Mammals

Got milk? No, not without mammals! Mammals are the only animals that have mammary glands to produce milk for their young. As mammals, we experienced this as infants when our mothers breastfed us, and other mammals ranging from dolphins to hedgehogs do the same!

But, what else makes a mammal, a mammal? Let's find out!

• First, we will take a broad look at mammals by unpacking their definition and key characteristics.
• Then, we will learn how mammals are classified and cite examples of each group.
• Lastly, we will discuss how mammals evolved millions of years ago.

Mammals Definition

We know that mammals produce milk for their young, but let's look at a broader, more helpful definition of mammals.

Now, you might be unfamiliar with some of these terms. What do we mean by endothermic vertebrates?

Mammals Characteristics

The two major characteristics that set mammals apart from other animals are body hair and mammary glands.

Figure 1. Mammals share these basic characteristics.

Every mammal has body hair at some point in its development. Body hair can take on various forms. Some are quite obvious, like fur and whiskers. Others are more surprising: horns, hooves, and nails. The function of the body hair differs depending on its form, but some general purposes include insulation from the cold, camouflage, and in the case of whiskers, gathering sensory information.

Every mammal also has mammary glands. As we discussed earlier, females have mammary glands that secrete milk that they use to nourish their young. Milk-producing tissue starts growing in female mammals as soon as they conceive offspring. Lactation is stimulated when they are nursing their young. Milk is generally rich in fats, sugars, proteins, minerals, and vitamins, so it is a balanced diet for their young.

Mammals are also characterized by having a lower jaw hinged directly to the skull rather than through a separate bone (which is the case for other vertebrates). They also have three tiny bones in the middle ear that transmit sound waves.

Furthermore, while reptiles have teeth that are mostly uniform in size and shape, the mammalian jaw bears teeth with varying sizes and shapes adapted for consuming various kinds of food:

• incisors and canine teeth are modified for shearing,

• while premolars and molars are modified for crushing and grinding.

Classification of Mammals

There are about 5,300 extant species of mammals on Earth. Mammals can be broadly classified into three distinct lineages:

1. Monotremes

2. Marsupials

3. Eutherians

Figure 2. Mammals can be classified as monotremes, marsupials, or eutherians.

Monotremes

Let's start by looking at the characteristics of monotremes.

So, how do they provide milk to their offspring? Monotremes secrete milk through glands on the belly of the mother, while the newly hatched baby sucks milk from the mother’s fur.

Marsupials

Up next, we have marsupials.

Unlike monotremes, marsupials have nipples and are viviparous (meaning they bear live young rather than laying eggs). The marsupial embryo develops inside the mother’s uterus. The lining of the uterus and the membranes outside the embryo form a placenta, which is a structure that diffuses nutrients from the mother’s blood into the embryo.

What distinguishes marsupials from eutherians-- the next group we are going to discuss-- is that marsupials give birth to premature offspring, which complete their embryonic development while nursing outside the mother’s body. Most species of marsupials keep their nursing young in a maternal pouch called marsupium.

Eutherians

Now, let's define eutherians.

The eutherian placenta enables their nursing young to complete their embryonic development within their mother’s uterus. Because their young complete their embryonic development within the mother’s uterus, eutherians tend to have longer pregnancies than marsupials.

• Like marsupials, eutherians also have nipples through which their mammary glands open.

Evolution of Mammals

Now you may be wondering, how are mammals so different from one another yet have key similarities?

Before true mammals existed, there were synapsids.

Around 201 to 145 million years ago, during the Jurassic period, the first true mammals began to appear.

Throughout the Jurassic and Cretaceous ages, the first mammals coexisted with dinosaurs. Still, these first true mammals were different from the mammals we are familiar with. The majority of them were small, measuring no more than a meter, probably because dinosaurs filled the ecological niches of large-bodied animals.

In the late Cretaceous period (around 100 to 66 million years ago), non-avian dinosaurs died out. This led mammals to undergo adaptive radiation.

Adaptive radiation gave rise to mammals with diverse characteristics. That is why mammals as we know them today range in size from bats that weigh less than a gram to blue whales which can weigh over 180 metric tons. They also live in all major habitats, as there are mammals that swim, run, fly, burrow, glide, or climb. They also have a wide range of feeding habits, with some feeding on plants and insects, while others feeding on larger prey.