Reproduction mechanism of mammals
Mammals are dioecious.
The reproductive system is formed by paired testes in males and ovaries in females.
In males, spermatozoa are formed in the testes (which are removed from the testes through the vas deferens through the penis).
Females produce eggs in the ovaries that are much smaller than those of other vertebrates.
Fertilization in mammals is internal. Mammals are viviparous (with the exception of the platypus and echidna).
In the early stages of development, the embryos of mammals are in many ways similar to the embryos of amphibians and reptiles. They have a notochord, gill slits, and other features.
The embryo develops inside the mother’s body in a special organ of the female – the uterus, where it is protected from the effects of adverse environmental factors.
The place in the uterus where the mother’s blood vessels come in contact with the embryo’s blood vessels is called the placenta. The embryo is connected to the placenta by the umbilical cord, in which the blood vessels pass.
In the placenta, the numerous blood vessels of the embryo are in close contact with the blood vessels of the mother’s body. Through the walls of blood vessels, the developing embryo receives nutrients, oxygen and is freed from carbon dioxide and other substances unnecessary for it.
The duration of the development of embryos in the uterus (pregnancy) is different in different mammals. Usually, the smaller the mammal, the shorter the gestation period: small animals (mice, hamsters) carry babies in the womb for 11–15 days; medium-sized (rabbits) – 1 month; large (moose, cows) – about 9 months.
After the cub is fully formed, the mother begins labor. The walls of the uterus are greatly reduced and the fetus is pushed out of it through the birth canal. The baby born into the world begins to breathe on its own. After that, the umbilical cord, which provides the connection between the embryo and the placenta, is torn or gnawed by the female.
At first, the cub is fed by mother’s milk, which is formed in the mammary glands of the female at the time of birth. Milk has a high nutritional value and contains all the substances necessary for the growth and development of offspring.
In different species of mammals, newborn cubs are developed unequally.
Mammals that live in open places, do not have shelters and flee from enemies, as a rule, give birth to sighted cubs, covered with wool and able to independently move after their mother (horses, zebras, goats, sheep, saigas, gazelles, etc.).
Mammals, making burrows or nests, in which their offspring are protected from enemies, give birth to helpless, naked and blind cubs (rabbits, squirrels, foxes, dogs, cats). The mother protects and feeds them for a long time.
Caring for offspring
No other animals surround their young with such care and spend as much time raising them as mammals. The instinct of caring for offspring is especially developed in those mammals whose young are born helpless. Mothers warm them with the warmth of their bodies, lick them, protect them from enemies, teach them how to find food. Some mammals (bats, koalas) carry babies on them.