Asymmetric interspecific competition drives shifts in signalling traits in fan-throated lizards

Interspecific competition can occur when species are unable to distinguish between conspecific and heterospecific mates or competitors when they occur in sympatry. Selection in response to interspecific competition can lead to shifts in signalling traits—a process called agonistic character displacement. In two fan-throated lizard species—Sitana laticeps and Sarada darwini—females are morphologically indistinguishable and male agonistic signalling behaviour is similar. Consequently, in areas where these species overlap, males engage in interspecific aggressive interactions. To test whether interspecific male aggression between Si. laticeps and Sa. darwini results in agonistic character displacement, we quantified species recognition and signalling behaviour using staged encounter assays with both conspecifics and heterospecifics across sympatric and allopatric populations of both species. We found an asymmetric pattern, wherein males of Si. laticeps but not Sa. darwini showed differences in competitor recognition and agonistic signalling traits (morphology and behaviour) in sympatry compared with allopatry. This asymmetric shift in traits is probably due to differences in competitive abilities between species and can minimize competitive interactions in zones of sympatry. Overall, our results support agonistic character displacement, and highlight the role of asymmetric interspecific competition in driving shifts in social signals.     

The effects of competition on fitness depend on the sex of both competitors

In intraspecific competition, the sex of competing individuals is likely to be important in determining the outcome of competitive interactions and the way exposure to conspecifics during development influences adult fitness traits. Previous studies have explored differences between males and females in their response to intraspecific competition. However, few have tested how the sex of the competitors, or any interactions between focal and competitor sex, influences the nature and intensity of competition. We set up larval seed beetles Callosobruchus maculatus to develop either alone or in the presence of a male or female competitor and measured a suite of traits: development time, emergence weight; male ejaculate mass, copulation duration, and lifespan; and female lifetime fecundity, offspring egg–adult survival, and lifespan. We found effects of competition and competitor sex on the development time and emergence weight of both males and females, and also of an interaction between focal and competitor sex: Females emerged lighter when competing with another female, while males did not. There was little effect of larval competition on male and female adult fitness traits, with the exception of the effect of a female competitor on a focal female's offspring survival rate. Our results highlight the importance of directly measuring the effects of competition on fitness traits, rather than distant proxies for fitness, and suggest that competition with the sex with the greater resource requirements (here females) might play a role in driving trait evolution. We also found that male–male competition during development resulted in shorter copulation times than male–female competition, a result that remained when controlling for the weight of competitors. Although it is difficult to definitively tease apart the effects of social environment and access to resources, this result suggests that something about the sex of competitors other than their size is driving this pattern.     

Plasticity in male mating behavior modulates female life history in fruit flies

In many species, intense male-male competition for the opportunity to sire offspring has led to the evolution of selfish reproductive traits that are harmful to the females they mate with. In the fruit fly, Drosophila melanogaster, males modulate their reproductive behavior based on the perceived intensity of competition in their premating environment. Specifically, males housed with other males subsequently transfer a larger ejaculate during a longer mating compared to males housed alone. Although the potential fitness benefits to males from such plasticity are clear, its effects on females are mostly unknown. Hence, we tested the long-term consequences to females from mating with males with distinct social experiences. First, we verified that competitive experience influences male mating behavior and found that males housed with rivals subsequently have shorter mating latencies and longer mating durations. Then, we exposed females every other day for 20 days to males that were either housed alone or with rivals, and subsequently measured their fitness. We found that females mated to males housed with rivals produce more offspring early in life but fewer offspring later in life and have shorter lifespans but similar intrinsic population growth rates. These results indicate that plasticity in male mating behavior can influence female life histories by altering females’ relative allocation to early versus late investment in reproduction and survival.     

Female preference for dominant males in the Mexican mojarra cichlid fish,Cichlasoma istlanum

It is widely assumed that female preference and male competition operate simultaneously during sexual selection. Dominance is likely an honest indicator of male quality, and females can identify and choose the dominant male to reproduce with individuals with greater competitive abilities, thus improving the quality and competitiveness of their offspring. In this context, few studies have investigated female preference in relation to male fighting ability. The Mexican mojarra, Cichlasoma istlanum, is a cichlid species native to the Balsas River basin. It is territorial during reproduction and provides parental care. Males commonly engage in territorial defence, whereas females care directly for offspring. This study examined whether females prefer dominant males that exhibit more aggressive behaviour. The authors conducted experiments using groups of two males and one female to test competitive ability in males and female preference. They also quantified the time during which the female associated with the dominant male and the subordinate male after observing the outcome of a fight between the two males. They found that Mexican mojarra females preferred dominant males and that the time females spent associating with males was positively related with their aggressive behaviours during competition. These results indicate that dominant males were more attractive than subordinate males to female Mexican mojarra. The relationship between female preference and male dominance in the Mexican mojarra demonstrates the importance of male competitive ability for future parental care in reproduction.     

