One of the mammals' unique features is their mammary glands. All mammals possess them. In females they produce milk, in males they are usually non-functional. Only one male mammal, the Dayak fruit bat, is known to produce milk (men have been known to very occasionally produce milk, but this is due to a hormone imblalance).
Mammary glands are very like sweat glands, and in some mammals (the platypus and the two species of echidna) the milk simply oozes from the skin in the same way as sweat. Other mammals have teats, which help deliver the milk directly into the mouths of the young. Having teats, though, tends to limit the number of young that a female can suckle, and so most female mammals have twice as many teats as their average litter to make sure that none starve. A few (such as the Virginia opossum) give birth to more young than they can suckle, and so the 'slowest' young die from starvation.
Milk is a liquid made up of water, proteins, fats, carbohydrates and minerals, but the milk of different species has a different balance of each of these, tuned to the needs of the youngsters. High protein milk can help the young to grow fast while high fat milk helps them gain weight and put on blubber fast. Mammals that live in cold environments tend to produce very fat-rich milk to help their young develop insulation. Many seals and sea lions produce milk that is over 50 per cent fat, and hooded seal pups grow about 5kg per day, being weaned in only 4 days.
Kangaroos often have two youngsters of different ages suckling at once, one tiny and one nearly weaned. Therefore they produce two different compositions of milk from the two teats used by the youngsters. The very young kangaroo will be firmly attached to one teat constantly for the first few weeks and will be provided with milk with very little fat in it. The older youngster, though, will receive milk with almost 20 per cent fat from the other free teats.
The milk also contains antibodies, passed on from the mother to her offspring, which provides them with immunity to many diseases.
Producing milk for her youngsters puts a lot of strain on the mother's resources. She has to produce an ideal, balanced diet for the growing babies from her own diet. Often females have to metabolise their own body tissues to produce the milk, especially in hard times. If a drought or period of starvation continues too long, a mother will eventually stop producing milk and the young will die, but this may help her to survive and breed again when conditions have improved.
Camels store fat in their humps, which can be broken down to provide it with water, energy and Carbon Dioxide.