A listener wrote to us with an interesting question. She wondered what triggers lactation in monotremes.
What Happens In Placental Mammals?
In humans and other placental mammals, the detachment of the placenta triggers hormonal changes in a new mom that lead to milk production. But monotremes, like the platypus, lay eggs and don't have placentas. So how does the mother's body know when to start lactating?
As in humans, milk production in monotremes is triggered by a hormone called oxytocin. In mammals with placentas, such as humans, oxytocin increases during pregnancy, but high levels of the hormones estrogen and progesterone inhibit milk production until the baby or babies are born.
In contrast, monotremes may begin milk production as soon as their eggs are laid, rather than when the babies hatch. Why make milk for babies still in their eggs? To answer that question, we need to go back 250 million years, when milk production was just starting to evolve.
Fossil and molecular evidence suggests that lactation evolved during the Triassic period. Like modern monotremes, our early mammalian ancestors did not have nipples, but produced secretions from glands in their skin.
Evolution Took Place
One hypothesis is that these secretions evolved as a protective coating for eggs with soft, thin, parchment like shells, such as those of modern monotremes. This hypothesis is supported by the observation that during incubation monotreme eggs are covered with a moist sticky substance.
Unfortunately, monotremes are notoriously secretive breeders, so no one has yet identified whether the sticky coating is produced by the mother's mammary glands or not.
So until we learn more about the reproductive behavior of these rare and secretive mammals, we can't know for sure how and when lactation begins in the monotremes.