Sperm maturation and male tactic-specific differences in ejaculates in the plainfin midshipman fish Porichthys notatus

Using the plainfin midshipman fish Porichthys notatus, a species with alternative reproductive tactics (ARTs), we investigated how sperm maturation shapes sperm competitive abilities. We compared sperm performance and morphology before and after final sperm maturation by sampling sperm from the testes and stripped ejaculates of guarders and sneakers. In accordance with sperm competition risk theory, ejaculates from sneaker males had three times as much sperm as ejaculates from guarder males and sneaker males produced faster swimming sperm than guarder males, but this was only the case after final sperm maturation had occurred. Additionally, fully mature sperm found in ejaculates had larger heads and midpieces than sperm found in the testes. These results emphasize the important role played by non-sperm components of an ejaculate in mediating sperm performance and potentially also morphology.     

The relative costs and benefits of territorial defense and the two conditional male mating tactics in the Comanche Springs pupfish (Cyprinodon elegans)

We examined the aggressive costs and reproductive benefits of territorial defense and its alternatives in a population of the Comanche Springs pupfish (Cyprinodon elegans). The breeding system was characterized by three different male mating tactics: territorial defense, satellite positioning, and sneak spawning. The mating tactic adopted by males reflected the males' sizes. Territorial residents were the largest, satellites were medium-sized, and sneakers were the smallest adult males observed. Consistent with the hypothesis that primary mating tactics are relatively high-cost, high-benefit strategies, we found that territorial males engaged in a number of aggressive encounters but had higher reproductive successes than any other males. However, our observations did not support the premise that conditionally breeding males engage in relatively low-benefit, low-cost tactics. Medium satellites and small sneakers acquired fewer spawns than did territorial males, but both satellites and sneakers were involved in as much aggression as territorials. That is, the data supported the prediction that satellite or sneaker males with the inability to compete for territories would attempt to accrue some reproductive opportunities in the presence of territorial males despite the high costs of spending time on the breeding grounds. Adopting conditional tactics appeared to allow satellites and sneakers to make the best of a bad situation. During a subsequent breeding season, large males were absent from the population, and medium-sized males established territories. The aggressive behavior exhibited by medium residents was similar to the previous year, but these males did not acquire higher reproductive successes than medium satellites had. The implications of switching tactics and the influence of operational sex ratios on the costs and benefits of the male tactics are briefly addressed. Electronic Publication     

Dimorphic ejaculates and sperm release strategies associated with alternative mating behaviors in the squid

Sperm competition is a powerful postcopulatory selective force influencing male adaptations associated with increasing fertilization success, and it is usually related to the evolution of different strategies of ejaculate expenditure between individuals. Ejaculates may also be influenced by additional selective pressures associated with sperm competition, such as timing between insemination and fertilization, female reproductive tract morphology, and fertilization environment. Also, males that adopt alternative mating tactics may face distinct sperm competition pressures, which may lead to the evolution of intraspecific diversity in ejaculates. In loliginid squids, males with alternative reproductive tactics (sneakers and consorts) differ not only in mating behavior, but also transfer spermatophores into two distinct sites within the female. Here, we compared structure and functioning of spermatophores between sneakers and consorts in the squid Doryteuthis plei applying microscopy techniques and in vitro experiments. Sneakers and consorts exhibit differences in spermatophore structure that lead to distinct spermatophoric reactions and spermatangium morphologies. Moreover, in sneakers, sperm release lasts longer and their sperm show an aggregative behavior not detected in consorts. Slow sperm release may be a strategy to guarantee longer sperm provision, given the wide interval between sneaker mating and egg release. For consorts, in turn, intense and quick sperm discharge may be advantageous, as timing between mating and egg‐laying is relatively short. Within the complex squid mating system, factors such as (i) different fertilization sites and (ii) interval between mating and egg release may also influence sperm competition, and ultimately shape the evolution of divergent ejaculates between dimorphic males.     

