature conditions, it is the offspring of females mating with non-attractive males who mature faster; these offspring are heavier as well. Thus, the relationship between male attractiveness and offspring development is not environmentally robust, and support for a good genes model of mate choice in A. grisella is dependent on conditions. These findings suggest genotype–environment interactions and emphasize the necessity of testing sexual selection models under a range of natural environments.     

Female mate choice across spatial scales: influence of lek and male attributes on mating success of blue-crowned manakins

Lekking males compete for females within and among leks, yet female choice is expected to work differently at each of these spatial scales. We used paternity analyses to examine how lek versus male attributes influence mate choice in the blue-crowned manakin Lepidothrix coronata. We tested the hypotheses that females prefer (i) to mate at larger leks where a larger number of potential mates can be assessed, (ii) to mate with unrelated or highly heterozygous males expected to produce high-quality offspring, (iii) to mate with males that display at higher rates, and that (iv) display honestly reflects male genetic quality. Our results show that (i) males at larger leks are not more likely to sire young, although females nesting close to small leks travel further to reach larger leks, (ii) siring males are not less related to females or more heterozygous than expected, (iii) within a lek, high-display males are more likely to sire young, and (iv) both male heterozygosity and display rate increased with lek size, and as a result display does not reliably reflect male genetic quality across leks. We suggest that female mate choice in this species is probably driven by a Fisherian process rather than adaptive genetic benefits.     

Sexual selection is influenced by both developmental and adult environments

Sexual selection is often assumed to be strong and consistent, yet increasing research shows it can fluctuate over space and time. Few experimental studies have examined changes in sexual selection in response to natural environmental variation. Here, we use a difference in resource quality to test for the influence of past environmental conditions and current environmental conditions on male and female mate choice and resulting selection gradients for leaf‐footed cactus bugs, Narnia femorata. We raised juveniles on natural high‐ and low‐quality diets, cactus pads with and without ripe cactus fruits. New adults were again assigned a cactus pad with or without fruit, paired with a potential mate, and observed for mating behaviors. We found developmental and adult encounter environments affected mating decisions and the resulting patterns of sexual selection for both males and females. Males were not choosy in the low‐quality encounter environment, cactus without fruit, but they avoided mating with small females in the high‐quality encounter environment. Females were choosy in both encounter environments, avoiding mating with small males. However, they were the choosiest when they were in the low‐quality encounter environment. Female mate choice was also context dependent by male developmental environment. Females were more likely to mate with males that had developed on cactus with fruit when they were currently in the cactus with fruit environment. This pattern disappeared when females were in the cactus without fruit environment. Altogether, these results experimentally demonstrate context‐dependent mate choice by both males and females. Furthermore, we demonstrate that simple, seasonal changes in resources can lead to fluctuations in sexual selection.     

Benefits of polyandry: Molecular evidence from field‐caught dung beetles

When females mate with multiple males, they set the stage for postcopulatory sexual selection via sperm competition and/or cryptic female choice. Surprisingly little is known about the rates of multiple mating by females in the wild, despite the importance of this information in understanding the potential for postcopulatory sexual selection to drive the evolution of reproductive behaviour, morphology and physiology. Dung beetles in the genus Onthophagus have become a laboratory model for studying pre‐ and postcopulatory sexual selection, yet we still lack information about the reproductive behaviour of female dung beetles in natural populations. Here, we develop microsatellite markers for Onthophagus taurus and use them to genotype the offspring of wild‐caught females and to estimate natural rates of multiple mating and patterns of sperm utilization. We found that O. taurus females are highly polyandrous: 88% of females produced clutches sired by at least two males, and 5% produced clutches with as many as five sires. Several females (23%) produced clutches with significant paternity skew, indicating the potential for strong postcopulatory sexual selection in natural populations. There were also strong positive correlations between the number of offspring produced and both number of fathers and paternity skew, which suggests that females benefit from mating polyandrously by inciting postcopulatory mechanisms that bias paternity towards males that can sire more viable offspring. This study evaluates the fitness consequences of polyandry for an insect in the wild and provides strong evidence that female dung beetles benefit from multiple mating under natural conditions.     

Is egg carrying attractive? Mate choice in the golden egg bug (Coreidae,Heteroptera)

In several species of fish, females select males that are already guarding eggs in their nests. It is a matter of debate as to whether a female selects a good nest site for her offspring (natural selection) or a male for his attractiveness (sexual selection). The golden egg bug, Phyllomorpha laciniata Vill, resembles fish in the sense that mating males carry more eggs than single males, but in the bugs, female mate choice is decoupled from egg site choice. The sexual selection hypothesis predicts that if females select males using male egg load as a cue for male quality, they should not mate with a male when eggs are removed, regardless of his mating attempts. When individual females were enclosed with an egg-loaded male and an unloaded male, they mated equally often with both males, although the loaded males courted more. In addition, when only successful males were used, females mated equally often with the loaded male and the unloaded male irrespective of sex ratio. Male choice rather than female choice affected mating frequency when sex ratio was equal. Therefore, females do not select the male by the eggs he carries, but successful males may receive many eggs due to egg dumping by alien females while they mate or as a consequence of mate guarding.     

Do female leaf beetles Galerucella nymphaeae choose their mates and does it matter?

