Perhaps the U tribe represents a matrilineal society. At the very least, it represents one where the females seem to retain the power of sexual selection in a way that most human tribes do not.
Closest literary example that comes to mind is Heinlein's `The Moon Is A Harsh Mistress`; in the lunar society, women called all the shots sexually when it came to choosing or rejecting partners. Males were more passive, though they still held an active role in society and government.
