Rival Males Research Articles

Sexual coercion and courtship by male western gorillas.

Sexual coercion and courtship are possible explanations for why male primates may direct agonistic behavior towards females. If so, then in species where females exhibit mate choice by transferring between males: (a) females who are not lactating (potential migrants) should receive more agonistic behavior than other females, (b) males should exhibit more agonistic behavior towards females during intergroup encounters than when no rival males are nearby, and (c) males should show more herding behavior when their group contains potential migrant females. We tested those hypotheses in a population of approximately 150 western gorillas at Mbeli Bai, northern Congo. We also tested whether difference in male phenotypic traits influenced their rates of agonistic behavior towards females. Of the 332 observed cases of male agonistic behavior towards females, 29% represented feeding competition, 7 % involved interventions in conflicts between females, and the remaining 64 % were considered potential evidence of sexual coercion and/or courtship. After excluding the cases of feeding competition and intervention, a multivariate analysis indicated that potential migrant females received agonistic behavior at a statistically significantly higher rate than other adult females. Females also received agonistic behavior at a significantly higher rate during intergroup encounters than at other times. Herding occurred during 22% of the 292 dyadic interunit encounters, and was significantly more likely to occur when the group contained a potential migrant female, but was not influenced by the number of adult females or the type of group encountered. Males with shorter body lengths had significantly higher rates of aggression, but phenotypic traits were not significantly correlated with herding. Collectively, our results are consistent with sexual coercion and/or courtship as an explanation for male-to-female agonistic behavior, but we are unable to distinguish between those two male mating strategies. Both types of behavior are likely due to a combination of perceived risks of female transfer and opportunity to advertise protector ability.

Size does not matter for male Marbled Salamanders (Ambystomaopacum)

Understanding the phenotypic attributes that contribute to variance in mating and reproductive success is crucial in the study of evolution by sexual selection. In many animals, body size is an important trait because larger individuals enjoy greater fitness due to the ability to secure more mates and produce more offspring. Among males, this outcome is largely mediated by greater success in competition with rival males and (or) advantages in attractiveness to females. Here we tested the hypothesis that large male Marbled Salamanders (Ambystoma opacum (Gravenhorst, 1807)) mate with more females and produce more offspring than small males. In experimental breeding groups, we included males chosen specifically to represent a range of sizes. After gravid females mated and nested freely, we collected egg clutches and genotyped all adults and samples of hatchlings with highly variable microsatellite markers to assign paternity. Size had little effect on male mating and reproductive success. Breeding males were not bigger than nonbreeding males, mates of polyandrous females were not smaller than those of monogamous females, and there was no evidence for positive assortative mating by size. Although body size did not matter for male Marbled Salamanders, we documented considerable fitness variation and discuss alternative traits that could be undergoing sexual selection.

Group synchrony and alternation as an emergent property: elaborate chorus structure in a bushcricket is an incidental by-product of female preference for leading calls

Many acoustic animals exhibit temporally structured chorusing, and in some cases, groups of calling males display elaborate forms of synchrony and/or alternation. Such temporal structure has traditionally been explained as an adaptation by which chorusing males preserve critical call features, maximize the attractiveness of their local group to females, or improve their ability to detect, evaluate, and/or evade rival males or predators. However, an alternative possibility is that synchrony and alternation simply emerge as incidental by-products of basic pairwise signal interactions between male neighbors. Thus, females may not be influenced by synchrony and alternation, and males may not benefit per se from the very chorus that they collectively produce. We studied chorusing in the bushcricket Ephippiger diurnus, a species that sings in both synchrony and alternation, by presenting natural and modified chorus stimuli to females in a series of playback tests. We found that females responded readily to the various stimuli, but we did not observe an elevated response to the natural chorus stimuli in any experiment or in any of the several E. diurnus populations tested. Our results demonstrate for the first time how elaborate forms of synchrony and alternation can represent emergent properties of choruses as opposed to specialized group displays that afford particular advantages to the individual singers.

