Male-biased Operational Sex Ratio Research Articles

Positive association of queen number and queen-mating frequency Myrmica ants: a challenge to the genetic-variability hypotheses

Detailed knowledge of the mating system in specific social insect populations is essential for testing general evolutionary hypotheses of multiple paternity in eusocial Hymenoptera. We have studied the mating frequency of queens in a polygynous population of the red ant Myrmica sulcinodis. Genetic mother-offspring analysis showed that double mating occurred at a considerable frequency, but that the effective number of queen-mates remained close to one. After quantifying the effects of multiple maternity (polygyny) and multiple paternity (polyandry) on the genetic diversity of workers, we conclude that multiple paternity in M. sulcinodis did not evolve as an adaptation to increase genetic variation within colonies. Contrary to the predictions from `genetic variability' hypotheses, we found a positive correlation between colony-specific queen number and the average number of mates per queen. Such positive association of queen number and frequency of multiple mating was also found after analysing comparative data across six species of Myrmica ants. These results suggest that resticted dispersal of young queens may be a common factor promoting both polygyny and polyandry at the same time, and that moderate degrees of multiple mating may be an unselected consequence of (1) mating at low cost when mating occurs close to the nest and (2) mating in swarms with a highly male biased operational sex ratio. Future comparative tests of genetic-variability hypotheses should therefore not include species with such evolutionary derived mating system characteristics.

Mating ecology of the nonpollinating fig wasps ofFicus ingens

The males of many fig wasps fight fatally for mating opportunities. The concentration of females in space has been proposed as one factor selecting for their aggressive behaviour. We studied the temporal distribution of receptive females to obtain a clearer impression of the operational sex ratio in figs. Females of nonpollinating species emerged from figs over a protracted period of time and this resulted in an extremely male-biased operational sex ratio, conducive to extreme fighting. Since there were so few receptive females at any one time, a male could defend an eclosing female. Consequently, the largest,Otitesella longicaudamale in a fig had a much higher mating success than smaller males. This suggests that larger males have a larger fitness advantage than larger females and a Trivers–Willard effect could have important implications for sex allocation. Apterous and seemingly nondispersing males routinely left their figs. Such dispersal can affect both (1) sex allocation by reducing the degree of local mate competition between brothers and (2) male dimorphism by reducing the mating opportunities of males with a dispersing morphology. We show that the wingless digitata males of theOtitesella digitataspecies group disperse on to leaves close to their natal fig. An extremely male-biased sex ratio resulted in almost allO. longicaudafemales being mated. These findings suggest that the classical concept of the fig wasp mating system is too simplistic and that important assumptions of sex allocation models are violated.

Temperature modulates competitive behaviour: why sand goby males fight more in warmer water

An operational sex ratio is defined as the ratio of males to females ready to mate. Temperature has been shown to affect patterns of mating competition over the breeding season, as ambient water temperature differentially influences the potential reproductive rates of male and female sand gobies, Pomatoschistus minutus (Pallas), creating an unbiased operational sex ratio in colder water, and a male biased operational sex ratio in warmer water. Here, the direct effect of water temperature on frequencies of inter- and intrasexual interactions was studied in a short (24 hr) time perspective in aquaria with an unbiased adult sex ratio. Males were found to interact significantly more with other males, and also tended to interact more with females, in a warm (16 °C) water treatment, as compared with a cold (8 °C) treatment, even though there was no general increase in total number of interactions in the warm water treatment. Female-initiated interactions were not significantly affected by temperature. An altered operational sex ratio is unlikely to account for the observed difference in male-male interactions between treatments, for two reasons: (1) the operational sex ratio, which was estimated after the last behavioural recording in each replicate, was slightly, but equally, male biased in both treatments; (2) the operational sex ratio of each replicate was not correlated with the number of male-male interactions recorded from that replicate. Instead, these results suggest that temperature may influence the intensity of male mating competition directly, and I propose that individual fish use water temperature as a proximate cue for estimating and forecasting operational sex ratios.

Mate Selection by Male Winter Moths Operophtera Brumata (Lepidoptera, Geometrldae): Adaptive Male Choice or Female Control?

