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.
Mammals are endothermic vertebrates characterized by the presence of hair and the production of milk for their offspring.
Vertebrates are animals with a backbone, whereas invertebrates are animals that lack a backbone.
Now, you might be unfamiliar with some of these terms. What do we mean by endothermic vertebrates?
An organism that is endothermic (or “warm-blooded”) is capable of regulating its body temperature through metabolic processes.
In contrast, an organism that is ectothermic (or "cold-blooded") relies on its external environment to maintain a stable body temperature. This explains why you might find reptiles like lizards basking in the sun.
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.
The specific composition of milk varies among species.
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.
At some point in your life you may have wondered, why are whales and dolphins considered mammals?
Whales and dolphins are part of a group of marine mammals called cetaceans. Like other mammals, cetaceans also have hair. If you are wondering why you haven't seen pictures of whales and dolphins with hair, it's because they are born with hair but lose them as they grow older. Cetaceans also have mammary glands with which they produce milk for their young.
Are cetaceans related to land mammals, by any chance? Yes, cetaceans are thought to have evolved from terrestrial mammals like hippopotamuses, pigs, and cows! Fossil evidence shows that extinct cetacean ancestors’ pelvis and hind limb bones gradually shrank over time until it disappeared completely and developed into flukes and flippers.
Classification of Mammals
There are about 5,300 extant species of mammals on Earth. Mammals can be broadly classified into three distinct lineages:
Monotremes
Marsupials
Eutherians
Figure 2. Mammals can be classified as monotremes, marsupials, or eutherians.
Let’s go through the defining characteristics and cite some examples of each one.
Monotremes
Let's start by looking at the characteristics of monotremes.
Monotremes are egg-laying mammals. Like other mammals, they have hair and produce milk, but they do not have nipples.
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.
To date, monotremes are made up of only one species of platypus and four species of echidnas (or spiny anteaters).
Marsupials
Up next, we have marsupials.
Marsupials are mammals that give birth to live young which complete their development outside of the mother's body.
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.
There are over 330 species of extant marsupials, including kangaroos, bandicoots, koalas, opossums, and wombats. Around 250 species of marsupials are found in Australia, New Guinea, and neighboring islands, while around 70 are found in South and Central America. One species is native to the United States and Canada.
Eutherians
Now, let's define eutherians.
Eutherians–which are often called placental mammals–are mammals that give birth to live young and have a placenta that is more complex than those of marsupials.
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.
Eutherians are the most diverse and widespread group of mammals. Examples of eutherians include rodents, primates, elephants, and whales. Humans like you and me are also classified as eutherians.
Evolution of Mammals
Now you may be wondering, how are mammals so different from one another yet have key similarities?
The answer lies in its evolutionary history.
Before true mammals existed, there were synapsids.
Non-mammalian synapsids were four-legged terrestrial animals that did not have hair, had a sprawling gait, and laid eggs. Each side of their skulls contained a single temporal fenestra, which is a hole found behind the eye socket.
In the span of 100 million years, mammalian traits slowly began to appear in the jaw of early non-mammalian synapsids. These changes include the incorporation of the two bones that made up the jaw bone into the middle ear.
Synapsids developed into gigantic herbivores and carnivores during the Permian period (299-252 million years ago), and for a while, they ruled over the tetrapod community. During the Triassic period (around 252-201 million years ago), their diversity dramatically decreased, but more and more synapsids that resembled mammals started to appear. These synapsids were still quite different from the mammals we know today: they were little, hairy, egg-laying, insect-eating animals with a fairly high metabolic rate.
Around 201 to 145 million years ago, during the Jurassic period, the first true mammals began to appear.
They underwent a period of diversification into numerous brief lineages.
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.
It was during this period when monotremes, marsupials, and eutherians began to emerge.
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 is the process in which a group of organisms diversifies into forms to fill different ecological niches.
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.
Mammals - Key takeaways
- Mammals are endothermic vertebrates characterized by the presence of hair and the production of milk for their offspring.
- Mammals can be broadly classified into three distinct lineages: monotremes, marsupials, and eutherians.
- After the extinction of non-avian dinosaurs, mammals underwent adaptive radiation.
- Adaptive radiation is the process in which a group of organisms diversifies into forms to fill different ecological niches.
References
- J. Knox Jones , Don E. Wilson, and David M. Armstrong, "Mammal". Encyclopedia Britannica, 17 Aug. 2022.
- The Ocean Portal team, "Whales." Smithsonian Ocean, March 2020.
- Jane B. Reece et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
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