Sperm Competition

Sperm Competition

Performance & security by Cloudflare

You cannot access www.evolvingyourman.com. Refresh the page or contact the site owner to request access.
Copy and paste the Ray ID when you contact the site owner.

Ray ID:

74712d163ffe0726

74712d163ffe0726 Copy

URL: https://www.sciencedirect.com/science/article/pii/B978008045337800190X
URL: https://www.sciencedirect.com/science/article/pii/B9780128002865000018
URL: https://www.sciencedirect.com/science/article/pii/B9780128096338012917
URL: https://www.sciencedirect.com/science/article/pii/B9780123725684000069
URL: https://www.sciencedirect.com/science/article/pii/B9780121005436500337
URL: https://www.sciencedirect.com/science/article/pii/B9780128195581000117
URL: https://www.sciencedirect.com/science/article/pii/B9780121005436500362
URL: https://www.sciencedirect.com/science/article/pii/B9780121005436500428
URL: https://www.sciencedirect.com/science/article/pii/B9780080887838000115
URL: https://www.sciencedirect.com/science/article/pii/B978012100543650026X
Sperm competition is defined as direct competition among males for a limited number of unfertilized eggs.
Sperm competition occurs whenever females mate multiply and ejaculates from different males overlap and compete for fertilization, and is virtually ubiquitous throughout the animal kingdom. Transfer of many sperm is advantageous in sperm competition, but the cost of sperm production favors male ejaculate tailoring. It has also promoted the evolution of ‘manipulative’ ejaculates (mating plugs, nonfertile sperm, cooperative sperm, and seminal proteins) that suppress female receptivity and reduce the risk of sperm competition. Some of these adaptations are costly to females since they interfere with their reproductive physiology and have led to a variety of female counter-behaviors and strategies to minimize these costs.
Samantha Leivers , Leigh W. Simmons , in Advances in the Study of Behavior , 2014
Sperm competition is the competition between the sperm of two or more males to fertilize the ova of a single female. Over the past few decades, the extent to which sperm competition has acted as a selective pressure throughout human evolution has been hotly contested. This review aims to assess the current evidence for sperm competition in humans, the limitations of that evidence, and directions for future research. We conclude that humans have primarily evolved defensive adaptations in response to the risk of sperm competition. Thus, men exhibit behaviors that anticipate and address their partner's infidelity, the success of which may have relaxed selection on physiological and morphological adaptations to tackle sperm competition offensively. However, the extent to which humans can perform offensive tactics has been sorely understudied and requires considerable further research before firm conclusions can be drawn.
Sperm competition occurs whenever females mate multiply and ejaculates from different males overlap and compete for fertilization, and is virtually ubiquitous throughout the animal kingdom. Transfer of many sperm is advantageous in sperm competition, but the cost of sperm production favors male ejaculate tailoring. It has also promoted the evolution of “manipulative” ejaculates (mating plugs, nonfertile sperm, cooperative sperm, and seminal proteins) that suppress female receptivity and reduce the risk of sperm competition. Some of these adaptations are costly to females since they interfere with their reproductive physiology and have led to a variety of female counter-behaviors and strategies to minimize these costs.
Tommaso Pizzari , Geoff A. Parker , in Sperm Biology , 2009
Sperm competition is the competition between the ejaculates of different males for the fertilization of a given set of ova. Charles Darwin (1871) proposed sexual selection as a process that operates on variation in male ability to compete with other males for access to reproductive opportunities, and which promotes traits that confer an advantage in reproductive competition. In most taxa individual females may copulate (or spawn) with multiple males (i.e., are polyandrous). As a consequence, the ejaculates of different males may co-occur around a set of ova at the time of fertilization, resulting in sperm competition. Sperm competition introduces variation in male reproductive success determined by the relative competitive fertilizing efficiency of the ejaculates of different males, and generates postcopulatory, intrasexual selection, which promotes traits that increase the fertilization success of an ejaculate under competitive conditions. A second consequence of polyandry is the potential for intersexual selection to continue after copulation through mechanisms that enable females (or ova) to bias the outcome of sperm competition in favor of the sperm of certain males, a process known as sperm selection or cryptic female choice. The past three decades have seen an explosion of interest in postcopulatory sexual selection that has highlighted the importance of sperm competition and cryptic female choice as engines of evolutionary change. This chapter reviews recent empirical and theoretical advances to discuss various ways in which sperm competition may shape the evolution of sperm and ejaculate traits.
