Japanese Macaques: the Wide Variety of Their Lives

Akihiro Eguchi


            Most primates live and have evolved in tropical area of the world. However, there are several exceptions like Macaca fuscata, which people often call Japanese Macaques or the snow monkey. They are primates which live in Japan, northern most part of the world where non-human primates live. The main islands of Japan lie between the latitudes of 24° and 46°N, and they have four distinct seasons. The summer time brings a high-temperature and humid climate resulting in many rainfalls, and in the winter time, the temperature drops down, and it snows mainly on the west coast.

Japanese Macaques live all over the islands, including many small neighboring islands, except the north island Hokkaido. Their body mass is about 12 kilograms in males and eight kilograms in females (Hamada, Hayakawa, Suzuki, Watanabe, & Ohkura, 2003). Similar to the Rhesus macaque, they have a red face, but have a short tail. They have brownish-red colored hair on the back, and gray hair on the neck to the abdomen, the inside of the limbs, and on the back of their tail (Aimi & Takahata, 1994).  In order to adjust to the varying weather in Japan, they developed unique behaviors depending on the regions in which they live. The groups which stay in places that get cold in winter, instead of hibernating, developed a thermoregulation behavior by staying in the sunshine if it is a nice day and huddling together if it is a cloudy day (Hanya, Kiyono, & Hayaishi, 2007).  The groups that live at Akaishi Mountains climb the mountain seeking young leaves and green shoots, which eventually reach up to 3,000 meters in altitude (Izumiyama, Mochizuki, & Shiraishi, 2003). Some other groups mimic and acquire new behavior from the interaction between humans, like washing potatoes with seawater (Yamagiwa, 2008) or soaking in a hot spring (McGrew, 1998). In addition to the innate behaviors common in similar species, they have developed many unique cultures by adjusting to each different environment in Japan.

            Although Japanese Macaques are living all over Japan, according to Yamagiwa and Hill (1998), their habitats can be divided into three different categories: provisioned groups, habituated groups in natural habitats, and wild un-habituated groups in natural habitats. In Japan, one reason why Primatology had matured rapidly from late 1950s was that researchers started to provision the Macaques by periodically feeding them at many places in Japan. The number of the site became more than 30 at the end of 1970's, which resulted in forming a new social structure by increasing their group size and infants' survival rate. As a result, these provisioned groups in habitats like Koshima, Takasakiyama, and Arashiyama may not represent typical Japanese Macaques in natural habitats anymore (Yamagiwa et al., 1998).

Therefore, research has shifted to be conducted over habituated groups in natural habitats like in Yakushima and Kinkazan both of which are in National Parks. Yakushima is an island that is 500 square kilometers in an area located 60 kilometers south of the main island (35°30'N, 130°30' E) within the Subtropics (Sprague, Suzuki, Takahashi, & Sato, 1998), where yearly rainfall is about 2,600 millimeters and the average temperature is 21 degrees Celsius (Suzuki, Hill, & Sprague, 1998). More than 20 troops of Macaca fuscata yakui, an endemic sub-species of Japanese Macaques, live densely without any natural predators (Suzuki et al., 1998). On the other hand, Kinkazan is a smaller mountainous island that is 10 square kilometers in area located in the northern part of Japan (38°8' N, 141°4' E) within the cold-temperate climate zone (Sprague et al., 1998), where the yearly rainfall is about 1,500 millimeters, and the average temperature is 11 degrees Celsius (Suzuki, 1998).  The macaques living there are habituated around people so that researchers can observe their natural habitat, which is a lot different from other provisioned groups by artificial feedings. Since the detailed research with un-habituated groups in natural habitats is difficult, most of the research is conducted based on the data collected at those habituated but natural sites.

            Hill (1997) reported that the annual diet of Japanese Macaques in Yakushima consists of 35.1% on leaves and shoots, 30.2% on fruit, 13.2% on seeds, 10.3% on animal matter, 5.6% on flowers, and 4.6% on fungi. According to him, there is a distinct seasonal change in the amount and the proportion of the diet; for example, in the spring time, they consumed the most by eating an abundant amount of mature leaves. During the summer time, on the other hand, they minimize the number of food items by foraging items that have many calories and nutrients like fruits, seeds, and animal matter (Hill, 1997; Go, 2010). Leaves are still abundant during the summer time but do not provide sufficient energy for their daily requirement in summer (Mori, 1979). Insects are also abundant and can be a source of protein, but it is time- and energy-consuming to catch them (Suzuki, 1998). Thus, a hot summer may result in poor production of foods, with high energy leading to a high rate of mortality in the monkeys (Hanya et al., 2004). Hill (1997) also observed that in the winter time, they eat a moderate amount of seeds and fruits, which are less abundant than in summer time, but they do not eat as many leaves. Therefore, a low fruit-food production in this mating season results in a low birth rate in the following year (Noma, Suzuki, & Izawa, 1998). These seasonal changes in diet lead to changes in feeding sites. They decide the feeding sites based on a temporal and spatial limitation of the food so that they can simultaneously and efficiently consume other abundant foods like leaves (Tsuji & Takatsuki, 2004; Go, 2010).

            In terms of demographics, Hashiba (1989) reported that from the late 1950’s the distribution range of Japanese Macaques expanded to mostly southern parts of Japan as the area of natal broadleaf forest decreased. The estimated population also grew from about 15,500 in the 1950’s to 114,431 in 1981. They usually form groups consisting of at least two adult males and females, but the size of each group varies from location to location. At the provisioned site like Takasakiyama, the group size doubles every seven years, and most groups under provision tend to exceed the group size of 100. On the other hand, groups under natural conditions keep a mean group size of 55 by frequent fissions (Yamagiwa & Hill, 1998).

