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 gamete-transfer 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 bulbo-urethral 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.
that will enable her male progeny (i.e., son) to produce more children (i.e., increased fitness).
Mate guarding and defense following mating is a simple way for a male to reduce the odds of another male copulating with a female. However, the trade-off is obvious: staying with one mate precludes males from courting others, and strategies that allow males to protect their paternity without being present would yield great advantages. Sperm competition is one such process that can be simply summarized as any event that leads to sperm of two or more males being present at once inside the reproductive tract of a female. Males that release seminal fluids with greatest number of sperm, and sperm with the greatest mobility are thus more likely to fertilize female eggs.
Other strategies also exist for males to ensure paternity. In some species of primates such as the Senegal bush baby (Galago senegalensis) or ring-tailed lemur (Lemur catta), males have a penis that is highly spinuous, and the function of these spines is to alter the reproductive tract of the mated female so that she become less receptive to subsequent mating from other males. In carnivores such as wolverines (Gulo gulo) or American mink (Mustela vison), the penis bone may also play a role in causing enough stimulation for females to abort the first set of fertilized eggs, thus allowing males with larger penis bones to father more offspring. This also would allow females to compare male quality via the size of males' penis bones inside the reproductive tract instead of by classic displays.
In many species of rodents such as brown rats (Rattus norvegicus), primates, or bats, some of the seminal fluids will form a copulatory plug. This plug is formed soon after copulation and it appears that its main function is to prevent leakage of sperm from the female reproductive tract following
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.