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.     

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.     

Butterflies tailor their ejaculate in response to sperm competition risk and intensity

Males of many insects eclose with their entire lifetime sperm supply and have to allocate their ejaculates at mating prudently. In polyandrous species, ejaculates of rival males overlap, creating sperm competition. Recent models suggest that males should increase their ejaculate expenditure when experiencing a high risk of sperm competition. Ejaculate expenditure is also predicted to vary in relation to sperm competition intensity. During high intensity, where several ejaculates compete for fertilization of the female''s eggs, ejaculate expenditure is expected to be reduced. This is because there are diminishing returns of providing more sperm. Additionally, sperm numbers will depend on males'' ability to assess female mating status. We investigate ejaculate allocation in the polyandrous small white butterfly Pieris rapae (Lepidoptera). Males have previously been found to ejaculate more sperm on their second mating when experiencing increased risk of sperm competition. Here we show that males also adjust the number of sperm ejaculated in relation to direct sperm competition. Mated males provide more sperm to females previously mated with mated males (i.e. when competing with many sperm) than to females previously mated to virgin males (competing with few sperm). Virgin males, on the other hand, do not adjust their ejaculate in relation to female mating history, but provide heavier females with more sperm. Although virgin males induce longer non-receptive periods in females than mated males, heavier females remate sooner. Virgin males may be responding to the higher risk of sperm competition by providing more sperm to heavier females. It is clear from this study that males are sensitive to factors affecting sperm competition risk, tailoring their ejaculates as predicted by recent theoretical models.     

Description of alternative male reproductive tactics in a shell-brooding cichlid, Telmatochromis vittatus, in Lake Tanganyika

Telmatochromis vittatus (Cichlidae) is a Tanganyikan substrate brooder which spawns in the gastropod-shell nests of a cichlid, Lamprologus callipterus. We describe male reproductive tactics of T. vittatus in and around the shell nests, where males of various sizes were found. Based on utilization patterns of the shell nests, interactions among males, and spawning behaviors, males could be categorized into four types based on reproductive tactics and in order of body size: sneaker males, satellite males, territorial males and piracy males. Size range of males in tactic groups rarely overlapped. Territorial males defended shell nests harboring multiple females, but during pair-spawning they were occasionally taken over by large piracy males that visited several nests repeatedly. Small sneaker males darted to pair-spawning territorial males and might ejaculate sperm. Satellite males did not perform parasitic spawning but pair-spawned in a single shell outside the nests. Spawning of satellite males was infrequently parasitized. The largest gonado-somatic index (GSI) was found in sneaker males followed by piracy males, territorial males and satellite males, suggesting that gonadal investment of males using the four tactics may be consistent with intensity or risk 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.     

Cannibalism and protective behavior of eggs in Arctic charr (Salvelinus alpinus)

From video recordings of spawning events, we quantified protective and cannibalistic behavior of Arctic charr occurring immediately after spawning. The number of fish cannibalizing on stray eggs was examined regarding (a) whether more than one male shed milt during the spawning event, that is, whether sperm competition occurred, (b) whether the sperm competition included few or many males, that is, the intensity of sperm competition, and (c) the density of fish at the spawning site. Response behavior toward egg cannibalism was also examined among females and dominant males in order to determine any parental investment toward protecting the eggs after spawning. Cannibalistic behavior was seen in almost 50% of the spawnings, and the multiple spawning events showed the highest numbers of fish cannibalizing on eggs. Both the number of males releasing milt and the number of fish approaching the spawning site were positively correlated with egg cannibalism. Sperm competition was, however, not a prerequisite for egg cannibalism. Although we also observed partial filial cannibalism, protective behavior of eggs was seen both among dominant males and females, suggesting that charr actually conduct parental care.     

