The Boss of the Jigokudani Monkey troop (named Togura 95, nicknamed Little Black because of his dark fur) hasn’t been seen for almost three weeks now. It is not unusual for male monkeys to separate from the troop. Leaving for a bit is just part of the monkey nature so the troop does not seem confused that their Alpha male is not there.
If he were to just take a break from going down to the Jigokudani Monkey Park, the direct family members of the Alpha would also not appear in the public eye. However, in this case, the only monkey who has been absent is the Boss.
Usually, if the alpha instinctively knows his time has come mentally and physically, he often decides to leave the troop voluntarily. This seems to be the case. Togura 95 might have figured there are many strong, young monkeys in the troop and decided to take his leave from his troop, for his and their own protection, after a reign of 6 years.
It’s still too early to judge whether or not he has really gone from the troop, but the people from the Park seem to think it is highly possible.