Japanese monkey (English spelling)

Japanese: ニホンザル（英語表記）Japanese monkey

A monkey found in Honshu, Shikoku, Kyushu and the surrounding islands of Japan. Its southern limit is Yakushima (Kagoshima Prefecture) and its northern limit is Shimokita Peninsula (Aomori Prefecture). Of the approximately 300 non-human primate species that live on Earth, the Japanese macaques of the Shimokita Peninsula live at the northernmost point and are famous as the monkeys in the northern limit. Japanese macaques are also known as snow monkeys because they are rare among non-human primates and live in areas with heavy snowfall. Scientific name: Macaca fuscata .

[Ecology] Adult males and females weigh 8-13kg and 6-12kg, respectively. Japanese macaques are seasonal breeders that mate from autumn to winter, give birth from spring to summer after a gestation period of 170 to 180 days. Usually, they give birth to only one child at a time, and twin births are very rare, accounting for 0.07% of births. Females give birth for the first time at 5-6 years old, and then give birth once every 2-3 years. After the age of 20, their reproductive ability decreases, and most individuals die by around 25 years old. To date, the age at which a female's last child was recorded to be 26 years old, and the oldest recorded age was 34 years old. Males reach adulthood and become reproductively capable 1-2 years later than females. The oldest recorded age for a male in the wild is 30 years old.

[Society] Japanese macaques form groups of 10 to 100 individuals. When food is abundant, such as through feeding, groups can exceed several hundred individuals. Females spend their entire lives in the group in which they were born and raised, but males leave their natal group when they become adults. Therefore, groups are formed by a small number of adult males, a large number of adult females, and their children. After leaving their natal group, males may repeatedly move in and out of other groups, but they may also become solitary roaming monkeys (or single monkeys) or form male groups. Females who have a matrilineal blood relationship, such as mother and daughter, sister, grandmother and granddaughter, or aunt and niece, often rest, feed, and groom themselves in close proximity. In addition, when fighting, they often support their relatives. There is a clear ranking system among adult females that is heavily influenced by blood ties. Kawamura Shunzo (1958) recorded the ranking relationships of 13 adult females in the Minoh population (Osaka Prefecture) and reported for the first time in the world that 1) mothers are dominant over daughters, and 2) younger sisters are dominant over older sisters (youngest child dominance) (Kawamura's principle). This principle was also confirmed in the Arashiyama population (Kyoto Prefecture), which has more than 50 adult females, and it was also revealed that females belonging to the same bloodline are ranked together and that the ranking relationships among all adult females are linear (Koyama Naoki, 1967). However, when the population becomes large, with around 100 adult females in the population, the Kawamura's principle does not match and the linear ranking relationships break down.

There is also a linear hierarchy of dominance among adult males. Males who migrate into a group from outside become the lowest ranking males, and their ranking rises whenever a higher ranking male dies or leaves the group. In general, kinship does not influence the ranking of males. However, in feeding groups, it has been reported that some males of higher kinship do not leave the group, but remain in their original group and become the higher ranking males. With the establishment of paternity determination techniques using genetic analysis, it has been found that the highest ranking male does not necessarily have the most offspring. The highest ranking male was once called the boss monkey. However, because they rarely actually display behaviors that would be inferred from the word boss, such as settling fights within the group or deciding the direction of the group's movements, the term alpha (α) male, which simply indicates the number one male or the highest ranking position, is now used.

Grooming not only serves a hygienic function, such as removing monkey lice eggs from monkeys' hair, but also serves a social function, such as building and maintaining affinity between individuals and repairing relationships that have been temporarily disrupted by fights. Grooming between adults is mostly female-female, and rare between male-male and male-female. Each female tends to focus grooming interactions on one specific female, who is usually her mother or daughter. The average number of grooming partners for an adult female is about 10, even when the group size is large. As a result, 60-70% of female-female grooming occurs between closely related individuals, such as mothers and daughters, sisters, and aunts and nieces, and grooming between distantly related individuals and between non-related individuals is rare. Among non-related individuals, grooming from subordinate individuals to dominant individuals is more common than the reverse. However, not all females prefer grooming to closely related individuals; some prefer grooming to unrelated individuals, and there have been cases of unrelated female pairs whose grooming relationship has continued for more than 10 years.

Japanese macaques produce more than 30 different vocalizations, with coo being the most frequently produced sound. Coos are often exchanged between individuals who have a friendly relationship, and they can recognize who has made the coo. Females produce coos more often than males, and the frequency of cooing is particularly high for top-ranking females.

Sweet-potato washing, the behavior of washing sweet potatoes scattered at feeding sites on the beach of Koshima Island (Miyazaki Prefecture) with water and then eating them, is a cultural behavior known worldwide. According to Kawai Masao, this behavior, which was started by a 1.5-year-old female monkey, spread to her baby playmates and then to some of their mothers, and was then transmitted from mother to baby (Kawai, M., 1965). It has been reported that the behavior of playing with stones, which involves rolling and hitting pebbles, seen in some Japanese macaque groups, was transmitted from baby monkey to baby monkey, and then from adult individuals to baby monkeys.

