Reproduction is possible only when the animals reach puberty. The timing of its occurrence varies greatly among species, but, as a rule, in small forms with a short life-span, the ability to reproduction occurs earlier. Some small rodents are able to bring offspring as early as a month old, not yet reaching the size of an adult animal. Hares reach puberty at the age of about a year, tigers at 5, rhinoceroses at 12-15, elephants at 15-20.
The males and females of many species of animals become sexually mature at the same time, which to a large extent is due to hormonal suppression on the part of older and dominant individuals.
The process of reproduction is of a cyclic nature, i.e. is associated with the periodic recurrence of a complex of specific phenomena. During the sexual cycle, a number of changes occur in the animal's body. These changes are the maximum during the breeding season in the female. The sexual cycle must end with fertilization, but if this does not happen, the female remains single. Sex cycles are repeated until the female is capable of reproduction. After reaching a certain age, this ability is lost, the so-called menopause occurs.
In terms of the number of sexual cycles, the species polycyclic, are distinguished in which the sexual cycles of females are repeated several times a year, and monocyclic, having one cycle in a year, timed to a certain season.
Sex cycle in males is somewhat weaker. It consists mainly in changes in the intensity of spermatogenesis, as well as in the behavior of animals.
The sexual system is very sensitive to changes in the environment. As research has shown, the rhythm of the sexual cycles is influenced by many factors, in particular food, temperature, light, etc.
The whole range of phenomena associated with the process of reproduction in animals, consists of several stages:
1) preparation for reproduction;
2) sex and intercourse;
5) suckling period and rearing of offspring;
6) the disintegration of families and the resettlement of the young.
Let's consider some features of these stages.
Preparing for reproduction
The first stage of the sexual cycle - the stage of rest, is characterized by the balance of the general state of the organism necessary for accumulation of forces and preparation for reproduction. However, the "rest", in which the organism is at this time, is relative: the sex glands of many species continue to gradually produce sex cells. In fact, this definition is only the external manifestations of the relationship between the sexes.
At the end of the mating period, the excitement of males and females decreases. If there is no pregnancy, then in the sexual cycle of females comes the stage of inhibition, characterized by extinction of sexual tone, a decrease in ovaries. The organism of the animal, thus, again returns to a state of rest.
Most of the individuals after the mating period are beginning to prepare for growing offspring.
As the reproduction period approaches, the stage of rest is replaced by the stage of excitation. This stage is characterized by a number of changes in the female genitalia and the general excited state. Under the influence of increased production of the anterior lobe of the pituitary gonadotropic hormone in the ovaries, intensive development of follicles occurs. Ripening follicles release a follicular hormone into the blood of the female, which causes (through the central nervous system) genital flushing, an increase in the size of the ovaries, and a more or less pronounced attraction to the male.
Simultaneously with the changes occurring during the estrus in the sexual organs of females, the sexual sphere of males also changes accordingly. Preparation for reproduction in them consists mainly in intensive maturation and the formation of male sex cells - spermatozoa. Like the ovaries, the testes and their appendages also increase in size during the period of preparation for reproduction. The presence of a male, his appearance, voice, smell, act as a stimulant on the process of ovulation in the female. In an isolated female, the formation of follicles occurs slowly; such a female is often infertile after intercourse.
The whole aggregate of sensations perceived from the other sex excites the partners. In many species of animals, the newly-entered females begin to search for rapprochement with males, flirt with them, produce a series of characteristic movements, urinate more often, adopt a pose before the mating that facilitates this act. In other species, females are less active and externally remain calm. The males, driven by the sexual instinct, begin to persistently pursue the females, chasing them. In connection with this, this phase is often called the "gon".
Thus, gon, or mating period, - is a set of actions aimed at connecting different-sex individuals, i.e. on mating, which results in the act of copulation.
The implementation of copulation is provided by a number of unconditioned and conditioned reflexes, manifested in a certain sequence.
Methods of fertilization
There are different types of fertilization. In all higher vertebrates, ringed worms, gastropods and arthropods, the insemination internal, ie, occurs when the male's sexual organ is introduced into the genital tract of the female. Most invertebrates and inferior vertebrates reproduce by virtue of the external fertilization performed with the simultaneous separation of sexual products by the female and male into an external, usually aquatic environment. As a typical example, fish or amphibians, whose females lay unfertilized eggs (eggs), and males that are nearby, immediately impregnate them with seminal fluid (milk). Often males actively induce females to spawn, pushing, biting or squeezing the female with their paws. Thus, the males of many species of frogs in response to certain touches demonstrate a so-called embracing reflex, expressed in the clutching of the object that touches their abdomen. For such an external fertilization, the decisive moment is that the process itself occurs in water, which promotes motility of the spermatozoa. Many terrestrial and some aquatic invertebrates have a kind of remote fertilization. Males of similar species excrete sperm with dosed portions packed into thin-walled sacs - spermatophores, protecting spermatozoa from drying. Males leave spermatophores in places of accumulation of females, and females that find them use sperm for egg fertilization.
