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Sperm competition is a term used to refer to the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means an individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase males' chance of reproductive success. Sperm competition results in a sexual conflict of interest between males and females.〔 Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by physically removing another male's sperm prior to mating with a female. Sperm competition is often compared to having tickets in a raffle; a male has a better chance of winning (i.e. fathering offspring) the more tickets he has (i.e. the more sperm he inseminates a female with). However, sperm are not free to produce,〔Olsson et al., 1997; Wedell et al., 2002〕 and as such males are predicted to produce sperm of a size and number that will maximize their success in sperm competition. By making many spermatozoa, males can buy more "raffle tickets", and it is thought that selection for numerous sperm has contributed to the evolution of anisogamy with very small sperm (because of the energy trade-off between sperm size and number). Alternatively males may evolve faster sperm to enable their sperm to reach and fertilize the ovum first. Dozens of adaptations have been documented in males that help them succeed in sperm competition. ==Defensive adaptations== Mate-guarding is a defensive behavioral trait that occurs in response to sperm competition; males try to prevent other males from approaching the female (and/or vice versa) thus preventing their mate from engaging in further copulations.〔 Precopulatory and postcopulatory mate-guarding occurs in birds, lizards, insects and primates. Mate-guarding also exists in the fish species ''Neolamprologus pulcher'', as some males try to "sneak" matings with females in the territory of other males. In these instances the males guard their female by keeping them in close enough proximity so that if an opponent male shows up in his territory he will be able to fight off the rival male which will prevent the female from engaging in extra-pair copulation with the rival male. Organisms that are involved in polygynous mating systems are controlled by one dominant male. In this type of mating system, the male is able to mate with more than one female in a community. The dominant males will reign over the community until another suitor steps up and overthrows him.〔 The current dominant male will defend his title as the dominant male and he will also be defending the females he mates with and the offspring he sires. The elephant seal falls into this category since he can participate in bloody violent matches in order to protect his community and defend his title as the alpha male. If the alpha male is somehow overthrown by the new comer, his children will most likely be killed and the new alpha male will start over with the females in the group so that his lineage can be passed on. Strategic mate-guarding occurs when the male only guards the female during her fertile periods. This strategy can be more effective because it may allow the male to engage in both extra-pair paternity and within-pair paternity. This is also because it is energetically efficient for the male to guard his mate at this time. There is a lot of energy that is expended when a male is guarding his mate. For instance, in polygynous mate- guarding systems, the energetic costs of males is defending their title as alpha male of their community.〔 Fighting is very costly in regards to the amount of energy used to guard their mate. These bouts can happen more than once which takes a toll on the physical well- being of the male. Another cost of mate- guarding in this type of mating system is the potential increase of the spread of disease. If one male has an STD he can pass that to the females that he’s copulating with potentially resulting in a depletion of the harem. This would be an energetic cost towards both genders for the reason that instead of using the energy for reproduction, they are redirecting it towards ridding themselves of this illness. Some females also benefit from polygyny because extra pair copulations in females increase the genetic diversity with the community of that species. This occurs because the male is not able to watch over all of the females and some will become promiscuous. Eventually, the male will not have the proper nutrition which makes the male unable to produce sperm. For instance, male amphipods will deplete their reserves of glycogen and triglycerides only to have it replenished after the male is done guarding that mate. Also, if the amount of energy intake does not equal the energy expended, then this could be potentially fatal to the male. Males may even have to travel long distances during the breeding season in order to find a female which absolutely drain their energy supply. Studies were conducted to compare the cost of foraging of fish that migrate and animals that are residential. The studies concluded that fish that were residential had fuller stomachs containing higher quality of prey compared to their migrant counterparts With all of these energy costs that go along with guarding a mate, timing is crucial so that the male can use the minimal amount of energy. This is why it is more efficient for males to choose a mate during their fertile periods.