Mammals live everywhere on Earth, from the freezing poles and scorching deserts to dense forests and sprawling grasslands. Many species have also adapted to the skies and oceans. Moreover, mammals evolved particular diets and behaviors according to their habitat. We can see nature's variation and complexity as we explore the many types of mammals.
So what animal did mammals evolve from? Approximately 200 to 250 million years ago, mammals emerged from the reptile order Therapsida during the Triassic period. These creatures walked on all fours and showed distinct mammalian features such as specialized teeth and an opening in the temporal region of their skull.
In the following sections below, check out what characteristics an animal possesses to be a mammal, the types of mammals based on reproduction, and a selection of our favorite species.
The following are the physical characteristics shared among living mammals:
There are three main types of mammals based on their reproductive methods.
Diverging from other types of mammals around 200 million years ago3, monotremes are the only mammals that lay eggs. Only five species of these mammals live in Australia and New Guinea. Among them are the platypus and four types of echidnas, or spiny anteaters.
'Monotreme' comes from Greek: 'monos' means 'single' while 'trema' represents 'hole.' These animals only have a single opening for their reproductive system and waste disposal, called a cloaca, hence the name.
Moreover, like reptiles, these egg-laying mammals have lower body temperatures. Despite these similarities, monotremes are still true mammals since they possess other necessary characteristics.
Marsupials have a pouch on their belly, a nurturing environment for their offspring, known as joeys, born helpless and undeveloped. The pouch allows the joeys to receive sustenance from the mother's teats as they grow outside the womb. It's like a portable nursery, providing protection and enabling the joeys to develop into fully grown adults4.
There are 335 species of marsupials in the world; a significant number live in Australia and New Guinea. Some mammals under this category are ground-dwelling kangaroos, tree-hanging koalas, and burrowing wombats.
From tiny shrews to massive elephants, more than 5,000 placental mammals live in various habitats. As their name indicates, placentals have a placenta that nourishes the unborn young, allowing newborns to become independent soon after birth. This type comprises nearly 94% of all mammal species.
The platypus is one of the few monotremes. Like a combination of many animals, the platypus has a duck-like bill, a beaver-like tail, and webbed feet. This monotreme is so unusual it caused early European naturalists to doubt whether it was a real animal upon its discovery. Besides its peculiar appearance, platypus uses electroception to hunt by detecting electric fields underwater2.
Related Read: Platypus Facts.
A notable feature of the order Diprotodontia is their specialized "diprotodon" teeth, comprising two large incisors, which they use for digging, gnawing, and self-defense.
A famous member of the order Diprotodontia is the kangaroo, living exclusively in Australia and Papua New Guinea. Kangaroos are impressively athletic thanks to their muscular hind legs and oversized feet. However, their physical prowess means they can fatally injure a human adult with a well-placed foot swipe.
Related Read: Kangaroo Facts.
With big front teeth, rodents are hard to miss. With over 2,200 species, rodentia is the largest order of mammals living in almost every habitat except Antarctica.
While a few rodents, like rats, are pests, others, like beavers, are invaluable to the global ecosystem. For example, they contribute to seed dispersal and soil aeration. Additionally, beavers build dams along rivers, promoting regional biodiversity by transforming their habitats.
Related Read: Beaver Facts.
Bats have evolved specialized wings of thin membranes stretched between elongated fingers, making them the only type of mammal that can fly. Moreover, they can perform impressive aerial acrobatics and navigate in complete darkness using echolocation.
These flying mammals hold second place in order diversity, following rodents, with over 1,400 species. They live almost everywhere except for extreme desert and Arctic conditions. Additionally, they vary in size, from the tiny bumblebee bat to the giant flying foxes with one-meter wingspans.
Some bats use echolocation to capture bugs mid-air, consuming hundreds or thousands each night. Keeping insect populations under control, they help maintain the balance of our ecosystems. On the other hand, frugivore bats feed on fruits, and vampire bats live off bovine blood.
Compared to other mammals, primates have larger brains relative to their body size. This increased brain size enables the various types of monkeys to show sophisticated behavior, solve complex problems, and interact socially.
Monkeys typically have tails, unlike other primates, a feature absent in apes and humans. Their unique teeth structure with sharp incisors and canines caters to their varied diet, unlike the flat molars in herbivorous gorillas.
Moreover, monkeys are social creatures exhibiting collective living, unlike the more solitary lifestyle of orangutans or tarsiers.
Lacking gills, cetaceans are one of the mammalian orders that primarily live in the ocean. Known for returning to the surface to breathe, these marine mammals also create beautiful songs and whistles that enchant all of us.
