Sperm competition

From Sperm competition is the talk of the fight between sperm cells for an egg cell. Each man's ejaculation contains, for example, millions of sperm, whereby only one egg is ready for fertilization and the fastest, most vital and most mobile sperm determines fertilization for itself.

What is the sperm competition?

The sperm competition corresponds to the competitive behavior with which sperm fight for the fertilization of an egg cell. Men produce significantly more sperm than women have egg cells for fertilization. In humans, the uterus usually only provides one fertile egg in each menstrual cycle. However, when a man ejaculates, an average of several million sperm is released. The most motile and therefore fastest sperm reach their destination first.

Sometimes the term sperm competition also means the competition between sperm from different individuals. Geoffrey Parker documented this type of sperm competition in the 1970s as the cause of the extreme excess of sperm with a limited number of egg cells. He kept the closely successive attempts at copulation of different males on only one female and proved that males with a higher sperm count are far superior to their competitors in this situation and have a correspondingly higher probability of fertilization.

Function & task

During an ejaculation, up to five milliliters of sperm enter the female vagina and from there swim up the fallopian tube with the help of its tail (flagellum). Only on the way do the sperm become fertile, as female enzymes remove certain proteins from the sperm. The majority of all released sperm do not survive the acidic environment of the vagina. A few hundred sperm make it into the fallopian tube and migrate towards the fertilizable egg cell.

Sperm survive in the fallopian tube for several days and can therefore wait for a certain amount of time to ovulate. After ovulation, they move to the egg cell and are hormone-controlled, for example by progesterone, which changes the beat pattern of the flagella and thus provides a direction.

Over the egg cell lies the zona pellucida, a layer of various closely meshed glycoproteins through which the sperm must pass. Individual glycoproteins bind to the sperm heads and cause the acrosome and zona pellucida to fuse. The enzymes of the acrosome cause the zona pellucida to dissolve and allow the sperm to reach the plasma membrane below. These reactions affect the proteins in the sperm, which ultimately bind to the receptors on the egg cell's cell membrane using the lock and key principle.

The sperm and egg fuse upon contact and the egg membrane is depolarized so that no further fertilization takes place.

The mobility and vitality of the sperm determine which sperm wins the battle for the fertilization of the egg. This has evolutionary advantages. Fast and motile sperm usually come from healthier and "stronger" men than slow or immobile ones. This means that processes of natural selection already take place by means of sperm competition, which should result in the most healthy offspring possible.

However, ejaculations of a healthy man do not only contain motile and vital sperm. In every ejaculation, the man also gives off immobile sperm, which are supposed to block any foreign sperm on the way to the target or can even kill foreign sperm chemically.

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Illnesses & ailments

A spermiogram can be used to determine the competitiveness of male sperm and thus ultimately the man's fertility. The sperm are examined in the form of an ejaculate sample in the liquefied state. After two to three days of sexual abstinence, a sperm sample is obtained from the patient by means of masturbation and, after liquefaction, examined in the laboratory. The examination corresponds above all to an observation under the microscope.

Various parameters play a role in assessing the fertility and competitiveness of the sperm. The microscopic analysis focuses on mobility. At least 65 percent of the sperm should be normally motile and around 25 percent should be clearly motile. Level A of the WHO stands for rapidly progressive mobility. Level D for lack of mobility. In addition to mobility, the shape of the sperm is examined under the microscope. At least 65 percent of sperm should be of normal shape per ejaculation. Concentration is also crucial for fertility. The lower limit is 20 million per milliliter. In addition, the vitality, i.e. the proportion of living sperm, determines the competitiveness. A healthy man releases at least 50 percent of living sperm per ejaculation. Dead sperm are stained with eosin and can be counted under the microscope in this way.