Macropodid marsupials such as the tammar wallaby (Mocropus eugenii) give birth to an immature pouch young. During lactation these mammals provide milk which progressively changes in all the major constituents to provide the appropriate nutrition for extensive development of the dependent young. The tammar also can provide a small volume of dilute (phase 2) milk for a newborn pouch young from one gland, and greater amounts of concentrated (phase 3) milk from another more developed gland for a young at heel, a phenomenon termed asynchronous concurrent lactation (ACL). We review recent data which shows that milk protein gene expression changes asynchronously during lactation, such that genes expressed in adjacent ACL glands differs significantly. This capacity for differential milk protein gene expression in adjacent mammary glands may reflect their differing sensitivities to circulating prolactin, and possibly other endocrine influences known to be required for mammogenic and lactogenic responses in this species. Specific prolactin binding changed markedly during lactation, and differed significantly in ACL glands. Nevertheless, it is most likely that endocrine control of lactation is modulated by local factors within the mammary gland. Local mechanisms sensitive to frequency of completeness of milk removal have been identified in other mammals, and preliminary evidence suggests that a similar mechanism, involving feedback inhibition by a secreted milk protein, operates in the tammar. Therefore, the macropod`s unique lactational strategy may arise, at least in part, from adaptation of a mechanism shared by most if not all mammals.
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