Final answer:
Mammals feed their babies with milk produced from their mammary glands. The milk composition changes over time, initially being rich in immunoglobulins, transitioning into watery foremilk to quench thirst, and ending with creamier, fat-rich hindmilk to satisfy hunger.
Step-by-step explanation:
Mammals lactate to provide nourishment for their newborns through a specially adapted biological process. The liquid that mammal mothers feed their babies is called milk, and it is produced in their mammary glands. Guinea pigs are an exception, as their newborns can survive without milk from the beginning but will consume it if available. In contrast, young grey kangaroos are highly dependent on maternal milk, staying attached to a nipple within the mother’s pouch for an extended period after birth.
The composition of breast milk changes significantly during lactation and even during a single feed, adapting to the baby’s developing needs. Initially, the breast milk, known as colostrum, is rich in immunoglobulins which help to protect the newborn against infections and is critical for the infant's immune system. As breastfeeding continues, the milk transitions into what is called foremilk, which is watery and designed to quench the infant’s thirst, and rich in lactose and protein. Toward the end of the feeding, hindmilk is produced, which is creamier, higher in fat and designed to satisfy the infant’s appetite.