Sperm competition may occur more frequently among molluscs than commonly assumed. There exists a huge literature on the anatomy and morphology of reproductive systems in molluscs, although not all groups have received the same attention. However, the significance of these structures with respect to sexual selection and sperm competition is, in most cases, unclear. Several mollusc species may be well-suited for studies on sperm competition, but their potential as experimental organisms has not yet begun to be exploited by behavioural and evolutionary biologists.
Studies on sperm competition in gastropods and cephalopods should prove to be particularly rewarding. Both gastropods and cephalopods are unique because of several important features. Their elaborate mating behaviour may rival the complexity of those of various vertebrates. Do individuals assess the quality of potential mates during courtship? Is there a displacement of previous sperm in gastropods and cephalopods that show protracted copulation? Sperm selection (either in the form of a selective storage or digestion) might occur in both groups. This could provide a unique possibility of female choice in sperm competition, which deserves attention. Careful studies of the morphology of the female reproductive tract with respect to sperm storage and mating experiments using molecular techniques for paternity analyses could begin to answer some of these questions. Furthermore, the adaptive significance of sperm polymorphism is still not known. There is much to be learned in these most interesting groups of animals.
Michael D. Breed , Janice Moore , in Animal Behavior (Third Edition) , 2022
Sperm competition is a form of male–male competition that warrants its own section in a textbook because it is indeed so basic and important. 78 Many marine animals 79 release their gametes into the ocean (this is called spawning ), but this does not mean there is an absence of competition in mating, nor does it mean that the resulting zygotes are the results of random meetings of eggs and sperm. As with many topics in science, the absence of much evidence for a phenomenon–in this case, for competition among small objects in the vast sea–does not mean competition does not occur. Spawning is most common in aquatic or semiaquatic environments, where sperm can easily swim to eggs. In this system, animals that release the largest amounts of sperm with strong swimming abilities are most likely to sire young. Sperm competition was probably the first type of sexual competition to evolve, and as anisogamy evolved, sperm competition was central to the evolution of mating systems.
Discussion Point: The Absence of Evidence
As noted, it is difficult to document sperm competition in the vast ocean. Because of that, there is little evidence for it. The general public is frequently told that there is no evidence for this or that phenomenon, ranging from compounds that might cause cancer to correlates of deeply held ideologies. How might you interpret such statements?
Sperm competition, coupled with external fertilization, is the simplest and, by far, the most common type of fertilization across the broad spectrum of the animal world. In many species, male and female never need meet; the female deposits her eggs, and later a male finds and fertilizes them. Even under these uncomplicated conditions, two processes make mating nonrandom. First, there is variation in the ability of males and females to find appropriate locations to deposit their gametes. Second, variations in sperm swimming speed and energy resources affect which sperm reaches the egg first.
Sperm competition occurs among sperm after they are released from the male. The sperm from a single male may compete to reach an egg first; if the female has mated with more than one male, then sperm competition occurs among the males.
Sperm competition has been studied in many species of fish with external fertilization. Selection strongly favors all males that produce sperm with maximum speed and energy resources (ATP), but in many species there are still large enough differences among sperm from different males to affect fertilization probabilities. The amount of sperm, or sperm volume; the distance of sperm release from the eggs; and the timing of sperm release are also critical factors in sperm competition. The more sperm a male is able to produce, the higher the probability that one of his sperm will win the competition. Males that are able to get closer to eggs are generally more likely to be successful, and a few seconds’ difference in sperm release can make all the difference in whether a male’s sperm reaches the eggs first.