不同氮素水平下入侵种豚草与本地种黄花蒿、蒙古蒿的竞争关系

已有研究表明,土壤氮素增加可提高外来植物的入侵性,降低本地植物的竞争力.为揭示全球氮沉降对入侵种与本地种之间竞争关系的影响,我们于2010年5-8月在中国科学院北京森林生态系统定位研究站温室内,采用取代系列实验方法(standard replacement experiment),研究了3个氮素水平下入侵种豚草(Ambrosia artemisiifolia)与本地种黄花蒿(Artemisia annua)、蒙古蒿(Artemisia mongolica)的生长特征及种内、种间竞争关系的变化.实验采用双因素-随机区组设计,设置了低氮、中氮和高氮3个氮素水平,每一氮素水平分别设置豚草和黄花蒿、豚草和蒙古蒿组成的竞争实验,生长90 d后测量株高和生物量.结果表明:单栽情况下,随氮素水平的增加3个物种的株高均增加,而生物量均无显著变化;混栽情况下,3个物种株高和生物量随氮素水平的增加变化各异,豚草呈极显著增加趋势,而黄花蒿无明显变化,蒙古蒿则先增加后减少.豚草的快速生长使其在竞争中处于优势地位,对本地种黄花蒿和蒙古蒿产生明显的竞争效应.但不同氮素水平下,豚草对本地种的竞争力不同:低氮素水平下,豚草＜两个本地种;中氮素水平下,黄花蒿＜豚草＜蒙古蒿;高氮素水平下,豚草＞两个本地种.氮素添加显著提高了豚草的种间竞争力,改变了豚草与本地种之间的竞争关系,使竞争有利于入侵种.据此推测,在全球变化的背景下,氮沉降的增加将会促进外来种豚草的入侵,增加本地群落的可入侵性.     

Sex-role reversal revisited: choosy females and ornamented, competitive males in a pipefish

In the pipefish Syngnathus typhle sex roles are reversed, thatis, females compete more intensely than males over mates. However,competition over mates among individuals of one sex does notnecessarily prevent members of that same sex from being choosy,and choosiness in the other sex does not prevent competitionwithin it. In an experiment we allowed a female pipefish tochoose freely between two males, after which we released themales and let the three interact. Comparisons with earlier resultsshow that both sexes courted partners and competed with consexuals.However, females courted more often than did males, and courtshipwas more frequent in treatments involving large individualsthan in treatments with small individuals. Males competed amongthemselves for access to mates but for a shorter duration thanfemales in the same situation. Males displayed an ornament towardsfemales but not to males during mating competition. Females,however, used their ornament in both contexts. Females did notalways mate with the male of their previously made choice, whichwe interpret as females being constrained by male-male competition,male motivation to mate, or both. Thus, in this sex-role reversedspecies, mate choice in the more competitive sex may be circumventedand even overruled by mate competition and mating willingnessin the least competitive sex. Hence, sex roles should not beconsidered as sexes being either choosy or competitive but ratherthat males and females may exhibit different combinations ofchoice and competition.     

Variation in the relative magnitude of intraspecific and interspecific competitive effects in novel versus familiar environments in two<Emphasis Type="Italic">Drosophila</Emphasis> species

Models of competitor coevolution, especially the genetic feedback hypothesis, suggest that a negative correlation between intraspecific and interspecific competitive effects may be important in sustaining competitor coexistence, and can give rise to oscillatory dynamics with repeated reversals of competitive superiority. I reanalyzed previously published census data from an experiment in which populationsof Drosophila melanogaster andD. simulans underwent competitive coevolution in one familiar and two novel environments, to specifically look for any evidence of a negative relationship between intraspecific and interspecific competitive effects on population growth rates, and for any indication of short period cycling in the relative magnitude of intraspecific and interspecific competitive effects. While there was considerable variation in the relative magnitude of intraspecific and interspecific competitive effects over generations, among both populations and environments, there was no clear evidence supporting the genetic feedback hypothesis. Intraspecific and interspecific competitive effects on population growth rates were strongly positively correlated in novel environments, and uncorrelated in the familiar environment. Data from the familiar environment indicated that indices of competition of populations of the initially superior competitor,D. melanogaster, might be showing some cyclic behaviour, but I argue that this is likely to be transient, and not suggestive of sustained oscillatory dynamics predicted by the genetic feedback model. I discuss the results in the context of the importance of the genetic architecture of intraspecific and interspecific competitive abilities in determining the coevolutionary trajectory of competitive interactions.     

