Explore the remarkable diversity of mammals: How many types exist?
Introduction to Mammalian Diversity
Mammals are one of the most diverse and successful vertebrate groups on Earth. Mammals have adapted to almost every type of environment on Earth, from the tiny blue whale weighing up to 160,000 kg to the colossal bumblebee weighing only 2 grams. How many types of mammals are there?. This question is much more complicated and interesting than you might think. Mammal Diversity Database estimates that approximately 6,836 species of mammal will exist in the world by 2025. This includes both recently extinct and living species. This is a huge increase over the 5,416 mammal species that were recognized only two decades ago. It shows how much more we still know about mammalian diversity.
Mammalogy is the study of mammals. It continues to provide new insights about their evolutionary history, ecological role, and amazing adaptations. Mammals are important pollinators, dispersers of seeds, predators and prey in ecosystems. Their conservation is vital to the health of our planet. This comprehensive article will explore the various ways that mammals are classified. We’ll also examine the major groups and investigate what makes them unique. Finally, we’ll consider the challenges faced by scientists when categorizing the mammalian variety.
Mammalian Classification Systems – Traditional vs. Molecular Approaches
Traditional Classification Methods
Mammalogists have classified mammals for centuries based on morphological traits – physical characteristics such as tooth shape, bone arrangement and reproductive systems. Carl Linnaeus’ pioneering work in the 18th century laid the foundations for mammalian classification, which George Gay lord Simpson further developed in his 1945 classification system. This has remained the standard for decades. The traditional classification divided placental mammals by 17 orders, based on similarity in structure and function. This included groups like Rodentia (rodents), Chiroptera(bats), and Carnivora(carnivores).
The system was limited in its ability to group mammals by physical similarities. Convergent evolution is a phenomenon that groups together mammals with similar appearances and similar ecological niches, despite the fact that they evolved from different ancestors. The traditional classification did not always reflect the evolutionary relationships.
Molecular Phylogenetics Revolution
In the 20th century and 21st century, DNA analysis technology revolutionized mammalian classifying. Scientists discovered by comparing the genetic sequences of different species that certain traditionally defined groups included mammals who were not closely related. Other mammals with a very different appearance shared recent common ancestors.
The Molecular Evidence suggests that the placental mammal can be divided into four super orders. These diverged between 85-105 million years ago.
1. Afrotheria includes elephants, manatees and golden moles
2. Xenarthra includes armadillos, sloths and anteaters
3. Laurasiatheria includes bats, whales and hoofed mammal species.
4. Supraprimates includes primates, rodents, and rabbits
The phylogenetic system is continually being refined to reflect new genetic evidence, which provides a more accurate view of the evolution of mammals.
Monotremes are the largest mammalian group, followed by Marsupials and Placentals.
The reproductive strategies of mammals are the basis for categorizing mammals. This divides mammals into three main sub classes, namely: marsupials and placentals.
Monotremes (Prototheria).
Monotremes are the oldest living lineage of mammals. They possess a unique combination between reptilian, and mammalian characteristics. They are oviparous, meaning they lay eggs instead of giving birth to living young. The young are fed milk from the mammary glands of the mother after hatching. This is not done through the nipples, but by specialized patches on the skin.
Only five monotreme species are extant: the platypus, and four echidna species. They are only found in Australia and New Guinea, and make up a tiny 0.07% (or fewer) of all mammal types. Monotremes retain primitive characteristics, such as a cloaca, which is a single opening used for reproduction and excretion. This is similar to birds and reptiles.
Marsupials
Marsupials are capable of giving birth to live babies after an incredibly short gestation, sometimes as little as eight days. The newborns, which are underdeveloped and highly altricial, must finish their development by being attached to the nipples. This is usually done within a pouch called a’marsupium’.
About 334 marsupial types are still extant, which is about 5% of the total number of mammals. Although marsupials tend to be associated with Australia, where they dominate the mammalian flora, they can also be found in North and South America. Some of the most notable marsupial groups include:
Diprotodontia – Kangaroos and wallabies as well as koalas.
Dasyuromorphia – Tasmanian devils (quolls), numbas, and other species
Didelphimorphia – American opossums
Peramelemorphia – Bandicoots, bilbies
Marsupials are able to adapt to fill a wide range of ecological niches. They can be herbivores or insectivores, but also carnivores.
Placental Mammals
Placental mammals make up the majority of mammalian species (approximately 94%), with approximately 6,423 species extant. The placental mammals have longer gestational periods, during which the embryo is fed by a special organ called the “placenta”. The placenta allows the embryo to develop more fully in utero. This results in an advanced offspring when compared with marsupials.
The placental mammals are found on every continent and ocean. The size of these mammals ranges from the tiny shrews that weigh just a few grammes to the huge blue whales, the largest animals to ever inhabit Earth.
