Squaws in tune

There were three Indian squaws. 

One slept on a deer skin, one slept on an elk skin, and the third slept on a hippopotamus skin. 

All three became pregnant. 

The first two each had a baby boy. 

The one who slept on the hippopotamus skin had twin boys. 

This just goes to prove that... the squaw of the hippopotamus is equal to the sons of the squaws of the other two hides.