Life history of male Japanese macaques living on Koshima Islet

In the study of Japanese macaques, two types of male transfer between groups, defined by the age of transfer and known as bottom- or top-rank transfer, have received much attention, whereas the meaning of the period of solitary life has received very little. Male solitary life has been regarded simply as a transient state between group transfer. We found that male solitary life is restricted to a specific period in the life history of Japanese macaques living on Koshima Islet. This period started when the body weights of males and females began to diverge and ended when the body weight increase of males had stopped. Solitary lifestyle is related to the growth of adolescent and post-adolescent males. We hypothesize that a solitary lifestyle is a necessary step for adolescent and post-adolescent males to achieve their full adult growth by avoiding feeding competition with others, especially with adult females. Increments of body weight increase between 3 and 15 years were larger for the sons of lower- and middle-rank females than for those of higher-ranking females. Although the sons of lower- plus middle-ranking females grew later, they achieved a full adult body weight similar to that of sons of higher-ranking females by the age of 15. We believe that this body weight increase was achieved because of the solitary lifestyle of adolescent and post-adolescent males. Correlation between male body weight and life-span was found for body weights at 12 years, but no correlation was evident at 6 years.