In Vitro Fertilization Reveals Offspring Recognition via Self-Referencing in a Fish with Paternal Care and Cuckoldry

Male bluegill sunfish (Lepomis macrochirus) exhibit alternative life histories: some males (parentals) delay maturation for up to 7 yr, then build nests, court females, and care for the eggs and fry, whereas other males (cuckolders) mature precociously, attempt to steal fertilizations from parentals, and provide no parental care. Parental males could avoid misdirecting their nepotism (i.e. caring for unrelated young) by abandoning entire broods if they were sired mainly by cuckolders or by discriminating between offspring and non‐kin fry within broods for which they care. We tested for kin discrimination by obtaining sperm from parental and cuckolder males and eggs from several females, and using them to conceive fry in vitro. In ‘blind’ laboratory tests, parental males (but not cuckolder males) distinguished between sources of dripping water that had been conditioned by their own offspring vs. unrelated fry. Parental males that were in the best physical condition were especially choosy. Because the only referents available to our experimental subjects were chemical cues emanating from their own body, our results imply that parental males can use self‐referent phenotype matching for kin recognition. This mechanism enables males to make the adaptive, nepotistic adjustments in paternal care that have been documented in previous studies.     

Genetic evidence for cuckoldry in bluegill Lepomis macrochirus

In colonies of bluegill Lepomis macrochirus , some males provide parental care for broods in nests, whereas other males steal fertilizations and do not provide parental care. Using experimental pond populations of bluegill of known genotype (determined through protein electrophoresis), we demonstrate that cuckolder males successfully fertilize eggs in parental male nests. Using electrophoretic techniques to assess the fertilization success of nesting parental male bluegill in Lake Opinicon, Ontario, we demonstrate that paternity of these males ranges from 41% to 100% among four colonies studied. This difference among colonies is related to the density of cuckolder males.     

Body size of virtual rivals affects ejaculate size in sticklebacks

Sperm competition occurs when sperm of two or more males competeto fertilize a given set of eggs. Theories on sperm competitionexpect males under high risk of sperm competition to increaseejaculate size. Here we confirm this prediction experimentallyin the three-spined stickleback (Gasterosteus aculeatus). Inthis species, sneaking (i.e., stealing of fertilizations byneighboring males) can lead to sperm competition. Sneaking malesinvade foreign nests, and the owners vigorously try to preventthis intrusion. In such fights, male body size is assumed tobe an important predictor of success. Consequently, the riskof sperm competition may depend on the size of a potential competitor.We experimentally confronted males before spawning with eithera large or a small computer-animated rival. We show that malesejaculated significantly more sperm after the presentation ofthe larger virtual rival than after the small stimulus. In addition,the time between the initiation of courting and the spawningwas shorter in the large virtual male treatment. The resultssuggest that stickleback males tailor ejaculate size relativeto the risk of sperm competition perceived by the size of apotential competitor.     

Releasing small ejaculates slowly increases per‐gamete fertilization success in an external fertilizer: Galeolaria caespitosa (Polychaeta: Serpulidae)

The idea that male reproductive strategies evolve primarily in response to sperm competition is almost axiomatic in evolutionary biology. However, externally fertilizing species, especially broadcast spawners, represent a large and taxonomically diverse group that have long challenged predictions from sperm competition theory—broadcast spawning males often release sperm slowly, with weak resource‐dependent allocation to ejaculates despite massive investment in gonads. One possible explanation for these counter‐intuitive patterns is that male broadcast spawners experience strong natural selection from the external environment during sperm dispersal. Using a manipulative experiment, we examine how male reproductive success in the absence of sperm competition varies with ejaculate size and rate of sperm release, in the broadcast spawning marine invertebrate Galeolaria caespitosa (Polychaeta: Serpulidae). We find that the benefits of Fast or Slow sperm release depend strongly on ejaculate size, but also that the per‐gamete fertilization rate decreases precipitously with ejaculate size. Overall, these results suggest that, if males can facultatively adjust ejaculate size, they should slowly release small amounts of sperm. Recent theory for broadcast spawners predicts that sperm competition can also select for Slow release rates. Taken together, our results and theory suggest that selection often favours Slow ejaculate release rates whether males experience sperm competition or not.     

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.     