Abstract Mate choice is an important component of sexual selection. It is expected to evolve if the benefits of choice outweigh the costs. Yet, the relative importance of costs and benefits in the evolution of mate choice remain poorly understood. In this study we present experimental evidence for adaptive mate choice by males, but not females, in the winter moth. In a series of experiments we show that: (a) males have a higher probability of attempting to copulate, and consequently also a higher probability to copulate with a larger, more fecund female; (b) if males are given the choice between two females they are more likely to copulate with the larger female; and (c) females do not seem to show any mate discrimination. A sample of winter moths collected in copula in the field did not show any assortative mating for body size. This is the first demonstration of male choice in a moth species with chemical communication. This choice is possibly based on variation in female pheromone quality and/or quantity. We argue that the relatively higher variation in female quality and the limited number of male matings probably have led to male choosiness despite a strongly male biased operational sex ratio. This is consistent with recent studies indicating that choice and competition may occur more frequently in the same sex than previously thought.

Mating Behavior and Reproductive Potential in the Turnip Moth Agrotis segetum (Lepidoptera: Noctuidae)

We investigated the lifetime mating potential and the reproductive behavior of male and female turnip moths Agrotis segetum (Schiff.) under field and laboratory conditions. The sex ratio was 1 : 1 in a lab-reared population as well as in two wild populations. Males were capable of mating repetitively a relatively large number of times (mean of 6.7 ± 2.7 matings) when given access to new virgin females throughout their lifetimes. Females seldom mated more than once (mean ± 1.3 ± 0.6 matings), indicating a male-biased operational sex ratio. The mean potential lifetime mating was five times higher in males, while the coefficient of variance was lower in males. There was no differences in longevity between animals that were allowed to mate and animals not allowed to mate, indicating no direct costs or benefits of mating in physiological terms. In males, the number of matings was positively correlated with longevity, but this was not the case in females. Nor was there a correlation between the number of female matings and the number of fertilized eggs. There was a negative correlation between the number of eggs fertilized and the number of times males had previously mated, indicating that male ejaculates were limited. Male spermatophore size also decreased with number of achieved matings. Laboratory-reared females attracted males in the field throughout their lifetimes, with a peak at 3–7 days of age. Wild males, allowed to choose between pairs of caged females in the field, were attracted in equal numbers to females of different ages. Females did not show any mate-rejection behavior in the field. They mated with the first male that courted them. No incidence of mate replacement by males arriving later to already courted females were recorded.

Competition for Mates in the Mole Salamander, Ambystoma Talpoideum: Tactics That May Maximize Male Mating Success

The relatively brief mating periods that are typical of mole salamanders (Amphibia: Caudata: Ambystomatidae) have surely selected for males that are efficient in locating and mating with females in the presence of high densities of competitors. We analyzed quantitatively the competitive mating tactics of the mole salamander, Ambystoma talpoideum, in two laboratory studies. First, we observed competitive interactions in encounters staged between two males and a single female. Such interactions were frequent, with males scrambling to interrupt one another's courtship efforts in an apparent attempt to 'steal' inseminations (and, we assume, fertilizations). The competitive behaviour patterns exhibited by male A. talpoideum appear typical of many other ambystomatid salamanders. We then conducted an experiment to determine the influence of a male-biased operational sex ratio on male mating effort (comparing the performance of target males placed with females alone and in the presence of three potential rivals). Contrary to our expectation, target males decreased their mating effort when density was high and the operational sex ratio was male-biased; many simply abandoned any attempt to court and mate. We hypothesize that decreased male mating effort under a currently male-biased operational sex ratio will be favoured if there exist alternative opportunities for relatively competition-free encounters with females. We suggest a method of testing this hypothesis using comparative data.