Function of copulatory plugs in house mice: mating behavior and paternity outcomes of rival males

Polyandry is widespread across animal taxa, and subjects males to intense post-copulatory sexual selection which favors adaptations that enhance a male’s paternity success, either by decreasing the risk of sperm competition and/or by increasing the competitiveness of the ejaculate. Copulatory plugs deposited by males are thought to have evolved in the context of sperm competition. However, experimental studies that assess the function of copulatory plugs remain scarce. Moreover, most studies have used unnatural manipulations, such as ablating plug-producing male glands or interrupting copulations. Here, we investigated whether repeated ejaculation affects plug size in a mammalian model species, the house mouse. When males experience short periods of sexual rest we found that plug size decreased over repeated ejaculations so that time since last ejaculation can be applied as an approximation for plug size. We induced natural variation in plug size arising from variation in male sexual restedness, and investigated the behavior and paternity success of rival males. Male behavior in the offensive mating role (second) was influenced, albeit not significantly, by the sexual restedness of the first male-to-mate, and therefore the size of his plug. However, second males sired a significantly greater proportion of embryos when competing against a male that had recently mated compared to a male that had not. This supports a potential role of the plug in promoting a male's competitive fertilization success when remating occurs, which could be mediated both by delaying female remating and by ensuring efficient sperm transport through the female reproductive tract.

Variation in male courtship activeness between latitudinal populations of Northern medaka

It has been demonstrated from free mating experiments using multiple males and females of the medaka, Oryzias latipes species complex, that lower-latitude males exhibit more active courtship behavior than higher-latitude males. In this study, courtship activeness in males from two latitudinal populations of Northern medaka, Oryzias sakaizumii, was measured in the presence of only one male and one female, where male–male interactions were completely eliminated. We found that males from the low-latitude population performed quick circles (i.e., courtship dance) and approached females more frequently than males from the high-latitude population. Moreover, the low-latitude males tended to start courtships sooner after encountering a female than the high-latitude males. Thus, the latitudinal variation in male courtship activeness persisted even in the absence of rival males. These probably reflect that sexual selection pressures are stronger at low latitudes.

Web reduction by courting male black widows renders pheromone-emitting females' webs less attractive to rival males

Male adaptations that limit sperm competition include guarding females, applying mating plugs and chemically reducing the attractiveness or receptivity of females. In many web-building spider species, females attract males with silk-borne volatile pheromones. In widow spiders (Latrodectus, 30 species), the courting male often engages in web reduction behaviour during which he excises and bundles sections of the female's web and wraps them with his own silk. Hypothesized functions of this widespread behaviour include sexual communication (e.g. through dissemination of male sex pheromone) and/or decreasing the female's attractiveness to rivals. The latter function was previously demonstrated in a single spider species, Neriene litigiosa, but the extent to which web reduction may decrease male–male competition has never been quantified in the field. In a dense population of western black widows, Latrodectus hesperus, we ran mate attraction experiments to test the hypothesis that web reduction and/or male silk addition decrease web attractiveness to potential rivals. Webs reduced by males attracted three times fewer males than intact webs; webs with a similar proportion of silk experimentally removed attracted as many males as intact webs. However, the experimental addition of male silk did not affect the attractiveness of intact webs. We conclude that web reduction in black widows limits male–male competition by reducing the attraction of rival males to females' webs. This effect is probably mediated through targeted excision of pheromone-laden silk by courting males, possibly in combination with the male's silk forming a physical barrier to pheromone emission.

Prolonged matings in a ladybird, Menochilus sexmaculatus: A mate guarding mechanism?

Occurrence of postcopulatory mate guarding behavior is common in those insects where males directly defend their partners from rival males, avoid sperm competition and thus increase their paternity success. However, this has not yet been evaluated in ladybirds (Coleoptera:Coccinellidae). In the present investigation, mate disruption studies were conducted to establish postcopulatory mate guarding behavior in a ladybird, Menochilus sexmaculatus (Fabricius). The laboratory findings affirmed the presence of mate guarding in this ladybird species. Results revealed significant influence of mate guarding behavior on fecundity and egg viability of females. The findings of the present study revealed that an effective mating took a minimum duration of 2.50min, and prior to which sexual activities of a male were not effective to fertilize the eggs of a female. The results further demonstrate that males improved their paternity with a mating duration of above 4.50min, while fecundity increased with a mating duration of above 4.75min. Although fecundity increased continuously with an increase in mating duration from 4.75min to 30.00min, the egg viability increased with a mating duration of above 4.50min and up to a duration of 10.00min. For this reason, M. sexmaculatus males display postcopulatory mate guarding behavior after 30.00min, and mate partners remaining in copula with the male riding on the female's back with his aedeagus inserted.

