Sperm competition

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Sperm competition

Sperm competition is "competition

Competition (biology)

Competition can be defined as an Biological interaction between organisms or species, in which the fitness of one is lowered by the presence of another....
between sperm

Sperm

The term sperm is derived from the Greek word sperma and refers to the male reproductive Cell . In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell....
of two or more males for the fertilization of an ovum

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....
". Sperm competition is often compared to having ticket

Ticket

Ticket may refer to:*Ticket , a card or slip of paper used to gain admission to a location or event*Ticket , a single election choice which fills more than one political office or seat...
s in a raffle

Raffle

A raffle is a competition in which people buy numbered tickets. Originating in southern Italy, it is a popular game in numerous countries and is often held to raise funds for a specific event, charity, or occasion....
; 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 maximises their success in sperm competition.

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Encyclopedia

Sperm competition is "competition

Competition (biology)

Competition can be defined as an Biological interaction between organisms or species, in which the fitness of one is lowered by the presence of another....
between sperm

Sperm

Ovum

Ticket

Raffle

Selection

In the context of evolution, certain traits or alleles of a species may be subject to selection depending on the Pragmatics the user has with the word....
for numerous sperm has contributed greatly to the evolution of anisogamy

Anisogamy

Anisogamy refers to a form of sexual reproduction involving gametes of different sizes. The smaller gamete is considered to be male whereas the larger gamete is regarded as female ....
(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. These adaptations may be behavioural; for example, males frequently guard females to prevent other males from also mating with them and gaining paternity of the offspring (e.g. in harem-keeping mammals such as elephant seals, or insects such as damselflies). Some species even produce more than one sperm type, typically a fertile type and a "helper" or "worker" caste (see Sperm heteromorphism

Sperm heteromorphism

Sperm heteromorphism is the simultaneous production of two or more distinguishable types of sperm by a single male. The sperm types might differ in size, shape and/or chromosome complement....
) that may help males win (or avoid) sperm competition.

Empirical support

In a study, it is found that because of female choice, the sperm undergoes many variations in its size and shape to accommodate the shape and physiology of the female reproductive tract. However, it is difficult to understand the mechanism and activity that goes inside the tract after mating has occurred which allows for the competition of sperm. During the mating season, females mate with different male partners. Females have a specialized sperm-storage organ in their reproductive system, for which the sperm of different males compete. From the results of this experiment, it is concluded that the males that had longer sperm length out-competed the male which had shorter sperm length. Also, from the study done by Scott Pitnick, the longest sperm cell known so far is from a species belonging to a fruit fly called Drosophila bifurra and measures up to two inches in length when fully uncoiled.

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 , also called the long-tailed field mouse, is a common murid rodent that was recognized as a distinct species in 1894. 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 90mm in leng...
(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

Bonobo

The Bonobo , which, until recently, usually was 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, chimpanzee....
(Harcourt et al (1981)). 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

Spermatophore

A spermatophore 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

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

Cloaca

In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the alimentary tract and urinary tract of certain animal species....
in order to peck out the sperm of the previous male suitor.

It has also been suggested that some species, including humans, have developed sperm that exist to stop the sperm from other males from reaching the egg, e.g. by killing them with enzymes or by blocking their access. These "kamikaze sperm" were suggested to make up as much as 99% of the sperm released during ejaculation, leaving just 1% capable of fertilisation . However, claims of task specialisation in human sperm have not been supported by subsequent studies and the "kamikaze hypothesis" is unsupported by most specialists, although it continues to be widely accepted by the general public. There is also currently no evidence of killer sperm in any non-human animals (Swallow & Wilkinson 2002; Till-Bottraud et al. 2005), although certain snails have an infertile sperm morph ("parasperm") that contains lysozymes, hinting that they may digest rivals' sperm (Buckland-Nicks 1996).

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 mitochondrion. Most other DNA present in eukaryotic organisms is found in the cell nucleus....
) ; cytoplasmic DNA is inherited from the mother only and it is thought that this could represent a constraint in the evolution of sperm evolution.

The British biologist Geoffrey Parker

Geoffrey Parker

Geoffrey Parker may refer to:* Geoff Parker, biologist* Geoffrey Parker , historian*Geoff Parker See also*Jeff Parker ...
proposed the concept of sperm competition in a 1970 paper.

Sperm competition

Empirical support

See also

Further reading

External links