We know how important milk is for babies. It is a rich blend of nutrients and protective chemicals that is essential for a baby's development.
In the entire animal kingdom, only one group of animals produce milk for their young: the mammals, the group we belong to. Mammalian milk is called "true milk".
However, a few other animals produce secretions for their babies that strongly resemble milk.
This "false milk" does not the look like cow or human milk, and nor is it produced the same way. But it serves the same purpose: it nourishes the animals' babies until they are old enough to fend for themselves.
Pigeons are great at sharing parenting duties. Unlike mammals, where only the female breastfeeds, males and females both produce "milk" for their young. This liquid is called "crop milk".
The crop is a sac-like structure at the base of a bird's neck, which they normally use to store and moisten food before digesting it. Two days before the pigeon eggs hatch, both the parents' crops become engorged with fluid-filled cells.
The pigeon parents regurgitate this thick milky goop into the mouths of their squabs. This crop milk remains the squabs' only food for several days after hatching.
Pigeon milk is extremely rich in proteins and fats. When it was fed to chickens in a 1952 study, their growth rate shot up by 38%.
A few other birds, like flamingos and emperor penguins, also produce crop milk.
Yes, you read that right: some cockroaches feed their young a kind of milk. One such example is the Pacific beetle cockroach.
Most female cockroaches lay eggs in a sac called an ootheca, which they drop from their bodies before the eggs are about to hatch. After the young cockroaches hatch out of the eggs, they scamper about looking for food.
But the Pacific beetle cockroach female takes a different approach to childcare.
Instead of laying eggs, the embryos develop inside her brood sac, her version of a womb. Once the embryos have fully-formed guts, they start drinking "milk" produced by cells within the brood sac, and quickly put on weight.
Because the young cockroaches get a lot of nutrition while still within their mother's body, they are more developed and mature when they are eventually born.
Like Pacific beetle cockroaches, female pseudoscorpions produce a milk-like substance. But she secretes it from her ovaries instead of her womb.
The female carries her fertilised eggs in a sac-like structure attached to her belly. Once the infant pseudoscorpions hatch, they stay in the sac and feed on their mother's ovarian milk.
Even after they leave the sacs, they continue to hitch a ride on their mother's back until they are old enough to live independently.
Pseudoscorpions are only 2-3mm long. They are often found in rooms with dusty books, so they are sometimes called "book scorpions".
The "milk" of discus fish doesn't look milky at all. Their milk is in fact a mucus-based secretion that coats the bodies of both parents. It is rich in proteins and antibodies.
A few days after the young discus fish hatch from their eggs, they make their way to their parents, and feed on the mucus secretions coating their bodies.
For the first two weeks, they spend most of their time feeding their young. Feeding lasts 5-10 minutes, after which one parent flicks the young hatchlings onto the other parent.
From the third week onwards, the parents start "weaning" their young. They swim away from them for prolonged periods of time, forcing them to look for other sources of food.
This pattern, first nourishing their young and then slowly weaning them, is similar to how mammals care for their children.
Taita African caecilian
Caecilians are amphibians, like frogs and salamanders, but they have lost their limbs and look like worms. Most species guard their eggs until they hatch and then abandon them.
The Taita African caecilian from south-east Kenya has developed a more elaborate style of parenting.
When the young caecilians hatch from their eggs, they are immature and totally dependent on their mother. To feed her babies, the female caecilian transforms the top layer of her skin into a thick layer of protein and fat.
Instead of drinking the secretions, the infant caecilians scrape off this skin layer using modified teeth. The skin is so dense with nutrients that in just a week the young increase in length by about 11%.
This takes a big toll on the mother. After one week of feeding her young in this way, she loses about 14% of her body weight.