Male Intrasexual Competition Research Articles

Vibrational communication in Scolypopa australis: characterising spontaneous and intrasexual calls in an invasive orchard pest

AbstractVibrational communication in the Auchenorrhyncha is an avenue of growing interest, with many pesticide‐free vibrational pest control programmes against auchenorrhynchan agricultural pests continuing to be developed around the world. Passion vine hoppers (PVH), Scolypopa australis (Walker) (Hemiptera: Ricaniidae), are an economically damaging pest species in New Zealand, where they facilitate the growth of sooty mould in kiwifruit orchards. Scolypopa australis is already known to use vibrational signals, therefore a further understanding of the spectral and temporal characteristics of its calls will be an essential next step to developing vibration‐based pest management strategies. Here, we aimed to characterise the properties of spontaneous S. australis calls in solitary and intrasexual communication contexts. We used laser Doppler vibrometry to record vibrational signals produced by male and female S. australis (separately), when in groups of three or individually. We sorted calls and call sub‐units (syllables) into groups based on cluster analyses of various spatio‐temporal parameters. We define four male calls and three female calls, and newly describe a ‘two‐syllable’ calling structure that may be involved in male intrasexual competition. Certain syllables were more prevalent at the beginning or end of calls for males, and were more homogenously distributed throughout calls for females. The types of calls used differed between single males and groups, with more energetically intense call types observed in the group settings. Additional studies on the behavioural context of S. australis vibrational calls and their responses to playback of such calls will be necessary to assess the viability of vibration‐based pest management for this species.

Effects of Voice Pitch on Social Perceptions Vary With Relational Mobility and Homicide Rate

Fundamental frequency ( fo) is the most perceptually salient vocal acoustic parameter, yet little is known about how its perceptual influence varies across societies. We examined how fo affects key social perceptions and how socioecological variables modulate these effects in 2,647 adult listeners sampled from 44 locations across 22 nations. Low male fo increased men’s perceptions of formidability and prestige, especially in societies with higher homicide rates and greater relational mobility in which male intrasexual competition may be more intense and rapid identification of high-status competitors may be exigent. High female fo increased women’s perceptions of flirtatiousness where relational mobility was lower and threats to mating relationships may be greater. These results indicate that the influence of fo on social perceptions depends on socioecological variables, including those related to competition for status and mates.

Mismatch between calf paternity and observed copulations between male and female reindeer: Multiple mating in a polygynous ungulate?

In polygynous systems, such as that exhibited by reindeer Rangifer tarandus, mate choice can be difficult to disentangle from male intrasexual competition because male behavior may constrain female choice. Multiple mating may provide an avenue for female mate choice, though it is difficult to identify using behavioral estimators alone. Molecular techniques address this issue by affording ecologists an opportunity to reassess mating systems from a genetic perspective. We assessed the frequency and possible explanations for multiple mating in reindeer using a genetic approach to determine the success of observed copulations in a semi-domesticated herd in Kaamanen, Finland. Behavioral and genetic data were synthesized with population characteristics over a 7-year period to test the hypothesis that, if present, polyandry in reindeer is driven by sexual harassment from sub-dominant males. We observed multiple mating in 42% of females, with as many as 60% exhibiting multiple mating in certain years. We found no evidence that multiple mating resulted from sexual harassment by sub-dominant males, suggesting that it is likely a deliberate strategy among females. Conversion rate of copulations into paternities varied with male size, with smaller males more likely to experience mismatch than larger males. Female preference for larger males persisted despite the occurrence of multiple mating, possibly suggesting a mechanism for cryptic post-copulatory selection. We suggest further research to delineate the possible influence of cryptic post-copulatory selection and multiple mating to defend against infertility in exhausted males.