Male sperm expenditure under sperm competition risk and intensity in quacking frogs

In the frog Crinia georgiana, reproductive behavior comprisesa "guarding tactic," in which males defend spawn sites and attractfemales by calling, and a "sneak tactic," in which males joinspawning pairs. The aims of the present study were to (1) relateejaculate expenditure by "guarding" and "sneak" males to theirprobability of mating with other males present (sperm-competitionrisk), and (2) determine if males adjust their ejaculate expenditureaccording to the number of males involved in a spawning (sperm-competitionintensity). Theory predicts that because sneak males alwaysmate with other males present, they will experience a highersperm-competition risk and should release larger ejaculatesrelative to that of guarding males. However, as the proportionof sneaks in a population increases so does the risk of spermcompetition to guarders, so expenditure by each tactic shouldmove toward equality. Given that the incidence of sneak behavioris high in C. georgiana, guarders and sneaks were expected toexperience similar risks of sperm competition and show similarinvestment in spermatogenesis. Comparison of testes size andejaculate size showed no difference between tactics. Modelsof sperm-competition intensity predict that males should increasetheir ejaculate size when spawning in the presence of one othermale but decrease their ejaculate size when spawning in thepresence of multiple males. Here, males maintained a constantsperm number irrespective of whether a mating involved one,two, or three males. This result suggests that male C. georgianado not facultatively adjust ejaculate investment in responseto fluctuating intensities of sperm competition.     

Sperm competition games between sneaks and guards: a comparative analysis using dimorphic male beetles

Sperm competition is widely recognized as a pervasive force of sexual selection. Theory predicts that across species increased risk of sperm competition should favor an increased expenditure on the ejaculate, a prediction for which there is much evidence. Sperm competition games have also been developed specifically for systems in which males adopt the alternative male mating tactics of sneaking copulations or guarding females. These models have not yet been tested in a comparative context, but predict that: across species male expenditure on the ejaculate should increase with increasing probability of a sneak mating; within species, sneaks should have the greater expenditure on the ejaculate; and the disparity in expenditure between sneaks and guards should be greatest in species with moderate risk of a sneak mating, and decline toward parity in species with low or high risk. Beetles in the genus Onthophagus are often characterized by dimorphic male morphologies that reflect the alternative mating tactics of sneak (minor males) and guard (major males). We conducted a comparative analysis across 16 species of male dimorphic onthophagines, finding that testes size increased across species with increasing frequency of the minor male phenotype. Minor males generally had the greater testes size, but across species the disparity between morphs was independent of the frequency of minor males. We present data on testes allometry from two populations of O. taurus that have undergone genetic divergence in the frequency of minor males. Consistent with the comparative analysis, these data support the notion that the relative frequency of sneaks in the population influences male expenditure on the ejaculate.     

Male traits associated with alternative reproductive tactics in Gobius niger

Male black goby Gobius niger , adopting parental or sneaking tactics, differed in secondary sex traits (elongation of the 4th ray of the first dorsal fin and black nuptial colouration) thus allowing the classification of wild-caught males. Parental males were larger and older than sneaker males, suggesting that the mating tactic is an expression of an ontogenetic gradient. Males adopting alternative tactics differed also in primary sex traits, including their testes and their two pairs of accessory structures: the seminal vesicles and mesorchial glands. Sneaker males had a higher investment in testes, while parental males showed larger seminal vesicles and more developed mesorchial glands. Histological analyses also showed that seminal vesicles from parental males presented some functional differences from those of sneakers. In the former these organs were devoted solely to mucin secretion, while in the latter they stored sperm and had a lower activity of secretion. Seminal vesicle features influenced ejaculate (sperm trail) characteristics and performance. Parental male trails were richer in mucins, but poorer in sperm than trails deposited by sneakers. As a consequence, while sneakers produced trails that released a large amount of immediately active sperm, trails laid by parental males released less sperm more constantly over a long time.     

