THE BASICS
Mammals (class Mammalia) are a diverse group of vertebrate animals to which we, humans, belong. Mammals evolved 220-240 million years ago and all of the extant orders appeared in the fossil record approximately 55 million years ago.
There are a total of 27 extant orders in class Mammalia. The primate order, to which humans belong, also includes our closest living relatives; Pan (chimpanzees and bonobos), Gorilla (gorillas), and Pongo (orangutans). Most research suggests that our closest living relatives are chimpanzees who we share 96% of our DNA with. However, there has been one study to suggest that our closest relative is actually the orangutan, to whom we share at least 28 unique physical characteristics with.
From wings to flippers, live young to eggs, the mammals of the world vary wildly in their adaptations and behaviors. However, there are some characteristics that unify all mammals. All mammals are endothermic homeotherms with high metabolisms. They also have mammary glands for feeding their young. Interestingly, the word “mammal” is derived from “mamma” which is Latin for “teat”. Mammals also have fur or hair at some stage during their life (yes, even dolphins), and a secondary palate which allows them to eat and breathe at the same time.
How diverse are mammals?
Mammals can be found on every continent and in every ocean across the globe. They are a diverse group of organisms with unique adaptations to the ecosystems and climates in which they live and to the resources that they depend on. Some of these unique traits include flight, echolocation, blowholes, baleen plates, blubber, horns, opposable thumbs, and hibernation.
There are many extremes found within the class Mammalia. The smallest mammal, the Bumblebee bat (Craseonycteris thonglongya) weighs just 2 grams. and the largest, the blue whale (Balaenoptera musculus), weighs 181,437 kilograms. The fastest mammal, the cheetah (Acinonyx jubatus) can run at a speed of up to 120.7 km/h (75 mph) while the three-toed sloth (Bradypus) moves at a speed of just 0.24 km/h (0.15 mph).
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How do mammals reproduce?
Mammalian reproduction involves male sex organs (the penis and testes) and female sex organs (the vagina, uterus, and ovaries). Like other animals, sexual behavior usually occurs during estrus, when the female animal is most fertile. However, many mammals and some non-mammalian animals have sex for motivations other than reproduction. (Yes, like humans, other mammals have sex for pleasure too)!
While most mammals give birth to live young, monotremes like the platypus and echidna lay eggs. The marsupials, such as kangaroos and opossums, give birth to live young very early during their pregnancy, with their infant crawling up their mother’s body and attaching to a teat, commonly inside a pouch. The fetus of marsupials is nourished by a placenta, although only for a very short period of time.
Placental mammals provide nourishment to their fetus with a placenta for a much longer period of time than marsupials. Placental mammal mothers like humans, elephants, and bats, carry their fetus within their uterus until a very late stage of development.
Ecological roles and conservation
Mammals play critical roles in the ecosystems they are found in. Some mammals, such as American beavers (Castor canadensis), are ecosystem engineers who actively create and make changes in their habitats. Predatory mammals, like sea otters (Enhydra lutris) and grey wolves (Canis lupus), are vital for reducing the overpopulation of prey species, and therefore protecting plants, soils, and waterways.
Of the approximately 6,554 recognized mammal species, 25% are of conservation concern and 103 species have already become extinct. Habitat loss, fragmentation, encroachment, poaching, pollution, and climate change are some of the many threats that face mammal species. Some of the most breathtaking, and the most ecologically significant, mammals are listed on the IUCN Red List as endangered or critically endangered. These include the 3 remaining orangutan species (Pongo spp.), Hawaiian monk seals (Neomonachus schauinslandi), the Javan rhinoceros (Rhinoceros sondaicus) and many more.
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Afrosoricida
These shrew-like mammals are found in Madagascar and Western Central Africa. Tenrecs, otter shrews, and golden moles belong to this order. They have small, unspecialized teeth and several species display daily or seasonal torpor and semi heterothermy. Some of these species also use echolocation.
Artiodactyla
This order contains the even-toed ungulates categorized into three different suborders. Artiodactyla includes species like camels, vicunas, deer, and pronghorn. It also includes cetaceans, or whales. They are mostly herbivorous and many species have horns or antlers.
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Carnivora
A diverse group of mammals, Carnivora includes species like the dogs, bears, skunks, cats, meerkats, and seals. They have enhanced zygomatic arches and, despite the name, are mostly omnivorous. Species in the order Carnivora can live in a variety of habitat types and vary significantly in size.
Chiroptera
20% of all mammal species are found in the order Chiroptera, the 2nd largest order of mammals. This order contains bats, the only mammals capable of true flight. There are two suborders contained within Chiroptera, Microchiroptera, the mostly insectivore species capable of echolocation, and Megachiroptera, tropical fruit-eating bats with large eyes.