Generally, sperm volume is correlated with testicular size ( Figure 11.30 ); larger testicles are necessary for production of larger amounts of sperm. A basic prediction in the biology of mating systems is that males of species with more intense sperm competition will have larger testes. The evolutionary responsiveness of testicular size to sperm competition is an important example of the effects of sexual selection on morphology and anatomy. Sometimes sperm limitation, which occurs when one male’s ejaculate doesn’t contain enough sperm to fertilize all a female’s eggs, results in selection for polyandry, in which the female mates with more than one male.
Figure 11.30 . Large testicular size relative to body size may be an indication of an evolutionary history that includes sperm competition. Why might this male be displaying/exposing his scrotum?
Sperm may also cooperate. This seems odd in a situation that allows only one sperm to fertilize an egg, but isolated sperm are not efficient swimmers. An aggregate of a few sperm (5–10) swims together—working cooperatively, the sperm swim faster and more directly to the egg. When the egg is reached, competition takes over. 80
In many cases external fertilization is combined with courtship; even though the eggs and sperm are released into the environment, the male and female first have the opportunity to assess the quality of their potential mate before committing their gametes. Bluegill sunfish ( Lepomis macrochirus ), a common fish of lakes in North America, exemplify how sperm competition can play a role in a more complex mating system that involves sophisticated behavioral interactions. Bluegill males can adopt a variety of mating strategies ( Figure 11.31 ). Some, termed “parental” males, build nests in the bottom of the lake and court females, who spawn in the nest. Parental males then guard the eggs. “Sneakers” dash in and release sperm near the nests of parental males. And male “satellites” are able to come close to eggs because they mimic females. Sperm of all three types of males have similar speed and swimming ability, but even though sneaker males release their sperm farther from the eggs and later than parental males, they have an advantage over parental males. 81 This suggests that sneaker males produce higher-quality sperm. Satellites also have an advantage over parentals, but this could be explained by the fact that they can get closer to the eggs when they release their sperm.
Figure 11.31 . A male bluegill sunfish and nest. This parental male is advertising the resources it has to support eggs. Sneaker males attempt to spawn with females that are approaching parental males.
Internal fertilization facilitates mating in terrestrial environments and, compared to external fertilization, allows for more control over gametes by both males and females; most terrestrial animals have evolved some form of internal fertilization. With internal fertilization, males do not need to respond so much to the effects of dilution and distance from the eggs on their gametes, and both sexes can exert a greater degree of choosiness over their mates. However, sperm competition remains an important mechanism in animals with internal fertilization. Sperm from a male will compete among themselves for fertilization opportunities, but any time a female mates more than once within a short time period, sperm competition among the sperm from the different males is likely. 82 In some cases, males even remove the sperm deposited by previous males. 83
C.W. Petersen , R.R. Warner , in Sperm Competition and Sexual Selection , 1998
Sperm competition occurs when sperm from different males compete for fertilizations ( Parker 1970 ). The degree of sperm competition ranges widely in fishes, both interspecifically and intraspecifically, from males that never experience sperm competition to males that are in frequent competition with several males. The relative ease of observing fishes in natural populations, combined with the high diversity of mating systems, make them excellent subjects for studying sperm competition and conducting intraspecific and interspecific tests of sperm competition theory. However, only recently have comparative studies on sperm competition been published ( Stockley et al . 1996 , 1997 ), and in Smith's (1984) volume the single chapter on fish focused on a single family, the livebearing Poeciliidae ( Constantz 1984 ). In this chapter we review the theory and data on sperm competition in fishes, compare these data with predictions from sperm competition theory, and discuss the evolutionary consequences of sperm competition in this diverse group of vertebrates.
