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Olive baboon
Papio anubis

SOCIAL ORGANIZATION AND BEHAVIOR

Olive baboons live in groups or "troops" as they are often called, ranging in size from 15 to 150 individuals (Rowel 1966; Dunbar & Dunbar 1974; Ray & Sapolsky 1992). Within the troop, there are several adult males, numerous adult females and their offspring of various ages. Females almost always remain in their natal group for their entire lives whereas males disperse in order to mate (Packer 1979a; Smuts 1985; Barton & Whiten 1993; Barton et al. 1996). Because females do not emigrate but rather stay and breed with immigrant males, some females within a group are closely related. Mothers, daughters, grandmothers, aunts, and nieces associate with each other as a subgroup of the larger troop. A stable linear dominance hierarchy exists within and between these matrilineal subgroups (Smuts 1985; Barton &Whiten 1993). Rank among females is passed down through the mother so that daughters rank just below their mothers and groups of related females are ranked higher or lower than other groups of matrilineal kin. Female relatives exchange friendly behaviors like grooming and remaining in close proximity to one another as well as supporting one another during agonistic encounters with other troop members, both male and female (Smuts 1985; Barton & Whiten 1993; Barton et al. 1996). In addition to having high-ranking supporters, dominance confers benefits to females, including increased access to food and higher reproductive success (Barton & Whiten 1993). Females also interact with one or two male members of the group and form long-lasting, social relationships that have been characterized as "friendships" (Smuts 1985). These friendships between male and female baboons include frequent, relaxed grooming sessions, traveling and foraging together throughout the day, sleeping near each other at the sleeping site, defense from aggressive conspecifics, and support in caring for infants (Aldrich-Blake et al. 1971; Smuts 1985). Friendships are sometimes formed when females are sexually receptive and approach newly immigrated, strange males to solicit mating through presentation of their swollen rumps or through grooming, but the bond can also begin between anestrous females and males and can persist into the cycling period of the female (Smuts 1985; Bercovitch 1991). Once a male accepts a female, the relationship becomes long-lasting and the two preferentially groom and remain in close proximity with one another. When females come into estrus, they tend to decrease the time associated with specific partners, but still strongly interact with their male friends (Smuts 1985). The dissolution of friendships occurs when males transfer to another group or when estrous females form new friendships with younger, more newly transferred males while cycling (Bercovitch 1991).

Unlike females, males leave their natal group and join another group after they reach sexual maturity. Transfer occurs after puberty, between six and nine years of age, when males have reached or are near their adult size (Packer &Pusey 1979; Smuts 1985). After leaving their natal group, males will transfer several more times over the course of their lives, to avoid breeding with their offspring (Packer 1979a; Sapolsky 1996). Because adult males within a group of olive baboons are not related, they often compete with each other, either directly or indirectly, for access to females (Smuts 1985). As a result, adult males generally associate with one another in tense, highly competitive ways (Aldrich-Blake et al. 1971; Smuts &Watanabe 1990). Older males, those past their physical prime or at least 14 years of age, generally have more cooperative, egalitarian, and relaxed relationships with each other compared to younger males, those that are in their physical prime and between the ages of eight and 11 years, and they work together, forming coalitions to displace younger males and prevent them from gaining mating opportunities (Smuts &Watanabe 1990). Even though they are capable of ganging up on younger group members, older adult male baboons do not rank highly in the dominance hierarchy that exists among males of the group. The hierarchy generally follows age, with the young adult males, both natal and transfers, ranking higher than older, long-term resident males (Smuts 1985). Both physically prime young adults and older adult male olive baboons outrank subadult males, those between the ages of about five to eight years old. Rank is contested, defended, and reinforced through agonistic interactions as well as other, friendlier interactions. Fights between adult males occur almost daily and injuries are common. In addition to wounds sustained during physical confrontations, subordinate males suffer higher levels of stress merely in the presence of higher-ranking males which can lead to health problems over time (Sapolsky &Share 2004). The dominance hierarchy among males is separate from the female dominance hierarchy within the group (Packer &Pusey 1979). Females do not always prefer high-ranking males as mates, but rather choose familiar males that are older and have been in the group for longer periods of time and with which they have spent time (Packer &Pusey 1979; Smuts 1985). Older male olive baboons also form coalitions with each other and work together to prevent a younger male from mating with an estrous female. (Smuts &Watanabe 1990). The social patterns exhibited by olive baboons vary across study groups and across sites such that at some sites, the dominance hierarchy is arranged differently and the effect of rank on access to cycling females may be more noticeable among some populations compared to others (Palombit pers. comm.).