            Social structure of Japanese Macaques also varies depending on the type of habitats, and according to Yamagiwa and Hill (1998), those social characteristics can be broadly classified into several different types. One type includes the population in Yakushima, a habituated but not provisioned environment in a warm-temperate forest.  The mean group size is only 32.3 in spite of the fact that the population in a warm area tends to form bigger troops. They live in small home areas, and are the only type that show territorial behaviors. According to Suzuki et al. (1998), they rank members based on linear hierarchy and relative dominance relationships are stable during the tenure, that usually last about 3 years. The rank changes mostly only when the higher-ranking male leaves or when a new male joins the troop. However, in this type of population, about 43% of the time, males, which are mostly fully grown adults, join the group as higher-ranking males, and others, usually young males, join the group at the bottom of the hierarchy, and this causes the change in the hierarchy (Sprague, Suzuki, Takahashi, & Sato, 1998; Suzuki et al., 1998). Therefore, it seems that age is an important key in their hierarchy system. It sometimes results in a fission entry if a female finds a non-troop male physically attractive (Sprague, Suzuki, Tsukahara, 1996; Suzuki et al., 1998).

Another population in a habituated natural environment, in a cool-temperature environment like Kinkazan, is classified into a different type by Yamagiwa and Hill (1998). They form small to medium size groups with relatively low density. They do not have a specific territory so sometimes play with neighboring groups, and usually non-troop males join a group as the bottom ranking member without any conflict. Therefore, it takes a long time to climb up to be the high-ranking male in this type of group (Yamagiwa & Hill, 1998). Females also have a unique ranking system. According to Yamagiwa (2008), females usually acquire their ranks based on the following two rules: first, a daughter is ranked as high as her mother, and second, a strong daughter, which is usually the youngest, is ranked highest among other sisters. Another distinctly different type is a population in provisioned habitats like Takasakiyama, Koshima, and Arashiyama, which are isolated from other natural environments. They tend to form a really big group. Usually, troop fission occurs during non-mating season when the troop size gets to more than 100 (Yamagiwa, 2008); however, the maximum recorded group size is 1,255 at Takasakiyama (Yamagiwa & Hill, 1998).

            These different types of groups share many social characteristics as well. The age of male natal dispersal is one example of a stable feature all over groups because it relates to their sexual maturity (Sprague et al., 1998). Yamagiwa (2008) reported that males leave their natal groups at the age of around four years old, prior to their sexual maturation. Some males become solitary males for a while, and some other males join a male band. Other males join another group, but neither of them come back to join the natal group. Researchers think that this behavior naturally occurred in order to avoid interbreeding.  The characteristic of female-kin groups is another common feature. Different from the males, in Japanese Macaques’ society, females spend their life in their natal group unless the group becomes a fission group; therefore, they are all matrilineal kin, forming a stable linear hierarchy in their dominant relationship (Yamagiwa & Hill, 1998).

            Japanese Macaques are primates that have a clear annual cycle of reproduction. Their mating season is from late autumn to early winter, and in the following spring, they bear babies (Noma, Suzuki, & Izawa, 1998). In the mating season, high-ranking males are given the priority to copulate with estrous females; however, females have the choice in this system. According to Hayakawa (2007), although 56 % of females copulate with high-ranking males more often than with other males, there is evidence that shows females tend to prefer novel males, which can be either middle-ranking males or non-troop member males. Also, Koba, Izumi, and Nakamura (2009) found that Japanese Macaques have a clear sexual difference in facial parts. For example, females have chins and noses more close together, and each has a distinct outline of their upper faces. Researchers concluded that Japanese Macaques depend on the features to recognize other individuals, and this might play a key role in mate selection (Koba et al., 2009). Hayakawa (2007) also reported that injured high-ranking males have a lesser chance of mating success. Therefore, the chances of high-ranking males’ success in mating are influenced by many factors like the size of the troop, number of visiting non-troop males, and the preference of females.

The reason why higher-ranking males have more chances is explained by Matsubara and Sprangue (2004). According to them, for females, the cost of mating with higher-ranking males is much lesser than the cost of mating with other males. If females want to mate with other males, they have to spend time traveling to the boundary edge of the troop, losing the amount of time for foraging. They also have to engage in more sexual activities, which often results in receiving aggression from other males. On the other hand, if they mate with only high-ranking males, they can spend the same amount of time foraging as usual because they do not need to travel, and they do not have to bear with aggression. Additionally, Thomsen et al. (2006) say that because females are better keeping a certain body weight to reproduce, wasting much energy is detrimental to them. As a result, while high-ranking males mate with females and ejaculate up to six times in a day, other males ejaculate by masturbation if they cannot have a chance to mate with any female. However, as described before, since females yet have a tendency to like novel males, while high-ranking males are mating in the center of the troop, other males, including non-troop males, have a chance to mate at the boundary edge of the troop. As a result, 41% of mating is actually done by non-troop males on Yakushima (Domingo‐Roura, Marmi, Andrés, Yamagiwa, & Terradas, 2004).

             In the island located at the east end of Asia, Japanese Macaques have uniquely evolved by adjusting their behaviors in order to live with the very different climates from the one where most of the other primates live. More or less influenced by humans living there, their habitats and social structures vary from place to place on the small islands. Recently, many farmers have complained about Japanese Macaques that eat and damage crops and fruits in their fields. As a result, some local governments allowed farmers to hunt them. According to Hashiba (1989), the number of Macaques hunted had been increasing every year. However, it is important to recognize that they have started coming down on human habitats because they deprived Japanese Macaques of many habitats in deciduous forests. Therefore, people should have a responsibility to come up with a better solution for this problem as soon as possible.