Is diversification in male reproductive traits driven by evolutionary trade-offs between weapons and nuptial gifts?

Many male animals have evolved exaggerated traits that they use in combat with rival males to gain access to females and secure their reproductive success. But some male animals invest in nuptial gifts that gains them access to females. Both these reproductive strategies are costly in that resources are needed to produce the weapon or nuptial gift. In closely related species where both weapons and nuptial gifts are present, little is known about the potential evolutionary trade-off faced by males that have these traits. In this study, we use dobsonflies (order Megaloptera, family Corydalidae, subfamily Corydalinae) to examine the presence and absence of enlarged male weapons versus nuptial gifts within and among species. Many dobsonfly species are sexually dimorphic, and males possess extremely enlarged mandibles that they use in battles, whereas in other species, males produce large nuptial gifts that increase female fecundity. In our study, we show that male accessory gland size strongly correlates with nuptial gift size and that when male weapons are large, nuptial gifts are small and vice versa. We mapped weapons and nuptial gifts onto a phylogeny we constructed of 57 species of dobsonflies. Our among-species comparison shows that large nuptial gift production evolved in many species of dobsonfly but is absent from those with exaggerated weapons. This pattern supports the potential explanation that the trade-off in resource allocation between weapons and nuptial gifts is important in driving the diversity of male mating strategies seen in the dobsonflies, whereas reduced male-male competition in the species producing large spermatophores could be an alternative explanation on their loss of male weapons. Our results shed new light on the evolutionary interplay of multiple sexually selected traits in animals.

Assessment of rival males through the use of multiple sensory cues in the fruitfly Drosophila pseudoobscura.

Environments vary stochastically, and animals need to behave in ways that best fit the conditions in which they find themselves. The social environment is particularly variable, and responding appropriately to it can be vital for an animal’s success. However, cues of social environment are not always reliable, and animals may need to balance accuracy against the risk of failing to respond if local conditions or interfering signals prevent them detecting a cue. Recent work has shown that many male Drosophila fruit flies respond to the presence of rival males, and that these responses increase their success in acquiring mates and fathering offspring. In Drosophila melanogaster males detect rivals using auditory, tactile and olfactory cues. However, males fail to respond to rivals if any two of these senses are not functioning: a single cue is not enough to produce a response. Here we examined cue use in the detection of rival males in a distantly related Drosophila species, D. pseudoobscura, where auditory, olfactory, tactile and visual cues were manipulated to assess the importance of each sensory cue singly and in combination. In contrast to D. melanogaster, male D. pseudoobscura require intact olfactory and tactile cues to respond to rivals. Visual cues were not important for detecting rival D. pseudoobscura, while results on auditory cues appeared puzzling. This difference in cue use in two species in the same genus suggests that cue use is evolutionarily labile, and may evolve in response to ecological or life history differences between species.

Use of acoustic signals in mating in an eavesdropping frog-biting midge

The sensory systems of organisms are shaped by selective pressures imposed by their performance in a variety of contexts. Female frog-biting midges use the mating calls of anurans to locate their host to obtain a blood meal. Although the use of sound in foraging is well documented in this group, it is unknown whether sound is used in other contexts. To investigate the ability to use sound in nonforaging contexts, we experimentally tested the prediction that frog-biting midges (Corethrella appendiculata) use sound in mating. We recorded their wing beats while swarming, in controlled tethered conditions and during different- and same-sex pair interactions. Our results show sexual differences in the acoustic properties of flight tones, with male midges having higher frequency wing beats than females. Wing beats of free-flying individuals were significantly higher in frequency than those recorded from tethered individuals, cautioning the interpretation of recordings obtained following this widely used technique. In addition, interacting, tethered opposite-sex pairs altered their flight tones to match their upper harmonics, converging at the third and fourth harmonic frequencies of males and females, respectively. In male–male interactions, however, the frequency of their wing beats diverged. Therefore, flight tones in frog-biting midges may function as courtship signals attracting conspecific females and deterring rival males. We discuss the use of sound in multiple contexts in these midges and potential factors leading to their ability to eavesdrop on anurans.

