Copulatory Plug Displacement Evidences Sperm Competition in Lemur catta

Male displacement of copulatory (sperm) plugs from female vaginas provides further evidence for sperm competition in ring-tailed lemurs (Lemur catta), a gregarious prosimian species with a multimale, multifemale mating system. During two mating seasons, I studied two groups of free-ranging ring-tailed lemurs on St. Catherines Island, GA, USA. I observed 22 mating pairs in which males achieved penile intromission. Copulatory plug displacement by males occurred in 9 cases. Plugs were displaced during copulation by male penes upon withdrawl following deep vaginal thrusting. In every case of copulatory plug displacement, the male displacing a plug mated to ejaculation with the estrous female. In a mating system in which females typically mate with more than one male during estrous, often in succession, copulatory plug displacement may function to disrupt or preclude other males' successful insemination of estrous females. The effects of sperm plug displacement on paternity in Lemur catta are unknown, as no study had heretofore documented copulatory plug displacement in this species. The first-male mating advantage suggested for Lemur catta should be re-evaluated where mating order is known, and copulatory plug displacement during mating, or lack thereof, is identified. Because there is a tendency for first-mating males to mate-guard for longer periods of time in Lemur catta, the latency period between the first mate's ejaculation and that of subsequent mates may be an important determinant of male fertilization success.     

Sperm Competition and Optimal Timing of Matings in Microcebus murinus

Female promiscuity is common in mammals and leads to sperm competition: the sperm of ≥2 males compete for ova. Scientists understand the possible role of optimal insemination periods for male reproductive success in many species as well as the impact of monopolization of receptive females. Information from experiments combined with detailed observations from the field that allow determining the relative impacts of the elements in the same species are rare. We studied sperm competition and the role of optimal insemination periods in gray mouse lemurs (Microcebus murinus), a small solitary nocturnal primate from Madagascar. We used controlled matings to identify the relative impact of both contest and scramble competition, which characterize their mating system, on paternity. Fifteen females mated with 3–6 males in quick succession. Our experiments revealed that the optimal insemination period is during early receptivity. Early but not first mating males are more likely to sire offspring. Comparison with our field data indicate that the timing of male monopolization efforts correspond with the optimal insemination period.     

Low Numbers of Sperm Retained in the Spermatheca May Explain the High Values of Sperm Precedence in the Migratory Locust, Locusta migratoria (Latr.)

Four hypotheses about the temporal variation of the number of spermatozoa in the spermatheca of once-mated females were tested in the migratory locust, Locusta migratoria. The best fit provided a regression model that assumed a sudden drop in sperm numbers being indicative of sperm ejection by females. Thereafter, one-fifth of an average ejaculate is stored. Low numbers of sperm stored do not lead to female sperm limitation, as none of the fitness parameters measured, fertilization success, hatching success, and offspring sex ratio, were correlated with the number of sperm present. Fertilization success decreased with successive egg pods. The offspring sex ratio was slightly male biased and tended to increase throughout the laying period. By applying our sperm ejection model to an independent data set of Parker and Smith (1975), we show that the low numbers of sperm retained by the female and subsequent sperm mixing rather than direct replacement may explain the high P₂ values found in this species.     

Possible Female Choice During Copulation in Ozophora baranowskii (Heteroptera: Lygaeidae): Female Behavior, Multiple Copulations, and Sperm Transfer

The remarkable behavior of female Ozophora baranowskii of lightly tapping the male with the hind legs during copulation was studied in relation to multiple copulations, copulation length, and sperm transfer. Multiple copulations did not affect female fecundity, fertility, or longevity. The incidence of tapping was higher in shorter copulations, which did not result in sperm transfer to the female. Female tapping behavior may be a mechanism for choosing between males after copulation has begun.     

