What Are Mammals?

kangaroo on the beach
© John Crux/Shutterstock.com

Written by Krishna Maxwell

Updated: September 23, 2022

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Of all the different types of animals, mammals are the ones that humans can relate most closely to. This is because humans are just one of the many species of mammals. So what are their characteristics? How are they alike and what are their differences?

All mammals are vertebrates, but not all vertebrates are mammals. There are five classifications of vertebrates, including mammals. The other classifications include birds, fish, reptiles, and amphibians. While they share characteristics with other vertebrates, there are some features that are uniquely theirs.

There are over 5,400 species that make up the mammal classification. While this may seem like a tremendous number, it isn’t the largest class of vertebrates. There are around 10,000 species of birds and over 33,000 species of fish.

They Produce Milk

All mammals have mammary glands, which are used to provide milk for their young. Mammary glands are made up of glandular tissue and ducts and develop from the sweat glands. While many have nipples that allow their young to nurse, one family of mammals, known as monotremes, secrete milk through ducts rather than nipples.

In nearly all mammals, the female is the only one who feeds the young. This means that the nipples of the male, while present, are underdeveloped. In a few species of mammals, such as the Bismarck masked flying fox and Dayak fruit bat, both the male and female are capable of producing milk and both will help care for the young.

They Are Warm-Blooded

A warm-blooded animal is better able to regulate its internal temperature, making it more resilient to outside threats. While a cold-blooded reptile requires a certain external temperature to function, warm-blooded mammals can maintain their body temperature through diet and other methods.

Foxes have exceptional hearing that allows them to easily hunt their prey.

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They Have a Four-Chambered Heart

One characteristic of mammals that you cannot see is their four-chambered heart. Reptiles and amphibians have three-chambered hearts, while fish have hearts with two chambers. Birds and mammals are the two classifications of vertebrates that have four chambers.

With a four-chambered heart, the body can easily keep the deoxygenated blood heading to the lungs separate from the oxygen-rich blood heading away from the lungs. This means the animal has fully oxygenated blood available at all times. By having constant access to a well-oxygenated supply of blood, mammals are able to physically exert themselves more fully and without the need for frequent breaks.

They Have Similiar Ear Structure

It is hard to believe that elephants, bats, foxes, and humans all have the same ear structure, but that is one of the characteristics of mammals. All of them have three inner ear bones that move sound waves from the eardrum to the inner ear, translating the vibrations into neural impulses which are interpreted by the brain.

Most Replace Their Teeth Once Over Their Lifetime

With the exception of a few mammals, such as kangaroos and manatees, mammals replace their teeth once over their lifetime. Born with deciduous teeth, these fall out to make room for the primary teeth. If they lose a primary tooth, it doesn’t grow back. This contrasts with animals such as alligators and sharks, that can replace teeth throughout their life.

Their Lower Jaw is Made of a Single Bone

In other classes of vertebrates, the lower jaw is made up of several bones and is not attached to the skull. The mammal’s jaw is made of a single bone, attached to the skull. This solid structure gives the mammal’s jaw tremendous power.

Manatees can hold their breath for up to 20 minutes.

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They are Protected by Hair

All mammals have some form of hair or fur during some period of their life. Some, like dogs and cats, are covered in fur throughout their life. Others, such as dolphins, have a light covering of hair early in development.

Hair can provide camouflage, protection from the weather, and aid in exploring their environment.

They Have a Neocortex

The neocortex of the brain is responsible for decision-making, perception, reasoning, and other higher actions. Mammals have a neocortex which allows them to operate on a higher level and have more complex relationships than many other animals.

Types of Mammals

The majority of mammals are viviparous, which means they give birth to live young. The exception to this is monotremes, which lay eggs. Duck-billed platypuses and echidnas are monotremes.

Viviparous mammals are either placental, which means the mother has a placenta to provide nutrients and remove waste during gestation, or marsupials, where the newborn is not fully developed and continues to develop attached to the mother’s body. Kangaroos and opossums are both marsupials.

Placental mammals make up the majority of the mammal group. By having a placenta to carry nutrients to the embryo and filter away waste, the developing fetus can remain in the womb until it is more fully developed. While newborn placental mammals range from fully-formed but helpless, such as a newborn kitten, to one that is up and running within hours of birth, like a foal, all placental newborns are developed before birth.

When most people think of marsupials they may consider them a different classification from mammals, but they are not. They have a short gestation period and give birth to live young, but the young are not fully developed. The young need additional time to mature outside the womb and do that by attaching to the mother’s nipple, where they remain for weeks or even months.

Monotremes are a unique classification of mammals, as they lay eggs. This separates them from marsupial and placental mammals, but, like these other mammals, they nurse their young. This odd combination of egg laying and producing milk has led to debates about the classification of platypuses. While these animals are definitely unique they still share more traits with other mammals than any other classification. The fact that they do have mammary glands and do produce milk is what finally put them solidly in the mammal category.

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About the Author

Krishna is a lifelong animal owner and advocate. She owns and operates a small farm in upstate New York which she shares with three dogs, four donkeys, one mule, and a cat. She holds a Bachelors in Agricultural Technology and has extensive experience in animal health and welfare. When not working with her own animals and tending her farm, Krishna is helping other animal owners with behavior or management issues and teaching neighboring farmers about Regenerative Agriculture practices.

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