Why are Mammals Called Mammals: Breasts, A Swede, and the French Revolution

Why are mammals called mammals? The answer, which your biology textbook won’t tell you, is because a fussy scientist in the 18th century held very strong feelings about breasts.

The fussy scientist in question was Carl Linnaeus, who I’ve covered in some detail before. Linnaeus was a Swedish biologist with a life-consuming passion for classification. He invented a system of scientific naming called binomial nomenclature, which is still used today. Binomial nomenclature gives every species on Earth a two part name, consisting of a genus and species. These two part names are then structured into a hierarchy based on shared physical traits, creating the hierarchical system of naming you might’ve learned in grade school: Kingdom, Phylum, Class, Order, Family, Genus, Species.

Born Carl von Linne, changed his name to Carolus Linnaeus 'cause he loved his own system that much.

Born Carl von Linne, changed his name to Carolus Linnaeus ’cause he loved his own system that much.

This system allows taxonomists to easily compare relative relatedness among different species, and gives every species on Earth a unique identifier. For example, humans:

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Hominidae
Genus: Homo
Species: Sapiens

Linnaeus’s crowning achievement though was not necessarily the creation of this system, but his fanatical implementation of it. Over the course of his career, he named and classified some 4,400 species of animals, and nearly 8,000 species of plants. These names were collected in the Systema Naturae, a mammoth book which, by forcing itself into the public and scientific conscience, forever codified Linnaean taxonomy as “the way things are done.”

Linnaeus’s self-appointed position of “Namer-in-Chief” also gave him great power, which, as we all know, comes with great responsibility. Generally Linnaeus’s decisions were uncontroversial and immediately accepted. Birds, for example, where placed in the class Aves – simply Latin for ‘bird’. (Or I should I say bird is English for Aves?).

That’s not to say Linnaeus was above a little bit of fun. Being the arbiter of names also gave him ample opportunity for revenging himself upon his enemies. For example, Linnaeus named the small, ugly plant Siegesbeckia after a scientist who had criticised him.

It's pretty ugly. Credit:

It’s pretty ugly. Credit: M. Belov.

Passive aggressive? Perhaps. But also a compelling reason not to cross him — lest you be forever associated with a noxious smelling weed.

But Linnaeus’s most curious, most controversial, and most political-driven choice was in the naming of the class we now call ‘mammals’. Naming this particular group of animals has proven tricky ever since Aristotle first took a stab at it, and despite various deviations, that first Aristotelian attempt – Quadrupedia – stood until Linnaeus came along and opted to change it.

Linnaeus included two groups – whales and humans – in the Quadrupedia which made that name incompatible with the general theme, so he had to change the name. Natural historians had a few suggestions, based on physical traits shared by all animals of that grouping. Pilosa, they suggested, “the hairy ones”; or Aurecaviga, “the hollow-eared ones”. More recent anatomical research suggests that Neocorticia “the ones with a neocortex” would be appropriate too. But Linnaeus choose a different name, Mammalia – “the ones with breasts.” Specifically, the ones with mammary glands.

Breasts (meaning here, mammary glands), while undeniably a shared trait among a large group of animals, are a curious choice. They are present in only one-half of individuals (females), and even then are biologically functional for a relatively small portion of the time (lactation). In many mammals, they are shrunken and heavily reduced outside of pregnancy and lactation. For example, platypus and echidna do not have breasts, and instead have highly reduced internal mammary glands which exude milk through the animal’s skin during lactation. In the face of the ubiquity of hair, or the acknowledged anatomical fact of the three inner ear bones, mammary glands seemed to some biologists to be a strange choice of name.

Not 100% accurate, but reasonable enough. Credit: davezilla.com

Not 100% accurate, but reasonable enough. Credit: davezilla.com

But Linnaeus had his reasons — which may have been rooted in the gender politics of the 18th century.

