Male-male competition and mating success in the orb-web spider,Nephila clavata,with reference to temporal factors

Seasonal occurrence patterns of adults of both sexes, intensity of male-male interactions, and mating success in the spider,Nephila clavata, were examined in the field. Adult males began to attend female webs about 2 weeks before female maturation. Large adult males were abundant in the early breeding season, but small males increased later in the season. From the distribution of males among female webs and size relationship of males within a web, male-male interactions seemed to be more intense when most females were still subadult. This was verified by a field experiment in which males were artificially introduced to female webs that were attended by other males. It was found that the probability of introduced males remaining on subadult female webs was lower than that on adult webs. As mating occurred mostly in the period shortly after the female final molt and first male sperm precedence was known in all spiders reported so far, intense male-male competition on subadult female webs seemed to be reasonable. Male longevity had an important influence on the mating success of males with just-molted females. Mating success was also affected by the relative body size of males present in a given period. Since larger males occupied the position closest to females within a web and stayed there longer, relative body size appeared to influence mating success through male-male competition. Female body size at maturation declined with time; hence, males that attained sexual maturity earlier had the advantage of mating with larger and more fecund females. Therefore, early maturation as well as larger size seem to be two important trairs influencing the reproductive success of males.     

Male choice generates stabilizing sexual selection on a female fecundity correlate

We know very little about male mating preferences and how they influence the evolution of female traits. Theory predicts that males may benefit from choosing females on the basis of traits that indicate their fecundity. Here, we explore sexual selection generated by male choice on two components of female body size (wing length and body mass) in Drosophila serrata. Using a dietary manipulation to alter female size and 828 male mate choice trials, we analysed linear and nonlinear sexual selection gradients on female mass and wing length. In contrast to theoretical expectations and prevailing empirical data, males exerted stabilizing rather than directional sexual selection on female body mass, a correlate of fecundity. Sexual selection was detected only among females with access to standard resource levels as an adult, with no evidence for sexual selection among resource-depleted females. Thus the mating success of females with the same body mass differed depending upon their access to resources as an adult. This suggests that males in this species may rely on signal traits to assess body mass rather than assessing it directly. Stabilizing rather than directional sexual selection on body mass together with recent evidence for stabilizing sexual selection on candidate signal traits in this species suggests that females may trade-off resources allocated to reproduction and sexual signalling.     

A Life History Trade-Off Between Flight Ability and Reproductive Behavior in Male Field Crickets (<Emphasis Type="Italic">Gryllus texensis</Emphasis>)

Male field crickets (Gryllus texensis) that differ in flight ability incur a life history trade-off between flight ability and reproduction, where flight ability comes with a male fitness cost. In courtship trials, flight-capable males produced courtship song, a necessary signal for mating success, with a significantly lower probability than flight-incapable males. The trade-off was evident in young males, and a similar trend occurred in older males. Males that lost the ability to fly through histolysis of flight muscles produced courtship song with a similar probability as males incapable of flight for their entire lives. Time of day did not affect the expression of the trade-off. Neither male morph nor time of day influenced female mating behavior.     

Larval food limitation in butterflies: effects on adult resource allocation and fitness

Allocation of larval food resources affects adult morphology and fitness in holometabolous insects. Here we explore the effects on adult morphology and female fitness of larval semi-starvation in the butterfly Speyeria mormonia. Using a split-brood design, food intake was reduced by approximately half during the last half of the last larval instar. Body mass and forewing length of resulting adults were smaller than those of control animals. Feeding treatment significantly altered the allometric relationship between mass and wing length for females but not males, such that body mass increased more steeply with wing length in stressed insects as compared to control insects. This may result in changes in female flight performance and cost. With regard to adult life history traits, male feeding treatment or mating number had no effect on female fecundity or survival, in agreement with expectations for this species. Potential fecundity decreased with decreasing body mass and relative fat content, but there was no independent effect of larval feeding treatment. Realized fecundity decreased with decreasing adult survival, and was not affected by body mass or larval feeding treatment. Adult survival was lower in insects subjected to larval semi-starvation, with no effect of body mass. In contrast, previous laboratory studies on adult nectar restriction showed that adult survival was not affected by such stress, whereas fecundity was reduced in direct 11 proportion to the reduction of adult food. We thus see a direct impact of larval dietary restriction on survival, whereas fecundity is affected by adult dietary restriction, a pattern reminiscent of a survival/reproduction trade-off, but across a developmental boundary. The data, in combination with previous work, thus provide a picture of the intra-specific response of a suite of traits to ecological stress.     

