What is a Mammal?

There are approximately 4,260 different mammalian species that have been discovered to date, although this figure varies because not all scientists agree that certain organisms are a distinct species. In addition, new species are always being discovered, therefore this figure is always changing.

Mammals are all warm-blooded, and all mammals are vertebrates, but there are also other animals, like birds, that have these characteristics, so there are additional traits that set mammals apart.

Mammals have six key characteristics that can be seen in each and every mammal, and it’s these traits that set them apart from other types of creatures:

1. Mammals produce milk to feed their young. Female mammals possess a modified sweat gland – a mammary gland – that is activated by hormonal changes that occur with pregnancy. In fact, this trait is what inspired the term “mammal,” a derivation of “mammary.”
2. Mammals all have one single bone comprising their lower jaw. In all other animals, more than one bone comprises the jaw.
3. All mammals have three tiny bones in the middle portion of the ear.
4. Mammals all possess a diaphragm, which is a thin muscular wall that separates the upper and lower portions of the torso.
5. Hair or fur is a characteristic only seen in mammals. All mammals develop fur or hair at some point during their development, though not all keep their fur or hair throughout their lifespan.
6. The heart of a mammal is unique in that it has one primary artery leaving the heart bending to the left, whereas other animals either have multiple arteries or the main artery bends in a different direction.

Within the class of animals considered mammals, there are three categories: eutheria or mammals possessing a placenta; metatheria, also known as marsupials or pouch-bearing mammals, and prototheria, also known as monotremes or egg-laying mammals.

In addition, there are a few characteristics that are exclusive to mammals, although not every mammal has these traits.

• The vast majority of female mammals have a placenta, used to protect and nourish the offspring prior to birth. Marsupials and monotremes do not have a placenta.
• In their lifetime, a mammal will not have more than two sets of teeth. Typically, mammals grow one set of teeth as juveniles, and then a new permanent set grows in as they near adulthood.
• A mammal is warm blooded, meaning it has the ability to generate its own body heat and maintain a steady body temperature, despite ambient temperature changes.
• Mammals also have a separation between their mouth and nasal cavity. Other animals, like reptiles do not have an upper palate that allows the nasal cavity to remain open regardless of whether there is something inside the mouth.

In addition to the three categories of mammals, there was once a fourth category that is now completely extinct. Multituberculates are a category of mammal that arose during the late Jurassic period 160 million years ago and they survived up until about 35 million years ago.

They have no living descendants today, but fossil records indicate that they were similar to modern rodents.

Multituberculates were named for their teeth. These animals had one pair of incisors on the lower jaw and their molars had numerous cusps forming numerous rows of teeth. These mammals also lacked canine teeth on the upper jaw, like many rodents of today.

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