抄録
The observability of 68 males over three years of age in a free-ranging group of Japanese macaques (Macaca fuscata) at Katsuyama, Okayama Prefecture, was examined for five years (five birth and post-birth seasons and three mating and post-mating seasons). There were large individual differences in observability (percentage of days when the author observed the subject near the group at least once to the total observation days) among nine peripheral adult males, while six central males were constantly found near the group. Compared to peripheral adult males, the observability of younger males (three to seven years old at the start of the present study) gradually decreased with increasing age. On the other hand, the values for two sons and four grandsons of the α-female were much higher than those of other same-aged males, indicating that there was some delay in their leaving the natal group. The present results also suggest that observability during the six-month period that includes the mating season may be useful for assessing whether or not males will leave their natal group during the subsequent six-month period (birth season).
