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Rhesus macaque
Macaca mulatta

SOCIAL ORGANIZATION AND BEHAVIOR

Rhesus macaques live in large, multi-male/multi-female groups that have an average of 10 to 80 individuals, regardless of habitat type. Groups may number in the hundreds in mountainous areas and areas of high human food subsidization or agricultural habitats (Lindburg 1971; Seth & Seth 1986; Qu et al. 1993; Southwick et al. 1996). Rhesus groups are characterized by female philopatry and male dispersal; females remain in their natal groups and form dominance hierarchies according to their matrilineal kinship while males emigrate from their natal groups at the beginning of the breeding season shortly before puberty, and may transfer groups throughout their lives in search of mating opportunities (Melnick et al. 1984). Female rhesus macaques very rarely leave their natal groups (Fooden 2000).

Among females, rank remains relatively stable over a lifetime and is passed on to female offspring. Each female rises in rank above her older sister, and therefore when old, high-ranking females disappear or die, they are usually replaced by their youngest daughters (Seth 2000). One of the benefits of dominance for a rhesus macaque is priority access to food and space. High-ranking females have greater access to feeding sites because they displace lower-ranking females and they are less likely to be disturbed during feeding compared to subordinates (Deutsch & Lee 1991). Because they have cheek pouches, though, low-ranking females do not consume less food than high-ranking females, they simply store as much as they can into their cheek pouches and then move away from the group to eat (Deutsch & Lee 1991). This method of feeding is more energetically expensive than remaining in the same area while feeding, so low-ranking females may be consuming the same amount but using more energy to consume it (Deutsch & Lee 1991).

Dominance status and rank among males is not stable over a lifetime, compared to female rhesus macaques. Immature males inherit the rank of their mothers, but as they mature, their status changes based upon a combination of social and aggressive skills (Lindburg 1971; Berard 1999; Bercovitch pers. comm.). Aggression is sometimes used to establish and reinforce social position, though, and aggressive behavior seen in macaques includes slapping, pushing, pulling fur, tail yanking, and biting as well as other non-contact behaviors such as displays and threats (Lindburg 1971). Once males attain dominant status, they enjoy this rank for an average of two years before being displaced by another male (Bercovitch 1997).

REPRODUCTION

Females reach puberty around age three while males are sexually mature by age four (Rawlins & Kessler 1986b). The ovarian cycle lasts for 28 days and is characterized by the darkening of the skin surrounding the anogenital region accompanied by menstruation (Catchpole & van Wagenen 1975). Estrus lasts for eight to 12 days, with the day of ovulation occurring at the midpoint of the estrus period. Females have increased sexual activity during ovulation, exhibiting the highest number of copulations seen during the ovarian cycle (Fooden 2000). Females reproduce from three until about 20 years of age (Rawlins & Kessler 1986b). Males reach puberty between three and 3.5 years of age but do not reach adult body size until about eight years old (Dixson & Nevison 1997; Bercovitch et al. 2003). Though males are capable of reproducing by age four, they are not reproductively successful until after age eight, or when they reach adult size. During this time between becoming sexually mature and when they begin to mate, young rhesus macaques are learning the social skills, including fighting ability, that will influence their success throughout their lives (Bercovitch et al. 2003). Both males and females reach sexual maturity sooner in captivity (Catchpole & van Wagenen 1975).

There is marked birth seasonality in rhesus macaques, with the majority of mating occurring in October through December and births coinciding with the end of the rainy season, or during the period of highest food abundance (Lindburg 1971; Qu et al. 1993). At Cayo Santiago, the mating season is much longer and begins in July and lasts until December (Chapais 1986). High-ranking males have more opportunities to mate with females than low-ranking males, but do not always sire a disproportionate number of infants. Lower-ranking males may have similar reproductive success compared to high-ranking males because they are new immigrants and are more attractive to females because of this (Berard 1999). From one breeding season to the next, females will drastically reduce the amount of mating they do with familiar males and over a period of three years, they try not to mate with any familiar males given the opportunity to mate with unfamiliar males (Bercovitch 1997; Berard 1999).

During the breeding season, females enter into consortships with one or more males. An individual female will spend longer amounts of time in contact with, grooming, and mating with these males. Males and female rhesus macaques are promiscuous breeders, mating multiple times with multiple mates (Lindburg 1971). Both males and females initiate these consort relationships and competition for access to mates is related to the high levels of aggression seen in rhesus macaque groups during this time of year. Gestation lasts 164 days in rhesus macaques and the interbirth interval is between 12 and 24 months (Fooden 2000). If a female does not have a successful pregnancy or her infant dies in the first year of life, she is more likely to give birth the following season than a female who successfully rears an infant (Seth 2000).