The 18th century was awash in breasts — the maternal breast, in particular. Prior to the 18th century, the ideal breast was the sort found on Greek and Roman statues: high, round, young and decidedly unmotherly. A virginal breast. But in the 18th century, the maternal breast proved resurgent, rising in fascination in the culture of 18th century Europe. Its peak, perhaps, came during the French Revolution, when a maternal breast, heavy with milk, became a symbol used by delegates to the French National Convention.

Prior to the 18th century in Europe, you were likely to see this. Credit: Met. Museum of Art

Prior to the 18th century in Europe, you were likely to see this… Statue of Aphrodite. Credit: Met. Museum of Art

Unfortunately for women, what that flag was meant to symbolize was a return to ‘nature’ — and nature, in a society where the terms of citizenship were determined by men — meant a system where women were denied political agency, forbidden citizenship, and confined solely to a life at home. Breasts were used as a symbol to “legitimize the sexual division of labor in European society”, writes historian Londa Schiebinger. Philosophers, politicians, and natural historians (unsurprisingly, all men) used the breast, and the act of breast-feeding, to argue that women’s proper place was in the home.

But not so much this. Nami Island, Korea. Credit: fie-nuts.net

But not so much this. Nami Island, Korea. Credit: fie-nuts.net

In particular, they took issue with the common practice among upper and middle-class women of wet-nursing. Wet-nursing most commonly involved a wealthy mother having her offspring nursed by a poor woman who had lost her own infant, but was still lactating. Wet-nursing was a hotly debated issue. There was some evidence that it contributed to increased infant mortality, but it also allowed women the choice of continuing in public life while still having a newborn. It was also a useful source of income for poor women, who were paid for their time. The important thing was that women generally had a say: they could use a wet-nurse, or nurse their own offspring — they were given a choice.

Wet-nursing was unpopular with (male) commentators, including Linnaeus. As a practicing physician, and a firm believer in nursing by the mother, he published tracts condemning women who used wet-nurses. In writings that predated his System Naturae, Linnaeus contrasted ‘wicked’ wet-nursing with a wholesome and loving animal mother – whales, lions, tigers – that nursed their own young. Predicting our own contemporary specious arguments about poor people making poor parents, Linnaeus argued that the milk of lower-class wet-nurses could corrupt infants.

Erasmus Darwin (Chuck's granddad) once argued that cause of Caligula's nuttiness was being wet-nursed by a poor woman. Linnaeus quotes him appreciatively.

Erasmus Darwin (Chuck’s granddad) once argued that cause of Caligula’s nuttiness was being wet-nursed by a poor woman. Linnaeus quotes him appreciatively.

Linnaeus wrote strongly, and frequently, about the ‘natural’ role of women as a stay-at-home mom. In a heady culture rife with arguments over the meaning of nature, sexual division of labour, and whether or not women were deserving of citizenship and equal treatment under the law, is it any wonder that he chose Mammalia as a name? Schiebinger writes that Linnaeus “sought to render nature universally comprehensible, yet the categories he devised infused nature with middle-class European notions of gender.”

If you ask a biologist now why mammals are called mammals, they will likely tell you its because of the presence of mammary glands. But the underlying history — why mammary glands were chosen as the signifier instead of another shared trait — is less widely known. But that history is important as a reminder that science, no matter how much it would conceive of itself as disinterested and objective, can be, and often is, political.


Koerner, Lisbett. 2001. Linnaeus: Nature and Nation
Schiebinger, Londa. 1991. “The Private Life of Plants: Sexual Politics in Carl Linnaeus and Erasmus Darwin.” in Science and Sensibility.
Schiebinger, Londa. 1993. “Why Mammals Are Called Mammals: Gender Politics in Eighteenth-century Natural History.” The American Historical Review 98 (2): 382–411.

PS: Fun fact: mammalogy, “the study of mammals,” doesn’t mean what it thinks it means. The actual study of mammals would be “mammalology”. Mammalogy just means “the study of breasts.” I couldn’t find anywhere to include that naturally above, but since I live with a mammalogist, I felt obligated to include it here.

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