Vervet
Chlorocebus

SOCIAL ORGANIZATION AND BEHAVIOR

Like other Old World monkeys such as macaques (Macaca species), vervets live in multi-male/multi-female groups and exhibit female philopatry (Struhsaker 1967). They live in groups of seven to 76, but the minimum number of adults necessary to maintain a stable group is two. If only one adult remains in a group with juveniles, the group merges with a neighboring troop (Fedigan & Fedigan 1988; Isbell et al. 1991). The average group size at Amboseli National Park is 24, and the average group size across all study sites is 25 individuals (Struhsaker 1967; Fedigan & Fedigan 1988). The composition usually includes a ratio of 1.5 adult females to one male. There are generally between one and seven adult males and two and ten adult females plus their offspring (Cheney & Seyfarth 1990). On Barbados, the average group size is slightly smaller, with only 15.3 individuals per group (Horrocks 1986).

Females remain in their natal groups throughout their entire life; therefore the core of the social group is closely related adult females and their dependent offspring. A linear dominance hierarchy according to matriline exists within the group and high-ranking females and their close relatives (mothers, sisters, and daughters) are the most sought-after grooming partners and have preferential access to food resources (Isbell et al. 1999). High-ranking females enjoy priority access to food during foraging bouts and have access to higher quality food, which greatly increases their yearly food intake compared to lower- and mid-ranking females (Whitten 1983). Consuming more and higher quality food confers an advantage to females both in overall health and in higher reproductive success. With higher stores of body fat, females are less likely to experience amenorrhea induced by malnutrition and they are likely to have higher milk quality that contributes to faster infant growth and therefore decreased mortality in their larger infants, and decreased interbirth intervals (Whitten 1983). Furthermore, obvious signs of nutritional stress are less evident in high-ranking females. At Amboseli, 31.4% of the adult vervets showed hair loss and changes in skin color associated with limited food availability, but low-ranking vervets were more likely to exhibit these external signs of stress than their comparably well-fed high-ranking conspecifics (Isbell 1995).

Rank relationships between females can be seen when one adult female supplants another; when the higher-ranking female approaches the lower-ranking female, the one lower in rank moves away from the approaching female and is displaced (Isbell et al. 1999). In addition to approach-avoidance interactions, vervets can be placed within the dominance hierarchy based on fighting between individuals (Isbell 1995). Females are supported by their relatives in agonistic interactions and form coalitions that last a lifetime. Daughters inherit the rank of their mothers because of social support given to their daughters even after they reach adulthood (Fairbanks & McGuire 1986).

Males emigrate from their natal groups around the time they reach sexual maturity and change social groups several times throughout their lifetimes (Struhsaker 1967; Cheney & Seyfarth 1990). The age at which males first transfer is around five years. Transfer coincides with sexual maturity and one explanation for male transfer is to avoid breeding with their mother or sisters (Cheney & Seyfarth 1983). Emigration is risky for young males, not only because they increase their risk of predation when they are not part of a group, but also because adult female vervets often attack transferring males (Cheney & Seyfarth 1983; Isbell et al. 2002). Because of the geographic distribution of vervet territories, highly overlapping and with multiple neighboring groups, males are able to transfer into neighboring groups without crossing large expanses of land, but decreasing the risk of predation is not the only reason young vervets immigrate into neighboring groups; they chose their new groups non-randomly and based on the presence of closely related male relatives (Cheney & Seyfarth 1983). Furthermore, they do not transfer alone but rather with older brothers or peers. By transferring into a group with relatives (half-brothers or half-sisters) and by emigrating with a partner, young vervet males decrease their risk of predation during the transfer process as well as decrease potential conflict as they move into a group with a few relatives (Cheney & Seyfarth 1983). These factors are important enough to outweigh the risks of breeding with their nieces in their new groups, but statistical analysis has shown that vervets transferring with close relatives into a group with close relatives actually decreases the possibility of mating with closely related kin (Cheney & Seyfarth 1983). Males transfer multiple times over their lives in order to avoid breeding with their sexually mature daughters; the rate coincides with maturation rates among females, therefore males transfer groups every two years. At the time of secondary transfer, adult males do not transfer with a partner nor do they join groups with close relatives (Cheney & Seyfarth 1983). Most transfers occur during the breeding season, from April to June. During this time, females are most likely to tolerate a new male in the group because they are sexually receptive (Cheney et al. 1988).