Sperm competition in a fish with external fertilization: the contribution of sperm number, speed and length

The role of sperm number and quality in male competitiveness was investigated using in vitro fertilization experiments with bluegill (Lepomis macrochirus). Bluegill males use one of three mating tactics: 'sneakers', which streak spawn; 'satellites', which mimic females; and 'parentals', which are territorial. The in vitro experiments mimicked natural spawning by incorporating these males' mean proximity to eggs and timing of sperm release. Using a maximum-likelihood algorithm, raffle equations were fit to paternity data, which revealed a strong effect of sperm number on male competitiveness. There was no difference in sperm flagellum length, curvilinear swim speed or path linearity among the three male mating types, and these traits did not explain any additional variation in male competitiveness. It was estimated that, given closer proximity to eggs, satellites need release only 0.34 times as many sperm as parentals to obtain equal paternity. Despite being farther from the eggs and releasing sperm about half a second after parentals, sneakers need only release 0.58 times as many sperm as parentals to obtain equal paternity. Thus, the increased competitiveness of sneakers' sperm must come from a component of sperm quality other than speed or length.     

Sperm competition roles and ejaculate investment in a promiscuous mammal

Theoretical models of sperm competition predict how males should allocate sperm and seminal fluid components to ejaculates according to their mating role (dominant vs. subordinate). Here, we present a detailed analysis of ejaculate expenditure according to male roles in the bank vole (Myodes glareolus). Sperm competition occurs regularly in this species, and dominant males typically achieve higher fertilization success than subordinates. Contrary to theoretical predictions, we found that dominant male bank voles invest more sperm per ejaculate than subordinates, both absolutely and relative to body and testes mass. The testes of dominant males were also absolutely (although not relatively) larger than those of subordinates. However, we found no evidence that subordinate males compensate for lower sperm numbers per ejaculate by increasing ejaculation frequency or sperm velocity. Similarly, we found no evidence for differential investment in copulatory plug size according to male roles in sperm competition, although dominant males had significantly larger seminal vesicles (both absolutely and relative to body mass) compared with subordinates. We conclude that sperm competition roles can have significant but unexpected influences on ejaculate investment in mammals with clearly defined differences in male social status.     

Ejaculate of sneaker males is pheromonally inconspicuous in the black goby,Gobius niger (Teleostei,Gobiidae)

The black goby, Gobius niger, shows alternative male mating tactics, i.e., parental and sneaker males. Males release a sexual pheromone that attracts females and stimulates aggressive displays in males. This pheromone is produced by the mesorchial gland, a structure well developed in parental males but markedly undeveloped in sneakers. We measured the behavioral response of parental males to the ejaculates of males performing different reproductive tactics. Parental males reacted to the ejaculate of other parental males, with stereotypic aggressive behaviors, but not to the ejaculate of sneakers; consequently sneaker male ejaculate appears to be pheromonally inconspicuous.     

Sperm competition games played by dimorphic male beetles

Reproductive strategies often consist of two alternative tactics whereby males either compete for and guard females, or sneak copulations. By their nature, alternative tactics expose males to differing risks of sperm competition; sneaks will always be subject to sperm competition but guards will be subject to sperm competition with low probability, dependent on the number of sneaks. Recent game-theoretical models predict that males in the sneak role should have the greater gametic expenditure but that the disparity in expenditure should decrease with increasing numbers of sneaks. Male dung beetles in the genus Onthophagus can be separated into two morphs: major males have horns and guard females whereas minor males are hornless and sneak copulations. Here we compare testis size and ejaculate characteristics between these alternative morphs. We find that in O. binodis 30% of males are sneaks, and sneaks have larger testes, ejaculate volumes, and longer sperm than guards. In O. taurus 60% of males are sneaks and there are no differences in gametic traits. Our data thus provide empirical support for game-theoretical models of sperm competition.     

