I wrote a novella about a winged species and while I've loosely modeled them on bats, I have a spinoff story involving a child of the species and the story works better if they're egg layers. So I went in search of animals occupying the liminal space between mammals and birds, and discovered some fun facts about lactating egg layers that surprised me.
- Male emperor penguins — not the females — can produce milk to feed the chicks that hatch from the eggs they incubate, one at a time.
- Mammary gland secretions probably evolved to keep eggshells moist.
- Some cockroaches feed their young a kind of milk, produced by cells in the brood sac (similar to a mammalian womb).
- Baby discus fish nurse by eating a thick mucus produced by cells on the surface of the adult's bodies.
- Tsetse flies gestate their young internally, one at a time, and give birth to them live — the larvae are born encapsulated in a sac of milk.
Avian milk is called bird milk or crop milk. It has more protein and fat than human or cow milk, but lacks calcium or carbohydrates. Because English doesn't have enough homonyms, a bird's "crop" is a food storage organ located between the esophagus and stomach. That's where baby birds are aiming when they stick their beaks down their parents' throats.
The nutritional reserves in egg yolk come from vitellogenin proteins. Casein is the comparable "milk resource" in lactating mammals. Most mammals have non-functional "pseudogenes" for producing vitellogenin, but monotremes like the platypus still have one functional vitellogenin gene (as opposed to the three found in, say, chickens). There are three main lineages of mammals; placental mammals, marsupials, and monotremes.
Egg-laying monotremes like the echidna and platypus don't suckle; the females lack nipples and lactate through areolae glands. For a long time, paleontologists thought that suckling (and sucking, as through a straw) evolved after monotremes split off from the placental mammal tree, but it turns out that this might not be true; the suckling reflex might have been lost after the monotremes evolved to eat hard-shelled prey.
Some bird parents stop eating around the time their eggs hatch into chicks so that their milk doesn't get any seeds in it, since the babies' digestive systems aren't developed enough to digest "regular food" for at least a couple of days. This takes a couple of days for pigeons. Flamingos take about two months to develop enough to eat solid food.
📗 If you found this interesting, you may also enjoy my plausibility deep dive on herding giant vegetarian spiders.
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