The role of active female choice in sexual selection is frequently difficult to ascertain, and this is particularly the case for many insect species. Also, it is uncertain whether choosing between males would affect offspring viability. We designed an experiment to investigate the presence of female choice in a Coleoptera species (Galerucella nymphaeae). We also estimated whether mate choice would have any effect on offspring performance. Females were first placed with two males in a test arena to see which of the males copulated with the virgin female, and how quickly. Subsequently the loser male was offered a new virgin female to test for any change in latency time until mating. The two-male tests indicated that males with wider upper prothoraxes were more likely to mate with the female, and the latency time until mating was shorter when the winner male had relatively long wings. When the loser males were placed singly with females the latency time was not correlated with male size, and was the same as when two males were used. These results suggest that male-male competition is the most likely cause of sexual selection on size, and if females have any preferences they are not very strong. The seemingly passive female strategy may be sufficient to ensure that females mate with the most vigorous males, since in the field several males usually compete for access to each female. Finally, the benefits of female choosiness were estimated to be low and non-significant. The eggs of the winner males were no more likely to hatch, offspring survival into adulthood was no greater among descendants of winner males, and the offspring did not differ in adult size. Received: 3 March 1997 / Accepted: 13 October 1997     

Genetic quality and sexual selection: an integrated framework for good genes and compatible genes

Why are females so choosy when it comes to mating? This question has puzzled and marveled evolutionary and behavioral ecologists for decades. In mating systems in which males provide direct benefits to the female or her offspring, such as food or shelter, the answer seems straightforward — females should prefer to mate with males that are able to provide more resources. The answer is less clear in other mating systems in which males provide no resources (other than sperm) to females. Theoretical models that account for the evolution of mate choice in such nonresource-based mating systems require that females obtain a genetic benefit through increased offspring fitness from their choice. Empirical studies of nonresource-based mating systems that are characterized by strong female choice for males with elaborate sexual traits (like the large tail of peacocks) suggest that additive genetic benefits can explain only a small percentage of the variation in fitness. Other research on genetic benefits has examined nonadditive effects as another source of genetic variation in fitness and a potential benefit to female mate choice. In this paper, we review the sexual selection literature on genetic quality to address five objectives. First, we attempt to provide an integrated framework for discussing genetic quality. We propose that the term ‘good gene’ be used exclusively to refer to additive genetic variation in fitness, ‘compatible gene’ be used to refer to nonadditive genetic variation in fitness, and ‘genetic quality’ be defined as the sum of the two effects. Second, we review empirical approaches used to calculate the effect size of genetic quality and discuss these approaches in the context of measuring benefits from good genes, compatible genes and both types of genes. Third, we discuss biological mechanisms for acquiring and promoting offspring genetic quality and categorize these into three stages during breeding: (i) precopulatory (mate choice); (ii) postcopulatory, prefertilization (sperm utilization); and (iii) postcopulatory, postfertilization (differential investment). Fourth, we present a verbal model of the effect of good genes sexual selection and compatible genes sexual selection on population genetic variation in fitness, and discuss the potential trade-offs that might exist between mate choice for good genes and mate choice for compatible genes. Fifth, we discuss some future directions for research on genetic quality and sexual selection.     

Mating order-dependent female mate choice in the polygynandrous common lizard Lacerta vivipara

Recent studies indicate that directional female mate choice and order-dependent female mate choice importantly contribute to non-random mating patterns. In species where females prefer larger sized males, disentangling different hypotheses leading to non-random mating patterns is especially difficult, given that male size usually correlates with behaviours that may lead to non-random mating (e.g. size-dependent emergence from hibernation, male fighting ability). Here we investigate female mate choice and order-dependent female mate choice in the polygynandrous common lizard (Lacerta vivipara). By sequentially presenting males in random order to females, we exclude non-random mating patterns potentially arising due to intra-sexual selection (e.g. male–male competition), trait-dependent encounter probabilities, trait-dependent conspicuousness, or trait-dependent emergence from hibernation. To test for order-dependent female mate choice we investigate whether the previous mating history affects female choice. We show that body size and body condition of the male with which a female mated for the first time were bigger and better, respectively, than the average body size and body condition of the rejected males. There was a negative correlation between body sizes of first and second copulating males. This indicates that female mate choice is dependent on the previous mating history and it shows that the female’s choice criteria are non-static, i.e. non-directional. Our study therefore suggests that context-dependent female mate choice may not only arise due to genotype-environment interactions, but also due to other female mating strategies, i.e. order-dependent mate choice. Thus context-dependent female mate choice might be more frequent than previously thought.     

Reproductive success by large, closely related males facilitated by sperm storage in an aggregate breeding amphibian

The outcome of sexual selection on males may depend on female mate choice and male–male competition as well as postcopulatory processes such as cryptic female choice and sperm competition. We studied the outcome of sexual selection in the spotted salamander ( Ambystoma maculatum ), specifically examining the role of body size and relatedness on male reproductive success. Using controlled mating experiments in the field, we gave females access to three males of different sizes. We used seven microsatellite loci to determine paternity in the resulting larvae, estimate relatedness ( r ) between females and their mates, and calculate md ² (a measure of within-individual genomic divergence), heterozygosity, and standardized heterozygosity in the larvae. Both body size and relatedness to the female were significant predictors of male reproductive success. The relatedness of the males available to a female did not influence the amount of stored sperm she used to sire her larvae. Nonetheless, computer simulations showed that the average md ², heterozygosity, and standardized heterozygosity of the offspring were lower than expected by random mating. These differences are due to the use of stored sperm to fertilize some eggs; md ², heterozygosity, and standardized heterozygosity of larvae sired by stored sperm were significantly lower than those of larvae sired by the experimental males. These results suggest that relatedness may further influence a male's long-term reproductive success by determining whether his sperm is stored for later breeding seasons. Sexual selection in this salamander likely involves a complex interaction among many factors and may act over many seasons.