Ejaculate depletion patterns evolve in response to experimental manipulation of sex ratio in Drosophila melanogaster

We assessed the extent to which traits related to ejaculate investment have evolved in lines of Drosophila melanogaster that had an evolutionary history of maintenance at biased sex ratios. Measures of ejaculate investment were made in males that had been maintained at male-biased (MB) and female-biased (FB) adult sex ratios, in which levels of sperm competition were high and low, respectively. Theory predicts that when the risk of sperm competition is high and mating opportunities are rare (as they are for males in the MB populations), males should increase investment in their few matings. We therefore predicted that males from the MB lines would (1) exhibit increased investment in their first mating opportunities and (2) deplete their ejaculates at a faster rate when mating multiply, in comparison to FB males. To investigate these predictions we measured the single mating productivity of males from three replicates each of MB and FB lines mated to five wild-type virgin females in succession. In contrast to the first prediction, there was no evidence for differences in productivity between MB and FB line males in their first matings. The second prediction was upheld: mates of MB and FB males suffered increasingly reduced productivity with successive matings, but the decline was significantly more pronounced for MB than for FB males. There was a significant reduction in the size of the accessory glands and testes of males from the MB and FB regimes after five successive matings. However, the accessory glands, but not testes, of MB males became depleted at a significantly faster rate than those of FB males. The results show that male reproductive traits evolved in response to the level of sperm competition and suggest that the ability to maintain fertility over successive matings is associated with the rate of ejaculate, and particularly accessory gland, depletion.     

Sperm competition within a dominance hierarchy: investment in social status vs. investment in ejaculates

There is increasing recognition that male-male competition can take many forms, but as yet the form is not predictable a priori. Many recent studies have focused attention on how males in disadvantaged mating roles compensate through sperm competition. However, mating systems in which subordinate males are reproductively suppressed, particularly through the stress of social interactions, may limit the ability of males to respond by increasing investment in sperm quality. We examined the interaction between social status and ejaculate tactics in Nauphoeta cinerea, a cockroach that has a mating system with well-characterized dominance hierarchies. Both social experience with other males and social status influenced aspects of ejaculates. The stress of social interactions reduced the size of the ejaculate and number of sperm inseminated. In ejaculates formed prior to social experience, however, males that go on to become dominant inseminated more sperm than males that go on to become subordinate, suggesting innate differences among males. Our results show that though selection for increased success in sperm competition for subordinate males in a hierarchy can occur, both the traits and the way in which the balance between pre- and post-copulatory strategies is negotiated will depend on specific details of the mating system. These details will include how the physiological effects of social interactions may limit selection through male-male competition.     

Male meadow voles respond differently to risk and intensity of sperm competition

There are 2 models of male adjustment of sperm investment inthe ejaculate in relation to sperm competition. The "risk model"predicts that as "risk" of sperm competition increases, sperminvestment also increases. This prediction has been supportedin many species, including mammals. The "intensity model" involvesthe number of competing males copulating with the same femaleand predicts that males will allocate the highest sperm investmentat low sperm competition intensity (SCI) and then decreasingsperm investments as SCI increases. Two alternative outcomesare that sperm investment is unaffected by SCI and that sperminvestment increases as SCI increases. There are studies supportingall 3 possible outcomes in relation to SCI but no data on mammals.The present paper presents the first study of SCI in a mammalspecies, the meadow vole, Microtus pennsylvanicus. We used odorsof conspecific males to simulate low and high intensities ofsperm competition. We found that males allocate the highestsperm investment at low SCI and decrease significantly theirsperm investment at high SCI. We also found that males allocatethe lowest sperm investment at low sperm competition risk (SCR)and the highest sperm investment at high SCR. All these resultsagree with current theoretical models of sperm competition.     

Territorial male gobies respond aggressively to sneakers but do not adjust their sperm expenditure

We investigated whether mating behavior (sperm expenditure,courtship rate, and nest guarding) varied according to differentlevels of sperm competition in territorial males of two gobyspecies, the grass and the black gobies. We measured sperm expenditure(sperm released after 30 min from the beginning of the spawning),male courtship rate, and nest-guarding behavior in territorialmales of both species during simulated spawnings, in which wevaried the number of attending sneakers. Our results showedthat, in both species, territorial males adjusted their effortin nest guarding to the presence of rival sneakers by increasingthe time spent patrolling the territory and attacking the sneakers.In contrast, sperm expenditure and male courtship rate werenot influenced by the number of attending sneakers. These resultsare in agreement with those reported for other fish with alternativemating tactics and help to interpret previous inconsistenciesbetween theoretical predictions and measured levels of spermreleased at different levels of sperm competition by sneakersof the two gobiids studied here.     

