The reproduction strategies of mammals differed in such a way that, for the same problem (reproduction), solutions were found as different as egg laying and prolonged gestation within the maternal body.
Differences with respect to mammalian reproduction strategies are considered to be the most important in the evolutionary division of the Mammalia class, allowing the classification of mammals into three groups:
Representing this mammal subclass (subclass) Prototheria ) can only be found in Tasmania, Australia and New Guinea, with only three species: the platypus ( Ornithorhynchus anatinus ) and two species of anteater or echidna ( Tachyglossus sp.).
Monotremes have a cloaca – opening to the outside – common to the digestive, urinary and reproductive systems, such as reptiles. It is this characteristic that gives them their name, which means “one orifice”.
Unlike the other mammalian females, which have an ovary on each side, those of monotremes have only one ovary, the left, just like birds. As the eggs descend through the oviduct, where fertilization occurs, several glands add albumin and later a thin leathery shell. Eggs are laid 12 to 20 days after fertilization. During the incubation period, the embryo is fed by the yolk sac: in the case of the platypus, in the nest for about 12 days; in the anteater, for a week, in a temporary pouch that develops in the abdomen during the breeding season, to which females transfer the eggs after laying them. After hatching, the pups drink the milk that drips from the mother’s mammary glands to the tufts of abdominal hair on platypuses, or to the pouch on the echidnas.