The troop fission of Japanese monkeys has been observed in 11 troops, and the total number of fissions observed numbered 22. Examined cases of fission relative to season, amount of food, troop size and socionomic sex ratio of the main troop and to individuals that acted as nucleus in forming a branch troop and females, exhibited variance, not being uniform. But inquiry into troop fission as seen from the standpoint of the mechanism maintained by Japanese monkeys' society indicated (1) troop fission checks the growth of troop size and increases the socionomic sex ratio of the main troop, (2) troop fission is an effective mechanism for the maintenance of class structure among adult males, and (3) the possibility that troop fission functions to check inbreeding.
Thus, it must be said in general that troop fission, along with single desertions of males, plays a very important role as one mechanism for the maintenance of the society of Japanese monkey. However, the branch troop is often composed of a large number of males and a small number of females; in short, it is under very difficult conditions that a branch troop starts on its way to becoming a fully established Japanese monkey troop.
The troop fission is an effective mechanism for the maintenance of the main troop, but not too effective for that of the branch troop.
It cannot be said that there are abundant data on troop fission of primates other than Japanese monkey. But, from limited data, we can find various forms of troop fission according to each species, and it may be ultimately related to the maintenance mechanism of the primate society.