Once integrated into their new group, male vervets occupy the lowest rank. Like females, males also have a linear dominance hierarchy . This hierarchy is separate from the females' and includes all adult males within the group. Rank among males can be assigned based on displacement interactions as well as physical fighting (Isbell 1995). When males attain alpha status within the group, they hold this position between three and five years (Fairbanks & McGuire 1986). The alpha male in a vervet group inhibits the social behaviors and interactions with females of the lower-ranking males in the group (Keddy Hector & Raleigh 1992). He restricts their access to high-ranking females through aggression and prevents them from maintaining spatial proximity to, grooming of, or other affiliative behaviors with females. By doing so, he decreases the chances of subordinate males forming alliances with high-ranking females and he protects his position as a primary breeder within the group (Keddy Hector & Raleigh 1992). It is important to the alpha male to minimize the chance of relationships forming between dominant females and subordinate males because while males are dominant over females in one-on-one interactions, females form coalitions and several related females are dominant over any male. Females in these coalitions will support a male during a dominance interaction and can help determine the outcome of the male dominance hierarchy (Cheney 1983).

REPRODUCTION

Vervets are seasonal breeders. Among captive vervets in Kenya, the mating season lasts from July to September and births are grouped between November and January (Eley et al. 1986). In wild African populations, the breeding season lasts from April to June and the majority of births are concentrated from October to December (Cheney et al. 1988; Baldellou & Adan 1997). In the West Indies, births occur throughout the year but most are concentrated from April to July (Horrocks 1986). One of the reasons seasonal breeding is thought to have evolved in vervets is the advantage of giving birth during times of food abundance. After the rainy season, food resources are much more plentiful compared to other times of the year. Lactating females and new infants are not as likely to be nutritionally stressed during this time of year, increasing the chances of survival of both the mother and her infant (Baldellou & Adan 1997).

Females reach sexual maturity around four years of age, in the wild, and usually give birth for the first time around age five (Cheney et al. 1988). In captivity, females mature more quickly and can give birth as young as two years of age (Fairbanks & McGuire 1985). On Barbados, female vervets reach sexual maturity at 2.8 years of age (Horrocks 1986). Female vervets have no external signs of ovulation (Eley et al. 1989; Cheney & Seyfarth 1990). Data from captive vervets provide information about reproduction. The ovarian cycle lasts 32.5 days, on average, and is characterized by menstruation. Peak sexual receptivity occurs around day 13 of the cycle (Else et al. 1986; Eley et al. 1989). Female vervets have cycles throughout the year, but ovarian cycles are irregular during the non-breeding seasons (Else et al. 1986). Males reach sexual maturity around five years of age, but do not achieve full adult weight until six years of age, limiting the opportunity for mating before then. Gestation lasts 163 to 165 days (Eley et al. 1986; Andelman 1987). The interbirth interval varies between one and two years, depending on if the mother had a surviving offspring the previous breeding season. If she has successfully raised an infant, she is more likely to skip a breeding season whereas a female that has lost an infant is more likely to reproduce the following year (Lee 1984; Cheney et al. 1988). In the West Indies, the interbirth interval is slightly less than one year (Horrocks 1986).

PARENTAL CARE

Infant mortality among wild vervets is exceptionally high during the first year of life. At Amboseli, 57% of all infants die in their first year because of predation or malnutrition and subsequent illness (Cheney et al. 1988). With such high levels of mortality among young vervets, it is essential that parental care is provided. Mother vervets are the primary caregivers to their infants. During the first three months of life, 90% of the infant's time is spent within two meters (6.56 ft) of its mother (Lee 1984; 1987). For the first two weeks, they are in almost constant contact with each other, with the infant nursing frequently throughout the day. During the first week of life, infant vervets wrestle gently with older female relatives in the group and begin to play socially by the third week of life. After six weeks, the percent of time spent at distances greater than two meters (6.56 ft) increases quickly, especially during weeks seven through nine (Lee 1984). At this stage, the mother begins to resist nursing attempts by the infant so that by 12 weeks, the mother is almost constantly rejecting the nursing attempts of her infant. Weaning is not complete at this time, however, as some young vervets are seen nursing into their second year of life if their mother has not reproduced again (Lee 1984). The percentage of time spent in close proximity to the mother decreases over time so that between four months and one year of age, young vervets only spend about 30% of their time within two meters of their mothers. After one year of age, this percentage drops to about 10% (Lee 1987).

Because female vervets remain in their natal groups for life and males do not transfer until the age of five, infants often have several siblings within the group. Between 17 and 20% of a young vervet's time is spent near its siblings (Lee 1987). Older sisters play an important role in providing care for their younger siblings, allowing them to practice mothering skills and adding to the protection of the infant from predators and other group members. Additionally, the mother is able to more efficiently forage when her infant is with an allomother, so this break in caretaking may provide and important boost to both the infant and the mother (Lee 1987). Other females in the group are attracted to infants and can act as allomothers as well. All group members are attracted to high-ranking infants more often than low-ranking infants and attempt to gain access to high-ranking females through their infants. Males use this tactic in order to form social relationships that might lead to future mating opportunities while subordinate females attempt to decrease aggression from dominant females within the group (Lee 1987; Keddy Hector & Raleigh 1992). The close association between siblings can also lead to alliances and support during agonistic interactions. Older siblings of both sexes are active in defending and protecting their younger brothers and sisters from the aggression of unrelated individuals (Lee 1987). These alliances are maintained as the animals age. Males that remain near their brothers often transfer as pairs when they leave their natal group (Cheney & Seyfarth 1983; Lee 1987).

