Why Are Mammals Called Mammals?

We know that we are classed as mammals, as are many other animals on the planet. With mammals sharing several other features apart from the ability to provide milk, why are we called mammals?

Mammals were named by Carl Linnaeus, the father of taxonomy. Linnaeus named mammals after the latin word mamma, meaning breast.

In this article I look further into the reasons why we and other mammals got this name and look at the history of the name.

Mammals all share several features that other animals don’t. They have hair or fur on their bodies, although some, like whales, only have this before they are born. 

Mammals’ ears have three bones that are unique to them while other animals don’t. They all have a diaphragm that separates their heart and lungs from their abdominal cavity. 


Most are viviparous, meaning they give birth to live young (although there are notable exceptions, like the platypus). Among a few other features, mammals produce milk from mammary glands to feed their young. 

If mammals all share these similarities, why are we named after the latin word for breast? Where did the name mammal come from in the first place, and why do we call them by that name today?

Aristotle – Pre-mammal Taxonomy

Long before mammals were called mammals, Aristotle was trying to come up with different classifications for the animals he observed. 

He divided animals generally into two types: those with blood, and those without blood, similar to the way in which we distinguish between vertebrates and invertebrates. 


Among the animals with blood, he distinguished five different varieties:

  • Viviparous quadrupeds 
  • Birds
  • Oviparous quadrupeds
  • Fish
  • Whales 

Viviparous quadrupeds are what today we would classify as mammals.  Viviparous means bearing live young, and quadrupeds move around on four feet. 

For centuries, Aristotle’s classifications of animals were what scientists and naturalists used to distinguish one from the other.

Conrad Gessner and John Ray – Challenging Aristotle

Although a few naturalists looked at Aristotle’s view of animal classifications over the centuries, Conrad Gessner was perhaps the first to really expand on what Aristotle had started. 

His multi-volume Historia animalium (which bore the same title as Aristotle’s work), consisted of over 4500 pages, and exhaustively explored the animal kingdom as it was known to his world in the mid 1500s. 

In the process of this magnus opum, he added greatly to the world’s understanding of different animals.  

John Ray, primarily a botanist, sought to classify living things in more detail by the mid 1600s, and is responsible for popularizing the term “species.” 

John Ray found fault with some of Aristotle’s classifications, noting that bloodless animals still have vital fluid, that animals giving birth to live young do this through the use of internal eggs, and finally, that whales and other similarly equipped aquatic creatures have more in common with viviparous quadrupeds than fish, and therefore, maybe the term quadruped was not the most accurate.

Enter Carl Linnaeus

Carl Linnaeus, often referred to as the “father of taxonomy,” was born in Sweden in 1707, the eldest of five children. 

As a child, he had a fascination with his family’s garden and in learning the complex Latin names of all of the flora and fauna he found there. He studied medicine and botany, traveled all over the country collecting plant specimens to study, and became a professor of botany in 1741. 

Linnaeus began classifying multitudes of plants and animals using a new taxonomical system, and even though he was appointed the physician to the king in 1747, he continued to study and publish treatises on various aspects of plant and animal life until his death in 1778, a few years after he retired. 

In his lifetime, Linnaeus named over 12,000 species of plants and animals, and his published works continue to be the blueprint for the naming of organisms today. 

The Binomial System

With so many plants and animals in the world, Linnaeus wanted to make sense of them all? The answer was classification. 

By grouping organisms into broad families, then into narrower and narrower groups of similar traits, the vast array of flora and fauna can be categorized and brought into an orderly fashion. 

Linnaeus devised a system whereby each plant and animal was given a Latin genus name and species name. These two names constitute the binomial system

For example, humans are Homo sapiens; our genus name is Homo, which categorizes us with other animals recognized as “man,” and our species name, sapiens, distinguishes us from other animals in the Homo genus, such as Homo habilus or Homo erectus

Linnaeus’ New Classifications

Linnaeus was still using the term quadrupedia in the first edition of his 1735 work Systema Naturae, but this was not to last. 

He realized that humans should be included in this group, and as we do not primarily walk on all fours, Linnaeus started searching for another term to describe all of the warm-blooded hairy creatures. 

