First appearing during the Mesozoic era 181 to 135 million
years ago, the first mammals were descendents of doglike
reptiles living in a world dominated by dinosaurs. Warm
blooded and furry, mammals are a familiar, charismatic group.
Distinctive characteristics of mammals include their fur, and
milk producing mammary glands. All mammals have hair or fur. Even whales have a few course
hairs around their mouths. Depending on its function, fur can
vary in its appearance such as a sheep’s thick, curly wool; a
porcupines’s sharp quills (yes, quills are hairs); or the
Mammals are able to survive in almost any type of habitat
from frigid tundra to a hot, humid, tropical rainforest. They
can live almost anywhere because the temperature inside their
bodies stays the same whether it’s cold or warm outside
(they are homeothermic). They can also make their own body
heat to keep themselves warm enough (they are endothermic).
Unlike most birds or most reptiles who lay eggs, most
mammals give birth to the “live young." Mammals can be
distinguished because they have mammary glands (which produce
milk for their young), ears are comprised of three ear bones,
they are endothermic, and they have fur. Mammal brains are
generally larger than other animals, and the largest animal in
the world is a mammal - the blue whale.
Monotremes are the only group of mammals that are oviparous
(meaning they lay eggs), laying one to three eggs. Monotremes resemble other mammals in producing milk to
nourish their young, in having three inner ear bones and a
single bone in the lower jaw. However, monotremes are highly
specialized feeders and the adults have no teeth.
Like marsupials, monotremes have lower metabolic rates than
other mammals but they are still endothermic and can
maintain their bodies at a constant temperature regardless of
environmental temperatures. They are one of the only two
groups of venomous mammals, shrews are the other group. Only
three species exist today - the duckbilled platypus and two
species of spiny anteaters.
Marsupials are distinguished by having live-born young that
crawl into a pouch (marsupium) that is equipped with
mammary glands. The young are born immature, much as the
hatchlings of monotremes. Brains are typically smaller
than placental brains.
While marsupials were once common through-out the world,
they have been displaced by placental relatives in most places
with the opossum being the only species that is found beyond
The placental mammals are a very diverse group with an
enormous range of body forms and complex social interactions.
The 3782 species are divided among 19 orders. They include the
terrestrial insectivores, edentates, ungulates, rodents,
carnivores and primates.
Placental mammals have adapted to an amazing diversity of
environments and many have highly specialized abilities - from
bats that are capable of
sustained flight aquatic seals to massive whales that live
their entire lives in the water.
Distinguishing characteristics of placental mammals include
embryonic development in uterus with the live birth of young
at advanced and relatively mature stages.
(Click here to learn more about different types of placental
A pine marten peeks out from