Ritualized greetings between males are the most common form of male-male social contact in olive baboons and happen in socially neutral situations; that is, they are seen when there are no resources such as food or females at stake (Smuts &Watanabe 1990). The greeting begins when one male approaches another with a rapid, stereotyped gait, looking directly at the other male, smacking his lips, squinting, laying his ears flat against his head, and finally presenting his hindquarters (Smuts &Watanabe 1990). These are all affiliative signals in baboon communication and the male being approached either responds with similar facial expressions or turns away from the presenting male, rejecting the greeting. These greeting rituals can be characterized as tense and tentative, but seemingly serve the purpose of reinforcing the dominance hierarchy during times of relative calm. Non-aggressive greeting behavior may be beneficial to males because if they are able to reinforce their rank in relation to one another, during times of resource competition and high social anxiety, less physical confrontation may be necessary (Smuts &Watanabe 1990). Greetings between older males may also be important in solidifying cooperation while younger, higher-ranking males rarely complete the greeting ritual and are more likely to fight for a position of dominance (Smuts &Watanabe 1990).

REPRODUCTION

Both males and females reach sexual maturity between four and six years of age. Males do not reach full adult size until they are about 10 years old in the wild and between seven and eight years in captivity (Smuts 1985; Bercovitch 1989). Sexual maturation in male olive baboons is characterized by enlargement of the testicles, growth of the canine teeth, which are much larger in males than females, a deepening of the voice, and an increase in body size and bulk (Jolly & Phillips-Conroy 2003). Females continue to grow and reach their adult size within three years after puberty and give birth one to two years after they begin cycling (Smuts 1985; Smuts &Nicolson 1989). The average ovarian cycle lasts 37.5 days in wild olive baboons and is characterized by sexual swelling common among Old World monkeys. Swelling lasts for about 18 days, and ovulation is probably during the final week of swelling, as the tumescence subsides (Bercovitch 1991). The skin in the anogenital region becomes swollen and bright pink or red and serves as a signal of reproductive quality to males. These swellings can increase a female's weight by up to 14% and can often be burdensome. Females cannot move or sit as easily when swollen and are subject to bacterial and parasitic infections if the skin is cut or torn (Domb &Pagel 2001). Displaying such vivid signs of receptivity has an important function for females. Females with larger swellings reproduce at a younger age, produce a larger number of offspring per year, and a larger number of those offspring survive. Males are more attracted to females with larger swellings, contest mating more aggressively, and are likely to suffer from multiple followers and intruders during mating with females that have large swellings (Domb &Pagel 2001).

Mating among olive baboons is promiscuous; both males and females have multiple mates. One characteristic of baboon reproductive behavior is the formation of mating consortships, defined as continuous close spatial association between a male and a sexually receptive female characterized by copulatory activity (Packer 1979a). Olive baboons form these exclusive relationships, lasting a few minutes or up to two weeks in length, in which they copulate frequently and where the male prevents other males from mating with his consort partner (Bercovitch 1983; 1991). During this time, the pair may separate themselves from the group during the day, traveling within the sight of, but separately from, the main troop (Bercovitch 1983; Forster &Strum 1994). Furthermore, the consorting male protects the female from other males who attempt to mate with her and severe fights between males can occur. This is especially true during the most fertile time of her cycle, during the final few days of tumescence, when male challenges are significantly higher than during other times of the female's cycle (Bercovitch 1989; Jolly &Phillips-Conroy 2003). Females are involved in consortships with an average of three to four males and ranging up to nine different males over the period of fertility.

There is no seasonality in baboon reproduction; females cycle and give birth throughout the year (Bercovitch & Harding 1993). Gestation lasts about six months (180 days). The average interbirth interval at Gilgil , Kenya is about two years (760 days), but can vary greatly depending on a number of factors (Smuts & Nicolson 1989). For example, high-ranking females have shorter interbirth intervals than lower-ranking females in a group. This may be linked to nutrition and food availability to low-ranking members of the group (Smuts & Nicolson 1989). The amount of time between births is also affected by the survival of the previous offspring. If a female loses an infant, she resumes cycling within a month and will reproduce sooner than if she successfully raised the infant. Age also affects interbirth intervals , with older females having a shorter time between births than younger females (Smuts & Nicolson 1989).

