Current thinking is highlighting the mammary glands and the process of lactation in the evolutionary success of mammals over and above the selective advantages provided by the nutritional and antimicrobial properties of milk. The extended period of contact between mothers and their young, necessitated by the regular and frequent transfer of milk, particularly characteristic of the primate strategy of reproduction and the primate mode of life, has been suggested to afford the offspring the opportunity for more learning and the eventual development of the levels of intelligence present in "higher" primates. Lactation offers the opportunity for maternal effects on development and the eventual phenotype of the offspring in addition to those that occur during pregnancy or from behavioral interactions. Lactation comes with high metabolic costs, which are manifested in parent-offspring conflict, and special physiological adaptations have evolved which match milk supply to demand by the young.