Genetic effects of demographic bottleneck and recovery in Kinkazan Island and mainland populations of Japanese macaques (Macaca fuscata).

Populations of Japanese macaques were significantly reduced in most areas from the 1900s to the 1960s and then recovered mainly in the northeastern part of Honshu. A drastic reduction in population size reduces genetic variability through a bottleneck effect. Demographic expansion after the reduction that accumulates new mutations can reduce the bottleneck effects or drive the recovery of genetic variability. We examined the genetic status of a small island population (Kinkazan Island) and a larger mainland population (southern Tohoku) of Japanese macaques that experienced recent demographic bottlenecks and recovery using eight microsatellite loci. The two populations were significantly genetically different from each other. The Kinkazan population exhibited lower genetic variability, remarkable evidence of bottleneck (i.e., significant heterozygosity excess and lower frequency of rare alleles), and a considerably smaller effective population size based on genetic data than based on the current census size. These results indicate that the genetic status has not completely recovered from the demographic bottleneck despite a full recovery in census size on Kinkazan Island. New mutations might rarely have accumulated because of the small carrying capacity of the island. Therefore, the genetic variability of the population would have been restrained by the severe bottleneck size, small carrying capacity, and long-term isolation. On the other hand, the bottleneck effect seems to be limited in the southern Tohoku population considering higher genetic variability, non-significant heterozygosity excess in many mutation conditions, and the highest frequency of rare alleles.

Fission-fusion dynamics in a wild group of Japanese macaques (Macaca fuscata) on Kinkazan Island caused by the repeated separation of an alpha male being followed by females.

The spatiotemporal cohesion of a group is an essential element of primate society. It is thus important to clarify the factors that influence the extent of variation in spatial cohesion and individual membership in a group over time, known as fission-fusion dynamics. During the mating season of 2019, the alpha male (TY) in a wild group of Japanese macaques on Kinkazan Island repeatedly disappeared from the group, and we observed fission-fusion dynamics caused by his movement. The group seemed to have split when TY left the group or its home range, and females who followed him had the most affiliative relationships with TY or were the relatives of those females. Although TY disappeared from the group in the post-mating season, these fission-fusion dynamics only occurred during the mating season probably because females had a more substantial need for protection against aggression from the other males during the mating season. These results indicate that, although rare, fission-fusion dynamics based on affiliative relationships between males and females can occur with the separation of alpha males from the group. More studies in other populations are needed to clarify the influence of group males on the spatial cohesion of groups.