Sperm competition

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Sperm competition is "competition between sperm of two or more males for the fertilization of an ovum". ^[1] 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 ^[2], and as such males are predicted to produce sperm of a size and number that maximises 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. 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) that may help males win (or avoid) sperm competition.

Contents 1 Empirical support 2 See also 3 References 4 Further reading 5 External links

[edit] 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. ^[3]

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 ^[4]. 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 (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). ^[5]

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 ^[6] .

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.^[7]

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 ^[8]. 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) ^[9]; 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 proposed the concept of sperm competition in a 1970 paper.

[edit] See also

• Sperm heteromorphism
• Sexual selection
• Sperm Wars

[edit] References

[edit] Further reading

• Alcock, John 1998. Animal Behavior. Sixth Edition. 429-519.

• Baker, Robin 1996. Sperm Wars: 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.

• Harcourt, A.H., Harvey, P.H., Larson, S.G., & Short, R.V. 1981. Testis weight, body weight and breeding system in primates, Nature 293: 55-57.

• 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.

• 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, 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 http://www.els.net

• Wedell, N., Gage, M.J.G, & Parker, G. A. 2002. Sperm competition, male prudence and sperm-limited females. Trends in Ecology & Evolution, 7: 313-320.

[edit] External links

• Sperm competition and the Kamikaze Sperm Hypothesis