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Cingulata
Armadillos are the only surviving members of the order Cingulata. These mammals are burrowers with dorsal armor and a cap of armor on top of their heads, over their shoulders and backs, and on their tails. They are opportunistic, eating insects, small vertebrates, and plants.
Dasyuro-morphia
74 extant species of mostly carnivorous Australian marsupials are found in the order Dasyuromorphia. Among the extinct species is the thylacine, a medium-to-large canid resembling a cat or wolf. Today, most species are small, ranging from the size of a mouse to the size of a domesticated cat.
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Dermoptera
Colugos are found in the order Dermoptera. They have naked bellies and large, forward-facing eyes. They are nocturnal and arboreal, clinging to trees and gliding.
Didelphi-morphia
American marsupials, like the Virginia opposum, are found within the order Didelphimorphia. There is only 1 family and over 60 species in this order. Most of these species have prehensile tails and no marsupium.
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Diprotodontia
Kangaroos, wombats, koalas, and wallabies are found in the order Diprotodontia. These mammals are herbivores and have webbed or syndactyly toes. They are marsupials that can be terrestrial, semi-fossorial, or arboral.
Eulipotyphla
Hedgehogs, gymnures, true shrews, and shrew-like moles are found in the order Eulipotyphla. These species primarily eat invertebrates and live underground.
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Hyracoidea
This order includes small herbivorous mammals called hyraxes. Densely furred with short tails, these mammals have a similar appearance to pikas or marmmots. However, they are more closely related to manatees and elephants. They lack incisors and rely on their molars to cut their food. They have hoof-like nails and rubbery footpads that help them grip steep, rocky surfaces.
Lagomorpha
Rabbits, hares, and pikas are contained within the order Lagomorpha. These mammals are terrestrial herbivores and have large hindfeet and two sets of upper incisors.
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Macroscelidea
African insectivores called elephant shrews make up the order Macroscelidea. These mammals are insectivorous and terrestrial. They have long, slender snouts that move. They are one of the fastest mammals on earth, capable of moving at speeds of 17.9 mph.
Microbiotheria
Monito del monte is the last extant species in this order, surviving in the South American countries of Argentina and Chile. These tiny opossums are semi-arboreal and nocturnal.
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Monotremata
Monotremes, echidnas, and platypods find themselves in this order. These mammals have some of the most interesting reproductive strategies with in the class Mammalia, laying eggs and sectreting milk from pores rather than teats.
Notorycte-morphia
Two species of marsupial moles are found in the order Notoryctemorphia. These small mammals have no pinnae and vestigial, blind eyes.
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Paucituber-culata
There are only seven living species left in this order, with at least 60 extinct species whom once roamed the earth. These South American marsupials are known as shrew opossums. They are known for their long, hairy tails, clawless hallux, and diprotodon jaws.
Peramele-morphia
Bandicoots and bilbies are found in this order. There are over 20 species of bandicoots and only one species of bilby extant today. These mammals are insectivores and have long muzzles and large ears.
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Perissodactyla
Odd-toed ungulates make up the order Perissodactyla. This includes horses, rhinoceroses, and tapirs. These ungulates are herbivorous and digest cellulose in their intestines rather than in stomach chambers.
Pholidota
Eight species of pangolin are found in the order Pholidota. These mammals are insectivorous, known for eating ants and termites. They have long, muscular tongues, long skulls, and no teeth.
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Pilosa
Sloths and anteaters are contained within the order Pilosa. These two mammals are split into two suborders, Folivera and Vermilingua. Species found within Folivera are arboreal and slow moving. They have coarse hair and a mutualistic relationship with algae that lives within their hairs. Species within the suborder Vermilingua have long skulls and no teeth. They can be terrestrial or arboreal and some are nocturnal.
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Primates
Primarily arboreal, the order Primates includes species that have hands and digits, enlarged braincases, and forward-facing orbits. Humans, aye-ayes, apes, lemurs, monkeys, and other primates are categorized into two suborders, Strepsirrhini and Haplorhini.
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Proboscidea
There are three living species remaining in this order, two African elephants and one Asian elephant. This order contains the largest living land mammals who have prehensile proboscises (trunks) and graviportal limbs. Their tusks are permanently growing upper incisors and they lack canines.
Rodentia
42% of all mammal species are found in the order Rodentia. Species within this order are mostly herbivorous with rootless, ever-growing incisors and no canines.
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Scandentia
Treeshrews are found in the order Scandentia. These omnivores eat insects and fruit and live in Southeast Asia. They have tails as long as their bodies and can be terrestrial, arboreal, or semi-arboreal.
Sirenia
This order includes several species of mammals commonly called sea-cows or manatees. They are fully aquatic herbivores, usually found in warmer waters. They are herbivorous and have flexible necks.
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Tubulidentata
Aardvarks are found in the order Tubulidentata. They are solitary and elusive with very long tongues and conical skulls. Their teeth are unique, lacking enamel, incisors, and canines.