PARENTAL CARE

While both male and female olive baboons take part in caring for infants, the majority of care provided is by their mothers. They are acutely dependent on their mothers for food and travel for the first several months of life. Infant olive baboons are born with bright pink skin and black coats. As they age, their skin darkens and they lose their natal coats. By six months, their coats have transitioned from black to the olive color characteristic of adults (Ransom & Rowell 1972). For the first few days of life, the mother physically supports the infant, which may have a difficult time grasping on to her for long periods of time. Within the first week of life, though, its grasp will strengthen and it will be able to support itself, clinging to the mother's fur for long periods of time. As soon as two weeks of age, the infant may break physical contact with the mother to investigate the ground and food items such as grass, but only for a few minutes at a time, and never far from the mother (Ransom & Rowell 1972). By three weeks, the infant may try to move away from the mother as it explores its surroundings, but it is quickly retrieved. For the first 10 months, the infant is within arm's reach of the mother at least 50% of the time, but the distance between mother and infant quickly increases as the infant ages so that by one year of age, the infant spends at least 50% of its time in distances greater than 8.5 m (27.9 ft) of its mother (Nash 1978).

As infants spend less time near their mothers, they increase the time spent playing by themselves or with their peers. For the first month of life, infant olive baboons nurse and sleep frequently, but as they age, their activity levels increase (Ransom &Rowell 1972). Nursing decreases over the first year of life, with the most significant decrease in time spent nursing occurring between 10 months and one year of age, when young baboons are weaned (Nash 1978). Mothers begin to reject nursing attempts by young olive baboons as early as six months of age. Though they do not nurse after being weaned, young baboons often take the nipple in stressful situations, which seemingly calms them. They can continue to do this until the mother has another infant or as long as the mother will allow it (Nash 1978). Infant baboons also depend on their mothers for transportation. While they initially cling to their mothers' ventrum, around three months of age they transition to riding "jockey style" on their mothers' backs (Ransom &Rowell 1972). Riding continues until the end of the first year in most instances and the mother rejects the infant's attempt to cling to her or climb on her after this age (Nash 1978).

It is advantageous for an adult natal male to care for an infant because he is likely related to it, either as a sibling, uncle, or cousin; caring for his young relative increases his inclusive fitness (Packer 1980). It is unusual for non-natal adult males to care for infants because, as is often seen in other nonhuman primate species where males and females mate promiscuously, males are not certain of their paternity and caring for an unrelated infant is a waste of time and energy (Packer 1980). While adult males protect infants that could be their offspring, thereby increasing their own fitness, the benefits of caring for unrelated infants also outweigh the costs incurred. Adult males decrease the probability of being threatened by other males if they are carrying infants (Packer 1980; Smith &Whitten 1988). This behavior is potentially hazardous for the infant, as the mere presence of an infant does not always deter a high-ranking male from directing physical aggression at a lower-ranking male. If two males begin to fight, the infant certainly is in danger of being injured or killed (Packer 1980). Additionally, the infant is often handled roughly as it is being picked up by the threatened male and vocalizes loudly, indicating its distress (Packer 1980; Smith &Whitten 1988).

Despite being exploited by adult males in some instances, infants benefit from a caregiving relationship with an adult male. Males groom infants, calming them during stressful periods and potentially decreasing ectoparasitic loads (Packer 1980). Another tangible benefit to infants is that adult males are capable of protecting infants from predators. At Gombe, Tanzania, where chimpanzees often hunt and capture young olive baboons, the mere presence of an adult male deters chimpanzees from attempting to catch a young baboon (Packer 1980). Additionally, adult resident males protect infants from newly immigrant males and other conspecifics. Infanticide by newly transferred males has been recorded in Tanzania and circumstantial evidence has been found at Gilgil, Kenya (Smuts 1985; Palombit 2003). When an infant dies, its mother resumes cycling much more quickly than if she had nursed and weaned the infant, beginning to cycle after the infant had gained independence. One hypothesis about infanticidal behavior is that male primates and other animals use infanticide as a way to induce cycling and increase their own reproductive success after joining a group (Hrdy 1974).

