What liquid do mammary glands produce to feed young mammals? The answer is milk.
Milk is a nutrient-rich liquid secreted by mammary glands during lactation. It supplies young mammals with energy, water, proteins, fats, sugars, minerals, vitamins, and, in many species, immune compounds. This feeding system is one of the defining features of Mammalia and explains the group’s name, which comes from the Latin mamma, meaning breast.
Milk varies greatly between species. Seal milk is exceptionally rich in fat, helping pups grow quickly in cold environments, while the milk of many hoofed mammals supports a different growth pattern. Human colostrum, the first milk after birth, contains concentrated immune factors before mature milk becomes established.
The answer is not blood, nectar, or yolk. Mammary glands are specialized exocrine glands, and their secretions reach the young through teats in most mammals. Monotremes such as platypuses and echidnas are an important exception: they produce milk but lack conventional nipples, so the milk is released onto skin and gathered by the young.