〔 Also, males will be more likely to guard their mate when there is a high density of males in the proximity〔 Sometimes organisms put in all this time and planning into courting a mate in order to copulate and she may not even be interested. There is a risk of cuckholdry of some sort since a rival male can successfully court the female that the male originally courting her could not do. However, there are benefits that are associated with mate- guarding. In a mating- guarding system, both parties, male and female, are able to directly and indirectly benefit from this. For instance, females can indirectly benefit from being protected by a mate. The females can appreciate a decrease in predation and harassment from other males while being able to observe her male counterpart.〔 This will allow her to recognize particular traits that she finds ideal so that she’ll be able to find another male that emulates those qualities. In polygynous relationships, the dominant male of the community benefits because he has the best fertilization success.〔 Communities can include 30 up to 100 females and, compared to the other males, will greatly increase his chances of mating success.〔 Males who have successfully courted a potential mate will attempt to keep them out of sight of other males before copulation. One way organisms accomplish this is to move the female to a new location. Certain butterflies, after enticing the female, will pick her up and fly her away from the vicinity of potential males. In other insects, the males will release a pheromone in order to make their mate unattractive to other males or the pheromone masks her scent completely.〔 Certain crickets will participate in a loud courtship until the female accepts his gesture and then it suddenly becomes silent. Some insects, prior to mating, will assume tandem positions to their mate or position themselves in a way to prevent other males from attempting to mate with that female.〔 The male checkerspot butterfly has developed a clever method in order to attract and guard a mate. He will situate himself near an area that possesses valuable resources that the female needs. He will then drive away any males that come near and this will greatly increase his chances of copulation with any female that comes to that area. In post- copulatory mate- guarding males are trying to prevent other males from mating with the female that they have mated with already. For example, male millipedes in Costa Rica will ride on the back of their mate letting the other males know that she’s taken. Japanese beetles will assume a tandem position to the female after copulation. This can last up to several hours allowing him to ward off any rival males giving his sperm a high chance to fertilize that female’s egg. These, and other, types of methods have the male playing defense by protecting his mate. Elephant seals are known to engage in bloody battles in order to retain their title has dominant male so that they are able to mate with all the females in their community.〔 Copulatory plugs are frequently observed in insects, reptiles, some mammals, and spiders.〔 Copulatory plugs are inserted immediately after a male copulates with a female, which reduce the possibility of fertilization by subsequent copulations from another male, by physically blocking the transfer of sperm.〔 The Indian Meal Moth (''Plodia interpunctella'') are restricted in engaging in further mating activities because the spermatacore serves as a copulatory plug immediately after copulation.〔 Bumblebee mating plugs, in addition to providing a physical barrier to further copulations, contain linoleic acid, which reduces re-mating tendencies of females. Similarly, ''Drosophila melanogaster'' males release toxic seminal fluids, known as ACPs (accessory gland proteins), from their accessory glands to impede the female from participating in future copulations. These substances act as an anti-aphrodisiac causing a dejection of subsequent copulations, and also stimulate ovulation and oogenesis.〔 Seminal proteins can have a strong influence on reproduction, sufficient to manipulate female behavior and physiology.〔 〕 Another strategy, known as sperm partitioning, occurs when males conserve their limited supply of sperm by reducing the quantity of sperm ejected.〔 In ''Drosophila'', ejaculation amount during sequential copulations is reduced; this results in half filled female sperm reserves following a single copulatory event, but allows the male to mate with a larger number of females without exhausting his supply of sperm.〔 To facilitate sperm partitioning, some males have developed complex ways to store and deliver their sperm. In the blue headed wrasse, ''Thalassoma bifasciatum'', the sperm duct is sectioned into several small chambers that are surrounded by a muscle that allows the male to regulate how much sperm is released in one copulatory event. A strategy common among insects is for males to participate in prolonged copulations. By engaging in prolonged copulations a male has an increased opportunity to place more sperm within the female's reproductive tract and prevent the female from copulating with other males. It has been found that some male mollies (''Poecilia'') have developed deceptive social cues to combat sperm competition. Focal males will direct sexual attention toward typically non-preferred females when an audience of other males is present. This encourages the males that are watching to attempt to mate with the non-preferred female. This is done in an attempt to decrease mating attempts with the female that the focal male prefers, hence decreasing sperm competition. