Vervet

TAXONOMY

Suborder: Haplorrhini

Infraorder: Simiiformes

Superfamily: Cercopithecoidea

Family: Cercopithecidae

Subfamily: Cercopithecinae

Genus: Chlorocebus

Species: Ch. aethiops, Ch. cynosuros, Ch. djamdjamensis, Ch. pygerythrus, Ch. sabaeus, Ch. tantalus Subspecies: Ch. p. excubitor, Ch. p. hilgerti, Ch. p. nesiotes, Ch. p. pygerythrus, Ch. p. rufoviridis, Ch. t. budgetti, Ch. t. marrensis, Ch. t. tantalus

Other names: grivet or savanna monkey; Bale Mountains vervet or djam-djam; vervet monkey; green monkey; tantalus monkey.

The classification of vervet monkeys was recently updated, moving all of the species from the genus Cercopithecus to a new genus, Chlorocebus. There are now at least six species of vervets recognized, but often in the literature, they are ubiquitously referred to as Chlorocebus aethiops (Ch. aethiops) by their former genus, Cercopithecus aethiops (C. aethiops).

They are sometimes lumped together with a group of primates called guenons, medium-sized arboreal African monkeys of the genus Cercopithecus.

Appearance

Typically, Ch. aethiops, Ch. pygerythrus, Ch. tantalus and Ch. cynosuros have a yellow to greenish-brown coat with white undersides and white fur on their brows and cheeks.

They have bluish skin on their abdomens while their faces, hands and feet are all black-skinned. Males of all species of vervets have bright blue scrotal areas contrasting with red penises. This striking coloration, when combined with the white fur of their undersides is highlighted in what is referred to as the "red, white, and blue display.

There is some variation in fur length and coloration among the species. Chlorocebus sabaeus has golden-green fur with pale hands and feet. The tip of the tail is golden yellow as are the backs of the thighs and cheek whiskers.

They do not have a distinguishing band of fur on the brow, like other species, and males have a pale blue scrotum.

Bale Mountains vervets, Ch. djamdjamensis, have much longer, thicker fur than other species and their fur is darker brown. They have dark gray hands and feet and white beards and there is only a faint white brow band in this species.

Infant vervets are born with black natal coats and pink faces that gradually change to the adult coloration by 12 weeks of age.

All vervet males and females are sexually dimorphic and wild adult males weigh between 3.9 and 8.0 kg (8.60 and 17.6 lb), averaging 5.5 kg (12.1 lb), and measure between 420 and 600 mm (1.37 and 1.97 ft), averaging 490 mm (1.61 ft) from the top of the head to the base of the tail. Wild adult females weigh between 3.4 and 5.3 kg (7.50 and 11.7 lb) and average 4.1 kg (9.04 lb), and measure between 300 and 495 mm (11.8 in and 1.62 ft), averaging 426 mm (1.40 ft) .

Vervets move quadrupedally and they are equally as comfortable on the ground as they are in the trees. They are semi-terrestrial and semi-arboreal, spending time feeding and traveling on the ground during the day and retreating to the trees to sleep at night (Fedigan & Fedigan 1988). They have the characteristic cheek-pouches like other members of the superfamily Cercopithecoidea which allow them to forage and store food to be eaten later.

Lifespan in wild vervets is difficult to characterize because of high rates of predation in the long-term study areas. One female was at least 13 years old when she died and females in captivity have lived this long. Between 11 and 12 years, serious health problems are seen in captive vervets and this is probably the upper limit of their lifespan.

RANGE

Vervets are the most widespread of the African monkeys and inhabit large parts of sub-Saharan Africa.

They are found broadly across the continent from Senegal to Ethiopia and north into Egypt and Eritrea as well as southward to South Africa.

Vervets are largely absent from the forests of the Congo Basin in west-central Africa, though some species inhabit the edges of these forests.The species are separated geographically but some areas of hybridization occur.

Political instability in some regions in eastern and western Africa has made censusing vervets difficult, but the range is reasonably defined.

Chlorocebus pygerythrus ranges from the Ethiopian Rift Valley in central Ethiopia eastward into Somalia, and southward into Kenya, northern Tanzania and eastern Uganda.

To the north, Ch. aethiops is found in Sudan, east of the White Nile River, Ethiopia, Eritrea, and probably into southeastern Egypt. In the southeastern part of its range, Ch. aethiops hybridizes with Ch. pygerythrus as well as with Ch. p. hilgerti at the Omo River in Ethiopia.

