Abstract
Core to Mammalia is the ability to synthesize mammary secretions (milk). Lactation is a complex trait that plays a central role in shaping mammalian life-history characteristics, morphology, and physiology, and the large demands associated with milk synthesis contribute to the high costs of reproduction in mammals. In this chapter, we synthesize information across the fields of ecology and evolution, organismal biology, integrative physiology, biomedicine, anthropology, and animal (dairy) science to provide a holistic view on lactation. While our aim is to provide a comprehensive primer on lactation using examples from species across mammals, the majority of work on lactation physiology and milk composition derives from few representative eutherian species of biomedical and agricultural importance. We review the ecological and evolutionary factors that led to the emergence of this adaptive feature and shaped the diverse lactation strategies seen in extant species. Because lactation relies on integration across the whole organism, we examine the integrative organismal physiology that supports and constrains milk synthesis and lactation performance. We then narrow our focus to a review on mammary anatomy as it relates to milk synthesis before giving an overview on the proximate physiological processes underlying each stage of lactation (mammogenesis, lactogenesis, galactopoiesis, milk ejection, and involution). Our chapter ends with a brief discussion on how milk shapes offspring development and phenotype.