Regional divergence in the skull morphometry of adult male Galapagos sea lion

Abstract The skull often reveals adaptation to ecological and environmental influences. Food availability affects the somatic growth of otariids, and population abundance can influence male intrasexual competition. We hypothesized there would be regional differences between Galapagos sea lion (GSL, Zalophus wollebaeki) rookeries as a result of variance in ecosystem metrics and population abundance. We used skull measures related to individual size and aggressiveness. We analyzed 19 linear measures for size and 15 for aggressiveness taken on 49 adult male skulls of GSL collected in four bioregions of the archipelago. Nine skull measures had a significant effect on individual clustering into bioregions (P < 0.001). The most discriminating measures were snout length, supraorbital process width, nasal length, and canine width. We found a significant relationship between skull grouping patterns and chlorophyll-α levels (proxy marine productivity and body growth) and GSL population abundance (proxy of agonistic behavior) of each bioregion, as well as with the interaction between both variables (P < 0.001). Our results suggest significant regional differences in GSL skull morphometry, which could be associated with resource availability and level of male intrasexual competition in the archipelago. This information is important for understanding the life history of this species.

Influence of operational sex ratio and male age on mating competition intensity in reindeer (Rangifer tarandus)

The operational sex ratio (OSR), the ratio of sexually active males to sexually receptive females, is one of the main measures used to predict the intensity and direction of mating competition, influencing the opportunity for sexual selection. Here, we conducted the first experimental study to investigate how OSR and male age impacts the intensity of mating competition in reindeer (Rangifer tarandus), under semi-natural conditions during the rut. We manipulated OSR on two levels in two enclosures, a female biased treatment (3♂:6♀ = OSR 0.5) and a sex balanced treatment (3♂:3♀ = OSR 1), over 2 years with males from two age groups, and with females of various ages. We found some support for prevailing OSR theory, notably with female intrasexual competition occurring at lower frequencies in OSR 1 than OSR 0.5, and male intrasexual competition occurring at higher frequencies in the older male age group. Courtship behaviour was found to occur at higher frequencies in OSR 1 than OSR 0.5; however, there was no effect of male age. To successfully pass on genes to the next generation, one needs access to mates and winning a competitive bout is not always indicative of successfully accomplishing gene flow. Studies on OSR have the potential to help us understand the drivers behind sexual competition and how best to predict breeding outcomes during a rut.

Influence of operational sex ratio and male age on mating competition intensity in reindeer (Rangifer tarandus)

The operational sex ratio (OSR), the ratio of sexually active males to sexually receptive females, is one of the main measures used to predict the intensity and direction of mating competition, influencing the opportunity for sexual selection. Here, we conducted the first experimental study to investigate how OSR and male age impacts the intensity of mating competition in reindeer (Rangifer tarandus), under semi-natural conditions during the rut. We manipulated OSR on two levels in two enclosures, a female biased treatment (3♂:6♀ = OSR 0.5) and a sex balanced treatment (3♂:3♀ = OSR 1), over 2 years with males from two age groups, and with females of various ages. We found some support for prevailing OSR theory, notably with female intrasexual competition occurring at lower frequencies in OSR 1 than OSR 0.5, and male intrasexual competition occurring at higher frequencies in the older male age group. Courtship behaviour was found to occur at higher frequencies in OSR 1 than OSR 0.5; however, there was no effect of male age. To successfully pass on genes to the next generation, one needs access to mates and winning a competitive bout is not always indicative of successfully accomplishing gene flow. Studies on OSR have the potential to help us understand the drivers behind sexual competition and how best to predict breeding outcomes during a rut.

Effects of Sex and Breeding Status on Skull Morphology in Cooperatively Breeding Ansell’s Mole-Rats and an Appraisal of Sexual Dimorphism in the Bathyergidae

African mole-rats of the genus Fukomys (Northern common mole-rats) combine a monogamous mating system and pronounced sexual size dimorphism; a pattern highly untypical for mammals. At the same time, they live in cooperatively breeding groups composed of reproductive and non-reproductive members of both sexes. How and to which degree sex and breeding status influence morphofunctional characters in eusocial mole-rats is not well characterized but essential to come to a comprehensive understanding of their peculiar social system. Here, we explore patterns of morphological differentiation in skulls of Ansell’s mole-rats (Fukomys anselli) by means of multivariate analysis of linear skull measurements combined with a 2D shape analysis of cranium and mandible. Compared to females, males display larger skulls relative to body size and show an expansion of the facial portion of the cranium, while reproductive status did not have an effect on any of the traits studied. We also show that species of Fukomys mole-rats display a scaling of relative sexual body size dimorphism in compliance to Rensch’s rule, which is deemed indicative of intense male intrasexual competition. For the bathyergid family as a whole, results of scaling analyses were more ambiguous, but also indicative of Rensch’s rule conformity. In line with genetic field data, our results point to a greater role of male-male conflicts in Fukomys than is traditionally assumed and support the notion that reproductive status does not correlate with morphofunctional segregation in these unusual rodents.