Effect of Drosophila melanogaster Female Size on Male Mating Success

In this study, we examined the influence of female size on mating success in Drosophila melanogaster. The results that were obtained from experiments performed in mating chambers allowed us to confirm the results of previous studies, demonstrating higher mating success of larger D. melanogaster males, and to conclude that female size also affects mating success, either when considering a single male or two competing males. We observed that the advantage for larger males depends on their size relative to that of the female, demonstrating a previously unknown role for female size in mating behavior studies. This effect of female size on mating success depends on various factors: males take longer to initiate courtship toward larger females, large females receive more wing vibrations from males prior to mating, and large females tend to keep moving for longer periods during male courtship. The importance of this finding is discussed in the context of recent reports on sexual conflict in D. melanogaster, in which males were observed to depress fitness in females as a result of intercourse.     

Is bigger better? Male body size affects wing‐borne courtship signals and mating success in the olive fruit fly,Bactrocera oleae (Diptera: Tephritidae)

Variations in male body size are known to affect inter‐ and intrasexual selection outcomes in a wide range of animals. In mating systems involving sexual signaling before mating, body size often acts as a key factor affecting signal strength and mate choice. We evaluated the effect of male size on courtship displays and mating success of the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae). Wing vibrations performed during successful and unsuccessful courtships by large and small males were recorded by high‐speed videos and analyzed through frame‐by‐frame analysis. Mating success of large and small males was investigated. The effect of male–male competition on mating success was evaluated. Male body size affected both male courtship signals and mating outcomes. Successful males showed wing‐borne signals with high frequencies and short interpulse intervals. Wing vibrations displayed by successful large males during copulation attempt had higher frequencies over smaller males and unsuccessful large males. In no‐competition conditions, large males achieved higher mating success with respect to smaller ones. Allowing large and small males to compete for a female, large males achieve more mating success over smaller ones. Mate choice by females may be based on selection of the larger males, able to produce high‐frequency wing vibrations. Such traits may be indicative of “good genes,” which under sexual selection could means good social‐interaction genes, or a good competitive manipulator of conspecifics.     

Ecological context and the importance of body and gnathopod size for pairing success in two amphipod ecomorphs

Ecological context generates interspecific variation in mating behavior by imposing differential constraints on the action of sexual selection, but operation of these constraints in nature is not well understood. We used field and laboratory studies to examine the importance of body size and size of sexually dimorphic appendages, the gnathopods, for pairing success in two freshwater amphipod species within the Hyalella azteca species complex. We focused on a large-bodied species found in habitats where ecological factors tend to favor large body size, and a small-bodied species in habitats where small body size is favored by size-selective predation. A field study indicated that although male pairing success was greater for larger males in both species, pairing success increased throughout the range of variation in male size in the large species, whereas, in the small species, pairing success was low for smaller individuals, but roughly constant across intermediate to larger sizes. A laboratory mate choice experiment was consistent with the field study, finding that females of the large species exhibited a preference for larger males that was independent of absolute male size, but females of the small species were indifferent when choosing between males of intermediate to larger size. Species also differed in the direction of sexual size dimorphism in the field, with males being the larger sex in the large species but the smaller sex in the small species, a pattern consistent with the species differences in mate preference. Large gnathopod size relative to body size was associated with enhanced pairing success across all body sizes for the large species, but, in the small species, large gnathopod size enhanced pairing success only in smaller males. Species differences in mating behavior appear consistent with change driven by differing forms of the interaction between sexual and natural selection.     

Sexual selection on male body size and mating behaviour in American toads,Bufo americanus

Mating behaviour of B. americanus was observed from 1985 to 1987. The population contained 38–45 males and 11–26 females, depending on the year. The breeding season of this ‘explosive breeder’ usually encompassed less than 48 h. Male reproductive success varied from zero to two matings per season and zero to an estimated 15 126 zygotes per season. All females mated once per season and variation in their zygote production was estimated to be 4017–11 624 zygotes per season. Body length explained 76% of the variation in zygote production of females. However, male body length was only weakly correlated with mating success in two of three seasons, and with zygote production in one of three seasons. Male arm length was predicted to correlate with male mating success because longer arms should facilitate remaining clasped to females when challenged by rival males. However, mating males did not differ from non-mating males in arm length, and the relationship between arm length and body length was the same for the sexes. Various male behaviours were measured using focal-animal sampling but only call rate correlated with male mating success. The pattern of size dimorphism (females larger than males) is consistent with the observed sex-specific relationships between reproductive success and body size.     