Evolution of delayed reproductive senescence in male fruit flies: Sperm competition

Populations ofDrosophila melanogaster that had been subjected to long-term selection favoring either delayed or rapid senescence were compared with respect to age-specific components of male reproductive success involving sperm competition. These components of reproductive success were divided into those related to sperm defense (protection of sperm from other males), and into those related to sperm offense (ability to mate with previously mated females and to displace the sperm of other males). Males were tested at four ages ranging from 1–2d to 5–6 wk after eclosion. Several aspects of sperm defense capability showed clear evidence of senescent decline. Furthermore, males from populations selected for delayed senescence were superior to males from control (rapid senescence) populations with regard to components of sperm defense. The superiority of males from populations with delayed senescence either increased as a function of male age, or was present at all ages tested. These results indicate that the rate of reproductive senescence in maleD. melanogaster can be altered in predictable directions by artificial selection. There were no differences between selection regimes with regard to sperm offense, and most components of sperm offense did not show clear evidence of senescence. The improved late-age reproductive success of males from populations selected for delayed senescence did not appear to entail any cost or trade-off at early ages with respect to the reproductive traits examined in these experiments.     

Adaptive control of copulation duration by males under sperm competition in the mite,Macrocheles muscaedomesticae

In a manure-inhabiting predatory mite, Macrocheles muscaedomesticae (Gamasida, Macrochelidae), when the female mates with two males, the first male takes nearly perfect fertilization priority (Yasui, 1988). The present study examined whether the first-male's sperm precedence is influenced by the copula-duration of the first and second males mating with the same female, and whether males control their copulation duration by assessing the probability that the mate has been inseminated by other males. Results of the artificial interruption of copulation showed that sperm precedence value, P₂ (the proportion of the offspring fathered by the second male), was negatively correlated with the copulation duration of the first male but positively correlated with that of the second male. There was a threshold (ca. 180–300 seconds) in the first-male's copulation duration beyond which P₂ decreased drastically; when length of the first copulation exceeded this threshold, the second males did not fertilize eggs, whereas they fertilized more than half of the eggs when the first-copulation duration was shorter than the threshold. Almost all males copulated for a longer period (average 509.8 seconds) than this threshold if the copulation duration of the previous male had not exceeded the threshold, but if it was longer than the threshold, second males had shortened their copulation (67.6 seconds). These results suggest that males are able to assess the insemination status of their mates and to adjust their copulation duration depending on the probability of fertilizing eggs by their own sperm. A mechanistic explanation for sperm precedence (i.e., plug-formation within sperm receptive organ of the females) is proposed.     

The Effect of Male Competition on the Courtship Song of Drosophila melanogaster

The courtship song of Drosophila has been extensively used as a model system for studies of sexual selection and species recognition. Traditionally, the courtship song has been recorded from males placed individually with a female. However, under natural conditions females are exposed to multiple courting males, and the effect of their joint signal on mate recognition by the female is not yet understood. Here, we recorded the courtship behavior of D. melanogaster males singing either individually to a female lpar;1:1) or in the presence of an additional male lpar;2:1). We compared the structure of the male song in the two experimental designs. Our results show that when two males courted a female their songs could overlap each other. Males produced a significantly different signal in the presence of competition; the duration of each song component was significantly shorter and the rate of singing was markedly lower. The present study demonstrates that male competition can dramatically alter the acoustic signal detected by the female.     

Sperm displacement behavior of the cuttlefish Sepia esculenta (Cephalopoda: Sepiidae)