Early growth,dominance acquisition and lifetime reproductive success in male and female cooperative meerkats

In polygynous species, variance in reproductive success is higher in males than females. There is consequently stronger selection for competitive traits in males and early growth can have a greater influence on later fitness in males than in females. As yet, little is known about sex differences in the effect of early growth on subsequent breeding success in species where variance in reproductive success is higher in females than males, and competitive traits are under stronger selection in females. Greater variance in reproductive success has been documented in several singular cooperative breeders. Here, we investigated consequences of early growth for later reproductive success in wild meerkats. We found that, despite the absence of dimorphism, females who exhibited faster growth until nutritional independence were more likely to become dominant, whereas early growth did not affect dominance acquisition in males. Among those individuals who attained dominance, there was no further influence of early growth on dominance tenure or lifetime reproductive success in males or females. These findings suggest that early growth effects on competitive abilities and fitness may reflect the intensity of intrasexual competition even in sexually monomorphic species.     

Interspecific competition in perennial sedentary organisms: An individual-based model

We investigated a mathematical model of the dynamics of the ecological system consisting of two competing perennial species, each of which leads a sedentary life. It is an individual-based model, in which the growth of each individual is described. The rate of this growth is weakened by competition from neighboring individuals. The strength of the competitors' influence depends on their size and distance to them. The conditions, in which the competitive exclusion of one of the competitors and the coexistence of both competitors take place are provided. The influence of the parameters responsible for the strength of competition, the degree of competitive asymmetry, and consideration of the importance of specific elements of the spatial structure of this ecological system on the results of the competition were analyzed. Both species co-exist when they are equal competitors. Permanent coexistence is possible only when interspecific competition is weaker than intraspecific. When interspecific competition is stronger, the coexistence of equal interspecific competitors is random. Both species have equal probability of extinction. If species are not equal competitors, the stronger one wins. This result can be modified by different strengths of intraspecific competition. The weaker interspecific competitor can permanently coexist with stronger one, when its individuals suffer stronger intraspecific competition.     

Reproductive interference explains persistence of aggression between species

Interspecific territoriality occurs when individuals of different species fight over space, and may arise spontaneously when populations of closely related territorial species first come into contact. But defence of space is costly, and unless the benefits of excluding heterospecifics exceed the costs, natural selection should favour divergence in competitor recognition until the species no longer interact aggressively. Ordinarily males of different species do not compete for mates, but when males cannot distinguish females of sympatric species, females may effectively become a shared resource. We model how reproductive interference caused by undiscriminating males can prevent interspecific divergence, or even cause convergence, in traits used to recognize competitors. We then test the model in a genus of visually orienting insects and show that, as predicted by the model, differences between species pairs in the level of reproductive interference, which is causally related to species differences in female coloration, are strongly predictive of the current level of interspecific aggression. Interspecific reproductive interference is very common and we discuss how it may account for the persistence of interspecific aggression in many taxonomic groups.     

Male competition reverses female preference for male chemical cues

Females must choose among potential mates with different phenotypes in a variety of social contexts. Many male traits are inherent and unchanging, but others are labile to social context. Competition, for example, can cause physiological changes that reflect recent wins and losses that fluctuate throughout time. We may expect females to respond differently to males depending on the outcome of their most recent fight. In Bolitotherus cornutus (forked fungus beetles), males compete for access to females, but copulation requires female cooperation. In this study, we use behavioral trials to determine whether females use chemical cues to differentiate between males and whether the outcome of recent male competition alters female preference. We measured female association time with chemical cues of two size‐matched males both before and after male–male competition. Females in our study preferred to associate with future losers before males interacted, but changed their preference for realized winners following male competitive interactions. Our study provides the first evidence of change in female preference based solely on the outcome of male–male competition.     

