Mammary glands are specialized glands unique to mammals. Along with several other traits, mammary glands set mammals apart from other members of the animal kingdom. These glands are capable of producing milk, a nutrient-rich substance which is used to nourish baby mammals. Depending on the species and the conditions, milk is produced for varying amounts of time, providing young mammals with the basic nutrition they need to thrive and grow until they are old enough to eat solid foods.
These glands are believed to have arisen from sweat glands. The groundwork for the mammary glands is laid during fetal development, and mammals of all genders are born with rudimentary mammary glands, laid out in pairs along the “milk lines” which run down the abdomen. Mammals such as humans produce only two mammary glands, in contrast with animals such as pigs, with an impressive 18. The difference in number usually reflects the number of young the mammal carries during a typical pregnancy.
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The glands stay relatively underdeveloped until the mammal begins to experience hormone changes which promote the further development of the mammary glands. This generally occurs in female mammals only. During pregnancy, additional hormone changes prepare the glands to actively produce milk, and when the baby is born, the mother can usually express milk very quickly. These milk producing glands work by connecting a series of hollows lined with milk-expressing cells to a system of ducts which drain to the nipple. As the baby develops and loses interest in milk, production will go down, and eventually the mammary glands will stop producing altogether.
It is sometimes possible for male mammals to produce milk from their mammary glands, although this is unusual, and may require the use of hormones to stimulate gland development and milk production. Several factors can influence the quantity and quality of milk produced. Diet is important, with malnourished mammals producing less milk, and diet can also affect the flavor of the milk in some cases. Stress can also be a factor.
Humans have developed a taste for the milk of several fellow mammals, including cows, sheep, and goats. These animals are raised for their milk and are routinely bred and weaned to keep up a consistent supply of milk. A wide variety of products can be made from animal milk, including cheese, butter, yogurt, kefir, and skyr, and many cultures have their own milk specialties, some of which are thousands of years old.