Sperm competition

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Sperm competition is a term used to refer to the competitive process between sperms of two different males to fertilize an egg of a lone female. Since females have decided to engage in promiscuous mating to increase their chances in producing more viable offspring, competition has evolved. Mating with more than one mate has generated a situation known as sperm competition; the presence of sperm competition has led to an evolutionary response; a response that has caused males to develop adaptations to help them increase their chances in succeeding in reproductive success. Sperm competition can be categorized with offensive and defensive roles; these categories are known to work antagonistically. Males have evolved several tactics that help them allocate their sperm effectively; this is dependent on how severe the influence of sperm competition on a given population is and is considered sperm competition that is on the defensive side. Adaptations and tactics such as mate-guarding, mating plugs, and releasing of toxic seminal substances to reduce female re-mating tendencies have advanced over time. The offensive form of sperm competition occurs when another male attempts to ruin or interrupt the reproductive success of the defensive male.

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, and as such males are predicted to produce sperm of a size and number that maximizes their success in sperm competition. By making many sperm, males can buy more "raffle tickets", and it is thought that selection for numerous sperm has contributed greatly to the evolution of anisogamy (because of the energetic trade-off between sperm size and number). 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; it is done to prevent the female from engaging in further copulations. Precopulatory and postcopulatory mate-guarding occurs in birds, lizards, insects, and primates; 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. Strategic mate-guarding is beneficial because males guard females during their fertile periods; strategically guarding these females allows the male to engage in both extra-pair paternity and within-pair paternity. Copulatory plugs are frequently observed in insects, reptiles, some mammals, and spiders. Copulatory plugs are inserted immediately after a male copulates with a female; they are intended to limit the possibility of subsequent copulations from another male. P. interpunctella are restricted in engaging in further mating activities because the spermatacore serves as a copulatory plug immediately after copulation. Bumblebee mating plugs contain a substance called linoleic acid; this substance lowers re-mating tendencies in females. Some males 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. In sperm partitioning males partition the amount and size of the sperm they produce; sperm partitioning is evident in Drosophila Melanogaster. In drosophila, ejaculation amount during sequential copulations are partitioned; this results in half filled female sperm reserves following a single copulatory event. Also in drosophila males may release toxic seminal fluids from their accessory glands to impede the female from participating in future copulations; the substances found in the seminal fluid function as an anti-aphrodisiac that dejects subsequent copulations and stimulates the occurrence of ovulation and oogenesis. The toxic seminal fluid that is released is called acps; acps is a huge constituent found in seminal fluids. Seminal proteins have a strong influence on reproduction; an influence that is strong enough to manipulate female behavior and physiology. Some males have developed complex ways to store and deliver their sperm; this can be seen in pair-forming birds. In the blue headed wrasse thalassoma bifsciatum 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..

Offensive adaptations

Offensive adaptation behavior differs from defensive behavior because it involves an attempt to ruin the chances of another male's opportunity in succeeding in copulation by engaging in an act that tries to terminate the fertilization success of the previous male. A male on the offensive side of mate-guarding may terminate the guarding male's chances at a successful insemination by brawling with the guarding male to gain access to the female. In Drosophila, males release seminal fluids that contain additional toxins like pheromones and modified enzymes that are secreted by their accessory glands intended to destroy the sperm that have already made their way into the female's reproductive tract from a recent copulation. Based on the "last male precedence" idea, some males can remove sperm from previous males by ejaculating into the female new sperm; this hinders successful insemination opportunities of the previous male because the newly introduced sperm has now gained precedence over the sperm that was already in the female.

Mate choice

The "good sperm hypothesis" is very common in polyandrous mating systems. The "good sperm hypothesis" suggests that a male's genetic makeup will determine the level of his competitiveness in sperm competition. When a male has "good sperm" he is able to father more viable offspring than males that do not have the "good sperm" genes. Females also tend to select males that have these superior "good sperm" genes because it means that their offspring will not only be more viable but they will inherit the genes and pass them on to future generations which will increase fitness levels for future generations as well.

Evolutionary consequences

There are several varieties in the morphology of specie penis. One evolutionary response to sperm competition is the shaping of the human penis. The human penis is known for displacing seminal fluids that were implanted within the female reproductive tract by a rival male. It does this by an action called thrusting. This thrusting occurs during intercourse; the thrusting manually removes seminal fluid out of the cervix area from a previous mating. Semen displacement is an example of an evolutionary consequence that resulted in response to sperm competition;Since males have evolved to increase their ejaculate size in the presence of sperm competition an evolutionary consequence in testis size has also occurred; large testis size can be found across the animal kingdom during sperm competition because since sperm is made in the testis, testis size need to evolve to become larger in size in order to make more sperm so larger ejaculates can be discharged. Males with larger testis have been documented to achieve higher reproductive success rates than males with smaller testis; evident in male yellow pine chipmunks. Male yellow pine chipmunks that had large testis fathered more offspring than males with smaller testis. In some insects and spiders the male copulatory organ breaks off or tears off at the end of copulation and remains within the female to serve as a copulatory plug. This broken genitalia is believed to be an evolutionary response to sperm competition. This damage to the male genitalia tells us that these males can only mate once. This breaking off of the genitalia to form a copulatory plug can be seen in Nephila fenestrate.