The Role of Chemical Communication in Reproduction
In all processes associated with reproduction, a crucial role is played by chemical, or olfactory, communication.
1. Individual smell. As we have already said, the smell is a kind of "business card" animal. It is purely individual, but at the same time species-specific. The individual smell of an animal is formed from a number of components: its sex, age, functional state, sexual cycle stage, etc. This information can be encoded by a number of odorous substances that make up the urine and secrets of specific glands. Sources of odor are products of incomplete anaerobic oxidation of secretions released to animals. A huge role in creating an individual smell is played by the microbial landscape.
In many species of mammals, the male during the rush acquires a very strong specific odor (deer, antelope, goat, wild boar). When mating, he reports a "male smell" and the female, than prevents fertilization by other males.
2. Pheromones and their role in sexual behavior. Microorganisms living in the animal's body take an active part in the synthesis of pheromones. During the breeding season, the secretion of musk glands increases in the females, while the sex hormones and specific sex pheromones appear in the secretion. In even greater amounts during estrus these substances are contained in the urine. They contribute to the creation of an odor that attracts males, in whom the susceptibility to the corresponding pheromones sharply increases during this period. Secretion of glands and marking activity is enhanced not only in females, but also in males. For example, during the breeding season, portions of the so-called beaver stream left by representatives of both sexes in beavers on elevated portions of the shore serve as one of the means facilitating the encounter of males and females. A huge role is played by the sense of smell and pheromones in the life of insects - we have already cited the example of males of some butterflies that capture the specific smell of a female in the presence of several molecules in a cubic meter of air.
3. The role of pheromones in regulating the processes associated with reproduction. In addition to attracting representatives of the opposite sex, pheromones have a huge impact on social and sexual behavior and regulate reproduction functions.
Urine of aggressive males contains a specific pheromone of aggression, which includes metabolites of male sex hormones, which can promote aggression in dominant males and subordination reaction in low-ranking individuals.
Laboratory studies have shown that the smell of urine of male mice in addition to aggression causes in individuals of the same species many other behavioral and physiological reactions. The smell of an unfamiliar male inhibits the study of the new territory by other males, attracts females, blocks pregnancy, triggers synchronization and acceleration of estrus cycles, accelerates the sexual maturation of young females and suppresses the normal development of spermatogenesis in young males. As indicated by VE Sokolov and EV Kotenkova, the pheromones that cause the above reactions have the same structure as the pheromones of aggression. Similar phenomena are described in many representatives of different taxonomic groups.
Hormonal suppression of sexual activity is observed among primates. So, for example, in small monkeys, a game of urine and other excretions of the dominant female, bearing its individual smell, depending to a large extent on the volatile molecules of pheromones, affects its adults and in principle are fully capable of reproducing daughters in such a way that they are in company with their own mother are unable to conceive. When the dominant female grows old and her pheromones lose strength, the roles of mother and daughter change. Now already one of the daughters acquires the status of a dominant and begins to bring offspring, suppressing the sexual functions of the mother and her younger sisters. Similar phenomena are observed among males of this species.
Absolutely amazing examples of regulation of sexual activity of individual individuals are given by social insects. Here is how the processes occurring in the family of honey bees are described, E. N. Panov: "The queen bee, unlike the female workers, produces special active substances, the so-called pheromones of the uterus. At least one of them, produced by its maxillary glands, has a number of powerful effects on the behavior of worker bees. In particular, it stimulates bee-workers to build honeycombs, but at the same time hinders their activity in the construction of queen cells. In addition, this "uterine pheromone" inhibits the development of ovaries in female workers, who are thus completely deprived of the ability to produce and lay eggs in the presence of the uterus. However, all these effects are possible only if the concentration of pheromone circulating among members of the community is high enough. Pheromone enters the working bees in several stages. Its main carriers are bees from the retinue of the uterus, which feed it and periodically feel their antennae antennae. The composition of the suite is changing: some wet-nurses leave the uterus, others occupy the place of the departed in the circle of the maids of honor. Former members of the suite exchange food with workers in other sectors of the nest, carrying on themselves insignificant doses of magical mother material. So the chemical signals about the presence and the state of the queen are spreading through the relay among all members of the collective, leaving no one unaware of the momentary situation.