Linking intra‐ and interspecific assortative mating: Consequences for asymmetric sexual isolation

Assortative mating is of interest because of its role in speciation and the maintenance of species boundaries. However, we know little about how within‐species assortment is related to interspecific sexual isolation. Most previous studies of assortative mating have focused on a single trait in males and females, rather than utilizing multivariate trait information. Here, we investigate how intraspecific assortative mating relates to sexual isolation in two sympatric and congeneric damselfly species (genus Calopteryx). We connect intraspecific assortment to interspecific sexual isolation by combining field observations, mate preference experiments, and enforced copulation experiments. Using canonical correlation analysis, we demonstrate multivariate intraspecific assortment for body size and body shape. Males of the smaller species mate more frequently with heterospecific females than males of the larger species, which showed less attraction to small heterospecific females. Field experiments suggest that sexual isolation asymmetry is caused by male preferences for large heterospecific females, rather than by mechanical isolation due to interspecific size differences or female preferences for large males. Male preferences for large females and male–male competition for high quality females can therefore counteract sexual isolation. This sexual isolation asymmetry indicates that sexual selection currently opposes a species boundary.     

Differential introgression of a female competitive trait in a hybrid zone between sex‐role reversed species

Mating behavior between recently diverged species in secondary contact can impede or promote reproductive isolation. Traditionally, researchers focus on the importance of female mate choice and male–male competition in maintaining or eroding species barriers. Although female–female competition is widespread, little is known about its role in the speciation process. Here, we investigate a case of interspecific female competition and its influence on patterns of phenotypic and genetic introgression between species. We examine a hybrid zone between sex‐role reversed, Neotropical shorebird species, the northern jacana (Jacana spinosa) and wattled jacana (J. jacana), in which female–female competition is a major determinant of reproductive success. Previous work found that females of the more aggressive and larger species, J. spinosa, disproportionately mother hybrid offspring, potentially by monopolizing breeding territories in sympatry with J. jacana. We find a cline shift of female body mass relative to the genetic center of the hybrid zone, consistent with asymmetric introgression of this competitive trait. We suggest that divergence in sexual characteristics between sex‐role reversed females can influence patterns of gene flow upon secondary contact, similar to males in systems with more typical sex roles.     

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.     

Male and Female Mating Behavior is Dependent on Social Context in the Butterfly Bicyclus anynana

Reproduction is often more costly to females than it is to males, leading to the evolution of ornamented or competitive males and choosy females. Reproduction costs to females, however, can be reduced through nuptial gifts provided by males. These gifts, by increasing female survival or fecundity, can promote the evolution of mutual mate choice, ornamentation, or competition in both sexes, as well as plasticity in mating behavior dependent on social context. We tested for plasticity in male and female mating behavior in a species of butterfly, Bicyclus anynana, where male spermatophore gifts contribute to female survival and fecundity, and where mutual mate choice and ornamentation were previously established. We examined the effect of a sexual competitor on male–female interactions by observing and comparing the behavior of male–female pairs with that of triads containing either an extra male or an extra female. In the presence of a sexual competitor both males and females copulated less than when in male–female pairs, regardless of the direction of sex-ratio skew. Active males increased their own likelihood to copulate, while active females increased their likelihood of being courted. In addition, there was an effect of social context on relative rates of male and female courting and flying. These results suggest that both males and females change their mating behavior in response to social context in the butterfly Bicyclus anynana.     

Sexual conflict and ecology: Species composition and male density interact to reduce male mating harassment and increase female survival

Sexual conflict is a pervasive evolutionary force that can reduce female fitness. Experimental evolution studies in the laboratory might overestimate the importance of sexual conflict because the ecological conditions in such settings typically include only a single species. Here, we experimentally manipulated conspecific male density (high or low) and species composition (sympatric or allopatric) to investigate how ecological conditions affect female survival in a sexually dimorphic insect, the banded demoiselle (Calopteryx splendens). Female survival was strongly influenced by an interaction between male density and species composition. Specifically, at low conspecific male density, female survival increased in the presence of heterospecific males (C. virgo). Behavioral mating experiments showed that interspecific interference competition reduced conspecific male mating success with large females. These findings suggest that reproductive interference competition between con‐ and heterospecific males might indirectly facilitate female survival by reducing mating harassment from conspecific males. Hence, interspecific competitors can show contrasting effects on the two sexes thereby influencing sexual conflict dynamics. Our results call for incorporation of more ecological realism in sexual conflict research, particularly how local community context and reproductive interference competition between heterospecific males can affect female fitness.     