[Development] From birth, baby Japanese macaques are able to cling to their mother (grasping reflex), seek out the mother's nipple (seeking reflex), and suck milk. The amount of milk consumed remains almost constant for the first 5 months of life, but after 6 months of age, the amount consumed drops to half the amount before, and breastfeeding continues for the first year of life. Weaning is determined by when the mother gives birth to the next child. Babies start putting solid food in their mouths from about 2 weeks after birth, but they do not actually start chewing or swallowing until about 5 weeks of age. Baby monkeys in their first year of life have a strong tendency to feed in the same way as their mother or baby monkeys of the same age who are nearby, and they also have a clear tendency to feed on the same foods. A similar tendency has been confirmed in baby monkeys in their second year of life. Therefore, baby monkeys appear to acquire a repertoire of food items by observing and learning the feeding behavior of their mothers during the first to second years of life.

Babies begin to walk crudely in the first week after birth, and their motor skills develop rapidly during the first two to three months of life. During this period, baby monkeys visually recognize their mothers from other individuals, and they move away from their mothers and increase their interactions with other individuals. For the first month after birth, the baby is carried by clinging to the mother's abdomen, but from about the second month, it also begins to be carried by clinging to her back. After the first year of life, the mother occasionally carries the baby on her back, even when the baby has acquired sufficient motor skills. As male baby monkeys grow, their interactions with their mothers decrease, but from the second year of life, they become more likely to interact with males of a similar age. For males, their relationship with their mothers ends when they leave the group. From about the age of three to four, baby females begin to maintain relatively frequent grooming and close contact with their mothers, and by the time they give birth for the first time, they establish a ranking among the adult females in the group that inherits their mother's ranking. A decline in cognitive ability in aged individuals has been revealed in learning experiments in captivity. However, inappropriate social behaviors that are assumed to be due to a decline in cognitive ability have not yet been confirmed in aged individuals living in groups. A case has been reported that supports the grandmother hypothesis, in which an elderly infertile female monkey acted as a mother to her grandchild, helping the grandchild survive. → Primates [Masayuki Nakamichi]

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Japanese:

日本の本州，四国，九州とその周辺の島々に分布するサル。南限は屋久島（鹿児島県），北限は下北半島（青森県）である。地球上に生息する約300種のヒト以外の霊長類の中で，下北半島のニホンザルは最北端に生息し，北限のサルmonkey in the northern limitとして有名である。ニホンザルはヒト以外の霊長類では珍しく，豪雪地域にも生息するため，スノーモンキーとしても知られている。学名Macaca fuscata。

【生態】　おとなの雄と雌の体重はそれぞれ8～13kg，6～12kgである。ニホンザルは秋から冬にかけて交尾をし，170日から180日の妊娠期間を経て，春から夏にかけて出産する季節繁殖動物である。出産は通常一産一子で，双子出産は出産の0.07％で，たいへん珍しい。雌は5～6歳で初産をし，その後2～3年に1回の割合で出産をし，20歳を過ぎると繁殖能力が減少し，多くの個体は25歳ころまでに寿命を終える。これまでの野生ニホンザルの雌の最終出産年齢は26歳，最高齢は34歳が記録されている。雄は雌より1～2年遅れておとなに達し，繁殖能力を有するようになる。野生場面での雄の最高齢記録は30歳である。

【社会】　ニホンザルは10頭から100頭ほどからなる集団を形成する。餌づけなどによって食料が豊富な場合，集団は数百頭を超えることもある。雌は生まれ育った集団で生涯を過ごすが，雄はおとなになるころに出自集団から離脱する。そのため，集団は少数のおとな雄と多数のおとな雌，そしてその子どもたちから形成される。雄は出自集団から離脱したのち，他の集団への移出入を何度か繰り返すが，単独で行動するハナレザル（または，ヒトリザル）になったり，雄グループを形成したりすることもある。母と娘，姉妹，祖母と孫娘，叔母と姪といった母系血縁関係matrilineal blood relationshipにある雌同士は，近距離での休息や採食，毛づくろいを頻繁に行なう。また，ケンカの場合には，血縁個体を支援することが多い。おとな雌の間には，血縁の影響を大きく受けた明瞭な順位rankが存在する。川村俊蔵（1958）は箕面集団（大阪府）の13頭のおとな雌の順位関係を記録し，①母は娘よりも優位である，②妹は姉よりも優位である（末子優位）ということを世界に先駆けて報告した（川村の原則）。この原則はおとな雌が50頭以上の嵐山集団（京都府）でも確認され，さらに，同じ血縁系に属する雌たちは互いにまとまって順位づけられること，すべてのおとな雌の間の順位関係が直線的であることも明らかになった（小山直樹，1967）。しかし，集団内のおとな雌の頭数が100頭前後の大型集団になると，川村の原則に一致せず，直線的順位関係も崩れてくる。

