Sperm competition
Sperm competition is a term used to refer to the competitive process between spermatozoa
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

 of two different males to fertilize an egg of a lone female. Competition occurs whenever females engage in promiscuous mating to increase their chances in producing more viable 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 from sexual conflict
Sexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...

 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 ticket
Ticket (admission)
A ticket is a voucher that indicates that one has paid for admission to an event or establishment such as a theatre, movie theater, amusement park, zoo, museum, concert, or other attraction, or permission to travel on a vehicle such as an airliner, train, bus, or boat, typically because one has...

s in a raffle
A raffle is a competition in which people obtain numbered tickets, each ticket having the chance of winning a prize. At a set time, the winners are drawn from a container holding a copy of every number...

; 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 spermatozoa, males can buy more "raffle tickets", and it is thought that selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...

 for numerous sperm has contributed greatly to the evolution of anisogamy
Anisogamy refers to a form of sexual reproduction involving the union or fusion of two dissimilar gametes — anisogamous, anisogamic,...

 (because of the energetic trade-off between sperm size and number). Alternatively males may evolve faster sperm to enable their sperm to reach and fertilize the ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 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 to prevent other males from approaching the female (and/or vice versa) thus preventing her 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, by physically blocking the transfer of sperm. P. 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 a substance called linoleic acid, which reduces re-mating tendencies of females.

Similarly, Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

 males release toxic seminal fluids, known as acps, from their accessory glands to impede the female from participating in future copulations. The substances found in the seminal fluid acts as an anti-aphrodisiac causing a dejection of subsequent copulations and stimulates ovulation and oogenesis. Seminal proteins can have a strong influence on reproduction, sufficient to manipulate female behavior and physiology.

Another strategy is known as sperm partitioning, where males spread out the amount and size of the sperm they produce. 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 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.

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.

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 new sperm into the female; hindering successful insemination opportunities of the previous male.

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 may select males that have these superior "good sperm" genes because it means that their offspring will be more viable and will inherit the "good sperm" genes which will increase their fitness levels when their sperm competes.

Evolutionary consequences

One evolutionary response to sperm competition is the variety in penis morphology of many species. For example, the shape of the human penis has been selected through sperm competition. The human penis has been selected to displace seminal fluids that were implanted within the female reproductive tract by a rival male. The thrusting action which occurs during sexual intercourse
Sexual intercourse
Sexual intercourse, also known as copulation or coitus, commonly refers to the act in which a male's penis enters a female's vagina for the purposes of sexual pleasure or reproduction. The entities may be of opposite sexes, or they may be hermaphroditic, as is the case with snails...

 manually removes seminal fluid out of the cervix area from a previous mating.

Evolution to increase their ejaculate size in the presence of sperm competition has a consequence on testis size. Large testis
The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...

 can produce more sperm required for larger ejaculates, and can be found across the animal kingdom when sperm competition occurs Males with larger testis have been documented to achieve higher reproductive success rates than males with smaller testis 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, for instance Nephila fenestrate, 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 repears that these males can only mate once.

Female choice for males with competitive sperm

Female factors can influence the result of sperm competition through a process known as "sperm choice". Proteins present in the female reproductive tract or on the surface of the ovum may influence which sperm succeeds in fertilizing the egg. During sperm choice females are able to discriminate and differentially use the sperm from different males. One instance where this is known to occur is inbreeding; females will preferentially use the sperm from a more distantly related male than a close relative.

Empirical support

It has been found that because of female choice (see sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

), morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 of sperm in many species occurs in many variations to accommodate or combat (see sexual conflict
Sexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...

) the morphology and physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 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
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

, mollusk and other phyla
Phyla, the plural of phylum, may refer to:* Phylum, a biological taxon between Kingdom and Class* Phylum , in linguistics, a large division of possibly related languagesPhyla, as a singular, may refer to:...

 have a specialized sperm-storage organ called the spermatheca
The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...

 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
Wood mouse
The wood mouse is a common murid rodent from Europe and northwestern Africa. It is closely related to the yellow-necked mouse but differs in that it has no band of yellow fur around the neck, has slightly smaller ears, and is usually slightly smaller overall: around 90 mm in length...