This comparison between humans and other animals enraged some people, who didn’t want to be classified alongside animals they considered inferior, so Linnaeus’ job was quite difficult. 

Linnaeus tried to focus not on how these animals moved, or even the amount of hair they had, but on something indisputably similar between species. 


Even though humans no longer walk on all fours, it was plain to even the skeptics about our ancestry to see that, like all other warm-blooded hairy animals, human females lactate. 

Due to this, Linnaeus latched on to the term Mammalia to describe us and our fellow mammals, from the latin mamma, meaning breast, in the tenth edition of his Systema Naturae in 1758. 

Some naturalists contemporary with Linnaeus kept using Quadrupedia, and other would-be taxonomists tried their own catchy names over the years, such as Pilifera, which means having hair. None of these caught on, and we are still called mammals today. 

Furthermore, Linnaeus’ insistence on linking humans to other mammals had a profound effect on the way we look at ourselves as a species. 

Rather than setting humans apart as something other than animal, it placed us squarely among our other mammalian family members. This attitude paved the way for other scientists and naturalists, such as Charles Darwin, to take those similarities further and help our understanding of human evolution.

Social Motive for Linnaeus’ Naming

Despite the link between humans and other mammals as regards lactation, it is a bit odd that Linnaeus chose to focus on that as the nomenclature for a huge chunk of the animal population. 

There are many clearer options for names (such as the pilifera that his French contemporary favored to highlight the hairiness of mammals), or more poetic names.  

Linnaeus coined “homo sapiens,” meaning “man of wisdom” in the same work as he coined “mammal,” One theory to explain this choice of name is that Linnaeus had other motives, particularly social ones. 

In his day, most women of the upper classes did not nurse their own children, but sent their newborns to a “wet nurse” for suckling (another mother who is lactating and has milk to spare), as breastfeeding was unfashionable. 

If you were stuck at home breastfeeding an infant, you couldn’t be out in society or wear the most fashionable clothing of the day. 

Wives of professional men such as lawyers sometimes helped run their business, and for them, it could be cheaper and easier to hire a wet nurse than to hire someone to help run the business while they themselves were nursing. 

Some physicians and scientists disagreed with this practice, including Linnaeus. In 1752, he wrote a treatise on the problems of wet-nursing. In addition to being a practicing physician, he and his wife had seven children, and so he had personal experience of children being nursed as well. 

In his 1752 treatise, he made many comparisons between nursing human mothers and nursing mothers of other species, perhaps foreshadowing his thoughts about this comparison when it came to naming them “mammals” six years later. 

Linnaeus’ efforts to champion mothers nursing their own children were not in vain; by the end of the 18th century and through the 19th, breastfeeding became much more common among upper class women due to changing attitudes about the health and welfare of babies and their mothers.

Does the Term Mammal Hold Up to Modern Classification?

Many of the 12,000 plants and animals Linnaeus named in his lifetime have had to be reclassified as more information is learned about their specific characteristics. 

So what about the term “mammal?” Can we still use this as a way to describe the whole class of these animals? 


While the vast majority of mammals do indeed lactate from breasts or nipples and feed their young with milk, mammals don’t all do this in the same way. 

Monotremes (including platypus, echidna, and anteaters), for example, do have mammary glands, but no teats; their milk is just excreted onto their bellies near special hairs for their young to find. 

Males of all the mammalian families have only vestigial nipples, if any at all. Many stallions (male horses), for example, do not even have vestigial nipples, which was an argument presented in Linnaeus’ time against using the breast as a means of nomenclature. 

Despite these exceptions (and the all males one is, arguably a big one), there is not a major reason that the term “mammal” should not continue to be used today. 



Information on Aristotle:


Why Mammals Are Called Mammals: Gender Politics in 18th Century Natural History:


The Linnean Society:


A History of Infant Feeding:


Vertebrate Diversity:


Why Do Men Have Nipples?:


Bryan Harding

Bryan has spent his whole life around animals. While loving all animals, Bryan is especially fond of mammals and has studied and worked with them around the world. Not only does Bryan share his knowledge and experience with our readers, but he also serves as owner, editor, and publisher of North American Mammals.

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