The newborn mammal, called the neonate, is constantly and rapidly changing, both structurally and physiologically.

In the uterus, the fetus is living in a warm, moist, protected environment, and receiving all its needs from the mother. Major metabolic and physiological changes occur in the transition from a fetus to the newborn.

At birth, most neonates -

have limited fat stores

the fat stores which are present are not readily available for metabolism

use up their limited glycogen stores rapidly after birth

have poor gluconeogenic capacity (synthesis of glucose by the liver)

are agammaglobulinemic (they have very low concentrations of immunoglobulin in their blood)

neonate of many species have low iron stores

have stucturally immature intestines

have immature digestive capabilities, including:

low activities of all pancreatic enzymes

low activities of stomach pepsin

low activities of many intestinal enzymes

immature stomach acid generating mechanism (stomach pH is ~3.5)

They do have -

high rennin activity - precipitation of casein, curd formation in the stomach

increasing lactase activity - breakdown of lactose in the intestine

high salivary lipase activity - breakdown of milk triglycerides

In addition, the composition of mammary gland secretions is constantly changing, particularly over the initial 3 to 5 days postpartum (see below on Colostrum Formation and Immunoglobulin Transport in the Mammary Gland; also see the Introduction and Milk Composition Lesson). There is considerable inter- and intra-species variation in how the colostrum changes during the initial days after parturition.

What controls the yield and composition of mammary secretions or the transition of colostrum to "mature" milk ?

- The physiological state or state of differentiation of mammary epithelia.

- Repeated removal of milk from the gland.

So, there is an effect of mammary function on neonate survival and vitality, and an effect of the neonate on mammary function.