Residual reproductive value and male mating success: older males do better

Evolution via sexual selection has traditionally been viewed as isolated from life-history constraints. As additionally reproductive resource allocation in males is underexplored, it is rather unclear how life-history factors have shaped lifetime investment into male sexually selected traits. Against this background, we here investigate male butterfly mating success in relation to age, nutritional status, assay condition and wing damage. As predicted, based on a low residual reproductive value, older males had a considerably higher mating success than younger males. Comparisons between virgin and once-mated males suggest that this pattern is related to age per se rather than differential ratings of the resource receptive female. We found no evidence for male body size or condition being important, supporting the notion that in weaponless animals intrinsic motivation is more important for mating success than the differences in physical properties (such as body size or condition). Flight cage experiments suggest that such differences in motivation may be masked under more natural conditions, where flight performance, having a clear impact on mating success (as evidenced by wing manipulation experiments), is likely to be crucial. We conclude that the life-history perspective is a fruitful one for gaining a better understanding of the evolution of sexually selected characters and the predictions derived from contest theory do also apply to male mating success.     

Differences in size and energy content affect the territorial status and mating success of a neotropical dragonfly

In male odonates, both size and fat content are related to territory defence and mating success. Males that are larger and have higher energy reserves win relatively more disputes for territory and attract more females. Wing colour has also been regarded as a mechanism that influences agonistic behaviour between males, as wing pigmentation might be regarded as a sign of male quality. In this study, we analysed whether a set of male physical (body size and wing colour), physiological (body fat content) and behavioural (disputes between males) characteristics were involved in the territory defence and mating behaviour of the neotropical dragonfly Zenithoptera lanei Santos, 1941 (Anisoptera: Libellulidae). Males were characterised as territorial whenever they warded‐off other males and remained in the same place within the pond for two consecutive days. In general, these territorial males were larger and had more abdominal and thoracic fat, engaged in pursuits more frequently, spent more time on sexual behaviour and female guarding, and mated more in comparison to subordinate males. By evaluating whether the percentage of wing area covered by black ink influenced male behaviour, we found that territorial males tended to act aggressively towards other males whose wings were partially painted, and sexually towards females irrespective of wing area painted. In Z. lanei, both body size and fat content play a role in defining territoriality. By subduing competitors and dominating preferred locations within high‐quality sites, these males are likely to be visited by females and engage in mating.     

DOES LARGE BODY SIZE IN MALES EVOLVE TO FACILITATE FORCIBLE INSEMINATION? A STUDY ON GARTER SNAKES

A trend for larger males to obtain a disproportionately high number of matings, as occurs in many animal populations, typically is attributed either to female choice or success in male-male rivalry; an alternative mechanism, that larger males are better able to coercively inseminate females, has received much less attention. For example, previous studies on garter snakes (Thamnophis sirtalis parietalis) at communal dens in Manitoba have shown that the mating benefit to larger body size in males is due to size-dependent advantages in male-male rivalry. However, this previous work ignored the possibility that larger males may obtain more matings because of male-female interactions. In staged trials within outdoor arenas, larger body size enhanced male mating success regardless of whether a rival male was present. The mechanism involved was coercion rather than female choice, because mating occurred most often (and soonest) in females that were least able to resist courtship-induced hypoxic stress. Males do physically displace rivals from optimal positions in the mating ball, and larger males are better able to resist such displacement. Nonetheless, larger body size enhances male mating success even in the absence of such male-male interactions. Thus, even in mating systems where males compete physically and where larger body size confers a significant advantage in male-male competition, the actual selective force for larger body size in males may relate to forcible insemination of unreceptive females. Experimental studies are needed to determine whether the same situation occurs in other organisms in which body-size advantages have been attributed to male-male rather than male-female interactions.     