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sperm competition is a term used to refer to the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means an individual male has decreased chances of producing offspring.Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase males' chance of reproductive success. Sperm competition results in a sexual conflict of interest between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by physically removing another male's sperm prior to mating with a female.Sperm competition is often compared to having tickets in a raffle; a male has a better chance of winning (i.e. fathering offspring) the more tickets he has (i.e. the more sperm he inseminates a female with). However, sperm are not free to produce,Olsson et al., 1997; Wedell et al., 2002 and as such males are predicted to produce sperm of a size and number that will maximize their success in sperm competition. By making many spermatozoa, males can buy more "raffle tickets", and it is thought that selection for numerous sperm has contributed to the evolution of anisogamy with very small sperm (because of the energy trade-off between sperm size and number). Alternatively males may evolve faster sperm to enable their sperm to reach and fertilize the ovum first. Dozens of adaptations have been documented in males that help them succeed in sperm competition.==Defensive adaptations==Mate-guarding is a defensive behavioral trait that occurs in response to sperm competition; males try to prevent other males from approaching the female (and/or vice versa) thus preventing their mate from engaging in further copulations. Precopulatory and postcopulatory mate-guarding occurs in birds, lizards, insects and primates. Mate-guarding also exists in the fish species ''Neolamprologus pulcher'', as some males try to "sneak" matings with females in the territory of other males. In these instances the males guard their female by keeping them in close enough proximity so that if an opponent male shows up in his territory he will be able to fight off the rival male which will prevent the female from engaging in extra-pair copulation with the rival male.Organisms that are involved in polygynous mating systems are controlled by one dominant male. In this type of mating system, the male is able to mate with more than one female in a community. The dominant males will reign over the community until another suitor steps up and overthrows him. The current dominant male will defend his title as the dominant male and he will also be defending the females he mates with and the offspring he sires. The elephant seal falls into this category since he can participate in bloody violent matches in order to protect his community and defend his title as the alpha male. If the alpha male is somehow overthrown by the new comer, his children will most likely be killed and the new alpha male will start over with the females in the group so that his lineage can be passed on.Strategic mate-guarding occurs when the male only guards the female during her fertile periods. This strategy can be more effective because it may allow the male to engage in both extra-pair paternity and within-pair paternity. This is also because it is energetically efficient for the male to guard his mate at this time. There is a lot of energy that is expended when a male is guarding his mate. For instance, in polygynous mate- guarding systems, the energetic costs of males is defending their title as alpha male of their community. Fighting is very costly in regards to the amount of energy used to guard their mate. These bouts can happen more than once which takes a toll on the physical well- being of the male. Another cost of mate- guarding in this type of mating system is the potential increase of the spread of disease. If one male has an STD he can pass that to the females that he’s copulating with potentially resulting in a depletion of the harem. This would be an energetic cost towards both genders for the reason that instead of using the energy for reproduction, they are redirecting it towards ridding themselves of this illness. Some females also benefit from polygyny because extra pair copulations in females increase the genetic diversity with the community of that species. This occurs because the male is not able to watch over all of the females and some will become promiscuous.Eventually, the male will not have the proper nutrition which makes the male unable to produce sperm. For instance, male amphipods will deplete their reserves of glycogen and triglycerides only to have it replenished after the male is done guarding that mate. Also, if the amount of energy intake does not equal the energy expended, then this could be potentially fatal to the male. Males may even have to travel long distances during the breeding season in order to find a female which absolutely drain their energy supply. Studies were conducted to compare the cost of foraging of fish that migrate and animals that are residential. The studies concluded that fish that were residential had fuller stomachs containing higher quality of prey compared to their migrant counterparts With all of these energy costs that go along with guarding a mate, timing is crucial so that the male can use the minimal amount of energy. This is why it is more efficient for males to choose a mate during their fertile periods. Also, males will be more likely to guard their mate when there is a high density of males in the proximity Sometimes organisms put in all this time and planning into courting