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Most mammals are viviparous, giving birth to live young. [1] However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. [2] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian ...
Large mammals develop at an absolute slower rate compared to small mammals. Thus, the large mammal tend have longer gestation periods than small mammal as they tend to produce larger neonate. [32] Large mammals require a longer period of time to attain any proportion of adult mass compared to small mammals. [33]
The young are born at a very early stage, and can be classified as a "larva" in the biological sense. [18] In placental mammals, the egg itself is void of yolk, but develops an umbilical cord from structures that in reptiles would form the yolk sac. Receiving nutrients from the mother, the fetus completes the development while inside the uterus.
Oviparity: fertilisation is internal, but the female lays zygotes as eggs with a substantial quantity of yolk to feed the embryo while it remains in the egg. The egg is not retained in the body, or only for a limited time. [1] Oviparity is found in insects, birds. Among mammals, the monotremes are oviparous.
Monotremes (/ ˈ m ɒ n ə t r iː m z /) are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tract ...
The egg is not retained in the body for most of the period of development of the embryo within the egg, which is the main distinction between oviparity and ovoviviparity. [1] Oviparity occurs in all birds, most reptiles, some fishes, and most arthropods. Among mammals, monotremes (four species of echidna, and the platypus) are uniquely oviparous.
The class Mammalia is divided into two subclasses based on reproductive techniques: egg-laying mammals (yinotherians or monotremes - see also Australosphenida), and mammals which give live birth . The latter subclass is divided into two infraclasses: pouched mammals ( metatherians or marsupials ), and placental mammals ( eutherians , for which ...
In the case of whales, dolphins and porpoises, the single calf is normally born tail first which minimizes the risk of drowning. [7] The mother encourages the newborn calf to rise to the surface of the water to breathe. [8] Large mammals which give birth to twins is much more rare, but it does occur occasionally even for mammals as large as ...