Aggregative Root Placement: A Feature During Interspecific Competition in Inland Sand-Dune Habitats

Segregation of roots is frequently observed in competing root systems. However, recently, intensified root growth in response to a neighbouring plant has been described in pot experiments [Gersani M, Brown J S, O'Brien E E, Maina G M and Abramsky Z 2001. J. Ecol. 89, 660–669]. This paper examines whether intense root growth towards a neighbour (aggregation) plays a role in competitive interactions between plant species from open nutrient-poor mid-European sand ecosystems. In a controlled field-competition experiment, root distribution patterns of intra- and interspecific pairs as well as single control plants of Corynephorus canescens, Festuca psammophila, Hieracium pilosella, Hypochoeris radicata and Conyza canadensis were investigated after one growing season. Under intraspecific competition plants tended to segregate their root systems, while under interspecific competition most species tended to aggregate roots towards their neighbours even at the expense of root development at the opposite competition-free side of the target. Preference of a root aggregation strategy over the occupation of competition-free soil in interspecific competition emphasizes the importance of contesting between individuals in relation to mere resource acquisition. It is suggested that in the presence of a competitor the plants might use root aggregation as a defensive reaction to maintain a strong competitive response and exclusive access to the resources of already occupied soil volumes.     

Patterns of sperm precedence are not affected by female mating history in Drosophila melanogaster

In promiscuously mating species, there is strong selection on males to maximize their share of paternity through both defensive and offensive means. This has been most extensively examined using the Drosophila melanogaster model system. In these studies, sperm competition has been examined by mating a virgin female to two consecutive males and then determining the fertilization success of both the first male (defending, P1) and the second male (offending, P2). Recent evidence suggests that male defense may be influenced by female mating history (i.e., virgin versus nonvirgin). Here, by mating females to males with three different genotypes, we show that female mating history does not affect male defensive or offensive abilities in sperm competition. We also show that, although female lifetime fecundity was not correlated with the number of times that she mated, it was reduced by increased exposure to males. These data indicate that measures of P1 and P2 previously reported in D. melanogaster may be robust to the specific mating history of the females used in these studies.     

Male pipefish prefer dominant over attractive females

Animals may obtain information guiding their choice betweenpotential partners from observing competitive interactionsand displays between them, or from displays directed at thechoosing individual. In the sex-role reversed pipefish Syngnathustyphle females display a temporary ornament (a color pattern)to other females as well as to males. We have previously shown that display of female ornaments per se is attractive to males.Here we show that information from competitive displays canoverride such direct attraction displays as signals in thepartner choice process. In a mate choice experiment, an enclosedmale could choose between two females. On the first experimentalday, females could interact freely, while on the second daythey were isolated from each other. When female-female competitionwas allowed, the ornament display was directed more to theother female than to the male: Time competing, rather thantime courting the male, correlated with ornament display duration.However, ornament display under competition and ornament displayin the absence of competition did not correlate significantly.In fact, females competing more intensively on day one displayedthe ornament less on day two. Furthermore, the ornament displayduring the first, but not the second, day predicted male matechoice on the second day. Thus, males remembered previous informationfrom competitive displays and used it rather than immediateinformation from displays in the absence of female-female competition.We suggest that competitive displays more reliably signal female quality as compared to noncompetitive ones, and that males benefitfrom mating with dominant females.     

Precopulatory but not postcopulatory male reproductive traits diverge in response to mating system manipulation in Drosophila melanogaster

Competition between males creates potential for pre‐ and postcopulatory sexual selection and conflict. Theory predicts that males facing risk of sperm competition should evolve traits to secure their reproductive success. If those traits are costly to females, the evolution of such traits may also increase conflict between the sexes. Conversely, under the absence of sperm competition, one expectation is for selection on male competitive traits to relax thereby also relaxing sexual conflict. Experimental evolution studies are a powerful tool to test this expectation. Studies in multiple insect species have yielded mixed and partially conflicting results. In this study, we evaluated male competitive traits and male effects on female costs of mating in Drosophila melanogaster after replicate lines evolved for more than 50 generations either under enforced monogamy or sustained polygamy, thus manipulating the extent of intrasexual competition between males. We found that in a setting where males competed directly with a rival male for access to a female and fertilization of her ova polygamous males had superior reproductive success compared to monogamous males. When comparing reproductive success solely in double mating standard sperm competition assays, however, we found no difference in male sperm defense competitiveness between the different selection regimes. Instead, we found monogamous males to be inferior in precopulatory competition, which indicates that in our system, enforced monogamy relaxed selection on traits important in precopulatory rather than postcopulatory competition. We discuss our findings in the context of findings from previous experimental evolution studies in Drosophila ssp. and other invertebrate species.