PARENTAL CARE

While the majority of parental care is the responsibility of the mother, rhesus infants are also handled by close female relatives and protected by adult males. In the first few days, the infant is carried ventrally and protected from other group members by the mother. Ventral clinging is the position most frequently adopted during travel for the first four months of life, but rhesus infants begin to ride dorsally for short periods during the second week (Lindburg 1971). By six weeks of age, locomotor skills are developed enough for the infant to move independently, but they do not move very quickly at this age, and if the mother is traveling too quickly, she will pick up the infant and carry it (Lindburg 1971). Some young rhesus are carried until they reach one year of age, though it is rare. During early infancy, rhesus macaques nurse exclusively for the first two weeks of life, after which they begin to experiment with solid food. At about four months of age, rhesus mothers begin to resist the attempts of their offspring to nurse, and young rhesus macaques are fully weaned by the birth of their next sibling (Fooden 2000).

Exploration off of the mother begins as early as five days old and continues to increase so that by the third week, the infant breaks physical contact with the mother as frequently as possible (Lindburg 1971). During this time, juvenile and adolescent females are intensely interested in the infant and will approach the mother and groom her in an attempt to get near the infant. When an infant is off the mother, a young rhesus female will touch the infant and try to carry it, but the mother is watchful of this interaction and any sign of distress from the infant may elicit an aggressive response from the mother towards the younger female (Lindburg 1971; Berman 1986). This practice of "aunting" behavior seen in young female rhesus macaques will influence their ability to successfully raise infants (Seth 2000).

Mother rhesus macaques show differential investment in their offspring depending on the sex of the infant. Maestripieri (2001) argued that rhesus mothers invest more energy into rearing daughters than sons as is evidenced by a female-biased birth ratio and longer interbirth intervals following the successful rearing of daughters compared to sons. He hypothesized that because mortality rates are higher among infant males, then in order for a mother rhesus to maximize her reproductive output, she should invest the most amount of energy into the infant that is more likely to survive. Females have higher survival rates than males, possibly due to lower disease or stress levels than males, an adult female should not invest as heavily in a male that is more likely to die (Maestripieri 2001). On the other hand, Bercovitch and Berman (1993) found that on Cayo Santiago, mothers who had sons had a delay in the next reproduction, and therefore there is a higher cost in producing males, not females.

COMMUNICATION

Vocal and gestural communication is important in rhesus macaques. Facial expression, body postures, and gestures are all forms of non-vocal communication among rhesus macaques and are important in interactions between individuals at short distances (Partan 2002). One facial expression that is seen throughout macaque species and is one of the most common expressions in rhesus macaques is the "silent bared teeth" face (Maestripieri 1999). Among rhesus macaques this is seen between individuals of differing rank with the lower-ranking or submissive animal performing the "silent bared teeth" face to the dominant animal (Flack et al. 2000). Another common facial expression used in dominance interactions include a "fear grimace" accompanied by a scream, heard in frightened animals and used to appease or redirect aggression (Rowe 1996). Dominant animals use a silent "open mouth stare" as a threat to other animals; this is accompanied by the tail sticking straight out behind the body with the monkey standing quadrupedally (Partan 2002). Another common visual communication signal is the "present rump," where the tail is raised and the genitals are exposed to the more dominant individual (Maestripieri 1999).

Vocalizations of rhesus macaques include "coos" and "grunts," which are commonly heard expressions during group movement, during affiliative interactions, and when one animal approaches another to groom (Hauser 1998). "Warbles," "harmonic arches," and "chirps," are heard in the context of finding high-quality, rare food items. The most common alarm call heard among rhesus macaques, the "shrill bark," is emitted in threatening situations and is consists of a single, loud, high-pitched sound (Lindburg 1971). Vocalizations made during aggressive interactions include "screeches," "screams," "squeaks," "pant-threats," "growls," and "barks" (Lindburg 1971). Infants have their own repertoire of vocalizations which include "geckers," which are harsh staccato sounds heard during weaning conflict. It is usually heard along with convulsive jerks of the body, and looks and sounds much like a human child's temper tantrum (Lindburg 1971; Partan 2002).

LISTEN TO VOCALIZATIONS

Macaca mulatta: Rhesus Vocalizations (Primate Cognitive Neuroscience Laboratory)

Content last modified: July 20, 2005

Written by Kristina Cawthon Lang. Reviewed by Fred Bercovitch.