Mechanisms of Hantavirus Transmission in Oligoryzomys longicaudatus.

The cricetid rodent Oligoryzomys longicaudatus is the species host of Andes virus (ANDV) which causes hantavirus pulmonary syndrome in southern Argentina and Chile. Population density, behavioral interactions, and spacing patterns are factors that affect viral transmission among wild rodents. We predict that the highest prevalence of hantavirus antibody positive would be found among wounded, reproductive males and that, at high population densities, wounded, reproductive males would be dispersers rather than resident individuals. The study was conducted seasonally from October (spring) 2011 to October (spring) 2013 in a shrubland habitat of Cholila, Argentina. During each trapping session, we classified captured O. longicaudatus as resident or disperser individuals, estimated population density, and recorded wounds as an indicator of aggression among individuals. We obtained blood samples from each individual for serological testing. We used generalized linear models to test the statistical significance of association between antibody prevalence, and sex, resident/dispersal status, wounds and trapping session. The highest proportion of seropositive O. longicaudatus individuals was among wounded reproductive males during periods of the greatest population density, and the characteristics of seroconverted individuals support that transmission is horizontal through male intrasexual competition. A positive association between dispersing individuals and hantavirus antibody was detected at high population density. Our study design allowed us to obtain data on a large number of individuals that are seroconverted, enabling a better understanding of the ecology and epidemiology of the ANDV host system.

Female sexual maturity as a determining factor of size-assortative pairing in the protandrous spider Manogea porracea (Araneae, Araneidae)

Protandry occurs when males mature prior to females in the breeding season. There may be a fitness benefit for males if they are able to copulate and defend virgin and more fecund females. Therefore, fitness benefits can be investigated by studying assortative mating during mate guarding of virgin females before and after female sexual maturation. The asynchrony of female development in the protandrous spider Manogea porracea (Araneidae) provides an opportunity to investigate how female sexual maturity (subadult and adult females) influence size-assortative pairing. We investigated size differences between couples composed of males and subadult females, and of males and adult females. We hypothesized that size-assortative pairing is stronger in couples with adult females. Adult females were larger than subadults, but the size of males paired with females from both groups was similar. We found assortative pairing within all couples involving an adult female. However, this pattern was not observed among couples with subadult females. It is possible that the production of sexual pheromones by adult females after their last molt intensifies male intrasexual competition, promoting positive assortative pairing. We discuss the potential mechanisms affecting male mate choice and mate competition during pairings in M. porracea.

Explaining marriage patterns in a globally representative sample through socio-ecology and population history: A Bayesian phylogenetic analysis using a new supertree

Abstract Comparative analyses have sought to explain variation in human marriage patterns, often using predictions derived from sexual selection theory. However, most previous studies have not controlled for non-independence of populations due to shared ancestry. Here we leverage a phylogenetic supertree of human populations that includes all 186 populations in the Standard Cross-Cultural Sample (SCCS), a globally representative and widely-used sample of human populations. This represents the most comprehensive human phylogeny to date, and allows us not only to control for non-independence, but also to quantify the role of population history in explaining behavioral variation, in addition to current socio-ecological conditions. We use multiple imputation to overcome missing data problems and build a comprehensive Bayesian phylogenetic model of marriage patterns with two correlated response variables and eleven minimally collinear predictors capturing various socio-ecological conditions. We show that ignoring phylogeny could lead to both false positives and false negatives, and that the phylogeny explained about twice as much variance as all the predictors combined. Pathogen stress and assault frequency emerged as the predictors most strongly associated with polygyny, which had been considered evidence for female choice of good genes and male intra-sexual competition or male coercion, respectively. Mixed support was found for a polygyny threshold based on variance in male wealth, which is discussed in light of recent theory. Barring caveats, these findings refine our understanding of the evolution of human marriage systems, and highlight the value of combining population history and current socio-ecology in explaining human behavioral variation. Future studies using the SCCS should do so using the present supertree.