The dominance hierarchy is established even for the youngest members of a vervet group. Among related and unrelated males, the dominance hierarchy is strictly age-graded, with older brothers dominant over their younger male siblings and younger unrelated males in the group. Older brothers are also dominant over their younger sisters (Lee 1987). Between young females, younger sisters are dominant over older sisters. Rank among unrelated juvenile females is determined according to maternal rank; daughters of high-ranking females are dominant over daughters of low-ranking females (Fairbanks & McGuire 1985; Lee 1987).

COMMUNICATION

Vocal communication in vervets has been well studied because of their intricate system of predator-sensitive alarm calls. Vervets give separate, distinct vocalizations for certain predators that elicit predator-appropriate responses from their conspecifics (Seyfarth et al. 1980a; Enstam & Isbell 2002). In the wild, leopards, martial eagles, and pythons pose serious predatory risks to vervets (Cheney & Seyfarth 1990; Isbell & Enstam 2002). Leopards crouch and pounce from the tall grass, eagles swoop down while flying and can use their talons to grab an infant off of a mother's back, and pythons move through the grass and attack from the ground (Seyfarth et al. 1980a). Alarm calls serve the purpose of alerting other group members to the presence of danger. When an adult vervet sees one of these predators and gives an alarm call, the rest of the group respond appropriately. When the leopard call is given, the monkeys run up into the trees, where they are safer from the ambush style of attack typical of big cats. When the eagle alarm call is sounded, vervets look to the sky and run into dense brush or low-lying bushes in order to hide from the swooping raptor . Finally, when the snake call is given, vervets respond by looking down around them; this response is appropriate because vervets often work as a group to mob dangerous snakes (Seyfarth et al. 1980a). Adult vervets properly alert group members to the potential predator while infant and juvenile vervets often misidentify predators and give inappropriate alarm calls (Seyfarth et al. 1980b). Though the infants are able to assign the suitable class of predator, for example giving an eagle alarm call when any bird flies over, they need to learn the appropriate context in which to give alarm calls (Seyfarth et al. 1980b). The vervet sounding the alarm call conveys information about the perceived threat and the other members of the group can interpret the call and respond appropriately. This form of semantic communication was long thought to be unique to humans and the other great apes, but researchers Dorothy Cheney, Robert Seyfarth, and Peter Marler effectively proved that vervets are capable of referential communication.

LISTEN TO VOCALIZATIONS

• Chlorocebus aethiops: Vervet Monkey Vocalizations  (Primate Cognitive Neuroscience Laboratory)

Visual communication is also used between vervets to signal the presence of a predator. In the West Indies, where the only threat to vervets are dogs and humans, visual signals are used by an adult male to communicate to the rest of the group of a threat in the area (Horrocks & Hunte 1986). On Barbados , crop-raiding behavior is prevalent among vervets and involves all but one adult male venturing into a cultivated field and feeding. The adult male positions himself conspicuously while the rest of the group forages and instead of alarm calling in the presence of a potential predator, he simply moves from his place of prominence, signaling to the other members of the group, that are watching him as they forage, to move into cover (Horrocks & Hunte 1986). By silently signaling, the sentinel does not draw attention to himself or to the fact that his group is nearby and the predator will not be alerted to their presence. In the event that the predator has already detected the sentinel, he will vocalize loudly to warn other group members (Horrocks & Hunte 1986). Because the vervets of the West Indies have spent over 300 years in close proximity to human predators, these behaviors have evolved separately from the alarm call system seen in African vervets but are still an impressive system of communication (Fedigan & Fedigan 1988).

Another form of visual communication seen among male vervets are the "splaylegged" and "red, white, and blue" displays. Both of these capitalize on the brightly colored genitalia of male vervets and serve in communicating information about status between males (Henzi 1985; Gerald 2001). The "splaylegged display" is a sign of aggression seen when a vigilant male detects males of another group nearing the territory boundary. Either sitting on a branch or on the ground, the displaying male sits with his knees apart, exposing his genitalia. This display serves as a warning to nearby males and may act as a deterrent for males attempting to enter the group (Henzi 1985). The "red, white, and blue display" is seen when a dominant male approaches a subordinate, encircles him, and lifts his tail, exposing his brightly colored anogenital area to the lower-ranking male (Gerald 2001). This display reinforces the dominance hierarchy among males in a less energetically expensive manner than physical fighting.

Content last modified: January 3, 2006

Written by Kristina Cawthon Lang. Reviewed by Karin Enstam.

Cite this page as:
Cawthon Lang KA. 2006 January 3. Primate Factsheets: Vervet (Chlorocebus) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/vervet/behav>. Accessed 2010 February 9.