Why Mammals Have a Monopoly On Milk

It all started with an egg

rhino nursing
A mother rhino breastfeeding her baby Ted via Flickr (CC BY-SA 2.0)

It’s something most kids learn in grade school science classes: all mammals produce milk. But why do mammals breastfeed, anyway? Researchers are slowly piecing together the story of lactation’s evolution, reports Shreya Dasgupta for the BBC— and what they’re finding might surprise you. 

For a start: lactation probably came long before mammals evolved. “Even though we now consider lactation as a characteristic of mammals, and it is clear that we are the only living existing creatures that have mammary glands, I believe that mammary glands have an older origin, Olav Oftedal of the Smithsonian Environmental Research Center in Maryland told the BBC.  In fact, writes Dasgupta, lactation started with critters that secreted extra water and nutrients through porous eggshells.

The first eggshells laid on land were vulnerable to drying out. The ancestors of birds and reptiles solved this problem with thicker, harder eggshells that prevented moisture loss. The ancestors of mammals, Oftedal says, may have gone a different route, keeping their eggs moist by secreting water from glands in their skin instead. Some frogs do this still — the male coqui (Eleutherodactylus coqui) cuddles with its eggs to keep them from drying out.

Dasgupta writes about other examples:

In some other amphibians – like the worm-like caecilians – the skin of females thickens with nutritious, fat-rich deposits. Once the tiny hatchlings come out of the eggs, they scrape this nutrient-rich skin off using specialised teeth.

In all these cases, the parents are actively transferring nutrients to their young through skin secretions. Oftedal argues they are comparable to breastfeeding.

The genes that encode for important components of milk — casein proteins — appear to be older than mammals, too. The three main groups of caseins all appeared before mammals split into their three main groups (monotremes, marsupials and eutherians). Gradually, those milk component genes replaced genes for producing egg yolks. This switch-over is apparent in mammalian genetics, writes Dasgupta. In fact, all mammals still carry three genes for egg yolk production, though they are switched off. The egg-producing monotremes like the duck-billed platypus, however, have kept one gene still active.

Eventually the not-yet-a-mammal ancestors that did produce milk died out, leaving milk production to the mammals instead. A few other animals do actually produce milk-like substances these days: for example, pigeons, flamingoes and emperor penguins have a thick, nutritious fluid that flows out of sacs near their throats. But this ‘milk’ has a completely different composition and looks more like pale yellow cottage cheese than fluid milk. A few types of insects including cockroaches and bat flies also produce a nutritional secretion for their young. But mammals can feel secure in their unique classification as the only true milk producers.

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