Intensity of male-male competition predicts morph diversity in a color polymorphic lizard.

Sexual selection is one of the main processes involved in the emergence and maintenance of heritable color polymorphisms in a variety of taxa. Here, we test whether the intensity of sexual selection, estimated from population sex ratio, predicts morph diversity in Podarcis muralis, a color polymorphic lizard with discrete white, yellow, orange, white-orange, and yellow-orange male and female phenotypes (i.e., morphs). In a sample of 116 Pyrenean populations and 5421 lizards, sex ratios (m/f) vary from 0.29 to 2.5, with the number of morphs for each sex ranging from 2 to 5. Male-biased sex ratios are associated with increased morph diversity as measured with Shannon's diversity index. The main factor accounting for this relationship is male morph richness (i.e., the number of morphs). In contrast, female morph diversity is not related to sex ratio. These results suggest a relationship between the intensity of male intrasexual competition and male morph diversity. While other selective forces may interact with sexual selection in maintaining the color polymorphisms in P. muralis, this evidence suggests a complex evolutionary scenario possibly involving frequency-dependent selection of alternative reproductive tactics and/or complex balancing selection.

Lack of Evolution of Sexual Size Dimorphism in Heteromyidae (Rodentia): The Influence of Resource Defense and the Trade-Off between Pre- and Post-Copulatory Trait Investment

One paradoxical finding in some mammals is the presence of male–male intrasexual competition in the absence of sexual size dimorphism. It has been a major goal of evolutionary biologists for over a century to understand why some species in which large males can monopolize multiple mates while excluding smaller competitors, exhibit little or no sexual dimorphism. In this paper I examine three of the main hypotheses that have been proposed to explain this conundrum using as study case the Heteromyidae, a rodent family with subtle sexual size dimorphism. Using a phylogenetic comparative approach, I address the potential influence of (1) fecundity selection, (2) covariation between pre- and post-copulatory traits, and (3) environmental constraints (resource shortage) in explaining patterns of body size and sexual size dimorphism (SSD) across 62 heteromyid species. Baculum size, a proxy of the strength of post-copulatory sexual selection, and SSD were negatively correlated suggesting that heteromyid rodents balance their reproductive investment between pre- and post-copulatory traits, which may prevent the evolution of extensive SSD. Results also support a role for resource competition in moderating SSD. The amount of SSD correlated negatively with latitude. This can be explained if high productivity relaxes the level of intrasexual competition among females, leading to more male-biased dimorphism since forces acting on both sexes are not cancelled. In line with this argument, territorial species exhibited a higher dimorphism in comparison with social species. No support was found for the fecundity selection hypothesis. Overall, this study provides insight into the factors driving observed patterns of sexual dimorphism in this iconic group and highlights the need to consider a broader framework beyond sexual selection for better understanding the evolution of dimorphism in this family.

Behavior and activity budgets of wild breeding polar bears (Ursus maritimus)

AbstractWe quantify the first complete description of breeding behavior and activity budgets of an undisturbed pair of adult polar bears, observed 24 h/d for 13 d from 2 to 15 May 1997, at Radstock Bay, Devon Island, Nunavut, Canada. The male herded the female to an area of 1–2 km2, where we observed them throughout the observation period. All behaviors were documented from when the adult female and her 2.5‐yr‐old cub were first observed being followed by an adult male, through separation of the cub from its mother, a week of intense interactions preceding several days with copulation, after which they parted. They mated for 51, 86, 66, and 150 min on 9–10, 12, 13, and 14 May, respectively, and parted on 14–15 May. The male deterred three challengers. The peak breeding season for polar bears runs from early April through mid‐May, although additional mating behavior has been documented in June. Timing of mating and duration of copulations in the wild were similar to reports from zoos. Induced ovulation, male intrasexual competition, female fitness, the mating system, and potential consequences of climate warming are discussed with insights made possible by documentation of the reproductive behavior of wild polar bears.