Male crickets adjust ejaculate quality with both risk and intensity of sperm competition

Sperm competition theory predicts that males should increase their expenditure on the ejaculate with increasing risk of sperm competition, but decrease their expenditure with increasing intensity. There is accumulating evidence for sperm competition theory, based on examinations of testes size and/or the numbers of sperm ejaculated. However, recent studies suggest that ejaculate quality can also be subject to selection by sperm competition. We used experimental manipulations of the risk and intensity of sperm competition in the cricket, Teleogryllus oceanicus. We found that males produced ejaculates with a greater percentage of live sperm when they had encountered a rival male prior to mating. However, when mating with a female that presented a high intensity of sperm competition, males did not respond to risk, but produced ejaculates with a reduced percentage of live sperm. Our data suggest that males exhibit a fine-tuned hierarchy of responses to these cues of sperm competition.     

Body Size Correlates with Fertilization Success but not Gonad Size in Grass Goby Territorial Males

In fish species with alternative male mating tactics, sperm competition typically occurs when small males that are unsuccessful in direct contests steal fertilization opportunities from large dominant males. In the grass goby Zosterisessor ophiocephalus, large territorial males defend and court females from nest sites, while small sneaker males obtain matings by sneaking into nests. Parentage assignment of 688 eggs from 8 different nests sampled in the 2003–2004 breeding season revealed a high level of sperm competition. Fertilization success of territorial males was very high but in all nests sneakers also contributed to the progeny. In territorial males, fertilization success correlated positively with male body size. Gonadal investment was explored in a sample of 126 grass gobies collected during the period 1995–1996 in the same area (61 territorial males and 65 sneakers). Correlation between body weight and testis weight was positive and significant for sneaker males, while correlation was virtually equal to zero in territorial males. That body size in territorial males is correlated with fertilization success but not gonad size suggests that males allocate much more energy into growth and relatively little into sperm production once the needed size to become territorial is attained. The increased paternity of larger territorial males might be due to a more effective defense of the nest in comparison with smaller territorial males.     

Increased performance of offspring sired by parasitic males in bluegill sunfish

Males of many species are characterized by alternative matingtactics. In bluegill sunfish (Lepomis macrochirus), some malesdelay maturation and become "parentals" while other males matureprecociously and become "cuckolders." Parentals use an overt,territorial mating tactic, defending a nest and courting females.Cuckolders instead use a sneaking tactic to parasitize parentals.It has previously been shown that parentals that are heavilycuckolded provide less care to their young, yet females do notappear to discriminate against cuckolders, and they may actuallyrelease more eggs when a cuckolder is present than when spawningonly with a parental. Here I examined growth rate of fry ofknown paternity through the yolk-sac stage of development usingcomplementary laboratory and field studies to assess a potentialindirect benefit for females that mate with cuckolders. Comparisonof maternal half-siblings sired in vitro shows that cuckolderoffspring grow faster and to a larger size than parental offspringwhile feeding endogenously on their yolk sac. Because both foodresource and maternal genes are equivalent across treatments,these data indicate a genetic difference in growth between thetwo male life histories. In the field, fry from nests that haveproportionately more cuckolder offspring are larger when theyemerge from the nest. This increased size can lead to threefoldhigher survivorship for cuckolder offspring than parental offspringfrom Hydra canadensis predation, a major predator of bluegillfry. These results are discussed in the context of mate choicefor direct and indirect benefits and in the context of the evolutionof alternative mating tactics.     

Can minor males of Dawson's burrowing bee, Amegilla dawsoni (Hymenoptera: Anthophorini) compensate for reduced access to virgin females through sperm competition?