Amphisexual parental behavior of a terrestrial breeding frog Eleutherodactylus johnstonei in Guyana

Parental care in Elmtherodactytus johwtonsi, a terrestrially breeding frog, was uniparentaL but care was provided by either sex. I tested Maynard Smith's evohitionarily stable strategy, continuous breeding model HI, by ^ramining social and ecological conditions under which die sexes provided parental care in Georgetown, Guyana, South America. During the long wet season most males chosen by females as mates provided parental care. No males that acquired females through satellite behavior and displacement competition provided care. Females provided parental care in most cases when males failed to do so, and some returned to nests for 72 h, even after being repeatedly driven away by attending males. Females initiated maternal care up to 72 h after ovipoarion following temporary removal of males. Male-biased operational sex ratios were signtfirandy different for wet and dry months, and no females provided care when monthly rainfall was <200 mm. Additionally, operational sex ratio multiply regressed on paternal and maternal care and rainfall indicated that paternal and maternal care were significantly correlated with operational sex ratio. Rainfall was not related to operational sex ratio. During the dryer months only males provided care. Males provided less care during wetter months, when more gravid females were available as mates. Significant mortality of eggs and embryos occurred in clutches with no parental care, and females and males did not differ in the ability to protect progeny. Although there was support for Maynard Smith's model of evolution of amphisexual care from biparental care, it was difficult to ascertain dearly whether amphisexual parental behavior evolved from either biparental behavior or no care. This uncertainty ensues because the behavior of females and males and development of progeny may have become coadapted after the initial evolution of care in E. johnstontL

Reproductive strategies of Atlantic salmon: ecology and evolution

Atlantic salmon (Salmo solar, Salmonidae) show a diversity of life history, behavioural and morphological adaptations for reproduction which have evolved as an outcome of competition to maximize reproductive success. Reproductive traits of females have been shaped principally by natural selection for offspring production and survival, those of males by sexual selection for access to matings. Female Atlantic salmon invest approximately six times more energy in offspring production (i.e. gonads) than males and face an important trade-off between number and size of eggs to produce that will maximize the number of surviving offspring. Timing of breeding and the construction of nests appear adapted to increase offspring survival. The most important determinant of female breeding success is body size because it affords high fecundity, access to breeding territories and decreased probability of nest destruction. Asynchronous female spawning and the male ability to spawn rapidly and repeatedly results in male-biased operational sex ratios that generate intense male competition for mates. This has likely been responsible for the evolution of elaborate male secondary sexual characters associated with fighting and status signalling. Furthermore, it has given rise, through frequency-dependent selection, to two alternative male breeding phenotypes: (1) large, anadromous males; and (2) small, mature male parr. Anadromous males invest heavily in behavioural activity on the spawning grounds, searching and fighting for mates and courting them, with body size being an important determinant of their breeding success. This behavioural activity carries a heavy cost, as anadromous males have significantly reduced survival relative to females. In contrast, mature male parr invest proportionally more in testes for sperm competition and attempt to ‘sneak’ access to matings. While this behaviour also carries costs in terms of subsequent growth and survival, male parr are more likely to breed again, either prior to or following a migration to sea, than anadromous males. While knowledge about the breeding of Atlantic salmon is detailed, we are only beginning to understand the ultimate causes and/or functional significances of their reproductive strategies. Predictive models of the life history variation are developing, focusing on the need for empirical study and testing of life history and reproductive patterns.

Female-biased mortality induced by male sexual harassment in a feral sheep population

In contrast to most populations of sexually dimorphic ungulates, a 3-year study revealed a tertiary sex ratio (the number of males per female at sexual maturity) strongly biased towards males in the Île Longue feral sheep (Ovis aries) population (Kerguelen subantarctic archipelago). This population presents a main winter lambing season and a secondary summer lambing period. The sex ratios of lambs and lamb carcasses were even. The skewed sex ratio of adults was therefore attributed to shorter longevity of females. We showed that (i) male monthly mortality followed a bimodal distribution with peaks corresponding to the two rutting periods. Male mortality was, however, highest during the winter even though less rutting occurred in this season than in the summer, suggesting that harsher winter conditions enhance the cost of mating competition; (ii) female mortality was related to the rutting activity of males but not to lambing. Female mortality peaked in the summer despite plentiful food resources. This period corresponds to the main rutting peak, suggesting a high survival cost of mating for females. Female mortality due to male harassment was confirmed by direct observations and examination of females injured during rutting periods. This situation may be due to a strongly male-biased operational sex ratio in the population, associated with an inability of males to establish a stable dominance hierarchy within a large flock.