Further evidence for links between facial width‐to‐height ratio and fighting success: Commentary on Zilioli et al. (2014)

Recent research has reported an association between facial width-to-height ratio (fWHR) and both fighting performance and judgments of formidability in a sample of mixed martial arts (MMA) combatants. The results provide evidence of fWHR being associated with sporting performance and aggression in men. However, it has been argued that the effect of fWHR might be a by-product of associations between body size and behavioral measures. Here we tested whether fWHR is associated with perceived aggressiveness, fighting ability and success in physical confrontation, while controlling for body size, also in a sample of MMA fighters. We found that perceived fighting ability was predicted by weight but not by fWHR. In contrast, both fWHR and body weight independently predicted perceived aggressiveness. Furthermore, we found positive associations between fWHR and fighting performance which appear to be independent of body size. Our findings provide further support for the proposal that fWHR is associated with fighting ability and perceived aggression, and that these effects are independent of body size. Therefore, fWHR might be considered as a viable and reliable marker for inference of success in male intra-sexual competition.

Sexual size monomorphism in the crested porcupine (Hystrix cristata)

Amongst mammals, female-biased sexual size dimorphism (SSD) is rare and it occurs mostly in species where reduced male intrasexual competition is present. Reverse SSD has been reported for Old World porcupines Hystrix spp. We compared weight and six metric body measurements of 40 male and 42 female crested porcupines from Southern Tuscany, Italy. No significant difference was observed between sexes. The monogamous mating system of porcupines, sharing parental care, together with no evidence of territoriality, militate against previous claims of SSD presence, probably due to small sample size and inappropriate statistical analyses.

Social competition masculinizes the communication signals of female electric fish

Male intrasexual competition and female choice explain the evolution of male ornaments. Except in sex-role-reversed taxa, female ornaments have been regarded as an epiphenomenon of genetic correlation, with no female-specific function or independently selected basis. Females from species with conventional sex roles may still experience some degree of female–female competition and male choice that could explain the persistence of female ornaments. We studied the effect of female competition on the expression of a sexually dimorphic communication signal. In the electric fish Brachyhypopomus gauderio, both sexes produce an electric signal pulse for communication and electrolocation. Male electric pulses are longer in duration and greater in amplitude than those of females. As competition increases, males further enhance their signals in response to elevated androgen levels. We explored whether females respond to social competition as males do, by enhancing their communication signals and increasing androgen levels. We measured amplitude and duration of the electric signal pulse, and testosterone levels in female B. gauderio in their natural habitat in Uruguay and estimated social competition by calculating population density and adult sex ratio (ASR). In the lab, we manipulated ASR and population density independently to separate these factors and eliminate seasonal confounds. Under both field and lab conditions, signal pulse amplitude increased with population density, while pulse duration increased with female bias in ASR. In the field, but not the lab, androgen levels increased when ASR was female biased. Our findings indicate that the socially mediated mechanism of signal regulation is shared by the sexes, although whether androgens regulate this signal plasticity in females remains unclear.

Sexual interference in the golden snub‐nosed monkey (Rhinopithecus roxellana): a test of the sexual competition hypothesis in a polygynous species

Sexual interference (SI), which is defined as any disturbance directed to a mating pair by other individuals, has been reported in several primate species. It is widely suggested that successful harassers experience improved mating success by increasing their access to reproductive partners as well as by reducing the mating success of rivals. Although theories of primate sexual conflict highlight male intra-sexual mating competition, females also are reported to actively disrupt copulations between mating partners. In this study, we investigated SI in a multilevel troop of Golden snub-nosed monkeys (Rhinopithecus roxellana) inhabiting the Qinling Mountains, China. Behavioral observations of 11 one-male units (OMU) that comprised the multilevel troop were conducted from September 2007 to May 2008. During this period 17.1% of 652 documented intra-OMU sexual encounters were characterized either by mild or aggressive forms of harassment. Sexual harassment was typically performed by a single individual (91.9%), and in 75.7% of cases the harasser was an adult or sub-adult female. The frequency of female harassment was positively correlated with the number of adult and sub-adult females residing in an OMU, and resulted in a significant decrease in matings ending in ejaculation. We found that the amount of SI a female received was not a significant predictor of her reproductive success. However, females who conceived during the mating season directed higher levels of harassment at other females than females who did not conceive. We evaluate the strength of the sexual competition hypothesis and the hormonally modulated aggression hypothesis in explaining patterns of SI in female Golden snub-nosed monkeys.

