Sperm Competition

= Sperm Competition =

 

Definition
Sperm competition has been defined as the "competition between sperm of two or more males for the fertilization of an ovum."1 One common analogy for sperm competition is the comparison to tickets in a raffle. In a raffle, a participant's chances of winning increase with the number of tickets s/he has; similarly, a male animal (human or non-human) has higher odds of fathering offspring the more sperm he uses to inseminate his female mate.

Several male animals have shown adaptations that help them succeed in sperm competition. These documented adaptations can be either behavioral or biological. Behaviorally, for example, male animals such as elephant seals or damselflies will guard females (as in harem keeping) to prevent other males from gaining sexual access to and mating with the females, and thereby gaining paternity of that female's offspring. Biologically, some species have even been said to produce more than one sperm type; these types are usually described as a fertile sperm type ("eusperm") and another "helper" or "worker" type ("parasperm"). This adaptation is called sperm heteromorphism.2 [See "Debates" below for more on sperm heteromorphism.]undefined

It should also be noted that because sperm require energy to produce, sexually reproducing male animals are predicted to maximize their success in sperm competition by making smaller sperm in greater numbers. When they do this, males can send out more sperm in every ejaculation, and it is believed that natural selection for numerous sperm has played a large role in the evolution of anisogamyby finding a balance in the energy trade-off between the number and size of sperm.3

Example(s) of use in context
One study conducted by Scott Pitnick found that female sexual selection influences sperm to undergo multiple variations in both size and shape in order to better fit the shape and physiology of the female reproductive tract.4 During the mating season of the fruit fly Drosophila bifurra, females mate with different male partners, enhancing the pressure for sperm competition. The results of Pitnick's study found that in these fruit flies, the males whose sperm were longer out-competed the males whose sperm were shorter. This selective pressure for the male Drosophilia bifurra flies to have sperm adapted to reach through the females' whole reproductive tract resulted in these flies having the longest sperm cells known to science so far. When fully uncoiled, the Drosophilia bifurra sperm cells reach a length of two inches, which is one thousand times longer than the average human sperm cell.5

undefinedAnother example in flies is the black-winged damselfly. In this species, the females have been observed mating with several different males during the course of just a few hours; the females therefore have an adaptive organ called a spermatheca, which is a sort of storage area for the sperm. During the mating and copulation process, the male damselfly uses his own special adaptation in the form of a penis that is specialized in form and essentially acts like a scrubber brush, removing the sperm of another male. This adaptation has been proven to be very successful; and the male damselfly can remove 90-100 percent of the competing sperm on average.6

undefinedSimilar to the male damselfly's specialized adaptive penis, the male Dunnock bird has also developed a function to eliminate competing sperm; the Dunnock's adaptation, though, is behavioral rather than biological. In the Dunnock species, the female birds are polyandrous, mating with multiple males. In order to weed out the competition's seed, the male Dunnocks will get rid of the previous male's sperm by pecking at the female's cloaca.7

There is also evidence to suggest that sperm cells which are genetically similar to one another will cooperate in order to ensure the survival of the others, and therefore to ensure the success of their genotypes reaching fertilization. This sperm cooperation, or altruism, can be seen as a competitive advantage in several ways; it can include the incapacitation or inhibition of competing sperm by allowing the cooperating sperm to band together physically and reach the egg first. One morphological, or physical, adaptation to do this is the sperm of the Wood mouse, which possess a specialized hook to link onto other genetically similar sperm and form chains that increase motility and mobility inside the female reproductive tract.8

undefinedIn addition to adaptations revolving around the sperm itself and how to enhance one's own chances of fertilization while being detrimental to the chances of others, there are also selective pressures for biological adaptations regarding the production and storage of sperm in the male reproductive organs. Across species, the relationships between the frequency of polyandrous female mating habits and larger male testis size is well documented. Among primates, the following patterns have been recorded: female gorillas are fairly monogamous, so gorillas' testes are smaller than those of humans. Humans, though larger than gorillas, have smaller testes than bonobos, as bonobos are very promiscuous.9Male chimpanzees have a structured multi-male, multi-female community. Because of this, they need to have large testicles and be able to make more sperm, giving them better chances of fertilizing a female.10

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Scientific background &amp; History
The concept of sperm competition was originally described and proposed by Geoffrey Parker, a British biologist, in his paper, "Sperm competition and its evolutionary consequences in the insects." This paper was published in a 1970 issue of Biological Reviews, and has contributed to theories and debates about inter-sperm competition and sperm heteromorphism. Many studies have observed behavioral and biological examples of sperm competition among various species. Specifics of these studies are detailed above in the "Example(s) of use in context" section.

Debates
The main debates in the arena of sperm competition center around the concept of sperm heteromorphism. While it is not contested that sperm heteromorphism exists, it is debated to what extent this plays a role in sperm competition, and how many different morphological forms of sperm there truly are. In 1996, the theory of sperm heteromorphism first emerged and posited that some species, humans included, have a proportionately large number of specialized sperm that cannot fertilize an egg. Instead, these specialized sperm are thought to be used for stopping the sperm of other males. They do this by killing them with enzymes or by blocking their access.

In popular culture, these sperm came to be known as "kamikaze sperm" or "killer sperm." Most studies that were done to check this theory, though, have not been able to confirm or fully support the initial theory and its findings, especially in humans. On the other hand, though there is also little evidence for this type of sperm heteromorphism in non-human animals, some snail species have "parasperm," which are infertile sperm morphs containing lysozymes. This has led to the theory that these parasperm may have the ability to functionally break down sperm from competing males.

Even the highly controversial show Manswers (think Mythbusters for "guy" stuff; some questions answered include How Can You Kill A Bear With Your Hands?, What Smell Turns Women On The Most?, and Who Drives Better, Men Or Women?) weighed in on the issue of sperm heteromorphism. This show stated, while ambiguously citing 'scientific evidence' that human sperm came in three varieties: sprinters (go straight for the egg), blockers (taking up space around the egg to keep competing sperm away), and the aforementioned kamikaze sperm. Seeing such things cited on this sort of show makes the science seem deligitimized when shown alongside other junk science and irrelevant questions like How Big Do Breasts Have To Be To Crush A Beer Can?

Author
Danica Hecht

Image/Figure


[Figure 1: Microscopic view of human sperm cells.11]