Female choice for males with competitive sperm

When it comes to sperm competition females also have a say; female choice in sperm competition exists to decide which male sperm will fertilize the egg of a particular female. Female choice can take place via a process referred to as “sperm choice”. In the process of female “sperm choice” females are able to discriminate and differentially use the sperm from different males. Female choice has also been known to occur to avoid inbreeding; should a female receive sperm from two males and she is closely related to one male she will choose the sperm from the male she is not related to.

Empirical support

It has been found that because of female choice (see sexual selection), morphology of sperm in many species occurs in many variations to accommodate or combat (see sexual conflict) the morphology and physiology of the female reproductive tract. However, it is difficult to understand the interplay between female and male reproductive shape and structure that occurs within the female reproductive tract after mating that allows for the competition of sperm. Polyandrous females mate with many male partners. Females of many species of arthropod, mollusk and other phyla have a specialized sperm-storage organ called the spermatheca in which the sperm of different males sometimes compete for increased reproductive success.

Evidence exists that illustrates the ability of genetically similar spermatozoa to cooperate so as to ensure the survival of their counterparts thereby ensuring the implementation of their genotypes towards fertilization. Cooperation confers a competitive advantage by several means, some of these include incapacitation of other competing sperm and aggregation of genetically similar spermatozoa into structures that promote effective navigation of the female reproductive tract and hence improve fertilization ability. Such characteristics lead to morphological adaptations that suit the purposes of cooperative methods during competition. For example, spermatozoa possessed by the Wood mouse (Apodemus sylvaticus) possess an apical hook which is used to attach to other spermatozoa to form mobile trains that enhance motility through the female reproductive tract. Spermatozoa that fail to incorporate themselves into mobile trains are less likely to engage in fertilization.

Selection to produce more sperm can also select for the evolution of larger testes. Relationships across species between the frequency of multiple mating by females and male testis size are well documented across many groups of animals, notable primates: female gorillas are relatively monogamous, so gorillas have smaller testes than humans, which in turn have smaller testes than the highly promiscuous bonobos. Male chimpanzees that live in a structured multi-male, multi-female community, have large testicles to produce more sperm, therefore giving him better odds to fertilize the female. Whereas the community of gorillas consist of one alpha male and two or three females, when the female gorillas are ready to mate, normally only the alpha male is their partner. Other means of sperm competition could include improving the sperm itself or its packaging materials (spermatophore).

The male black-winged damselfly provides a striking example of an adaptation to sperm competition. Female black-winged damselflies are known to mate with several males over the span of a only a few hours and therefore possess a receptacle known as a spermatheca which stores the sperm. During the process of mating the male damselfly will pump his abdomen up and down using his specially adapted penis which acts as a scrub brush to remove the sperm of another male. This method proves quite successful and the male damselfly has been known to remove 90-100 percent of the competing sperm.

A similar strategy has been observed in the Dunnock, a small bird. Before mating with the polyandrous female, the male dunnock pecks at the female's cloaca in order to peck out the sperm of the previous male suitor.

A notion emerged in 1996 that some species, including humans, some significant fraction of sperm specialize in a manner such that they cannot fertilize the egg but instead have the primary effect of stopping the sperm from other males from reaching the egg, e.g. by killing them with enzymes or by blocking their access. This type of sperm specialization became known popularly as "kamikaze sperm" or "killer sperm", but most follow-up studies to this popularized notion have failed to confirm the initial papers on the matter. While there is also currently little evidence of killer sperm in any non-human animals certain snails have an infertile sperm morph ("parasperm") that contains lysozymes, leading to speculation that they might be able to degrade a rivals' sperm.

Sperm competition has led to other adaptations such as larger ejaculates, prolonged copulation, deposition of a copulatory plug to prevent the female re-mating, or the application of pheromones that reduce the female's attractiveness. The adaptation of sperm traits, such as length, viability and velocity might be constrained by the influence of cytoplasmic DNA (e.g. mitochondrial DNA); mitochondrial DNA is inherited from the mother only and it is thought that this could represent a constraint in the evolution of sperm.

References

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External links

Sperm competition and the Kamikaze Sperm Hypothesis

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