Marsupial, immature mammals and birds, Mature mammals...

Marsupials

In the marsupials, the cub begins to develop in the uterus, but it is almost unconnected with the walls of the uterus. This circumstance also contributes to his early birth. The duration of pregnancy of marsupials is very small, especially in primitive forms (for example, in opossum or in marsupial cats - from 8 to 14 days, in koala - 35, and in kangaroos - 38-40 days). The degree of development of the embryo at the time of birth is somewhat different in different representatives of marsupials: its size does not exceed 25 mm in a large gray kangaroo - the largest representative of the detachment; in primitive insectivores and predators, it is even smaller - about 7 mm. To attach to the nipple, the newborn marsupial cub should get into the mother's bag, where its further development takes place. The newborn moves independently in the bag. He moves with the front legs with claws, wriggling like a worm, and twisting his head around. The space along which it creeps is covered with wool; this, on the one hand, hinders him, but on the other hand it helps: he firmly clings to the wool, and it is very difficult to shake it off. Finding the bag, he immediately climbs inside, finds the nipple and attaches to it. Between the moment of birth and the time when the calf is attached to the nipple, the marsupials usually pass from 5 to 30 minutes. During the time it takes a newborn to get to the bag, the mother occupies a special position and does not move. Kangaroos usually sit on the tail, passing between the hind legs and forward, or lie on their side. The mother keeps her head as if she is watching the baby all the time. Often she licks it - immediately after birth or while going to the bag. Sometimes she lick her hair towards the bag, as if helping the baby move in the right direction. Initially, the mummy nipple has an elongated shape. When a calf attaches to it, a thickening develops on its end, apparently connected with the release of milk; it helps the cub to keep on the nipple, which he always presses with his mouth. It is very difficult to separate it from the nipple, without ripping his mouth or damaging the gland. The marsupial calves passively receive milk, the amount of which is regulated by the mother with the help of contractions of the musculature of the milky field. Gradually, the baby begins to break away from the nipple and poke its head out of the bag. The first time when he wants to get out, he is not allowed by his mother, who can adjust the size of the outlet of the bag. Different types of marsupials spend in the bag a different period - from several weeks to several months. The stay of the baby in the bag ends, as soon as it becomes able to feed not milk, but other food. A mother usually looks for a nest or a lair in advance, where the children first live under her supervision.

Immature mammals and birds

The young of this group are born incapable of independent life. The degree of their immaturity can be different. For example, the young of many rodents (rats, mice, hamsters) are born naked, blind and deaf. The nestlings of most small birds also look the same.

Predators of predators are usually also blind and deaf, but covered with children's fur; young primates are born with covered fur, with open eyes, however their visual and auditory analyzers are still underdeveloped. With open eyes, covered with thick down, hatching birds of prey birds and gulls hatch from eggs.

Mature mammals

In this group the cubs are born with covered hair, capable of independent movements and with formed visual and auditory analyzers. They almost immediately after birth are able to move around the mother and along with the mother's milk to consume the food, typical for adult animals. Such, for example, are young ungulates, hares, guinea pigs, etc., and also chicks of brood birds.

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