As a result of studying this chapter, the student must:
• principles of the formation of sexual behavior;
• the role of different communications in the sexual behavior of animals of different taxonomic groups;
• similarity and difference of ways of interaction of animals during communication in groups and at reproduction;
• features of the formation of sexual behavior in ontogeny;
• the main ways of evolution of caring for the offspring;
be able to
• Analyze the behavior of animals in the process of preparing for reproduction during direct observation and viewing of video materials;
• Analyze the concepts of the founders of the behavioral science;
• the skills of conducting a comparative analysis of the sexual behavior of animals of different systematic groups;
• Methods for conducting observations of animals during the breeding season.
Reproduction of animals
Reproduction is the most important biological process, ensuring the maintenance and increase in the number of species, the possibility of its dispersal and, ultimately, the success of the struggle for existence.
In the animal world, there are a number of ways of asexual reproduction, for example direct division or budding, characteristic of invertebrates, as well as parthenogenesis, noted even in higher vertebrates. It is quite obvious that asexual reproduction is the simplest and least energy-intensive way of increasing the number of individuals. Nevertheless, in the process of evolution a complex process of sexual reproduction, which involves many problems and conventions, arose.
A number of invertebrates, as well as many vertebrates, have a phenomenon such as parthenogenesis, in which females lay eggs or give birth to live young without the participation of males. Reproduced in this way only females, and it is interesting that in nature there are entire local populations of similar species. For example, a parthenogenetic population of rock lizards was discovered in Armenia. In other parts of its habitat this species reproduces in the usual way. Most often, such populations arise in sufficiently stable and favorable conditions of existence.
Parthenogenesis in experimental conditions is possible even in mammals. To do this, it is necessary in some way to stimulate an unfertilized ovum to divide, which can be done in different ways, for example, by simply applying a needle with a micro needle.
Sexual reproduction appears in animals already at the lowest stages of the evolutionary ladder. So, even in the simplest unicellular microorganisms - infusorians, propagating by direct division, there is a so-called conjugation , which is an analog of the sexual process. In the process of conjugation, two infusorians seem to fuse for a time, during which they exchange hereditary information. Then the infusorians separate, and after each continues to divide by itself.
A number of invertebrate animals have hermaphroditism , in which each individual has both male and female gonads. Among well-known animals hermaphrodites are, for example, earthworms, leeches and many kinds of snails. However, despite the fact that in such animals each individual produces both ovules and spermatozoa, they mate with each other, producing a mutual exchange of germ cells. Large, shellless aplisias, or sea hares, which live in the coastal zone of the sea, can participate simultaneously in the process of fertilization up to 10-12 individuals that play the roles of both males and females.
In some animals, including a number of fish species, sex change occurs with age. Thus, in well-known aquarists, the swordfish fish are fairly common is the transformation of older females into males. In this case, they grow a xiphoid process on the tail, which is found only in males, and they begin to exhibit typical sexual behavior, successfully fertilizing females. A similar phenomenon is observed in small fish of sea cadets living in the coastal zone of the Black Sea. In some species of fish, the reverse process is also noted: the transformation of males into females.
Invertebrates such as coelenterates, worms of different types, echinoderms, in parallel there is both sexual and asexual reproduction. For a multitude of invertebrates, alternation of sexual and asexual generations . For example, coral polyps, which are actively growing due to vegetative reproduction, periodically produce a huge number of mobile males and female sex cells. Fertilization occurs in the water column. From the resulting zygotes, mobile larvae develop, which settle on a suitable substrate and give rise to new colonies of corals. Similarly, all attached forms multiply: sponges, bryozoans, hydroids polyps, etc.
Some species alternate parthenogenesis with normal sexual reproduction. For example, aphids multiply throughout the summer by live birth, in which unfertilized females give birth only to females. Young female aphids begin to give birth to the following females at the age of several hours. Thus, during the summer there is a huge succession of asexual generations. Zoologists calculated that if all the descendants of one aphids survived, then in one summer it could cover the whole globe with a continuous blanket. Fortunately, since aphids feed on many other animals, this does not happen. It would seem that this method of propagation of aphids is very successful and does not require anything better. However, in autumn many aphids lay eggs, from which males are derived, fertilizing a relatively small number of females. While in autumn most aphids, having completed their life cycle, die, fertilized females successfully survive the winter, and in the spring they lay eggs. Hatchlings of winged aphids hatch from the eggs, they fly in different directions, then, settling on suitable forage plants, lose their wings and begin to reproduce with parthenogenesis. Obviously, to successfully survive in the conditions of the continental climate, it is necessary for the aphids to expand the spectrum of variability in the genome, provided by sexual reproduction.
Thus, the main value of sexual reproduction is not simply to increase the number of individuals, but to expand the gene pool that furthers natural selection.
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