Exotic Animals

I left off with a discussion of how to define animal groups.  Today I want to talk about mammals and birds, and how to recognize them.

“Recognizing” mammals and birds may sound silly, but it’s not.  After all, if you ask, “What is a bird?” most people will include flight in their definition.  However, not all birds can fly.  So what is a bird?

To start with, all birds have feathers, and front legs that cannot support the body- all birds are obligatory bipeds, walking on two legs is not a choice for them.  Usually, the front limbs are modified as wings, but some birds (penguins) use their wings as flippers, and others (kiwis) lack them entirely.  In addition, all birds have a toothless beak, and a respiratory system that uses air sacs to force air through the lungs in one direction only.  This is connected with birds’ warm-blooded metabolism, since it makes them very efficient at getting oxygen into the bloodstream.  Finally, all birds have a hyper-shortened tail, and reproduce by laying eggs.  You’ll notice that, from this list, only feathers and the air-sac system are unique to birds; other animals have some of these traits, but only birds have them all.

Warm-bloodedness is a trait that birds share with mammals.  Mammals, however, have many more unique traits than birds do.  Fur or hair, mammary glands to produce milk for their young, 3 bones in the inner ear, and a jawbone that connects directly to the skull are the diagnostic characters of mammals.  Bearing live young works for most, but not all, mammal species: the platypus, and two spiny anteater species, all from Australia and New Guinea, lay eggs.

All of this may sound pretty obvious, but remember, a lot of it was not worked out until the 18th and 19th Centuries, when scientists began to rigorously classify the world in a way that we would recognize as modern.  I’ll define more groups later, as this blog grows.  The next couple of posts will be devoted to mammals and birds.