Sperm competition, mating rate and the evolution of testis and ejaculate sizes: a population model

There are various ways to estimate ejaculate expenditure. Ejaculate size or sperm number (s) is an absolute number of units of ejaculate. Relative ejaculate expenditure (E) is the expenditure on the ejaculate as the proportion of the total expenditure on all aspects of the mating, including finding and acquiring a female, and so on. Relative testis size or gonadosomatic index (sigma) is testes mass divided by body mass; it is assumed to reflect the product of mating rate (M) and ejaculate mass (s). In a new model, where mating rate, sperm competition and sperm allocation interact, and where the female's inter-clutch interval is assumed to be independent of s or M, we show that sigma is directly proportional to the mean E for a species; across species sigma and E increase monotonically with sperm competition. However, the relation between s and sperm competition across species depends on the range of sperm competition (low risk or high intensity): s increases with sperm competition at low risk levels, but decreases with sperm competition at high intensity levels. This situation arises because s alpha E/M; both E and M increase with sperm competition, but E increases differently with sperm competition in its two ranges.     

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.     

Brainless but not clueless: earthworms boost their ejaculates when they detect fecund non-virgin partners

In many animals in which females store sperm, males may detect female mating status and, in order to outcompete rival sperm, increase ejaculate size when copulating with non-virgin females. Although most studies have been restricted to organisms with separate sexes, theoretical models suggest that sperm competition should also be an important selective agent shaping life-history traits in simultaneous hermaphrodites. Nevertheless, the empirical support for ejaculate adjustment in a mating opportunity is scarce in hermaphrodites. In the present study, we performed a double-mating experiment to determine whether earthworms (Eisenia andrei) detect the mating status of their partners and whether they respond by adjusting their ejaculate. We found that earthworms triplicated the donated sperm when mating with a non-virgin mate. Moreover, such increases were greater when the worms were mated with larger (more fecund) partners, indicating that earthworms perform a fine-tune control of ejaculate volume. The results of the present study suggest that, under high intensity of sperm competition, partner evaluation is subject to intense selection in hermaphrodite animals, and donors are selective about to whom they donate how much sperm.     

Alternative male mating tactics in Gobius niger

In the field, parental males of the black goby Gobius niger were always >9 cm L _T, showed a conspicuous elongation of the first dorsal fin, and were darkly coloured. Larger parental males did not occupy larger nests or obtain more eggs, suggesting that over a threshold size reproductive success was not correlated with male size. The mating system was polygynous with different egg clutches simultaneously present in a nest. Smaller sexually mature males were observed around and inside nests in which spawning was occurring. These opportunistic males ranged in body size between 6 to 8 cm L _T. In contrast to parental males, they did not show an exaggeration of the first dorsal fin and were cryptically coloured, thus resembling small females in their external morphology. Aquarium experiments confirmed that smaller males perform a sneaking behaviour, releasing sperm when parental males spawn. As a consequence, it is possible to define three male types: type I are small, without an elongated dorsal fin and adopt a sneaking mating tactic; type III are large, have a pronounced elongation of the dorsal fin and perform parental behaviour; type II are intermediate both in size and elongation of the first dorsal fin and behave as either as sneakers or, possibly, as parental males according to nest availability and male competition.     

The impact of ovulation mode on sperm quantity and quality in mammals

Ovulation generally takes two forms: spontaneous and induced. These two ovulatory modes are predicted to cause different levels of male–male competition due to differences in males’ ability to predict the timing of ovulation of females that use each mode. Ovulation mode has been shown to be correlated with differences in testis size, but it is not known whether ovulation type influences ejaculate traits, specifically sperm quantity (sperm concentration, ejaculate volume, total motile sperm per ejaculate) and/or sperm quality (% sperm motility, % normal sperm). Using a cross-species comparative analysis of 130 mammals, we found that sperm concentration and total motile sperm per ejaculate were significantly greater in spontaneous ovulators. In contrast, ejaculate volume, % sperm motility and % normal sperm were unrelated to ovulation type. These data show that ovulation alters sperm quantity by increasing sperm concentration but not ejaculate volume, and that sperm quality does not vary with ovulation mode.     