　おとな雄間にも直線的優劣順位が存在する。外部から集団に移入してきた雄が雄の中で最下位になり，上位雄が死亡したり集団から離脱したりするごとに順位が上昇する。一般に，雄の順位決定には血縁系の影響がない。しかし，餌づけ集団では，上位血縁系の一部の雄が集団を離脱せず，出自集団にとどまり，上位雄になる場合も報告されている。遺伝子解析による父子判定技術の確立によって，最上位雄が最も多くの子を残しているとは限らないことがわかっている。雄の中で最も順位の高い個体をかつては，ボスザルと称していた。しかし，集団内で生じたケンカを鎮める，集団の移動方向を決めるなどのボスということばから類推されるような行動を実際に示すことが少ないため，現在では，単に第1位雄や最上位であることを示すアルファ（α）雄の呼称が用いられている。

　毛づくろいgroomingはサルの毛に付くサルジラミの卵の除去といった衛生的機能だけでなく，個体間の親和関係の構築や維持，さらにはケンカなどによって一時的に崩れた個体関係の修復などの社会的機能も有する。おとな間での毛づくろいは，雌-雌間が多く，雄-雄間，雄-雌間は少ない。どの雌も毛づくろいのやりとりを特定の1頭の雌に集中する傾向があり，その相手は母あるいは娘であることが多い。おとな雌の平均の毛づくろい相手数は集団サイズが大きくなっても約10頭である。結果的に，雌-雌間の毛づくろいの60～70％が母と娘，姉妹，叔母と姪などの近縁個体間で行なわれ，遠縁個体間，非血縁間での毛づくろいは少ない。非血縁個体間では，劣位個体から優位個体への毛づくろいがその逆方向よりも多くなる。しかし，すべての雌が近縁個体への毛づくろいを選好するのではなく，非血縁個体との毛づくろいを選好する個体も存在する。さらに，毛づくろい関係が10年以上継続する非血縁雌ペアがあることも確認されている。

　ニホンザルは30種以上の音声を表出し，クーCooは最も頻繁に表出される音声である。親和関係にある個体間でのクーの交換が多く，だれが発声したクーかを互いに認識できる。クーは雄よりも雌が多く発し，とくに第1位雌の発声頻度が高い。

　幸島（宮崎県）の浜辺の餌場にまかれたサツマイモを水で洗って食べるイモ洗いsweet-potato washing行動は文化的行動として世界的に知られている。河合雅雄によると，1歳半のメスが始めたこの行動は遊び仲間の子ザル，さらにその一部の母ザルに広まり，その後は母ザルから子ザルに伝播した（Kawai,M.，1965）。一部のニホンザル集団で見られる小石を転がしたり，打ち鳴らしたりする石遊び行動では，子ザルから子ザルに伝播し，その後は，おとなになった個体から子ザルに伝播したことが報告されている。

【発達】　ニホンザルの赤ん坊は誕生時から母ザルにしがみつき（把握反射），母ザルの乳首を探し（探索反射），ミルクを吸引することができる。生後5ヵ月間の摂乳量はほぼ一定であるが，6ヵ月齢以降はそれまでの半分の摂乳量となり，生後1年間は授乳が継続する。離乳は母ザルの次の子の出産時期によって決まる。赤ん坊は生後2週ころから固形物を口に入れるが，実際に咀嚼や嚥下を始めるのは5週齢ころからである。生後1年目の子ザルは近距離にいる母ザルや同年齢の子ザルが採食しているときに，同じように採食する傾向が強く，さらに，同じ食物を採食する傾向も顕著である。生後2年目の子ザルでも同様の傾向が確認されている。したがって，子ザルは生後1～2年の間に母ザルらの採食行動を観察学習して，採食品目レパートリーを獲得しているらしい。

　赤ん坊は生後1週目に稚拙な歩行を開始し，生後2～3ヵ月の期間に急速に運動能力を発達させる。この時期には，子ザルは母ザルを他個体から視覚的に認識し，母ザルから離れて他個体とのかかわりを増加させる。生後1ヵ月間は母ザルの腹部にしがみついて運搬されるが，2ヵ月目ころから背中にしがみついての運搬も始まる。母ザルは生後1年を経過し十分な運動能力を獲得した子ザルに対しても背中での運搬を時折行なう。雄の子ザルは成長に伴い，母ザルとのかかわりが減少し，逆に，生後2年目からは年齢の近い雄とのかかわりが頻繁になる。雄は集団からの離脱によって母ザルとの関係が終焉する。雌の子ザルは3～4歳ころから，母ザルとの比較的頻繁な毛づくろいや近接などを維持するようになり，初産ころまでに母ザルの順位を継承した順位を集団のおとな雌間の中に確立する。老齢個体における認知能力の減退が飼育下での学習実験で明らかになっている。しかし，集団の中で暮らす老齢個体においては，認知能力の減退に基づくと推測される不適切な社会行動はまだ確認されていない。高齢で老年期不妊の雌が孫の子ザルに母親行動を行ない，孫の生存に役立ったというおばあさん仮説grandmother hypothesisを支持する事例が報告されている。　→霊長類
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