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Pigtail macaque
Macaca nemestrina

SOCIAL ORGANIZATION AND BEHAVIOR

The general social structure for macaque species is multi-male/multi-female with distinct dominance hierarchies within each sex (Dittus 2004). Pigtail macaques exhibit this social organization in groups of up to 81 animals (Oi 1990a). Within these groups, the sex ratio is biased toward females, which remain in their natal groups their entire lives (Caldecott 1985; Oi 1990a). Males emigrate from their natal groups between five and six years of age and temporarily associate with other social groups before ranging solitarily or permanently immigrating into a group (Oi 1990a). Adult males are dominant over adult females and males often attack females and displace them at feeding sites. Groups of females are capable of attacking adult males, though, and individual females sometimes initiate aggressive encounters towards lower-ranking males with the help of female relatives (Oi 1990b). One reason this between-sex aggression may occur is decreased tolerance for competitors. Females may see low-ranking males not as potential mates but as competitors for food resources. Aggressive behaviors are even more severe between males than between males and females, with the highest-ranking male directing aggression toward lower-ranking males and solitarily ranging males that attempt to join the social group (Oi 1990b). The highest-ranking females in a pigtail macaque group are likely sisters and share similar rank and are highly tolerant of one another. Tolerance is exhibited through grooming, kissing, and feeding together (Oi 1990b).

Though there are multiple males within a group of pigtail macaques, there is a linear dominance hierarchy among the males and there is usually a dominant male within the group. When young males immigrate into a new group, they are the lowest-ranking males and will work their way up the dominance hierarchy, attempting to secure the position of alpha male. In captivity, if an adult male is successful at assuming the position of alpha male after he is placed in a group, he often is responsible for killing any infants within the group. This infanticidal behavior has only been recorded in captivity, not in the wild (Clarke et al. 1995).

REPRODUCTION

Pigtail macaques are not seasonal breeders and mating occurs year-round though there is a slight peak from January to May (Crockett & Wilson 1980; Rowe 1996). Females reach sexual maturity at three years of age, which is accompanied by the characteristic anogenital swelling during estrus, but compared to M. leonina, M. nemestrina females have much larger sexual swellings (Maestripieri pers. comm.). Males mature later, reaching puberty between three and 4.5 years (Sirianni & Swindler 1985). A female solicits a male by approaching him from behind and then passing directly in front of his face and presenting her rump while looking back over one shoulder. Females mate with multiple males, both adolescents and adults regardless of dominance rank, during estrus (Oi 1996). If only a few females in a group are in estrus, the two top-ranking males are able to monopolize mating with these females because it is possible for them to keep other males from mating with one or two females. In fact, if a low-ranking male attempts to solicit or mate with a female, the alpha male will interfere and act aggressively toward both the male and female. The higher-ranking males have difficulty monopolizing estrous females if there are more than two in a group at one time, and lower-ranking males have more opportunities to mate when more estrous females are present (Oi 1996). In one study with captive pigtail macaques, genetic tests of captive pigtail macaques have revealed that dominance rank did not influence paternity, that is, the highest ranking individuals did not have the most offspring in the group (Gust et al. 1996). These findings cannot be generalized to all pigtail macaque groups and genetic tests in wild groups of pigtail macaques could lead to new insight into reproductive success and dominance rank.

While male rank does not guarantee higher reproductive output among male pigtail macaques, female rank is a factor both in reproductive output and sex of offspring, according to one captive study. After a gestation period of 170 days, high-ranking females were more likely to gave birth to female infants while low-ranking females more often give birth to male infants (Maestripieri 2002). Furthermore, this bias increased with age in both high and low-ranking females, so that as they age, high-ranking females were even more likely to give birth to females while low-ranking females are even more likely to give birth to sons (Maestripieri 2002). One possible reason this pattern was seen could be that daughters are more energetically expensive to raise as they nurse more frequently and depend on their mothers for protection for longer because they remain in their natal group. Males, on the other hand, nurse less frequently and spend less time near their mothers as they age, eventually leaving their natal group (Maestripieri 2002). Higher-ranking females have more to gain by investing in daughters, who will remain in the group and form coalitions with their female relatives to maintain dominance. Low-ranking females do not gain as much by having daughters because they are likely to be the target of aggression from other females and females are energetically expensive to rear (Maestripieri 2002). They give birth to sons that will leave the group and have a chance of attaining a higher rank than would be possible if they were a female remaining in her natal group. This is one way that female pigtail macaques increase their inclusive fitness. More research on dominance rank and sex ratio is needed to test these hypotheses. The interbirth interval for wild pigtail macaques is between 18 and 24 months, depending on resource availability (Oi 1996).