a mate in order to copulate and she may not even be interested. There is a risk of cuckholdry of some sort since a rival male can successfully court the female that the male originally courting her could not do.However, there are benefits that are associated with mate- guarding. In a mating- guarding system, both parties, male and female, are able to directly and indirectly benefit from this. For instance, females can indirectly benefit from being protected by a mate. The females can appreciate a decrease in predation and harassment from other males while being able to observe her male counterpart. This will allow her to recognize particular traits that she finds ideal so that she’ll be able to find another male that emulates those qualities. In polygynous relationships, the dominant male of the community benefits because he has the best fertilization success. Communities can include 30 up to 100 females and, compared to the other males, will greatly increase his chances of mating success.Males who have successfully courted a potential mate will attempt to keep them out of sight of other males before copulation. One way organisms accomplish this is to move the female to a new location. Certain butterflies, after enticing the female, will pick her up and fly her away from the vicinity of potential males. In other insects, the males will release a pheromone in order to make their mate unattractive to other males or the pheromone masks her scent completely. Certain crickets will participate in a loud courtship until the female accepts his gesture and then it suddenly becomes silent. Some insects, prior to mating, will assume tandem positions to their mate or position themselves in a way to prevent other males from attempting to mate with that female. The male checkerspot butterfly has developed a clever method in order to attract and guard a mate. He will situate himself near an area that possesses valuable resources that the female needs. He will then drive away any males that come near and this will greatly increase his chances of copulation with any female that comes to that area.In post- copulatory mate- guarding males are trying to prevent other males from mating with the female that they have mated with already. For example, male millipedes in Costa Rica will ride on the back of their mate letting the other males know that she’s taken. Japanese beetles will assume a tandem position to the female after copulation. This can last up to several hours allowing him to ward off any rival males giving his sperm a high chance to fertilize that female’s egg. These, and other, types of methods have the male playing defense by protecting his mate. Elephant seals are known to engage in bloody battles in order to retain their title has dominant male so that they are able to mate with all the females in their community.Copulatory plugs are frequently observed in insects, reptiles, some mammals, and spiders. Copulatory plugs are inserted immediately after a male copulates with a female, which reduce the possibility of fertilization by subsequent copulations from another male, by physically blocking the transfer of sperm. The Indian Meal Moth (''Plodia interpunctella'') are restricted in engaging in further mating activities because the spermatacore serves as a copulatory plug immediately after copulation. Bumblebee mating plugs, in addition to providing a physical barrier to further copulations, contain linoleic acid, which reduces re-mating tendencies of females.Similarly, ''Drosophila melanogaster'' males release toxic seminal fluids, known as ACPs (accessory gland proteins), from their accessory glands to impede the female from participating in future copulations. These substances act as an anti-aphrodisiac causing a dejection of subsequent copulations, and also stimulate ovulation and oogenesis. Seminal proteins can have a strong influence on reproduction, sufficient to manipulate female behavior and physiology. Another strategy, known as sperm partitioning, occurs when males conserve their limited supply of sperm by reducing the quantity of sperm ejected. In ''Drosophila'', ejaculation amount during sequential copulations is reduced; this results in half filled female sperm reserves following a single copulatory event, but allows the male to mate with a larger number of females without exhausting his supply of sperm. To facilitate sperm partitioning, some males have developed complex ways to store and deliver their sperm. In the blue headed wrasse, ''Thalassoma bifasciatum'', the sperm duct is sectioned into several small chambers that are surrounded by a muscle that allows the male to regulate how much sperm is released in one copulatory event.A strategy common among insects is for males to participate in prolonged copulations. By engaging in prolonged copulations a male has an increased opportunity to place more sperm within the female's reproductive tract and prevent the female from copulating with other males.It has been found that some male mollies (''Poecilia'') have developed deceptive social cues to combat sperm competition. Focal males will direct sexual attention toward typically non-preferred females when an audience of other males is present. This encourages the males that are watching to attempt to mate with the non-preferred female. This is done in an attempt to decrease mating attempts with the female that the focal male prefers, hence decreasing sperm competition.」の詳細全文を読む スポンサード リンク
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