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Mothers and Offspring

Japanese macaques have annual mating and breeding seasons that vary between troops (Hiraiwa 1981). Gestation period lasts between 170-180 days (Nigi, 1971 as cited in Enomoto 1974). In a study of one free-living troop of macaques in Takagoyama, Japan, young mothers (5-9 years) were never observed to give birth in successive years. Middle aged mothers (10-14 years) sometimes gave birth successively, and older mothers over 20 years did so quite often (Hiraiwa 1981). A troop’s birth and infant mortality rates can be affected by the amount of resources available for pregnant mothers and offspring. The infant mortality rate may also be affected by the quality of the troop’s social interactions that influence the frequency of alloparenting behavior in young macaques (Ikeda 1982). If the infant dies, its mother will not come into estrus again until the next mating season (Hiraiwa 1981).

Mothers usually begin weaning their infants at 7 months (the weaning period is somewhat late because the cold winter temperatures and winter food scarcity in the macaques' habitat would not provide an environment conducive to weaning). If the female can wean and become pregnant again in a successive year, she has the chance to increase her reproductive success. Multiparous mothers (those who have given birth previously) seem to engage in this strategy. However, for primiparous mothers (those who have not previously given birth) it may be more beneficial to nurse the first infant for a longer period of time to help insure its survival. If the mother does become pregnant again, the older juvenile will not attempt to nurse after the new infant is born (Hiraiwa 1981).

Adult Japanese macaques have light brown fur with small amounts of gray, but young macaques' fur is dark brown and gradually changes to adult coloration later in life. Infants are born with their eyes closed but are almost immediately able to grasp their mother’s fur and to nurse. An infant's teeth begin to emerge at about 7 days old, and it starts eating solid food at about 5 weeks. Young macaques seem to learn their food repertoire from both their mothers and from other troop members, especially juveniles and adolescents (Hiraiwa 1981). Young Japanese macaques in the northern portions of their habitat sometimes make snowballs. This behavior has no apparent adaptive significance for fighting or feeding, so it is considered simply social and "fun” (MacDonald 2001).

Mothers and offspring share the strongest social bond among Japanese macaques, and mothers protect their young vigorously during the first few months of life (MacDonald 2001). Mothers carry their infants ventrally until they are about 4 weeks old, and dorsally thereafter. When the infant is very young, the mother restricts its movement and often retrieves it if it moves too far away (Hiraiwa 1981). The mother is the primary care-giver of offspring, but the entire troop takes some measure of responsibility for the young (Beacham 1998). Young infants will cling to adult females other than their mothers until they are about 6 months old, but after this age they will cling only to their mothers. Younger adult females act more positively to other females' infants than older adult females do, by allowing the infants to cling to them or spend time near them. Sometimes, two mothers may accidentally switch infants for brief periods. However, primiparous mothers seem more prone do so than multiparous mothers are (Lehman 1961 as cited in Hiraiwa 1981).

In the Takagoyama troop, infant mortality was greatest among primiparous, orphaned mothers. Orphaned mothers (primiparous and multiparous) showed frequent aggressive behavior toward their infants, but this behavior often decreased as the mothers raised more offspring and gained maternal experience. Primiparous mothers held their infants awkwardly, neglected them and left them behind more frequently, and carried them upside-down more often than multiparous mothers did. Because of these differences, primiparous females are sometimes considered less adequate mothers than multiparous females (Lehrman 1961 as cited in Hiraiwa 1981). Primiparous mothers in the Takagoyama troop also tended to wait longer to wean their infants than multiparous mothers did because of the weaning strategies discussed above (Hiraiwa 1981).

Observational learning in nulliparous, juvenile females seems to play a very important role in gaining maternal experience. Infants interact with juveniles and adolescents by 2 weeks of age. Alloparental care by young female macaques occurs throughout the year, and its frequency increases with the infant's age (partially because the mothers drive alloparents away until the infant is a month or so old). However, as the infant gets older, the percentage of time that its alloparent is an older sibling decreases. (Hiraiwa 1981).

The frequency of alloparental care by males is much lower than that of females. Infants come into contact with adult males by 4 weeks old, but the males are largely uninterested in the infants. The adult male's attitude toward the infant is not affected by the rank of the infant's mother. Males usually move away from an approaching infant or push it away. At 3-6 months of age, juveniles sometimes form groups and sit near adult males. The males usually tolerate this behavior, but they never initiate it. However, adult males have been observed caring for orphaned infants and other infants during the weaning period (Hiraiwa 1981). In the Arashiyama troop, each of the adult males has demonstrated strong ties to one or two juvenile females (less than 2 years of age), but these ties dissipated once the juveniles aged to maturity. Most of these interactions occurred with orphaned juveniles or with those who were part of a kingroup with which the particular central male had high levels of interaction (Grewal 1980).

Photograph courtesy of phototravels.net

Male young remain with their mothers and matrilineal groups until sexual maturity. At this stage (usually at the age of 3-7), they emigrate from their natal troop (Sugiyama 1976, as cited in Grewal 1980). However, they do not fully develop their secondary sex characteristics (canine teeth and red scrotum) until 8 years of age (Enomoto 1981). Male emigration is most likely an incest avoidance mechanism (Enomoto 1974).

Female young remain in the troop, and their bonds with their matrilineal group remain strong throughout their lives (Koyama 1967, 1970, as cited in Grewal 1980). Females reach sexual maturity and experience their first estrus cycle at about 3.5 years old. Once a female comes into estrus for the first time, she begins mating with males in her troop. However, young females do not often become pregnant as a result of their first mating attempt (Hiraiwa 1981). All females over 3.5 years come into estrus and mate with males, regardless of whether or not they have an infant (Hiraiwa 1981). However, it has been observed that females currently raising infants display the lowest levels of sexual activity of all females (Enomoto 1974).

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