M:Now let's see professor Stone spent a lot of time talking about how animals walk and run.I remember she started by describing how humans move, and later we went over the gaits of different two legged and four legged animals.Like cats, camels, lizards...

W:Wait.I think I missed the class when you went over how lizards move.At least, I do not have any notes on it.

M:Well, we talked in class about how modern reptiles like lizards run.I think I even copy the diagram the professor drew on the board.Here it is.Now, look at this.Lizards run with what'scalled as sprawling gait, their feet are well out on either side of their body and they move diagonally opposite feet together.The left fore foot with the right hind foot and the right fore foot with the left hind foot.They use this way of moving when they 're going slowly or quickly.When they move, they bend their bodies from side to side.And they time this bending so that they increase the length of their steps.
W:Okay.That makes sense.But I remember reading that, in general, animals evolves so that their feet are right under their bodies.But I do not see why?

M:That's because even the sprawling gaits suits lizards pretty well, there's a major disadvantage to this arrangement.Reptiles can not run and breathe at the same time.They stop briefly between short breaths of anywhere from 2 to 12 strides because when they move from side to side,the muscles of the left and right sides contract at different times.To breathe though, these muscles have to contract at the same time.Mammals do not have this problem because they do not move from side to side.Their feet are in the different position.