Another species of vervet found in Ethiopia is Ch. djamdjamensis, restricted to the Bale Mountains region and surrounding highland areas east of the central Rift Valley.

Chlorocebus tantalus is found in Sudan, Uganda, and northwestern Kenya around Lake Turkana, one of Africa's Great Lakes, and its range stretches west into Togo, Benin, Nigeria, Niger, Chad, Cameroon, Equatorial Guinea, Central African Republic, Congo, and into Ghana where it is restricted by the Volta River. It hybridizes with Ch. p. rufoviridis in Uganda along the northern and western shores of Lake Victoria (Isbell & Enstam under review).

The westernmost species of vervet is Ch. sabaeus, found from Senegal to the west bank of the Volta River in Ghana and ranging in Mauritania, Mali, Gambia, Guinea-Bissau, Guinea, Sierra Leone, Liberia, Côte d'Ivoire, and Burkina Faso. Chlorocebus cynosuros is found in northern Namibia, Angola, southern Democratic Republic of Congo, Botswana, and in Zambia. Chlorocebus is also found in Rwanda, Burundi, South Africa, Lesotho, Zimbabwe, Swaziland, Malawi, and Mozambique.

In addition to being found in 39 African nations, one species of vervet is also found on the Cape Verde islands off the horn of Africa in the Atlantic Ocean and on several Caribbean islands in the West Indies. Chlorocebus sabaeus was introduced in the late 1600s to the islands of St. Kitts, Nevis, and Barbados when ships involved in the slave trade traveled from to the Caribbean from West Africa.

Green monkeys traveled as pets or as items to be traded and sold and have since adapted well to life on these islands where they are sometimes considered a pest species.

Dorothy Cheney and Robert Seyfarth have conducted long-term research on vervets at Amboseli National Park in southern Kenya since 1977. Lynne Isbell has also been conducting long-term research on vervets at Amboseli and with Karin Enstam at a private conservation area, Segera Ranch, in north-central Kenya since 1992. Vervets have also been studied in the Caribbean for several decades. Most of the current published information about vervets is from studies done at these sites.

HABITAT

Vervets are habitat generalists, as is obvious from their widespread range in Africa and the success of introduced populations. They are tolerant of a wide variety of habitats and can live in humid rainforests, semi-desert environments, or swamps from sea level to elevations up to 4500 m (14,764 ft); their only limitation seems to be water availability and the presence of sleeping trees. Because of this limitation, they are especially prevalent in riverine forests bordering savannas. They are also able to exploit areas near cultivated fields because they are adept at raiding crops.

Additionally, vervets can survive quite well in urban areas. Vervets are rarely found in the depths of dense forests, but rather utilize the edges of tropical rain forests, lowland evergreen forests and montane forests. They seem to prefer wooded rather than heavily forested areas, such as dry deciduous forest, scrub forests and gallery forests, which are composed of both trees and shrubs (Nakagawa 1999). They are not found in open grassland with no trees, but they spend some time in open savannas moving between wooded areas. Data on climate have been provided for study sites within their range. In Senegal, there are two seasons; the dry season lasts from November to May and the rainy season lasts from June to October. The mean annual rainfall is 954 mm (3.14 ft), and almost all of it falls in the months from June to October. The dry season has maximum temperatures between 33° and 40° C (91.4° and 104° F) while the rainy season is slightly cooler, with average maximum temperatures ranging from 30° to 33° C (86° to 91.4° F) (Harrison 1984). In Cameroon, the wet season lasts from May to September while the rest of the year is dry. The average annual rainfall is 497 mm (1.63 ft) and average monthly temperatures range from 22.7° to 33.7° C (72.9° to 92.7° F) (Nakagawa 1999).

The climatological conditions in which vervets live in Kenya include a drier season lasting from September to January or March with average annual rainfall reaching 700 mm (2.30 ft). In the West Indies, vervets inhabit both densely populated areas in urban habitats as well as highly cultivated agricultural habitats with lower human population density. Because agriculture and fuelwood extraction have significantly decreased in the last 30 years, vervets also inhabit thickly wooded ravines of secondary forest. They can be found in mangrove swamps, in stands of sea grape, a tropical evergreen, and in pasturelands bordering wooded areas. The rainy season lasts from July to September and the dry season stretches from December to May. The average temperature on St. Kitts is 23.9° C (79° F).

