Indian maths

The chief of an Indian tribe in Western Nebraska was faced with a dilemma.
He had three daughters.
The oldest was one of the most beautiful women who ever lived. She was smart as well as beautiful.
The second daughter was also very beautiful, but not as striking as the first.
The third was very, very plain. (The tribe was a group of Plains Indians.)
There were three young Indian warriors vying for the affections of the chief's daughters. To solve the dilemma and get his daughters married, the chief proposed that the three Indian braves go out into the forest and return with evidence of their hunting prowess.

All three braves agreed.

The first Indian, the best warrior and hunter in the tribe, immediately spotted a ferocious mountain lion. Taking his bow and arrow, he shot the lion, skinned it and returned to camp with the pelt. The chief was impressed, and asked the young warrior if he wanted to marry his oldest, most beautiful, most accomplished daughter. The brave replied that he did. The chief said, "Take your lion skin to your tepee, lay it on the ground, and my daughter will join you. Then you may consummate your
marriage, and tomorrow, with all the rites and traditions of the tribe, I shall perform the sacred marriage ceremony." The young Indian did as he was directed.

The second Indian, not as lucky as the first, took two days to find prey worth shooting to prove his hunting prowess. He spotted a bear. He shot it with his bow and arrow, skinned it, and returned to camp with the bearskin. The chief was impressed, and he asked him if he wanted to marry his second daughter. The brave replied that he did. The chief said, "Take the bearskin to your tepee, lay the pelt on the ground, and my daughter will join you. Then you may consummate your marriage, and tomorrow, in all the rites and traditions of our tribe, I shall perform the sacred marriage ceremony." The Indian did as he was directed.

The third Indian was, to be frank, a bumbling, inept fool. He was no warrior, he couldn't hunt, and he could barely build a fire. He had been a failure at "Indianing" since anyone could remember. He went out to the forest, but couldn't find a lion or bear. Finally, he did manage to find a sleeping hippopotamus wallowing in the river. (Probably an escaped hippopotamus from the circus or zoo, to explain its presence in western Nebraska.) He shot it, skinned it, and brought the hippo skin back to camp. The chief was impressed, and he realized that although this was no fierce forest animal, the young brave had fulfilled his hunting obligation.
"Would you like to marry my third, plain daughter?" the chief asked. The brave replied that he did. "Take the hippo skin to your tepee, lay it on the ground, and my daughter will join you and you may consummate your marriage. Tomorrow, with all the rites and traditions of the tribe, I shall perform the sacred marriage ceremony." And the young brave followed instructions.

Oh, wonder of wonders! Nine months later, three wonderful events occurred.
The first Indian brave and his beautiful wife had a baby boy.
The second Indian brave and his beautiful wife had a baby boy.
But the bumbling Indian who was good at nothing, and his plain wife, who wasn't much better, had twin boys!
The tribe was amazed, none more than the chief, because twins portended something special in Indian folklore, and nobody could understand how this had happened to the third daughter and the third brave.
Finally the chief gathered the people around him and said, "I believe I know the reason for this mighty and wonderful event.
Remember, ... the sons of the squaw on the hippopotamus is equal to the sons of the squaws on the other two hides."