All male tribes. The uke's tribe holds a competition to determine the roles in their tribe (man or woman), the winners will be the man and the losers will be the women. Losers can manifest women' characteristics. The seme is from another tribe ans has a unrequited love with the uke. The uke was hunting when he got lost and was attacked by a tiger; uke ended up being injured. Seme saved him and they had seggs since their blood was compatible with each other. Turns out they were both sons of their tribe's chief and were next in line for the position. Uke was offered to the seme's tribe and they are now married.
Samezoku e no sasagemono