Sexual differences in response to larval food stress in two nuptial feeding orthopterans – implications for sexual selection

The extent to which a trait deteriorates in response to stress can indicate its fitness importance. Food limitation is a naturally-occurring stress in the katydid Kawanaphila nartee (Orthoptera: Tettigoniidae) and should cue larvae as to the level of sexual competition expected; hunger drives adult females to compete for copulations and thus acquire spermatophore meals. This increases sexual selection on females relative to males for increased body mass. As predicted, under experimental food-stress larval female K. nartee showed little loss of mass whereas male mass decreased. In contrast, the sizes of body parts less critical to mating success showed similar decreases in males and females. For katydids such as Conocephalus nigropleurum , where reversals in mating roles do not occur, larval food stress should result in a greater preservation of male body mass, an important trait in male mating success. This prediction was supported; male mass decreased less than that of females.     

Fitness components of male and female winter moths (Operophtera brumata L.) (Lepidoptera, Geometridae) relative to measures of body size and asymmetry

In this article we present data from two experiments on theassociation between individual asymmetry and fitness in thewinter moth. We performed a mate selection experiment and comparedasymmetry and body size of mated and unmated males collectedin the field. Individual asymmetry was not associated with copulationprobability, adult life span, or body size, even though body sizeis a reliable indicator of larval and pupal survival, femalefecundity, adult life span, and thus expected fitness. Therewas only a weak positive effect of body size on mating success,contrary to the strong effect of female size on male choicefound in previous experiments. Both males and females were capableof repeated mating, and the number of matings was correlatedwith female size, but neither with male body size nor with adultasymmetry. Yet, females engaged in repeated matings more frequentlyif they were first mated to a more asymmetrical male. This mayindicate that more asymmetrical males lose paternity due tofemale remating, although direct paternity analyses need tobe carried out. In addition, repeated mating may be uncommonunder field situations. In conclusion, the relationship betweenindividual asymmetry and fitness seems to be at best weak inthe winter moth.     

Male Mating Success in the Mountain Dusky Salamander,Desmognathus ochrophaeus: Are Small,Young, Inexperienced Males at a Disadvantage?

Young, small, sexually-inexperienced males suffer lower mating success than older, larger, more experienced males in many species of animals. I compared the mating success of male Desmognathus ochrophaeus (Amphibia: Caudata: Plethodontidae) reared to maturity in the laboratory with that of males collected as adults in the field. Courtship trials were staged in the laboratory between single males and females. Laboratory-reared males were significantly smaller in body size than adult males collected in the field, were certainly inexperienced sexually and were probably younger. No difference was found between laboratory-reared males and field-collected males in any measure of mating success, although the former apparently produced smaller sperm masses. I conclude that newly-mature male D. ochrophaeus are not discriminated against by females in the context of mate choice, and do not need to “rehearse” courtship in order to obtain successful inseminations. However, newly-mature males are probably at a disadvantage in terms of aggressive and sperm competition with larger males.     

Body size, age and paternity in common brushtail possums (Trichosurus vulpecula)

Sexual selection should produce sexual size dimorphism in species where larger members of one sex obtain disproportionately more matings. Recent theory suggests that the degree of sexual size dimorphism depends on physical and temporal constraints involving the operational sex ratio, the potential reproductive rate and the trade-off between current reproductive effort and residual reproductive value. As part of a large-scale experiment on dispersal, we investigated the mating system of common brushtail possums inhabiting old-growth Eucalyptus forest in Australia. Paternity was assigned to 20 of 28 pouch-young (maternity known) genotyped at six microsatellite loci. Male mating success was strongly related to body size and age; male body weight and age being highly correlated. Despite disproportionate mating success favouring larger males, sexual size dimorphism was only apparent among older animals. Trapping and telemetry indicated that the operational sex ratio was effectively 1 : 1 and the potential reproductive rate of males was at most four times that of females. Being larger appeared to entail significant survival costs because males 'died-off' at the age at which sexual size dimorphism became apparent (8-9 years). Male and female home ranges were the same size and males appeared to be as sedentary as females. Moreover, longevity appears to be only slightly less important to male reproductive success than it is to females. It is suggested that a sedentary lifestyle and longevity are the key elements constraining selection for greater sexual size dimorphism in this 'model' medium-sized Australian marsupial herbivore.     

Size and mating success in males of the western harvester ant,Pogonomyrmex occidentalis (Hymenoptera: Formicidae)

Mating success in males of the lek mating ant species,Pogonomyrmex occidentalis, increases with increased body size. We estimated the magnitude of the selection coefficients on components of size by collecting males in copula and comparing their morphology to that of males that were collected at the lek but that were not mating. Four characters, body mass, head width, wing length, and leg length, were measured for a sample of 225 mating and 324 nonmating males and 225 females. Significant direct selection favors increased wing length and leg length. Multiple regression of transformed variables (principal components) indicated that the increased mating success of larger males is a function of all four characters. We found no evidence of positive assortative mating on the basis of any individual character or on the multivariate general size variable (the first principal component).     