The diversity of mating systems and life histories in fishes is impressive. Fertilization can occur in the environment by either the release of gametes into the water column (pelagic spawning) or the deposition of eggs on the substrate (demersal spawning). Fertilization can also take some rather unexpected twists, as in the case of the catfish Corydoras aeneus (Callichthyidae), where females drink sperm by putting their mouth around the abdomen of the male. The sperm quickly pass through the digestive system, exit the anus, and externally fertilize the female's eggs in a protected space formed by the female's pelvic fins ( Kohda et al. 1995 ). In internal fertilizers, sperm are introduced into the female's body with subsequent events ranging from internal development and viviparity to cases where fertilization and development are delayed until the eggs are released. Internal fertilization can also occur in males, as is the case of pipefishes and seahorses where the female transfers unfertilized eggs into the pouch of her mate. Where it occurs. parental care can consist of uniparental care of eggs, biparental care of eggs and mobile off-spring, attachment of eggs to the body, mouth-brooding, and viviparity ( Breder and Rosen 1966 ; Blumer 1979 ).
Are there special characteristics of fishes that set them apart from other animal groups when we consider sperm competition? Like all animals, sperm from fishes must access eggs through an aqueous medium, but because fishes themselves live in an aqueous environment, fertilization often occurs outside the female's body. Pelagic fertilization with highly mobile males and females is more characteristic of fishes than other animal groups and results in selective pressures on sperm and spawning activity unique to this group. The close alignment of the partners means that while eggs and sperm are shed into a three-dimensional medium and both are free to disperse, they are initially in very close proximity. This type of external fertilization offers a precise location and time where unfertilized eggs are ready for insemination, and the opportunity for further fertilization declines very rapidly. It is probably because of this limited window of opportunity that sperm in some fish species have extremely short lifespans ( Petersen et al. 1992 ). As outlined below, rapid, repeated mating and the detailed information available on each successive partner appears to have led to the evolution of mechanisms allowing precise control of ejaculate amount in several fishes (e.g. Shapiro et al. 1994 ).
While external fertilization has been proposed as a fundamental factor affecting sperm competition (see Chapter 6 ), it is not clear just how the short-term availability of eggs characteristic of many fish matings should play out in terms of sperm competition. External fertilization provides an opportunity that can be exploited by several males simultaneously, resulting in high levels of sperm competition. Often, this takes the form of smaller peripheral males contributing their sperm to a mating between a larger male and a female ( Taborsky 1994 ). Most of the documented sperm competition has indeed been noted in externally fertilizing fishes, and some males have gonads comprising more than 10% of their body weight. However, external fertilization may also make mate guarding easier, since it is not necessary to guard an inseminated female for prolonged periods until fertilization occurs. In fact, many fish species with external fertilization where males defend either mates or preferred spawning sites often do not experience sperm competition.
In addition to its effect on intrasexual competition, some forms of external fertilization may also limit the extent to which females can choose among potential mates. Group matlngs, in which many males simultaneously release sperm, effectively prevent choice of a particular male, although females may still control their matings according to some group composition criterion. In benthic or pelagic pair matings, even in the presence of peripheral male activity, a female may still exercise choice over the male who will fertilize at least the majority of her eggs. Because males are more often the sex providing parental care of benthic eggs in fishes, female allocation of paternity to a particular male may have profound implications for the subsequent care that the eggs receive (see chapter 4 ). Thus, sperm competition (and the resultant dilution of paternity) can potentially reduce the direct benefits that a female might receive from a male of her choosing.
T.R. Birkhead , A.P. Møller , in Sperm Competition and Sexual Selection , 1998
Sperm competition may result in the production of genetically more diverse offspring that, under certain environmental conditions, may have an enhanced probability of survival and reproduction ( Williams 1975 ; Loman et al. 1988 ; Ridley 1993 ; Brown 1997 ). Recombination in sexually reproducing organisms by itself results in the generation of immense genetic diversity, even in the absence of any sperm competition ( Williams 1975 ). Hence, sperm competition is certainly not the only way to produce genetically d
Iwa Outdoor
Amateur Webcam Masturbate
Sensual Jane New Video

Sperm Competition

Report Page