What is a Mammal?

  Mammals are animals.
Yes but so are Insects, Reptiles, Spiders, Sponges and Slugs. We need to do better than that.
Mammals have a back-bone.Yes but so do Fishes and Frogs and they are not mammals.
Mammals are warm-blooded. Yes but so are birds, and birds are not mammals either.
So how do we describe a mammal??

Obviously, what we need in order to define a mammal are some characters, or traits that are possessed by all mammals and are unique to mammals, i.e. they do not occur in fishes and /slugs etc.. Fortunately, scientists have already worked it all out for us.
So here are six solid indications that an animal is a mammal


Character One

The first characteristic that guarantees that an animal is a mammal is that it (if it is female) can produce milk to feed its young. This milk is produced by modified sweat glands called 'mammary' glands. It is from these glands that the whole group takes its name, 'Mammals.

Character Two

The second test is the possession of hair, something humans often have problems with but which they should respect more. No other animal has hair in the same form as mammals, and all mammals have some hair at least at the beginning of their lives - baby whales and dolphins are born with a moustache.

Character Three

The lower jaw in mammals is a single bone on either side. In all other vertebrates there are more than one bone on each side of the jaw.


Character Four

  The mammal middle ear, and only the mammal middle ear, contains 3 bones. The Stapes or (Stirrup), Incus or (Anvil) and the Malleus or (Hammer). Once these bones were part of the lower jaw, but during the early evolution of mammals they changed jobs and became a part of our hearing apparatus instead.

Character Five

In mammals the main artery leaving the heart curves to the left becoming the aortic arch. In birds it curves to the right and in all other vertebrates there are more than one main artery leaving the heart.

Character Six

Finally mammals have a diaphragm. A sheet of muscle and tendon that separates the body cavity into two sections. Heart and lungs before/above, liver, kidneys, stomach, intestines, etc, behind/below. No other animal has a diaphragm.

There are about 4260 named species of mammal in the world. I say about, because it is hard to get a list of species that all the experts agree on. So until you become an expert taxonomist and can solve the problems of what are or are not species, 4260 will do for 2000 AD, but more will be discovered.

Mammals, or Mammalia are a class in the Phylum Chordata and the Subphylum Vertebrata.

Kingdom >>>  Phylum >>>  Subphylum >>>  Class >>>  Subclass
Animalia Chordata Vertebrata Mammalia

There are 3 Subclasses within the class Mammalia:
               Prototheria or Monotremes (2 species)
               Metatheria or Marsupials (275 species)
               Eutheria or placental mammals (3982 species)

Within each Subclass are Orders, Families, Genera and numerous Species. For information on how this works see Mammal Classification, for a list of the 16 orders and their 133 families, see Orders.

Other characteristics which are sure indicators that an animal is a mammal, but which may not be present in all mammals include:

1. The possession of a placenta in the females which allows the mother's system to nourish the growing young without being attacked by the mother's immune system. This applies to all the Eutheria which is why they are called placental mammals. Cows, horses, dogs, cats and people are all placental mammals.

2. Mammals have only two sets of teeth. Reptiles and fish have many sets and can replace lost teeth with new ones all their lives.

3. Mammals are heterodontic, meaning that their teeth are different shapes, except those with no teeth at all. We have some or all of:- incisors, canines, molars and premolars. Reptiles and fish have teeth that are all basically the same, though they can vary in size throughout the mouth. See image above.

4. The Buccal Cavity (the mouth) has a false palate as a roof, meaning that the nostrils do not lead directly into he mouth. Effectively, this means that if your mouth is full of food you can still breathe, but a reptile has to breathe around its food.

5. The body is maintained at a constant temperature, meaning mammals are endothermic; they generate heat within their bodies metabolically and also have special cooling mechanisms. Not all mammals choose the same temperature, and some are not very good at it. Bats, for instance, tend to cool down considerably when at rest, this is partly because of the large surface area for heat loss supplied by their wings. Finally there are the Naked Mole Rats, Heterocephalus glaber which have lost this ability, probably because they do not need it anymore. Birds are also endothermic. For more information on this see - On Being Warm-blooded.

So now you have a better idea of what a mammal is. Ultimately though, mammals are part of the beauty and diversity of life that makes this world so good to live in. Unfortunately, many people in positions of power fail to appreciate this beauty and the precious fragility of life and many mammal species are currently in danger of disappearing for ever. So far 86 species and 24 subspecies, a total of 110 distinct taxa of mammals have already become extinct since mankind started keeping records. Apart from these, 241 taxa of mammal are currently on the IUCN critically endangered list. Another 436 taxa are listed as Endangered and a further 776 are classed as vulnerable. In total this means 1453 taxa or over 25% of all living mammals are listed in the IUCN Red Data book.


There is great essay analysis example on Latest Mammalian Species Discovered.

Who are the 21st Century Mammals?




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