Mating success in the spittlebug Cercopis sanguinolenta (Scopoli, 1763) (Homoptera, Cercopidae): the role of body size and mobility

In insects, a sexual size dimorphism commonly occurs, with larger females. However, as a deviation from this general rule, larger males are found in some species. In these species often sexual selection for large males has been presumed. The spittlebug Cercopis sanguinolenta exhibits a distinct sexual size dimorphism with larger males. Mating behaviour was studied in a field population in respect to mating success of males and females. The aim of this study was to examine the mechanisms that lead to the observed non-random mating pattern. The results showed a mating pattern without size-assortative mating. A correlation was found between mating success and body size in males. In females no such correlation was found. The mobility of males depends on their body size and mobility is high only when females are present. However, in an analysis of covariance it was found that male mating success is not correlated with mobility, when controlled for body size. The mating system of the spittlebug was classified as scramble competition polygyny. Electronic Publication     

No Evidence for Significant Effect of Body Size and Age on Male Mating Success in the Spot-legged Treefrog

In anurans, body size and age of individuals generally affect male mating success. To test whether body size and age have effects on male mating success in the foam-nesting treefrog Polypedates megacephalus, a species widely distributed in China, we analyzed differences in body size and age between mated and unmated males for three populations using a Generalized Linear Mixed Model(GLMM). The results showed that mated males did not exhibit larger body size and older age than unmated males, suggesting that large and/or old male individuals did not have greater mating success than small and/or young males. Moreover, we also found a non-significant size-assortative mating pattern for all populations. Our findings suggest that body size and age of the foam-nesting treefrog do not affect male mating success.     

Mating behaviour and male mating success in the green treefrog

Green treefrogs, Hyla cinerea, at a study site in eastern Georgia behaved as prolonged breeders: there were many more males present each night than females, the arrival of which was unpredictable. Individual males were usually consistent in their behaviour during a single night, and were most often observed calling. There was a weak, negative correlation between male size (snout-vent length, unless otherwise stated) and the frequency of satellite behaviour, defined as a non-calling male's being situated near a calling male. In 2 of 3 years, the mean size of males observed as satellites was less than that of the calling males with which they were associated on a given night; however, many satellites were the same size or larger than calling males. The distributions of matings per male during three seasons were not significantly different than Poisson distributions with the same means. Individual mating success was best predicted by attendance at the breeding site; mating success was negatively related to the frequency of satellite behaviour in some analyses, but very little of the variance was explained. An exhaustive series of analyses failed to show any pattern of size-dependent mating, including weight-assortative mating, even when males that were frequently satellites were excluded. The relative sizes of males and females in amplexus had little effect on fertilization success. On a seasonal basis, males were clumped in their spatial distribution, and matings tended to occur where male density was highest. Individual males were highly variable in their patterns of movement, which were not correlated with size or mating success. Mating success was also unrelated to the frequency (Hz) of the low-frequency spectral peak in the advertisement call of the male; male size and weight were negatively correlated with the frequency of the low-frequency peak.     

Altitude and life‐history shape the evolution of Heliconius wings

Phenotypic divergence between closely related species has long interested biologists. Taxa that inhabit a range of environments and have diverse natural histories can help understand how selection drives phenotypic divergence. In butterflies, wing color patterns have been extensively studied but diversity in wing shape and size is less well understood. Here, we assess the relative importance of phylogenetic relatedness, natural history, and habitat on shaping wing morphology in a large dataset of over 3500 individuals, representing 13 Heliconius species from across the Neotropics. We find that both larval and adult behavioral ecology correlate with patterns of wing sexual dimorphism and adult size. Species with solitary larvae have larger adult males, in contrast to gregarious Heliconius species, and indeed most Lepidoptera, where females are larger. Species in the pupal‐mating clade are smaller than those in the adult‐mating clade. Interestingly, we find that high‐altitude species tend to have rounder wings and, in one of the two major Heliconius clades, are also bigger than their lowland relatives. Furthermore, within two widespread species, we find that high‐altitude populations also have rounder wings. Thus, we reveal novel adaptive wing morphological divergence among Heliconius species beyond that imposed by natural selection on aposematic wing coloration.