An essential role of the arginine vasotocin system in mate-guarding behaviors in triadic relationships of medaka fish (Oryzias latipes).

To increase individual male fitness, males of various species remain near a (potential) mating partner and repel their rivals (mate-guarding). Mate-guarding is assumed to be mediated by two different types of motivation: sexual motivation toward the opposite sex and competitive motivation toward the same sex. The genetic/molecular mechanisms underlying how mate presence affects male competitive motivation in a triadic relationship has remained largely unknown. Here we showed that male medaka fish prominently exhibit mate-guarding behavior. The presence of a female robustly triggers male-male competition for the female in a triadic relationship (2 males and 1 female). The male-male competition resulted in one male occupying a dominant position near the female while interfering with the other male's approach of the female. Paternity testing revealed that the dominant male had a significantly higher mating success rate than the other male in a triadic relationship. We next generated medaka mutants of arginine-vasotocin (avt) and its receptors (V1a1, V1a2) and revealed that two genes, avt and V1a2, are required for normal mate-guarding behavior. In addition, behavioral analysis of courtship behaviors in a dyadic relationship and aggressive behaviors within a male group revealed that avt mutant males displayed decreased sexual motivation but showed normal aggression. In contrast, heterozygote V1a2 mutant males displayed decreased aggression, but normal mate-guarding and courtship behavior. Thus, impaired mate-guarding in avt and V1a2 homozygote mutants may be due to the loss of sexual motivation toward the opposite sex, and not to the loss of competitive motivation toward rival males. The different behavioral phenotypes between avt, V1a2 heterozygote, and V1a2 homozygote mutants suggest that there are redundant systems to activate V1a2 and that endogenous ligands activating the receptor may differ according to the social context.

Responses to Femoral Gland Secretions by Visually Adept Male and Female Collared Lizards

Although visually transmitted social signals are well documented in many diurnal iguanians, including collared lizards, secretory femoral glands also suggest a role of chemical signals in intraspecific communication. We conducted laboratory trials to test the extent to which male and female collared lizards responded by tongue-flicking femoral gland secretions, neutral (water), and odoriferous (cologne) control substances, males distinguished self-secretions from those produced by unfamiliar rival males, and females distinguish secretions from unfamiliar vs. familiar males. Both males and females spent similar amounts of time in four arena quadrants each with a Petri dish treated with one of the four test compounds. Males gave more tongue flicks/trial to secretions produced by unfamiliar rivals and cologne than they tongue-flicked their own secretions and water. By contrast, the number of tongue flicks by females on control substances and familiar and unfamiliar males was similar. Results support the hypothesis that femoral gland secretions function in intrasexual signaling among male collared lizards, perhaps allowing them to distinguish unfamiliar rivals. Females tongue-flicked secretions from familiar and unfamiliar males with similar frequency that was high relative to that of males, suggesting a possible role of secretions in assessment of males. Ours is the first evidence of a signaling role of femoral gland secretions in collared lizards and adds to a growing body of evidence that chemical signaling has evolved in diurnal lizards that also have highly developed visual-based signaling.

No evidence that within-group male relatedness reduces harm to females in Drosophila.

Conflict between males and females over whether, when, and how often to mate often leads to the evolution of sexually antagonistic interactions that reduce female reproductive success. Because the offspring of relatives contribute to inclusive fitness, high relatedness between rival males might be expected to reduce competition and result in the evolution of reduced harm to females. A recent study investigated this possibility in Drosophila melanogaster and concluded that groups of brothers cause less harm to females than groups of unrelated males, attributing the effect to kin selection. That study did not control for the rearing environment of males, rendering the results impossible to interpret in the context of kin selection. Here, we conducted a similar experiment while manipulating whether males developed with kin prior to being placed with females. We found no difference between related and unrelated males in the harm caused to females when males were reared separately. In contrast, when related males developed and emerged together before the experiment, female reproductive output was higher. Our results show that relatedness among males is insufficient to reduce harm to females, while a shared rearing environment – resulting in males similar to or familiar with one another – is necessary to generate this pattern.