Diversity of Mammalian Species
Mammals are divided into 27 orders that represent distinct evolutionary lines with unique characteristics. The table below highlights orders with the greatest species diversity.
Table: Mammal Orders by Species Diversity
Order Examples Number of Species Approximate Percentage Mammals
Rodentia Rats, mice, squirrels, beavers 2,606 38.1%
Chiroptera Bats 1,447 21.2%
Eulipotyphla Shrews, moles, hedgehogs 548 8.0%
Primates Monkeys, apes, lemurs, humans 515 7.5%
Artiodactyla Deer and cattle, pigs and hippos 356 5.2%
Carnivora Cats, dogs, bears, seals 304 4.4%
Diprotodontia Kangaroos, wallabies, koalas 150 2.2%
Didelphimorphia Opossums 127 1.9%
Lagomorpha Rabbits, hares, pikas 107 1.6%
Cetacea Whales and dolphins 94.1%
Other Orders Various 582 8.5%
Mammal Diversity Database (Mammal Diversity Database) (2025).
Just two orders, Rodentia (and Chiroptera), account for almost 60% of the diversity of mammalian species. This striking disparity shows how evolutionary processes can lead to explosive radiation of some lineages, while others have more modest diversity.
Key Characteristics Defining Mammals
All mammals, despite their diversity, share some common characteristics.
1.Mammary glands: Female mammals produce breast milk to feed their young. The class name comes from Latin, which means “breast”.
2.All mammals, at one time or another, have hair that provides sensory functions, insulation, and camouflage.
3.Three Middle Ear Bones. Mammals have a set of three unique bones in their middle ear (malleus incus and stapes), which amplify the sound vibrations.
4.Neocortex region of brain: Mammals are equipped with a highly-developed brain area that is responsible for higher functions such as sensory perception, spatial reasoning and language.
5. Diphyodont Dentition: All mammals have two sets (deciduous (“baby”) and permanent teeth) with different types of teeth (incisors canines premolars molars).
6. Endothermy: Mammals can adapt to diverse environments by maintaining a constant body temperature.
7. Diaphragm: is a specialized muscle that helps respiration by separating the abdominal and thoracic cavities.
These characteristics are what distinguishes mammals from other classes of animals, while also allowing us to observe the incredible diversity that we see across species.
Ongoing discoveries and classification challenges
We are constantly evolving in our understanding of mammalian biodiversity. It is surprising to learn that dozens of mammal species were discovered every year, and 1,079 new species have been added between 2005-2018. The discoveries are made in remote areas of the globe, through genetic analyses that reveal cryptic species – morphologically similar yet genetically different – and by reexamining museum specimens.
The use of molecular techniques has revolutionized the taxonomy of mammalian species, leading to some significant changes in classification. For example:
The Skunks have been moved from Mustelidae back to Mephitidae, their own family
It was determined that whales evolved from Artiodactyla, (even-toed unipeds).
The elephant shrews have been moved to their own order Macroscelidea from Insectivora
The Mammal Diversity Database, maintained by the American Society of Mammalogists, is the most up-to-date authority. It incorporates new research findings regularly to update classification.
Mammalian Diversity and Conservation
Understanding mammalian classification is more than merely scholarly exercise. It has implications for conservation efforts. Understanding distinct species and evolutionary relationships allows us to identify lineages that are worthy of conservation and more effective protection.
Many mammal species are threatened by habitat destruction, climate changes, pollution, hunting and other human activities. Mammal Diversity Database reports that 113 mammal specie have been extinct since 1500 CE. Many more are currently endangered. According to the IUCN Red List, approximately 25% of mammal systems are in danger of extinction.
To maintain healthy ecosystems, conservation efforts must take into account the diverse ecological roles that mammals play, including as seed dispersers and pollinators, predators and ecosystem engineers. To protect mammalian biodiversity, it is necessary to preserve habitats, fight climate change, regulate hunting, and address other threats.
Conclusion: Appreciating Mammalian Diversity
How many different types of mammal are there? The story of evolution, adaptation, and scientific discoveries is revealed. Mammals are a diverse group of organisms that inhabit virtually all habitats on Earth. There are approximately 6,836 species recognized, and more are discovered every year.
Mammals continue to amaze and delight us with their diversity. From the egg-laying monotremes, which defy traditional mammalian characteristics, to the gliding Colugos and echo locating bats to deep-diving Whales. Our understanding of mammalian diversity and relationships will continue to develop as research, especially in molecular biology and previously unknown regions, continues.
The protection of this incredible biodiversity is one of the greatest challenges of our times. Understanding and appreciating this incredible diversity of mammals that we share with our planet will help us to ensure future generations continue to enjoy and be amazed by these amazing creatures.