Sperm displacement behavior of cuttlefish (Sepia esculenta) was observed in a tank. Before ejaculation, male cuttlefish used their arms III to scrape out sperm masses attached to the buccal membranes of females. The removed sperm mass debris was directly visible and countable. Active sperm were present within the removed sperm debris, implying that the aim of this behavior is to remove competing male sperm. However, many sperm masses remained on the female buccal membrane even after the removal behavior, showing that sperm removal in S. esculenta is incomplete. The duration of sperm removal (an indicator of male investment in that process) was unaffected by the body sizes of mated pair, the duration of spermatangia placement at the current mating (for the hypothesis that the sperm removal serves to creat attachment space of spermatophores), or the estimated amount of sperm masses deposited from previous matings. Moreover, male S. esculenta performed sperm removal regardless of whether the last male to mate with the partner was himself, suggesting males remove not only the sperm of rivals but also their own. Although the number of removed sperm masses increased with the time spent on removal of sperm, male cuttlefish may shorten the duration of sperm removal to avoid the risk of mating interruption. We conclude that this time restriction would likely influence the degree of partial sperm removal in S. esculenta. A digital video image relating to the article is available at .This revised version was published online in April 2005 with corrections in the abstract.     

Behavior of ergatoid males in the ant,Cardiocondyla nuda

Summary Ergatoid males of the ant,Cardiocondyla nuda, attack and frequently kill young males during or shortly after eclosion. Smaller colonies therefore contain typically only one adult male, which may inseminate all alate queens which are reared in the colony over a few weeks. In larger colonies, several males may be present, however, fighting among adult males was not observed. We discuss the significance of male fighting behavior in ants.     

Copulation behavior within one-male groups of wild <Emphasis Type="Italic">Rhinopithecus roxellana</Emphasis> in the Qinling Mountains of China

This study was conducted using focal animal sampling on the west ridge group of the Sichuan snub-nosed monkey (Rhinopithecus roxellana) located in the Zhouzhi National Nature Reserve on the north slope of the Qinling Mountains, from 8 July 2003 to 24 May 2004. The difference in the average frequency of copulations for each focal male for each month was significant (F = 3.068, P = 0.016, one-way ANOVA test) with the majority of copulations occurring between September and November. Duration of intromission ranged from 2 to 39 s, with a mean of 16.0 ± 0.4 s. Females initiated 627 courtship attempts (96.2%), while males only initiated 3.8%. Both adult females (72.8%) and sub-adult females (27.2%) were involved in sexual interference acts. Females who gave birth in 2004 performed more sexual interference acts than would be expected by chance in the reproductive period of 2003 (X ² = 13.73 > X ² _0.005,2, df = 2, P < 0.005). Male response to female interference was equally divided into “solicitor mounting” and “interrupter mounting”. The resident males of one-male units were not observed to mount both the solicitor and the interrupter or mount neither following female solicitation interruptions. Three post-conception copulations were also observed in this study. These results suggest a skewed sexual competition, with multiple females competing for a single male, which was shown by courtship attempts and female interference.     

The Effects of Competition on Growth and Biomass Allocation in Nymphoides peltata (Gmel.) O. Kuntze Growing in Microcosm

Two experiments were designed to investigate the effects of competition on growth and biomass allocation in Nymphoides peltata. First, competition between N. peltata and Zizania latifolia was assigned with the densities of N. peltata to Z. latifolia ratios of 4:0, 4:2, 4:4 and 4:8. The increase of density of Z. latifolia resulted in apparent decrease of total biomass, relative growth rate (RGR), leaf area ratio (LAR) and mean leaf area per plant of N. peltata. N. peltata allocated above-ground biomass to shoots and roots and decreased the ratios of above-ground to below-ground biomass (A_b/B_b) with increasing density of Z. latifolia. Second, competitions between N. peltata and emerged Z. latifolia, floating-leavedTrapa bispinosa and submerged Myriophyllum spicatum were studied in the mean time. Total biomass, A_b/B_b and mean leaf area per plant of N. peltata were higher when competing with floating-leaved T. bispinosa than in N. peltata growing in the community with submerged M. spicatum and emerged Z. latifolia. There were no significant differences in RGR, net assimilation rate (NAR) and LAR of N. peltata when growing with each of the competitor species. Our studies indicate that the growth of N. peltata is strongly inhibited by the presence of Z. latifolia, and N. peltata can show certain competitive advantages over T. bispinosa and M. spicatum.     