Dawson's burrowing bees (Amegilla dawsoni) exhibit a conditional mating strategy with two alternative tactics. Large (major)males exclusively patrol emergence sites in search of about-to-emergefemales, whereas small (minor) males usually search the peripheryof emergence sites for females that escape patrollers. About80% of the male population are minors, despite the fact thatpatrolling emergence sites is apparently the more profitablemating tactic. We tested the hypothesis that minor males gainfitness by mating with nonvirgin females and engaging in spermcompetition with rival ejaculates. If the sperm competitionhypothesis applied, it would help explain why nesting femalesproduce so many minor sons. Contrary to this hypothesis, however,we found that minor males do not exhibit traits frequentlyassociated with sperm competition. Minor and major males didnot differ in testis mass after controlling for body size.Neither did they differ significantly in the duration or patternof copulation nor in the volume of ejaculate transferred. Inaddition, and also contrary to the sperm competition hypothesis,females apparently mated only once. Loss of female sexual receptivityoccurred quickly after the onset of copulation, and nestingfemales appeared completely unreceptive. Thus, all aspectsof the bee's mating system strongly indicate that sperm competitiondoes not occur in Dawson's burrowing bee, so that minors cannotcompensate even partially via sperm competition for their mating disadvantage with virgin females.     

Sperm competition and male ejaculate investment in Nauphoeta cinerea: effects of social environment during development

Selective pressure arising from sperm competition has been predicted to influence evolutionary and behavioural adjustment of ejaculate investment, but also may influence developmental adjustment of ejaculate investment. Immature males able to target resources strategically based on the competitive environment they will experience when they become sexually mature should be at a selective advantage. In our study we investigated how the presence of potential competitors or mates affects ejaculate and testes investment during development in the cockroach Nauphoeta cinerea, a species where males control female remating via their ejaculate size (large spermatophores prevent females from remating and therefore function to avoid sperm competition for males) and females store sperm. Our aim was to determine whether the social environment influences developmental adjustment of ejaculate investment and the relative importance of ejaculate components with different functions; avoidance of or engagement in sperm competition. We conclude that the social environment can influence developmental and behavioural flexibility in specific ejaculate components that may function to avoid or engage in sperm competition.     

Influences of sex, size, and symmetry on ejaculate expenditure in a moth

Although sperm fundamentally function to fertilize eggs, forcesarising from both sexes select for optimal ejaculate composition.Sperm competition is one recognized agent in the evolution ofsperm and ejaculate structure. Few studies, however, have examinedhow female factors influence ejaculate structure, despite somebehavioral evidence for male mate choice. Male Plodia interpunctella(Lepidoptera, Pyralidae) accrue all resources for reproductionas larvae. Adults emerge with a limited sperm complement andare therefore under intense selection to optimize gamete allocation.I detected no effect of male body weight on ejaculate size.However, female reproductive potential (ovary masses) was dictatedby body weight In addition, heavier females had greater spermathecalvolumes, but there was no such relationship with bursal size.Finally, heavier females showed a higher mating frequency. Ifound that mating males were sensitive to female size and producedlarger ejaculates when mating with heavier females. Males mayejaculate more sperm into larger females either because it paysthem to "spend" more reproductive resources on matings thatprovide greater reproductive potential, or because heavier (longerlived and more attractive) females mate more frequently andhave larger spermathecal volumes. Alternatively, females maycontrol spermatophore formation and "accept" an appropriateejaculate to maximize fertility. Males may therefore be alsoselected to ejaculate more sperm into larger females to counteractgreater risks of sperm competition associated with heavier females.There was no association between male or female femur asymmetryand ejaculate size. P.interpunctella may be selected to exercisemodulation of ejaculate size because males invest paternally,sperm for the single reproductive episode are limited, and femalefecundity and mating pattern vary between individuals and areassociated with body weight. More obvious variability in malereproductive behavior and choice may therefore be paralleledat the cryptic gametic level by plasticity in ejaculate allocation.