Topics related to childbirth, parenting, and family in Japanese Macaque
May 22, 2026
As of April 30, 2026, a total of 68 monkeys—32 males and 36 females—live together as a single group in the monkey enclosure at Tama Zoo.
April and May are the birthing season for Japanese Macaque, and six female macaques gave birth at Tama Zoo in April. Of the offspring, four were female and two were male.
A characteristic of Japanese Macaque species is that the sex ratio of offspring born converges to 1:1. This characteristic means that even if a temporary imbalance in the sex ratio occurs due to accidents or other factors within a region or group, the number of males and females will remain nearly equal in the long term, thus preserving the species and maintaining genetic diversity within the species.
However, the current group has four fewer males than females, and there has always been a tendency for the number of males to be low. A review of past breeding records at the zoo shows that the oldest females lived to be 32 years old, while the oldest males lived to be 22 years old, a significant difference. In males, injuries from fighting were often the cause of death, and they tend to have shorter lifespans compared to females due to accidental deaths. Therefore, this tendency can be said to be the reason why there are slightly fewer males in the group's sex ratio.
If we apply the concept of "family" from the perspective of a zero-year-old to the bonds between monkeys, the relationships become deeper in the order of "older brother < grandmother < older sister < mother."
These close "family" relationships do not include the father. Why? Because during the limited breeding season in the fall, the females all go into estrus at once and mate with multiple males many times.
Thus, it is clear that it is difficult for a mother to recognize the father of her infant because she mates with multiple males before and after conception.
The monkeys do not know their fathers, but in order to preserve the genetic diversity of individuals within the group, Wildlife Conservatio Center of Tokyo Zoological Park Society conducts genetic testing to determine paternity.
In the case of Japanese Macaque, when all of them are captured in early winter each year, veterinarians collect blood samples from individuals born that year, who are approximately 8 months old. DNA is extracted from the cells in the blood, and genetic analysis tests are performed to identify the base sequences of several characteristic regions for each individual.
This information is also useful for determining paternity. DNA has a double-stranded structure, and each animal inherits one strand of genetic information from both the mother and the father. Because the mother holds the pup to her chest immediately after birth, the individual can be identified through behavioral observation. Since the genetic information of the adult animals that will become parents can be determined from past analysis data, it is possible to estimate the father candidate from the other strand of genetic information that is not from the mother.
Zoos use this kind of scientific information and behavioral observation records for each individual animal to manage their populations and conserve the genetic diversity of the animals they keep.
[Tama Zoo Tama Zoological Park]