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Mammals are famous for the specialized fluid they produce to nourish their young, and they’re not the only ones that do it! This episode, we discuss the functions and evolution of Milk.
In the news
Cambrian apex predator Anomalocaris might have stuck to soft foods
New fossil alligator is related to modern Chinese gators
Puberty identified in the bones of marine reptile Keichousaurus
Fossil evidence shows Cambrian Explosion might not have been so explosive
Got Milk?
The word “mammal” comes from “mamma” or “mammary,” words that describe the milk-producing glands unique to this group of animals. All mammal species produce milk (a process called lactation), and all mammal young require milk for healthy development.
Top right: Deer nursing. Image by Scott Presnell, CC BY-SA 2.0
Bottom left: Pig nursing. Image by Marie Richie, CC BY 2.0
Bottom right: Elephant nursing. Image by Rkimpeljr, CC BY-SA 2.0
Milk is a special, complex fluid with many components, including carbohydrates, amino acids, a variety of proteins, sugars, fats, and elements important for the immune system. The exact composition of milk varies quite a lot from species to species, and it even changes over the course of the development of offspring. The delivery mechanism also varies; most mammals deliver milk via teats (or nipples) – the number of nipples on an individual is usually correlated to how many offspring they produce at once – although monotremes do not have nipples at all.
It’s unclear exactly when milk evolved in the evolutionary lineage of mammals. It might have been present in the ancestors of true mammals, which lived during the Triassic Period.
The anatomical origins of lactation are easier to understand. Mammary glands are structurally very similar to apocrine glands such as sweat glands, and the various components of milk (fats, sugars, proteins, etc.) are very similar to those found elsewhere in the body, where they help maintain healthy life functions. These structures seem to have been co-opted and repurposed during mammal evolution for the function of producing specialized substances to nourish developing young.
The evolution of lactation is even easier to comprehend when you consider that mammals aren’t the only ones that feed their young special bodily secretions. In fact, milk-like substances are surprisingly common!
Who Else Has Got Milk?
Top right: Some fish, including discus fish, produce mucus that newborns feed upon. Image by Bernard Spragg, CC0 1.0
Bottom right: Tsetse flies have special glands that produce milk-like fluids to nourish developing young. Image by David Bygott, CC BY-NC-SA 2.0
Many non-mammal species produce special substances to nourish their embryos or newborn young. Often, these milk-like substances have similar components to mammal milk, and they even vary in their composition over the course of the young’s development, just like mammal milk.
Many fish produce mucus that their newborns feed upon, providing the young with extra nutrients and possibly boosting their immune systems.
Some birds, including flamingos, pigeons, and emperor penguins, produce “crop milk.” This is a milky substance, rich in fats and proteins and carbohydrates, which they feed to their offspring. The composition of this crop milk varies from species to species.
Some sharks and rays are known to produce “uterine milk,” a milk-like substance – again, lots of fats and proteins – that is produced to help nourish developing embryos inside the uterus.
The ever-fascinating caecilians are also known to produce uterine milk, as well as provisioning newborn young with secretions from the parent’s cloaca and with a special layer of skin (which the newborns eat right off the mother’s body!).
Tsetse flies and certain cockroaches also produce milk-like substances through specialized glands to provide nutrients to their young. Tsetse fly “milk” in particular has a lot in common with mammal milk, including the timing of its production and certain very similar components.
One more example: jumping spiders! Some species have been observed producing a milk-like substance to feed their young over an extended period of development. Not only is this milk similar in composition to mammal milk, it also appears to be necessary for the little spiderlings to develop into adults!
One more thing: it’s interesting and unusual that we humans drink milk as adults. Mammal milk contains lactose, which can be broken down in the body by the enzyme lactase. In most mammal species, the body only produces lactase when the animal is young, and it stops as they mature. Thus, most mammalian adults are lactose intolerant – they can’t digest lactose, and therefore, drinking milk can make them sick.
If you aren’t lactose intolerant, it’s because you have a genetic mutation that stops your body from turning off lactase production. This type of mutation has originated and become widespread in humans multiple times over the last 10,000 years or so, and in multiple populations. This is what allows some of us adult humans to drink milk without issues!
We also drink the milk of other species, which is pretty weird. Humans are weird.
Learn More
Go Ahead, Try to Explain Milk (non-technical)
10 Animals That Make ‘Milk’ and Aren’t Mammals (non-technical)
How Non-Mammals “Nurse” Their Young (non-technical)
The evolution of milk secretion and its ancient origins (technical, open access)
Marsupial and monotreme milk (technical, open access)
Mucus provisioning behavior in teleost fishes (technical, paywall)
Lactose Intolerance (technical, open access)
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