 (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 bonobo
The bonobo , Pan paniscus, previously called the pygmy chimpanzee and less often, the dwarf or gracile chimpanzee, is a great ape and one of the two species making up the genus Pan. The other species in genus Pan is Pan troglodytes, or the common chimpanzee...

s. 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
A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...


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
The Dunnock, Prunella modularis, is a small passerine bird found throughout temperate Europe and into Asia. It is by far the most widespread member of the accentor family, which otherwise consists of mountain species...

, a small bird. Before mating with the polyandrous female, the male dunnock pecks at the female's cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...

 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
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...

); mitochondrial DNA is inherited from the mother only and it is thought that this could represent a constraint in the evolution of sperm.

Further reading

  • Alcock, John 1998. Animal Behavior. Sixth Edition. 429-519.
  • Baker, Robin 1996. Sperm Wars
    Sperm Wars
    Sperm Wars is a primarily nonfiction book by evolutionary biologist Robin Baker. Originally published in English in 1996, it has since appeared in 25 languages and in 2006 a 10th anniversary edition was published in the United States...

    : The Science of Sex
    ISBN 0-7881-6004-4.
  • Dowling, Damian K., Larkeson Nowostawski, Albert & Arnqvist, Göran 2007. Effects of cytoplasmic genes on sperm viability and sperm morphology in a seed beetle: implications for sperm competition theory? Journal of Evolutionary Biology 20: 358-368.
  • Eberhard, William 1996 Female Control: Sexual Selection by Cryptic Female Choice ISBN 0691010846
  • Freeman, Scott; Herron, Jon C.; (2007). Evolutionary Analysis (4th ed.). Pearson Education, Inc. ISBN 0-13-227584-8.
  • Olsson, M., Madsen, T. & Shine, R. 1997. Is sperm really so cheap? Costs of reproduction in male adders, Vipera berus. Proceedings of the Royal Society of London B 264: 455-459.
  • Ryan, Christopher & Jethá, Calcilda. Sex at Dawn: The prehistoric origins of modern sexuality
    Sex at Dawn
    Sex at Dawn: The Prehistoric Origins of Modern Sexuality is a book co-authored by Christopher Ryan, PhD and Cacilda Jethá, MD , first published in 2010 by the Harper imprint of HarperCollins...

    . New York: Harper, 2010.
  • Shackelford, T. K. & Pound, N. 2005. Sperm Competition in Humans : Classic and Contemporary Readings ISBN 0-387-28036-7.
  • Shackelford, T. K., Pound, N., & Goetz, A. T. (2005). Psychological and physiological adaptations to sperm competition in humans. Review of General Psychology
    Review of General Psychology
    , is the quarterly scientific journal of the American Psychological Association Division 1: The Society for General Psychology. Review of General Psychology publishes cross-disciplinary psychological articles that are conceptual, theoretical, and methodological in nature...

    , 9,
    228-248. Full text
  • Simmons, Leigh W. 2001. Sperm competition and its evolutionary consequences in the insects. Princeton University Press, ISBN 0-691-05988-8 and ISBN 0-691-05987-X
  • Singh S R, Bashisth N. Singh and Hugo F. Hoenigsberg (2002).Female remating, sperm competition and sexual selection in Drosophila.Genet. Mol. Res. 1 (3): 178-215
  • Snook, Rhonda R. Postcopulatory reproductive strategies. Encyclopedia of Life Sciences
    Encyclopedia of Life Sciences
    The Encyclopedia of Life Sciences is an encyclopedia that spans the entire spectrum of life sciences and is published by Wiley-Blackwell....

  • Wedell, N., Gage, M.J.G, & Parker, G. A. 2002. Sperm competition, male prudence and sperm-limited females. Trends in Ecology & Evolution
    Trends (journals)
    Trends is a series of scientific journals owned by Elsevier that publish review articles in a range of areas of biology. They are currently part of Elsevier's Cell Press group of journals....

    , 7: 313-320.

External links

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