Odd as it sounds, mammals are not the only organism to nurture their offspring with something analogous to milk. Yes, even non-mammals nurse their young, although admittedly, non-mammals don’t actually produce milk.
One of the best known examples comes from the world of birds. A few types of birds, notably pigeons, flamingos, and penguins, produce a substance from the lining of their crops (the muscular pouch near the throat where food is temporarily stored) known as crop milk. Both males and females produce crop milk. According to biologist David E. Blockstein, writing in the Wilson Bulletin regarding doves, crop milk is high in protein and fat. For the first few days after hatching, the babies are completely dependent on crop milk for sustenance.
Doves are also unusual in that regardless of species, they all produce crop milk and hardly ever lay more than two eggs in a brood. Crop milk contains a special growth factor in addition to the nutrients, so the hatchlings grow exceptionally quickly during the crop-milk dependent stage. This early period of growth may be essential for survival. Blockstein found that if an extra egg was added to a nest, the growth rate during the crucial early period declined precipitously. The ability to produce crop milk, then, might give baby pigeons and doves an early advantage. The birds’ usually small brood size may in fact be closely related to crop milk.
Thousands of miles to the south, in the Amazon basin, the discus (a genus of related fishes) has an equally unusual approach to feeding its young. Scientist W.H. Hildemann notes in American Naturalist that aquarists noticed that newly-hatched fry refused all food and frequently died when separated from their parents. Even before the eggs hatched, the adults took good care of the eggs, fanning them to provide oxygen and zealously guarding then nest. It was observed that after hatching, the fry soon moved over to their parents, where they fed on a thick mucus produced by cells on the surface of the adults’ bodies. Like any parents, discus fish need a break sometimes and flick the fry over to the other adult. While the fry begin eating other food after a week or so, they continue to feed off their parents’ mucus for several more weeks. While Hildemann didn’t have the means to analyze the mucus, it is now believed that it contains a variety of proteins valuable for growth and development, including immune protection.
In both mammals and non-mammals, “milk” aids in growth and protection from infection. So maybe other animals don’t literally produce milk, but the similarities are very strong.