Covariation in life-history traits, demographics and behaviour in ischnuran damselflies: the evolution of monandry

Abstract The apparent erratic variation in life history traits, coloration patterns, and behaviours that exists among species within the damselfly genus Ischnura is shown to be interpretable when the species are partitioned into three groups. One group consists of species whose males are missing a pair of stout basal spines on the penultimate segment of their accessory penes. These are the only ischnurans in which males, by tandem guarding females, prevent sperm displacement. The other two groups can be recognized by the relative frequency with which mating occurs: monandrous species mate infrequently, polyandrous species more often. Compared to polyandrous species, monandrous species contain smaller size individuals, have greater sexual size dimorphism, have shorter duration copulations, do not have male biased operational sex ratios at aquatic sites, and are more likely to contain monochromatic females. Females belonging to the monandrous species tend to develop a characteristic form of pruinescence at maturity that obscures their underlying colour, and mature at a younger age. We propose that copulation serves only for sperm addition in monandrous species, for both sperm addition and displacement in polyandrous tandem guarding ischnurans, and for contact guarding as well as sperm addition and displacement in polyandrous species that do not tandem guard.

Mate acceptance and guarding by male fiddler crabs Uca tetragonon (Herbst)

The fiddler crab, Uca tetragonon (Herbst) mates both on the surface near female-defended burrows and underground in male-defended burrows. In underground mating, which requires relatively high male investment, males attract both non-ovigerous and ovigerous females into their burrows by claw waving. Males aggressively expel some females soon after they enter their burrows and others after pair formation, but before females spawn. Finally males guard some females in their burrows until they spawn, which presumably ensures paternity. Males do not select mates of a particular body size, but they do differentially accept females with late-stage eggs, those about to release larvae and spawn another clutch. Except at the beginning of the reproductive season, few ripe non-ovigerous females are available because females spawn successively and only in moderate synchrony. By differentially accepting late stage ovigerous females, males may increase their fertilization rates because they minimize the time they spend guarding each of their mates to ensure their paternity. A male-biased operational sex ratio and a high last male advantage in sperm competition are two conditions that may have favored male choice based on female guarding time in this species.

Sex ratios, mating behavior and sexual size dimorphism of the northern water snake, Nerodia sipedon

Competition among males to mate is generally associated with male-biased size dimorphism. In this study we examine mating behavior in the northern water snake (Nerodia sipedon), a species in which males are much smaller than females despite substantial competition among males to mate. Competition among males was a consequence of a male-biased operational sex ratio due to slightly higher female mortality from a birth sex ratio of 1 : 1, and, in 1 year, more synchronous and longer mating activity by males. Approximately one-third of both males and females appeared not to mate in a given year. Larger males were generally more likely to attempt mating, but size did not explain the variance in the number of aggregations in which individual males participated. Within aggregations, males that were successful at achieving intromission were larger than unsuccessful males in 1 of 2 years. Variation in condition (mass relative to length) and relative tail length were not generally useful predictors of either mating effort or success in males. Because large size was often advantageous to males, sexual size dimorphism appeared not to be a consequence of sexual selection favoring smaller males. Because sexual dimorphism was evident at birth, and both males and females matured sexually at about 4 years, sexual dimorphism was not simply a consequence of one sex growing at the maximum rate for longer. Female fecundity increased with size, and sex differences in size-fecundity relations may underly the pattern of sexual size dimorphism. However, because multiple mating by females is common, sperm competition is likely to be important in determining male reproductive success. Therefore, allocation of energy to sperm rather than growth may also prove to be an important influence on male growth rates and sexual size dimorphism.

Intersexual conflict over precopula duration in mate guarding crustacea

Precopulatory guarding in Crustacea is usually analyzed as a male decision problem. We suggest an alternative possibility that precopula is established as a result of intersexual conflict over precopula duration. Such a conflict can be expected when the male optimum for precopula duration exceeds the female optimum. As a result, males should start precopulatory attempts earlier, while females should resist until close to receptivity. Our analysis reveals two potential sources of conflict: (1) sexual differences in survival probabilities before and during the mate-guarding; and (2) sexual differences in the probability of finding a mate. The latter is perhaps a more probable source of intersexual conflict, since male biased operational sex ratios are common in mate-guarding Crustacea. The former requires that female moulting cycle is synchronous, whereas the latter may operate in populations with asynchronous moulting cycles as well. We further studied the expected intensity of behavioural conflicts in terms of expected present and future fitness gains. In the beginning of the female moulting cycle, there is no conflict. Conflict arises as males start the guarding attempts and females are motivated to resist, and ceases with a decrease in the female's motivation to resist. Several assumptions and predictions of the model are discussed and compared with the behavioural patterns observed in the aquatic isopod Idotea baltica.