The Combined Effects of Pre- and Post-Insemination Sexual Selection on Extreme Variation in Male Body Size

Orb-weaving spiders of the genus Nephila are notable for their sexual size dimorphism, with dwarf males and giant females. However, less well known is the extreme size polymorphism of males that is characteristic of some species. For example, adult male body size in N. edulis varies by an order of magnitude. Previous experiments reveal that male mating behaviour covaries with body size, suggesting the size variation is maintained by opposing pre- and post-insemination sexual selection pressures. Here, we test this idea by allowing males of different sizes to compete directly and simultaneously for access to females. Using the sterile-male technique for paternity assessment, we show that two competing males drawn from the extremes of size variations, split paternity the same way as two males of the same size drawn from the intermediate sizes. The paternity of a large male dropped from 50% to 30% on average if he competed against two instead of one small male. The large male increased his mating frequency when there were more rivals but required a much lower total duration of copulation to achieve the same paternity share. These data are consistent with the idea that opposing pre- and post-insemination selection pressures at least partly explain the variation in male body size. Co-ordinating editor: L.D. Hurst     

Male Parasitic Spawning in Two Species of Triplefin Blenny (Tripterigiidae): Contrasts in Demography, Behaviour and Gonadal Characteristics

Axoclinus nigricaudus and A. carminalis are blennioid fishes from the Gulf of California, Mexico. Males show alternative mating tactics; territorial individuals attract females while non-territorial individuals parasitically spawn with territorial males' females. Demography and gonadal characteristics were related to the degree of parasitic spawning (sperm competition) within and between species. Males of both species showed bimodal size distributions that largely reflected the two tactics. Territorial males spawned much more frequently than non-territorial males, but parasitic spawning was significant. Non-territorial males were more common and rates of parasitic spawning were much higher in A. nigricaudus indicating that it experienced higher sperm competition. In agreement with sperm competition theory, relative testis weight was greater in A. nigricaudus. In A. nigricaudus, the majority of parasitic matings were by non-territorial males, whereas in A. carminalis, both male-types parasitised frequently. Accordingly, only in A. nigricaudus did non-territorial males have relatively heavier testis than territorial males. In both species the testicular gland, that produces accessory products for sperm transfer, was large in territorial males, but small or undeveloped in non-territorial males suggesting male-types differ in fertilisation mode.     

The adaptive significance of mate guarding in the soapberry bug,Jadera Haematoloma (Hemiptera: Rhopalidae)

Male soapberry bugs (Jadera haematoloma)face severe mating competition at the northern edge of their range due to male-biased adult sex ratios. Copulations lasting up to 11 days may serve a mate guarding function (encompassing four or more ovipositions), but copulation duration is highly variable, with some pairings lasting as little as 10 min. Data were gathered to describe factors that influence the reproductive costs and benefits of prolonged copulation. Estimated copulation durations (mean ± SD) were 20 ± 23 h in the lab and 50 ± 8 h in the field and were only weakly affected by sex ratio. Females mated for 5 min produced as many fertile eggs as those mated for 600 min laid; they became depleted of fertile sperm after about 25 days. In twicemated females, the first male's paternity was reduced by about 60%, and all females (N = 13) whose mates were removed experimentally mated again within an average of 6 min. The outcome of sperm competition on a perclutch basis was not highly predictable. The possibility of increased sperm displacement in longer copulations was not tested. Males often guarded females during oviposition and successfully defended them from intruding single males by recopulating. Such intrusions occurred in the majority of oviposition attempts observed in nature. Even though most females mated promiscuously, in a focal aggregation with a mean sex ratio of 2.2 ± 0.4 males/female, the interval between matings by males was commonly several days. Males appeared to respond facultatively to several aspects of the distribution and availability of females. The intensities of mating competition and sperm competition indicate that monogamous mate guarding should be favored over nonguarding in nature. Unpredicted brief. pairings may result from assessment by males of female reproductive value or of their own physical condition, or from female resistance.