COMMUNICATION

Vocal and visual communication is important to olive baboons and serves to convey affection, aggression, wariness, and intent (Strum 1987). One of the most common vocalizations given by olive baboons of all ages is the "basic grunt", heard throughout the day as the group spreads out to feed, while traveling, during amicable social interactions, and as they settle into the sleeping site in the evening (Ransom 1981; Smuts 1985). Other vocalizations are heard under more specific conditions. Some calls given by adults include the "roargrunt", "cough-bark", and "cough geck". The "roargrunt" is heard in displays by adult males, this call is given through closed lips and is deep in pitch and sounds like a low humming. Given in situations of low anxiety, alarm, or discomfort, the "cough-bark" and "cough-geck" may be heard in response to unknown humans or low-flying birds (Ransom 1981). Other calls given by all group members include "two-phased barks" or "wa-hoos", "broken grunting", "pant-grunts", "shrill barks", and "screams". The "wa-hoo" is a two-syllable call given in series, ranging from one to 20 minutes in length in response to predators, during times of distress, such as when a baboon is separated from the group and trying to regain contact, and at night at the sleeping site in response to neighboring groups' "wa-hoos" (Ransom 1981). "Broken grunting" is heard in all age groups except the youngest infants and is a low volume, rapid series of grunts given during low intensity aggressive encounters. If these aggressive encounters intensify, the vocalization shifts from "broken grunting" to "pant-grunts", much louder, two-syllable vocalizations in which the first syllable is an inhaled "huh" and the second syllable is an exhaled "uh" (Ransom 1981). "Shrill barks" are loud, explosive calls given in alarm to sudden appearances of potential threats such as humans and which elicit startle responses in other group members. Heard in the most stressful situations, "screams" are any high-pitched, continuous sound in response to intense emotion including pain, fear, and aggression (Ransom 1981; Strum 1987). There are a few calls given only by juveniles and infants including "moans", "basic gecks", and "panting". Both "moans" and "basic gecks" are distress calls given in times of discomfort, fear, separation from the mothers, weaning, and during tantrums. "Panting" is heard during rough-and-tumble play, presumably when young olive baboons are having fun, and sounds like breathy laughter (Ransom 1981).

In addition to vocalizations, baboons have a repertoire of visual displays that convey emotion, intent, and other information to conspecifics. General greeting signals include "lipsmacking", "tongue protrusion", "jaw-clapping", "ear flattening", "eyes narrowed", and "head shaking". These facial expressions are sometimes accompanied by a "rear present" (Ransom 1981). Like the "grunt" vocalization, "lipsmacking" is a common behavior heard in a variety of social contexts including grooming and sexual behavior. It conveys affiliation, reassurance, appeasement, submission, and content (Easley &Coelho 1991). Agonistic postures are also important in olive baboon communication as precursors to physical attacks. "Staring", "eyebrow raising", "yawning", and "molar grinding", are all threatening facial expressions, though they increase in the intensity of threat they intend to convey as they build on each other. The "eyebrow raising" display, a signal of mild threat or annoyance, is particularly notable because olive baboons have white eyelids, made visible as they raise their eyebrows and creating a striking display. Additionally, adult males give the "yawning" display, the length of their canines is exaggerated and when given in concert with "eyebrow raising" indicate increasing levels of threat (Ransom 1981; Strum 1987). When giving a threat display at a distance, olive baboons bob their heads, adopt a rigid quadrupedal stance known as a "stiff arm threat" and slap the ground or employ other objects such as branches or small trees to display aggression (Ransom 1981). In response to aggressive displays, there are also a suite of submissive displays given by olive baboons including "rigid crouch", "tail erect", and "fear grin" (Ransom 1981; Strum 1987).

SPECIAL NOTES

One unusual characteristic seen in young olive baboons in Nigeria is their ability to swim and dive. They have been observed swimming, with their faces submerged in a river, and diving from trees overhanging the river. They have not been observed foraging during these activities, and presumably swimming and diving is a form of play among these baboons (Pfeyffers 1999-2000). Furthermore, at the site where they are often observed swimming, hot springs join the river and human tourists also enjoy swimming here. Other baboons that range near this river have not been recorded swimming or even wading in the river and there are no other published reports from outside of Nigeria of this behavior in olive baboons; this behavior may represent cultural differences between neighboring groups of olive baboons (Pfeyffers 1999-2000).

Content last modified: April 18, 2006

Written by Kristina Cawthon Lang. Reviewed by Ryne Palombit.