Current Sperm Competition Determines Sperm Allocation in a Tephritid Fruit Fly

Sperm competition (SC) occurs when the sperm of two or more males compete for the same set of ova. Theoretical models and experimental observations indicate that the presence of rival males causes focal males to adjust sperm allocation in a given copulation. Males allocate more sperm when they perceive the presence of one rival male (SC risk), either before or during mating, or when they perceive the presence of multiple rival males before mating (previous SC intensity). Conversely, males are expected to allocate fewer sperm when they perceive the presence of rival males during mating (current SC intensity). Here, we varied male perception of SC by manipulating the number of rival males, both before mating (from emergence to mating) and during mating (at the time of mating) to examine their effects on mating latency, copulation duration, and sperm allocation in the South American fruit fly Anastrepha fraterculus. We showed that exposure to rival males at the time of mating decreased mating latency. However, in contrast to the theory, exposure to multiple rivals at the time of mating increased sperm allocation. Female and male size were significant predictors of mating latency, copulation duration, and sperm allocation. Our results showed that there is a plastic response of males to the level of perceived SC through the number of rival males. Current levels of SC intensity are important in shaping male responses to SC, although the patterns in this species are opposite to predictions from the existing theory. We propose that female preference for males forming leks could explain lower sperm counts when encountering only one or two males.

Blackcaps (Sylvia atricapilla) increase the whistle part of their song in response to simulated territorial intrusion

AbstractBird song is a sexually selected signal that serves two main functions, attracting a mate and deterring rivals. Different signal parameters may be important in advertising to females compared to advertising to rival males. Species solve the problem of this dual function in a variety of ways, one of which may be to have separate parts of song directed at male and female receivers. The blackcap song has two distinct parts, a complex warble, assumed to be directed at female receivers, followed by a louder and more stereotyped whistle putatively directed at males. We simulated territorial intrusions by broadcasting blackcap song in territories. Comparing songs sung prior, with those produced in response to playback, showed that the proportion of the whistle component of songs increased, but not the warble. This study thus provides empirical evidence that the whistle component of the blackcap song plays a prominent role in male–male competition. The warble component of the blackcap song may be directed at females, but this requires further testing.

Rival male chemical cues evoke changes in male pre- and post-copulatory investment in a flour beetle.

Males can gather information on the risk and intensity of sperm competition from their social environment. Recent studies have implicated chemosensory cues, for instance cuticular hydrocarbons (CHCs) in insects, as a key source of this information. Here, using the broad-horned flour beetle (Gnatocerus cornutus), we investigated the importance of contact-derived rival male CHCs in informing male perception of sperm competition risk and intensity. We experimentally perfumed virgin females with male CHCs via direct intersexual contact and measured male pre- and post-copulatory investment in response to this manipulation. Using chemical analysis, we verified that this treatment engendered changes to perfumed female CHC profiles, but did not make perfumed females "smell" mated. Despite this, males responded to these chemical changes. Males increased courtship effort under low levels of perceived competition (from 1-3 rivals), but significantly decreased courtship effort as perceived competition rose (from 3-5 rivals). Furthermore, our measurement of ejaculate investment showed that males allocated significantly more sperm to perfumed females than to control females. Together, these results suggest that changes in female chemical profile elicited by contact with rival males do not provide males with information on female mating status, but rather inform males of the presence of rivals within the population and thus provide a means for males to indirectly assess the risk of sperm competition.

Female Callosobruchus maculatus can maximize long-term fitness through polyandry

Females often mate with multiple males even when a single mating is sufficient to fertilize their eggs. Remating can lead to direct and indirect benefits for females, but these benefits trade-off against the potential costs associated with polyandry. There is a growing understanding that not all males are equal in their ability to deliver sperm to the site of fertilization. Such differences can arise both from environmental (e.g., mating history) and intrinsic (genetic) sources of variation. Most studies that investigate female fitness control male mating history to overcome such effects, but this control may bias interpretations of the benefits of polyandry. Here, we tried to avoid overestimating the benefits of polyandry that might accrue from mating repeatedly with virgin males. To do this, we quantified 6 components of fitness in female bruchid beetles, Callosobruchus maculatus , that were mated between 1 and 4 times to males with unknown mating histories, either in isolation or with rival males present. As copulation number increased, fecundity, offspring production, and egg-to-adult survival declined. In contrast to these short-term costs, the probability that a female remained completely infertile was highest at 1 copulation, and geometric mean fitness peaked after 3 copulations. Pericopulatory harassment by males did not affect female productivity. There was a significant interaction between this harassment and copulation number on longevity such that harassed females mating 2 or 3 times, and females mating twice in isolation, died soonest. Our results suggest that females may be selected to mate multiply despite short-term costs to fitness.