The top of mind for this order are whales with powerful tails and a layer of blubber that insulates them, enabling them to thrive in different habitats, from the Arctic to the tropics.
Moreover, whales are intelligent animals with sophisticated hunting techniques, communication skills, and tool usage capabilities. Finally, whales are also the largest animal on Earth.
Related Read: Whale Facts.
Unlike most mammals, the order Artiodactyls have an even number of toes on each foot, earning them the nickname "even-toed ungulates." The well-known species under this order are deers. Usually prey, deers have a highly developed sense of hearing and smell that helps them avoid predators. Additionally, the annual regeneration of their antlers distinguishes them from other animals.
Unlike the previous mammalian order, Perissodactyls have an odd number of toes on each foot. For example, equines have a single toe, while rhinoceroses and tapirs have three.
Under the equines are the zebras, instantly recognizable due to their distinctive black and white striped coat. Zebras are herbivores, primarily grazing on grass, and live in socially structured herds. They play a critical role in the ecosystem by controlling vegetation growth and being a vital food source for major African predators.
Related Read: Zebra Facts.
Known for their tusks and trunks, elephants are the largest terrestrial mammals on Earth. Interestingly, they are the only living species under the Proboscidea order. But the elephants' ancestral links to the mammoth and mastodon showed there were more members1.
Aside from their plant-based lifestyle and massive ears, they are also as intelligent as primates, able to experience emotions like grief, joy, and anger.
Related Read: Elephant Facts.
The order Cingulata encompasses mammals known for their unique protective, bony plates or shells. "Cingulata" translates from Latin as "that which is encircled."
Armadillos, representing this order, sport a protective shell of small, bony plates called scutes. They are primarily nocturnal, spending their days in burrows and nights foraging for food, using their sharp claws for digging. Their primary diet consists of insects which they grind using their tiny, peg-like teeth.
Related Read: Armadillo Facts.
The Felidae family comprises big cats that can jump multiple times their body length, enabling them to catch prey or traverse natural habitats. There are also smaller ones, including the beloved domestic cats.
Among the iconic felines are tigers which are the biggest among this mammal family. With their heightened hearing and sight, they hunt using their dagger-like canines. Moreover, tigers are dominant predators with exceptional climbing and swimming skills, which are rare in other animals. Like other big cats, except the lion, tigers are solitary creatures.
The Canidae family, which includes wolves and foxes, have up to 300 million olfactory receptors in their noses. Man's best friends, the dogs, are also included in this mammalian family.
Wolves, standing out due to their size and sociable behavior, still share physical features with their smaller cousins. These include an elongated snout, sharp teeth, strong jaws, non-retractable claws, and highly tuned senses, essential tools for hunting and communication.
Another famous carnivorous mammals are the bear of the Ursidae family. Despite their bulky physique, bears are expert swimmers, runners, and climbers.
While bears live everywhere on the planet, polar bears dominate the Arctic region. These marine mammals are the largest of all bears and mainly feast on seals. Unlike other bears, these snowy bears do not hibernate during winter. Instead, only pregnant polar bears stay in dens with their offspring.
To sum up, there are diverse types of mammals, ranging from hardworking beavers to massive whales. Each has a unique role in keeping nature balanced in its environment. The more we learn about each one - their habits, behaviors, and functions - the more we'll appreciate our planet's wide range of life. It's well worth exploring each species' story to see how amazing our animal world is.
Rohland N, Reich D, Mallick S, Meyer M, Green RE, Georgiadis NJ, et al. (2010) Genomic DNA Sequences from Mastodon and Woolly Mammoth Reveal Deep Speciation of Forest and Savanna Elephants. PLoS Biol 8(12): e1000564.
Manger, P. R., Pettigrew, J. D., & Körtner, G. (1998). Electroreception and the feeding behaviour of platypus (Ornithorhynchus anatinus: Monotremata: Mammalia). Philosophical Transactions: Biological Sciences, 353(1372), 1211-1219.
Musser, A. M. (2013). Review of the monotreme fossil record and comparison of palaeontological and molecular data. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 136(4), 927-942.
Tyndale-Biscoe, H. (2005). Life of marsupials. CSIRO Publishing.
Isabela is a determined millennial passionate about continuously seeking out ways to make an impact. With a bachelor of science degree in civil engineering with honors, Isabela’s research expertise and interest in artistic works, coupled with a creative mindset, offers readers a fresh take on different environmental, social, and personal development topics.