The Wonder of Milk

Mammary glands, the source of milk, are unique to mammals, as is milk itself, though pigeons produce something similar in their crops for their young. Mammary glands are what give our class of animals their name. We are the mammals or the mammary gland possessing animals.

Mammary glands are modified sweat glands and are not the same as the mamillae (nipples or teats) through which most young mammals obtain the milk secreted by the mammary glands. Though all mammals have mammary glands and produce milk, not all mammals have teats. The exceptions are the two monotremes the Echidna and the Platypus.


In monotremes the milk is secreted onto the surface of the skin like sweat and licked off the body hairs by the young. In later mammals the mammary glands became more complicated with the possession of special ducts which channel the milk into the teats. The number of teats varies from 2 in Primates and the Marsupial mole, to 19 in the Pale Bellied Oppossum. Milk contains proteins, fats, carbohydrates, minerals and water. The proportion of the various constituents is highly variable between different species of mammals.

The Approximate Constituents of the Milk of Various Mammals in ml/mg per litre
Animal Water Fats Proteins Sugars Ash K2O NaO2 CaO Mg P2O5
Human 870 40 15 70 ? 1.20 0.65 1.04 0.12 1.40
Horse 900 22 20 60 3.6 1.05 0.14 1.24 0.13 1.31
Cow 880 34 33 44 7 1.4 0.8 1.65 0.21 1.45
Goat 862 48 48 46 8.5 1.3 0.6 2 0.16 2.91
Pig 840 50 37 50 6.3 0.99 0.74 2.4 0.14 3
Dog 770 93 97 31 9.1 1.41 0.81 4.53 0.2 4.93
Reindeer 677 171 109 28 15 2.11 2.33 5.08 0.39 4.38
Harp Seal 437.9 428.2 119.8 ? 9.14 1.61 1.16 0.94 0.3 3.35
Blue Whale 471.7 381.3 127.9 ? 14.3 ? ? 4.26 ? 6.23

Milk doesn't just leak out whenever, it generally requires a chemical and physical stimulus. The chemical stimulus to release milk is the hormone oxytocin produced in the pituitary gland. The production of milk in the glands is stimulated by the lactogenic hormone also produced in the pituitary gland at the time of parturition (the act of giving birth). The physical release is normally the act of suckling. Sometimes kneading of the area containing the mammary glands is also required and you will often see young mammals (cats, dogs, sheep, pigs, cattle, etc) appearing to head butt their mothers while sucking on the nipple. Milk also contains antibacterial and antifungal agents.

The amount of protein in a mother's milk controls how fast her young grow. While a young horse takes 60 days to double its weight after birth a baby Harp seal doubles its weight in only 5 days after birth.


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