Mating Tactics in the Tropical Millipede Alloporus Uncinatus (Diplopoda: Spirostreptidae)

AbstractField and laboratory observations of mating behaviour in a population of the tropical millipede Alloporus uncinatus were carried out over one breeding season. Males obtained mates through random encounters and by forming triplet associations with copula pairs. The occurrence of triplet associations in the field was coincident with a highly male biased operational sex ratio. Mate acquisition by males was apparently stochastic and direct physical competition did not occur. In laboratory experiments mating was size-selective probably as a consequence of female choice. We consider the possibility that sperm competition has contributed to the evolution of the mating system in this species.

Breeding behaviour of the African frog, Chiromantis xerampelina: multiple spawning and polyandry

Breeding was observed in the foam-nest frog, Chiromantis xerampelina, in Hoedspruit, South Africa. The population had a 3-week breeding season with a male-biased operational sex ratio and asynchronous and unpredictable female arrival. Nests were built above water and at 14 out of 15 nests, from one to seven unpaired males (‘peripheral males’) gathered around the amplexing pair during nest construction. Those peripheral males closest to the pair competed with each other, and with the amplexing male, to position their cloacae aginst the female's cloaca during oviposition. It is suggested that multiple spawning was occurring. Of 30 males, 25 (83%) were observed as peripheral males, and 17 (56·7%) were observed both in amplexus and as peripheral males. Male mating success and participation at nests was unrelated to size or weight. Chorus participation was the best predictor of mating success and presence at nests. Nests were built in two to four sessions with females descending to the water between sessions. On 55% of 20 occasions amplexing males dismounted between sessions, but they had only an 11% probability of re-amplexing when the female returned to the nest. Of 15 females, 10 (66%) mated polyandrously with two or more males on one night.

The Behavioral Ecology of Libellula luctuosa (Burmeister) (Odonata: Libellulidae)

AbstractIn a pond dragonfly, Libellula luctuosa, the number of males at the mating site increased linearly during mating seasons. The number of fertilizable females in the population did not increase. An increasingly male biased operational sex ratio (OSR) therefore correlated with the increasing density of males. I predicted that this would cause increased male‐male competition and alter female mate choice.Competition for territories influenced mating success in L. luctuosa. Two different mating strategies were adopted by males of L. luctuosa: some flew and defended lengths of shoreline (territorial males), and some perched and avoided male‐male interactions (satellite males). Satellite males increased later in the season when all territories were occupied. However, no satellite ever switched to the territorial strategy and no territorial male ever became a satellite. At high male density, two males occasionally defended the same territory. The occupation of territories was nonrandom. The probability of any male mating depended on the time of year. Territory owners did most of the mating; satellite males rarely mated. Postmating guarding of females by males depended on the number of conspecific males present.Mating success of males was also influenced by female mating behavior, and female mating behavior was influenced by the number of males present. Females visited several males prior to mating. When fewer males were present females were more likely to reject male mating attempts. The length of copulation did not change during the mating season. At high male density, females spent less time ovipositing immediately after mating. Postcopulatory mate guarding by males increased the length of time the female spent ovipositing. These behavioral patterns may represent different strategies of mate choice, precopulatory mate choice at low male density and postcopulatory mate choice at high male density.

Fighting for females in the toad Bufo calamita is affected by the operational sex ratio

Fighting between male natterjack toads, Bufo calamita, for access to females was studied in relation to both the influence of male and female density each night and the operational sex ratio. Fighting males were larger than other males and were present in the breeding area on more nights but their average mating success was the same. Fighting between males was associated with not strongly male-biased operational sex ratios. The density of breeding individuals, however, was irrelevant. Take-over rate was very low with an insignificant effect on mating success.