It is clear that the larger the community, the smaller the dose of pheromone coming to each of its members. As the family grows, the "dilution" of the mother substance increases, its concentration decreases. This is what happens at the turn of spring and summer, when all the cells are occupied by the brood, which daily gives hundreds of young bees-recruits. In this situation, the queen is already struggling to find empty cells, so she simply under the pressure of circumstances has to sharply reduce the rate of egg laying. Forced uterine inactivity leads to a decrease in the size of its ovaries, which immediately affects the overall physiological state of the insect. In particular, some scientists believe that the production of uterine pheromone in the mandibular glands of the queen is slowing, and this magical substance almost completely loses its magical effect on the ever increasing number of working individuals.
The results of all these changes begin to manifest themselves immediately. The first thing the worker bees are starting is the construction of queen cells. Soon, some of the workers begin to increase their ovaries, and the number of these Ichel-treads in the nest grows rapidly. According to the observations of the Ukrainian entomologist PG Moskalenko, the whorls often behave extremely aggressively towards the uterus and even to the bees from her suite, bearing significant doses of uterine pheromone.
... The approaching cataclysms in the life of the family are indicated by other changes in the behavior of worker bees. Not experiencing the influence of the uterine pheromone, they stop the construction of honeycombs and spend most of the time in complete inaction. Clinging to each other and forming dense bunches, hundreds of bees hang in a state of idleness on the lower edge of the honeycomb. The appearance of such bunches in the nest is a clear harbinger of swift swarming. In less than a few days, tens of thousands of workers will leave the overpopulated nest with a solid mass, dragging the uterus - their progenitor (Figure 12.1).
In nature, a very important factor for reproduction is the density of the population and the degree of tension of the hierarchical relationships in it. The hierarchy in the population contributes to the stabilization of numbers and prevents unlimited reproduction, as only high-ranking animals reproduce in a stable population.
Fig. 12.1. Swarming of bees
4. Psychological castration. According to EN Panov, the low birth rate in wolf packs exists because of the open despotism of the most high-ranking individuals. Although a large group of wolves may have several animals ready for reproduction, only one pair usually breeds. The dominant female during the rutting expels other mature females. A rigid hierarchical ranking is observed in the wolf pack and among the males. The behavioral ritual of the main male gives him an almost exclusive right to participate in reproduction. It is important to note that if the existing hierarchical structure is violated (for example, if the main male or the main female is removed from the group), the birth rate increases.
It can be assumed that in addition to directly suppressing the activity of a part of individuals, the psychophysiological mechanisms of pregnancy blocking (for example, resorption of embryos in wolves) can play a certain role in limiting fertility.
The ability of dominants to eliminate sexual competitors by their presence is called psychological castration. Similar situations are described in wolves, hyena dogs, certain primate species and many other animal species.
5. Synchronization of reproduction. Hormones and pheromones promote synchronization of reproduction in different individuals in the population. It is obvious that males and females should be ready for reproduction simultaneously. Under the action of hormones, the secretion of which depends on the season and especially on the length of daylight, the female has an estrous state. But the final fit occurs when the male and female are together. Thus, the sexual maturation of a female accelerates the males' presence alone, even if they are enclosed in a cage. In male pigeons kept alone, for example, there is no secretion of "goiter milk", necessary for feeding chicks and entertaining the female in the process of courtship. To do this, they need the presence of another male or female, and even their own reflection in the mirror is enough. In the fish of thorns during mating, each reaction of the male causes the corresponding reaction of the female, and vice versa. Thus, the zigzag dance of the male makes the female approach it; this in turn forces the male to swim to the nest. The appearance of a male swimming to the nest causes the female to be willing to follow it, etc.
Thus, the processes described above, such as: oppression of sexual function with the help of pheromones, psychological castration, synchronization of reproduction, etc. - in fact, they are the mechanisms by which sexual, or more accurately, natural selection is carried out in animal populations.
Also We Can Offer!
- Argumentative essay
- Best college essays
- Buy custom essays online
- Buy essay online
- Cheap essay
- Cheap essay writing service
- Cheap writing service
- College essay
- College essay introduction
- College essay writing service
- Compare and contrast essay
- Custom essay
- Custom essay writing service
- Custom essays writing services
- Death penalty essay
- Do my essay
- Essay about love
- Essay about yourself
- Essay help
- Essay writing help
- Essay writing service reviews
- Essays online
- Fast food essay
- George orwell essays
- Human rights essay
- Narrative essay
- Pay to write essay
- Personal essay for college
- Personal narrative essay
- Persuasive writing
- Write my essay
- Write my essay for me cheap
- Writing a scholarship essay