Postcopulatory inbreeding avoidance in guppies.

In many species, the negative fitness effects of inbreeding have facilitated the evolution of a wide range of inbreeding avoidance mechanisms. Although avoidance mechanisms operating prior to mating are well documented, evidence for postcopulatory mechanisms of inbreeding avoidance remain scarce. Here, we examine the potential for paternity biases to favour unrelated males when their sperm compete for fertilizations though postcopulatory inbreeding avoidance mechanisms in the guppy, Poecilia reticulata. To test this possibility, we used a series of artificial inseminations to deliver an equal number of sperm from a related (either full sibling or half sibling) and unrelated male to a female while statistically controlling for differences in sperm quality between rival ejaculates. In this way, we were able to focus exclusively on postcopulatory mechanisms of inbreeding avoidance and account for differences in sperm competitiveness between rival males. Under these carefully controlled conditions, we report a significant bias in paternity towards unrelated males, although this effect was only apparent when the related male was a full sibling. We also show that sperm competition generally favours males with highly viable sperm and thus that some variance in sperm competitiveness can be attributed to difference in sperm quality. Our findings for postcopulatory inbreeding avoidance are consistent with prior work on guppies, revealing that sperm competition success declines linearly with the level of relatedness, but also that such effects are only apparent at relatedness levels of full siblings or higher. These findings reveal that postcopulatory processes alone can facilitate inbreeding avoidance.

Male calling activity in syntopic populations of Rana latastei and Rana dalmatina (Amphibia: Anura)

Rana latastei and Rana dalmatina are two explosive breeder amphibians whose mating seasons last less than 1 month. The two species have very different repertoires of vocalisations, as Rana dalmatina uses only one advertisement call while Rana latastei uses two different vocalisations with opposite structural features. In our research, we continually recorded the calling activity of the two species during a whole breeding season in a pond where they breed in syntopy in order to assess the possible functions of their vocalisations. Males of both species increased their activity in coincidence with the peak of activity of females, but Rana latastei males reached the peak 2–3 days before the deposition peak. By contrast, males of Rana dalmatina reached the peak at the same time as the deposition peak, and called at high intensity long after the deposition peak. These different acoustic patterns led us to infer different prevailing functions for the vocalisations of these species. Male vocalisations of Rana latastei are addressed only to males in order to gain and confirm their position within male hierarchy. By contrast, Rana dalmatina males might vocalise also in order to attract females, rather than to deter rival males. Thus, vocalisation in this second species might also play an intersexual function. The two call types of Rana latastei were used since the beginning of the breeding season. No difference was found also in daily activity, and both vocalisations were used preferentially during night time.

The role of landmarks in territory maintenance by the black saddlebags dragonfly, Tramea lacerata

Territoriality can reduce competition for resources, but territorial defense can be costly; therefore, any behavior that reduces territorial costs may increase the net benefit of territoriality. Some species will align their territory boundaries with conspicuous landmarks that may serve to reduce defense costs. Dragonflies, including black saddlebags (Tramea lacerata), defend territories at breeding sites, keeping rival males away to allow themselves access to females. We used three treatments to investigate whether T. lacerata used landmarks: constraining landmarks (an object that provided a physical barrier to flight), non-constraining landmarks (an object of the same dimensions and construction that did not impede flight), and a control without landmarks. We observed patrolling male black saddlebags and recorded the locations of turns at their territory boundary and interactions with other dragonflies. When either type of landmark was present, individuals placed their boundary at the landmark far more often than any other location. In addition, individuals that used landmarks had a significantly narrower range of turn locations than those that did not. Unlike other studies, the use of a landmark did not seem to reduce defense costs, and interestingly not all individuals used landmarks when they were provided. We hypothesize that in this species, landmarks may only reduce costs during territory establishment, rather than during territory maintenance. Alternatively, landmarks may serve as part of a spatial reference system that aids male dragonflies in efficiently searching for females, and thus may be more important in increasing benefits rather than decreasing the costs of territoriality.