Sex-specific spawning behavior and its consequences in an external fertilizer

Identifying the target of sexual selection in externally fertilizing taxa has been problematic because species in these taxa often lack sexual dimorphism. However, these species often show sex differences in spawning behavior; males spawn before females. I investigated the consequences of spawning order and time intervals between male and female spawning in two field experiments. The first involved releasing one female sea urchin's eggs and one or two males' sperm in discrete puffs from syringes; the second involved inducing males to spawn at different intervals in situ within a population of spawning females. In both, fertilization success was measured as the fraction of eggs fertilized and the paternity share of each male. The results indicate that spawning after females imposes a cost on males but only during sperm competition. Further, the optimal interval between the initiations of male and female spawning depends on degree of sperm competition, distance between males and females, and water velocity. The results show that sex differences in spawning timing of marine invertebrates can be explained on the basis of the differential costs and benefits of spawning out of synchrony with the other sex and that the result of sexual selection on external fertilizers may be behavioral rather than morphological differentiation of the sexes.     

Male-derived cuticular hydrocarbons signal sperm competition intensity and affect ejaculate expenditure in crickets

Female sexual promiscuity can have significant effects on male mating decisions because it increases the intensity of competition between ejaculates for fertilization. Because sperm production is costly, males that can detect multiple matings by females and allocate sperm strategically will have an obvious fitness advantage. The presence of rival males is widely recognized as a cue used by males to assess sperm competition. However, for species in which males neither congregate around nor guard females, other more cryptic cues might be involved. Here, we demonstrate unprecedented levels of sperm competition assessment by males, which is mediated via the use of chemical cues. Using the cricket Teleogryllus oceanicus, we manipulated male perception of sperm competition by experimentally coating live unmated females with cuticular compounds extracted from males. We found that males adjusted their ejaculate allocation in response to these compounds: the viability of sperm contained within a male's ejaculate decreased as the number of male extracts applied to his virgin female partner was increased. We further show that males do not respond to the relative concentration of male compounds present on females, but rather to the number of distinct signature odours of individual males. Our results conform to sperm competition theory, and show for the first time, to our knowledge, that males can detect different intensities of sperm competition by using distinct chemical cues of individual males present on females.     

Selection on female remating interval is influenced by male sperm competition strategies and ejaculate characteristics

Female remating rate dictates the level of sperm competition in a population, and extensive research has focused on how sperm competition generates selection on male ejaculate allocation. Yet the way ejaculate allocation strategies in turn generate selection on female remating rates, which ultimately influence levels of sperm competition, has received much less consideration despite increasing evidence that both mating itself and ejaculate traits affect multiple components of female fitness. Here, we develop theory to examine how the effects of mating on female fertility, fecundity and mortality interact to generate selection on female remating rate. When males produce more fertile ejaculates, females are selected to mate less frequently, thus decreasing levels of sperm competition. This could in turn favour decreased male ejaculate allocation, which could subsequently lead to higher female remating. When remating simultaneously increases female fecundity and mortality, females are selected to mate more frequently, thus exacerbating sperm competition and favouring male traits that convey a competitive advantage even when harmful to female survival. While intuitive when considered separately, these predictions demonstrate the potential for complex coevolutionary dynamics between male ejaculate expenditure and female remating rate, and the correlated evolution of multiple male and female reproductive traits affecting mating, fertility and fecundity.     

Steroid hormones in bluegill, a species with male alternative reproductive tactics including female mimicry

The proximate mechanisms underlying the evolution and maintenance of within-sex variation in mating behaviour are still poorly understood. Species characterized by alternative reproductive tactics provide ideal opportunities to investigate such mechanisms. Bluegill (Lepomis macrochirus) are noteworthy in this regard because they exhibit two distinct cuckolder (parasitic) morphs (called sneaker and satellite) in addition to the parental males that court females. Here we confirm previous findings that spawning cuckolder and parental males have significantly different levels of testosterone and 11-ketotestosterone. We also report, for the first time, that oestradiol and cortisol levels are higher in cuckolders than in parental males. The two cuckolder morphs did not differ in average levels of any of the four hormones. However, among satellite males which mimic females in appearance and behaviour, there was a strong negative relationship between oestradiol levels and body length, a surrogate for age. This finding suggests that for satellite males, oestradiol dependency of mating behaviour decreases with increasing mating experience. Although such decreased hormone dependence of mating behaviour has been reported in other taxa, our data represent the first suggestion of the relationship in fishes.