Patterns of reproduction are truly fundamental to mammal biology. This is at once apparent from the word mammal itself. In all species of the class Mammalia (monotremes, marsupials, and placentals), females suckle their offspring, and almost all of them have teats (mammae) to deliver the products gathered from the milk-generating glands. As defining features of the class Mammalia, mammary glands and milk production (lactation) are clearly central to mammalian evolution. Indeed, these features undoubtedly appeared at an earlier stage than the birth of live offspring (vivipary). Whereas marsupials and placentals give birth to live offspring after a period of development within the mother's body, monotremes (platypuses and echidnas) still lay large, yolk-rich eggs. Although the milk-generating glands of monotremes release their products through milk patches in the pouch rather than through a small number of teats, suckling of the offspring is clearly evident. Hence, suckling occurs in all extant mammals, and most species show a characteristic duration of this behavior (lactation period) as one of their reproductive hallmarks. Unfortunately for biologists concerned with the reconstruction of mammalian evolution, reproductive features are very rarely preserved in fossils. For this reason, the origin of mammals is defined for practical purposes by the emergence of a new jaw hinge between the dentary and the squamosal, replacing the original reptilian jaw hinge between the articular and the quadrate. Even in the dentition, however, there are features that reflect the mammalian pattern of reproduction. One defining dental feature of mammals is the presence of only two sets of teeth throughout life (diphyo-donty), contrasting with the typical reptilian pattern of continuous, wave-like replacement of teeth (polyphyodonty). Young mammals usually have an initial set of deciduous teeth containing only incisors, canines, and premolars, which is replaced by a permanent set of teeth in which molars are also added. It is in itself revealing that the deciduous teeth of young mammals are referred to as "milk teeth", although the replacement of teeth may continue well after the end of the lactation period.
An intriguing question that arises is why lactation and suckling of offspring are consistently limited to female mammals. In principle, it should be possible for male mammals to produce milk as well and thus contribute directly to the survival of their offspring. This question is all the more appropriate because most male mammals (including the human male) have teats that serve no apparent function. As a rule, anatomical structures that have no function tend to disappear in the course of evolution, one striking example being the reduction and eventual loss of eyes in cave-living animals that live in the dark. Yet teats are so widespread among male mammals that it seems quite likely that male teats were present in the common ancestor of the marsupials and placentals. So why are they still present in most male mammals today, after some 150 million years of evolution? We are still awaiting a satisfactory answer to this enigma. All that can be said is that the initial structure that eventually gives rise to teats and (in females) to functional mammary glands appears very early in fetal development in both sexes, in the form of so-called milk lines, one on each side of the ventral surface of the body. Such early appearance in development is, in fact, a further indication of the basic importance of lactation and suckling in the evolution of mammals. But we still have no explanation for the fact that milk lines develop not only in the female fetus but also in the male.
The development of mammary glands is linked to another universal feature of mammals, namely the possession of hair and associated sweat glands, both developed in connection with the evolution of a relatively constant body temperature (homeothermy). It seems highly likely that mammary glands were derived from sweat glands through a secondary conversion that enabled them to produce a nutrient fluid instead of sweat.
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