PARENTAL CARE

Macaque mothers care for their offspring with little or no help from other individuals. While the majority of care occurs in the first year of life, including nursing, transportation, and protection, macaque mothers groom and socially support their offspring, especially females, throughout their lives or until they leave the natal group (Maestripieri 2004). The infancy period lasts from birth to one year in pigtail macaques and juvenilehood lasts from one to 3.5 years (Oi 1990a). During the first month of life, pigtail mothers rarely break contact with their infants, but as they age, infants begin to spend time off of their mother, exploring the surroundings. This is very tentative at first, but increases after the fifth week of life (Maestripieri 1994a). By the third month of life, the natal coat of infant pigtail macaques begins to change from black to the adult olive brown coloration (Maestripieri 1994b).

During infancy, pigtail macaques are subject to being grabbed or seized by other adult females in the group. All females in the group show great interest in infants, and higher-ranking females often grab lower-ranking females" infants (Maestripieri & Wallen 1995). These "kidnappings" occur especially while the infant is moving independently from the mother. If infants are separated from the mother long enough, they are likely to die from starvation or dehydration (Maestripieri 1994b).

COMMUNICATION

Some researchers (i.e. Crockett & Wilson 1980 and Oi 1990a) describe pigtail macaques as uncharacteristically quiet monkeys that move through the forest almost silently, with the only sounds coming from juveniles rustling leaves. They have been observed being silent after fleeing or a group disturbance (perhaps after being startled by researchers) and are especially quiet after crop raiding and being chased into forested areas by farmers (Crockett & Wilson 1980; Caldecott 1986). Laboratory and other field studies show they are not always quiet, and do have a wide range of calls and vocalizations. "Coo" vocalizations are used as a pigtail macaque group forages and moves through the middle and upper canopy of the rainforest. These can be short or long in duration and are the most common calls heard in the wild (Caldecott 1986). Other vocalizations include "squeals," "screeches," "screams," "growls," or "barks." These are heard in harassed, threatened, or threatening individuals during agonistic interactions within the group (Caldecott 1986).

Pigtail macaques also have a variety of facial expressions and postural behaviors that communicate status and intent to other pigtail macaques. "Branch shaking" by males is one display thought to attract the attention of estrous females as well as threaten peripheral males (Caldecott 1986). A common facial expression unique to pigtail macaques is the "pucker" or "protruded lips face" seen in both males and females. Males direct "puckers" toward estrous females as well as other males, with very different results. When adult males "pucker" towards females, they often copulate directly following the exchange but when a "pucker" is directed toward another male, the submissive male retreats (Caldecott 1986). The most common facial expression, and one that is seen throughout macaque species, is the "silent bared teeth" face. Among pigtail macaques this is seen between males, with the male performing the "silent bared teeth" face always lower-ranking than or submissive to the male to which the signal is directed (Flack et al. 2000).

SPECIAL NOTES

In Thailand, pigtail macaques are used by coconut farmers to retrieve fruits from the crowns of tall palm trees on coconut plantations, a dangerous job for a human but a natural behavior for a macaque. They are taken from the wild as infants and are raised by humans. By the age of five years, pigtail macaques are trained to respond to verbal commands, how to choose coconuts in different phases of ripeness, and how to remove a coconut from the stem (Sponsel et al. 2002). After a pigtail macaque is trained, it receives food rewards for performing these tasks properly and an efficient macaque can harvest between 500 and 1000 coconuts per day from a coconut palm plantation. About half of a family's yearly income may be earned by the pigtail macaque retrieving the coconuts which are sold at market or used for food and a multitude of other products (Sponsel et al. 2002).

Content last modified: September 12, 2005

Written by Kristina Cawthon Lang. Reviewed by Dario Maestripieri.