Sexual dimorphism in primate aerobic capacity: a phylogenetic test

Male intrasexual competition should favour increased male physical prowess. This should in turn result in greater aerobic capacity in males than in females (i.e. sexual dimorphism) and a correlation between sexual dimorphism in aerobic capacity and the strength of sexual selection among species. However, physiological scaling laws predict that aerobic capacity should be lower per unit body mass in larger than in smaller animals, potentially reducing or reversing the sex difference and its association with measures of sexual selection. We used measures of haematocrit and red blood cell (RBC) counts from 45 species of primates to test four predictions related to sexual selection and body mass: (i) on average, males should have higher aerobic capacity than females, (ii) aerobic capacity should be higher in adult than juvenile males, (iii) aerobic capacity should increase with increasing sexual selection, but also that (iv) measures of aerobic capacity should co-vary negatively with body mass. For the first two predictions, we used a phylogenetic paired t-test developed for this study. We found support for predictions (i) and (ii). For prediction (iii), however, we found a negative correlation between the degree of sexual selection and aerobic capacity, which was opposite to our prediction. Prediction (iv) was generally supported. We also investigated whether substrate use, basal metabolic rate and agility influenced physiological measures of oxygen transport, but we found only weak evidence for a correlation between RBC count and agility.

Genetic Patterns of Paternity and Testes Size in Mammals

BackgroundTestes size is used as a proxy of male intrasexual competition, with larger testes indicative of greater competition. It has been shown that in some taxa, social mating systems reflect variance in testes size, but results are not consistent, and instead it has been suggested that genetic patterns of mating may reflect testes size. However, there are different measures of genetic patterns of mating. Multiple paternity rates are the most widely used measure but are limited to species that produce multi-offspring litters, so, at least for group living species, other measures such as loss of paternity to males outside the social group (extra group paternity) or the proportion of offspring sired by the dominant male (alpha paternity) might be appropriate. This study examines the relationship between testes size and three genetic patterns of mating: multiple paternity, extragroup paternity and alpha paternity.Methodology/Principal FindingsUsing data from mammals, phylogenetically corrected general linear models demonstrate that both multiple paternity and alpha paternity, but not extra group paternity, relate to testes size. Testes size is greater in species with high multiple paternity rates, whereas the converse is found for alpha paternity. Additionally, length of mating season, ovulation mode and litter size significantly influenced testes size in one model.Conclusions/SignificanceThese results demonstrate that patterns of mating (multiple paternity and alpha paternity rates) determined by genetic analysis can provide reliable indicators of male postcopulatory intrasexual competition (testes size), and that other variables (length of mating season, ovulation mode, litter size) may also be important.

Body Mass, Territory Size, and Life-History Tactics in a Socially Monogamous Canid, the Red FoxVulpes vulpes

Male-biased sexual size dimorphism is typical of polygynous mammals, where the degree of dimorphism in body mass is related to male intrasexual competition and the degree of polygyny. However, the importance of body mass in monogamous mammals is largely unknown. We investigated the effect of body mass on life-history parameters and territory size in the red fox (Vulpes vulpes), a socially monogamous canid with slight sexual dimorphism. Increased body size in males appeared to confer an advantage in territory acquisition and defense contests because heavier males held larger territories and exerted a greater boundary pressure on smaller neighbors. Heavier male foxes invested more effort in searching for extrapair matings by moving over a wider area and farther from their territories, leading to greater reproductive success. Males that sired cubs outside their own social group appeared to be heavier than males that only sired cubs within their social group or that were cuckolded, but our results should be treated with caution because sample sizes were small. Territory size, boundary pressure, and paternity success were not related to age of males. In comparison, body mass of females was not related to territory size, probability of breeding, litter size, or cub mass. Only age affected probability of breeding in females: younger females reproduced significantly less than did older females, although we did not measure individual nutritional status. Thus, body mass had a significant effect on life-history traits and territory size in a socially monogamous species comparable to that reported in polygynous males, even in the absence of large size dimorphism.