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Facts About Mammals


Adult ring tailed lemurs and baby
Geri Lavrov/ Photodisc/ Getty Images

In this article, you'll learn interesting facts about mammals and find out about their unique characteristics, their life cycle and their evolutionary history.

FACT: Mammals are divided into 21 groups.

Scientists do not yet agree on the exact number of mammal groups, but on this website we recognize 21 different types of mammals. Some of the better known mammal groups include the primates, marsupials, bats, cetaceans, carnivores, rodents, elephants, and ungulates. Lesser known mammals include the hyraxes, monotremes, pangolins, tree shrews, and insectivores.

FACT: There are between 4,500 and 5,000 species of mammals.

Of all mammal groups, the most diverse are the rodents which includes over 1,700 species. Other diverse mammal groups include bats (977 species), primates (356 species), insectivores (365 species), and marsupials (292 species). Mammal groups with the fewest number of species include the aardvark (1 species), dugongs and manatees (4 species), and the flying lemurs (2 species).

FACT: The first mammals appeared approximately 200 million years ago during the Jurassic Period.

The ancestors of mammals were a group of reptiles known as the therapsids. The first true mammals to have evolved diverged from the therapsids during the Jurassic Period. Of all the mammal groups alive today, the monotremes are the oldest, followed by the marsupials.

FACT: Mammals are tetrapods.

Mammals have four limbs, a characteristic that places them among the group of animals known as tetrapods. It should be noted that although some mammals such as whales, dugongs, and manatees have lost their hind limbs during the course of evolution, they are tetrapods by descent.

FACT: Mammals are warm-blooded.

Mammals are warm-blooded or 'endothermic' which means they generate their own, internal heat.

FACT: All mammals have hair.

Hair is a defining characteristic of mammals, no other organisms possess true hair and all mammals have hair covering at least part of their body at some time during their life. An individual hair consists of a rod of cells that are reinforced by a protein known as keratin. Hair grows from skin cells called follicles. Hair can take on several different forms including fur, whiskers, spines, or horns. Hair serves numerous functions. It can provide insulation, protect the skin, serve as camouflage, and provide sensory feedback.

FACT: Mammals are amniotes.

Amniotes are a terrestrial vertebrates whose eggs are characterized by having several layers of protective membranes (the amnion, chorion and allantois). Reptiles, mammals, and birds are all amniotes.

FACT: The Cenezoic Era is the 'Age of Mammals'.

The Cenezoic Era (65 million years ago until the present day) is considered to be the Age of Mammals because it represents the time period during which mammals diversified and became the dominant land vertebrates.

FACT: The largest mammal is the blue whale.

The blue whale is the largest mammal and is also the largest animal alive today. It may even be the largest animal ever to have lived. Blue whales weigh between 110–160 tonnes (110—176 tons) and mature individuals measure in the range of 20–30 m (66–98 ft). The fin whale (Balaenoptera physalus) comes in a close second in size to the blue whale, weighing 47–74 tonnes (50–82 tons) and measuring 19–22 m (62–72 ft).

FACT: The smallest mammal is the bumblebee bat.

The bumblebee bat, also called the Kitti's hog-nosed bat, is the smallest of all mammals, measuring just over an inch in length and weighing a mere 2g. The bumblebee bat is a vulnerable species that inhabits limestone caves in Thailand and Burma.

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