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Copulation and fertilization mammals

About mammals > Reproductive processes
The African lion

Once eggs are released from the female ovaries, they migrate down into the uterus. Eggs are not self-propelled, and migration occurs passively by gliding over cells that have minuscule sweepers (cilia). In contrast, sperm cells each have a long flagellum that provides mobility. But for sperm to reach the egg or eggs, copulation must first occur.

Copulation in most terrestrial species occurs as the male straddles the female from behind. Typical examples of this type of copulation occur in deer, elephants, mice, and cats. Most animals remain in this position, but some, especially dogs (Canidae), may then turn 180 degrees and continue a prolonged copulation in a copulatory lock, where the penis points 180 degrees away from the head, and both animals face in the opposite direction. Another situation occurs in Cetaceans (whales and dolphins) where copulation occurs as both animals lay or swim belly to belly. In primates, including humans, copulation positions are more flexible but most often consist of the ventral-dorsal mount or the ventralventral position.

The position assumed during copulation helps transfer the gametes from the male to the female tract, and physical constraints of size and position necessitate the use of a gametetransfer organ, the penis. Depending on challenges faced by each species, characteristics of the penis such as size, length, or structure will vary.

But all penes have the same function: to facilitate the transfer of the sperm cells from the male body to the female reproductive tract. Unlike fishes, which can release their sperm in water, mammalian sperm loses its mobility when exposed to air and internal copulation and fertilization maximizes the chances of successful transfer of viable sperm.

Both sexes have evolved adaptations to facilitate every step of reproduction, and copulation is no different. For example, females that come into heat will often produce thick vaginal secretions that not only attract males, but that also serve to lubricate the reproductive tract in preparation for copulation. Males also produce a lubricant via their bulbourethral glands to help facilitate copulation. In some species, including humans, the vaginal tract is highly acidic to serve as a barrier against diseases or microbial infections that may occur in or on the reproductive organs of males. To neutralize the acidity of the female tract that could damage or affect the motility of sperm cells, males have a prostate gland that secretes an alkali buffer to bring the pH of the female tract closer to neutral (pH = 7) so that sperm mobility and survival is optimized.

Once sperm cells reach the egg, a single sperm cell fuses with the egg cell. An enzyme then allows penetration of the genetic material of the sperm into the egg cell. Once this is done, the membrane of the egg becomes sperm proof to prevent inclusion of additional genetic material that would unbalance the process and possibly render the zygote non-viable.

Although this process seems simple, males release millions of sperm in each ejaculation so obviously not all sperm reach eggs. Sperm mobility may be affected by external conditions (such as acidity in the female tract), but also by their age, and older sperm become less mobile as they age. Strength and mobility of sperm cells is crucial, especially if females copulate with numerous males, and in these cases, sperm of multiple males may be present at the same time, forcing males to compete again for the available eggs, this time via sperm wars within the female’s reproductive tract.

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