Mammals are divided into three large groups in relation to reproduction, although all have separate sexes, fecundation is internal and pups are fed milk secreted by the female's mammary glands.
- monotremes - this group includes the platypus and echidna, animals that lay eggs similar to those of reptiles, from which a tiny embryo is born that moves into a pouch, where its development ends by licking milk produced by the mother, because there are no nipples the other two groups);
- marsupials - In this group, which includes kangaroos, among others, there is no placenta to nourish the embryo during its development in the womb. Thus, at birth, marsupials are not fully developed. Females have a "double" reproductive system with two uteri and two lateral vaginas.
The offspring are born through an independent central birth canal that forms before each birth and may or may not remain open. For this reason, in some species the male's penis is forked.
Most species end their development within an external pouch in the female body - marsupium. In many species, females mate again during pregnancy, but the embryo will develop only after the previous offspring leave the marsupium - embryonic diapause;
placentals - This is the largest group of mammals, totally dominating the current terrestrial class and habitat. Amniotic eggs are usually tiny and held in the female's uterus for development, with the help of a placenta that provides fixation and nutrients (oxygen and food). In the opposite direction pass the excretions of the embryo. At birth, placentals are in a higher state of development than marsupials.
Although this reproductive method involves fewer offspring, it is a great success because it greatly increases the odds of survival of the offspring.
The milk produced by mammalian females is very high in fat and protein, which makes it highly nutritious, but also provides antibodies that help the juvenile to develop healthy. Since young people do not need to look for their own food in the first few weeks, it allows for a safer start to life than in other vertebrate groups.
The litters can have up to 20 calves or only one, with gestation periods of only 12 days (bandicute, an omnivorous marsupial type) up to 22 months (African elephant).
Males have a copulatory organ (penis) and the testicles are usually in a scrotum external to the abdomen.
Mammals actively communicate with each other, either through odors produced by odorous glands (located on the face, paws or groins), urine or feces, or by body positions, facial expressions, touch and noise, which can form complex messages.
Socialization begins shortly after birth through signals between parents and offspring, continuing in youth with interaction between offspring (play). Some species only interact to mate, but the vast majority form groups, permanent or temporary.
Social unity has several advantages, including the safety and ease of obtaining food, but there are other important aspects. Generally space management implies that the group, couple or individual defend their territory from same-sex and same-sex intruders.
In some species, such as seals or elephants, the sexes live apart most of the year, with males living alone or in small single groups. In this case, the competition to mate is fierce, with the most successful males being the largest, strongest and best equipped (rods, horns or prey).
Other species, such as zebras, form small harems with a single male, and the rest are expelled to single groups unless they defeat the dominant male in combat, robbing them of females.
The most complex type of social group is made up of several males and several females, and almost without exception is reserved for social primates and carnivores. In primates there is usually a constantly changing hierarchy, with higher-ranking males being the first to mate. In lions, males (usually siblings) collaborate in defending females, not competing for mating. In wolves and mabecos, the packs are made up of an alpha couple, the only one that mates, and the children of previous years, who instead of forming new packs remain and help raise younger siblings.