ECOLOGY

In much of their range, seasonal availability of food and water necessitates the ability of vervets to forage for and consume a wide variety of foods. In fact, vervets, along with chimpanzees (Pan troglodytes and P. paniscus) and baboons (Papio species), have the most omnivorous diet of all primates. Vervets eat all of the types of food that any primate is known to eat including leaves, gum, seeds, nuts, grasses, fungi, fruit, berries, flowers, buds, shoots, invertebrates, bird eggs, birds, lizards, rodents, and other vertebrate prey. They have a strong preference for fruit and flowers, which are seasonal resources, and from month to month vervets vary their diet tremendously to cope with fluctuations in food availability.

Brushfires during the dry season often temporarily reduce vegetative cover so that little is available during the beginning and middle of the rainy season. Subsequently, vervets can suffer severe nutritional stress accompanied by hair loss and changes in skin color. In more developed and agricultural areas, vervets are less subject to nutritional stress and seasonal availability of food because they depend on sources of food provided by humans including cultivated fruits, vegetables, and cereal crops. On Barbados, vervets consume fruits and vegetables grown in subsistence gardens and on larger plantations. Some of the most damaged crops include corn, sweet potato, bananas, mangos, papayas, guavas, cherries, cucumbers, peanuts and yams.

They have become an increasing nuisance to farmers that grow these crops and one management option has been to trap crop-raiding vervets and sell them research facilities for use in biomedical research. In addition to varying their diet according to environmental conditions, vervets change their daily activity patterns depending on the season. Generally, they travel, feed, and sleep as a group. In the rainy season, vervets spend their mornings, from about 7:00 a.m. to 11:00 a.m., traveling, feeding and drinking water and the afternoons, from 11:00 a.m. to 3:00 p.m., are spent feeding, drinking water, resting, grooming and doing some traveling. The proportion of time spent doing each activity changes during the dry season so that less time is spent traveling and feeding and more time is spent drinking, resting, and grooming in the mornings.

During afternoons in the dry season, more time is spent drinking water, feeding and grooming than during the rainy season. The evenings, from 3:00 p.m. until 7:00 p.m., are spent feeding and traveling to the sleeping tree. Sleeping trees utilized by vervets average 7.7 m (25.3 ft) in height and are usually found in wooded areas rather than standing alone or in small clusters of trees amidst open grassland. One reason vervets use sleeping trees is to decrease the risk of predation. In their natural habitat, potential predators include lions, leopards, cheetahs, other felid predators such as African wild cats, servals and caracals, hyenas, black-backed jackals, raptors and baboons (Papio species).

In the West Indies, dogs are serious predators and in most areas throughout their range, humans hunt vervets either for meat or as a means to control the population. Like changes in their diet and activity pattern, the day range length and home range size of vervet groups changes seasonally as well.

In Senegal, vervets travel between 665 and 2670 m (.413 and 1.66 mi) during a single day. Home range size and population density varies based on the spread of resources available in each habitat. The smallest home ranges are seen among vervets on St. Kitts and average .06 km² (.023 mi²) and the largest are seen among vervets living in Senegal, where home ranges are as large as 1.78 km² (.687 mi²). Where there are few resources, like in Senegal, vervets need to range farther to survive and fewer animals are supported by the ecosystem. On St. Kitts, where agricultural foods are readily available and in condensed areas such as on plantations, large numbers of animals can be supported in a much smaller space.

Population densities range from 9 individuals per square kilometer (5.6 per square mile) at the Segera Ranch in Kenya to 255 individuals per square kilometer (158 per square mile) on St. Kitts. When groups encounter one another at the border or overlapping areas of their home ranges, interactions can be described as friendly, where the two groups temporarily merge, or as agonistic, with serious fighting between members of both groups. The expression and degree of territoriality among vervets is facultative and conditions such as habitat type, season, intensity of predation, distribution, abundance, and defendability of resources, and the history of relationships between the groups are all factors contributing to the nature of the interaction of two groups.

In environments with clustered, easily defendable resources, territoriality is economically sound, especially in highly seasonal environments; vervets have much to lose by allowing neighboring groups into their range to exploit their food resources, especially at certain times of year. In other types of environments, such as savanna woodlands, resources are widely distributed and would be difficult for one group to defend without serious energetic costs, therefore, territorial behavior is not expected under these conditions.

SPECIAL NOTES

Vervets are among the most used primates in biomedical research in the United States and abroad. Because they are small, easily handled, nonendangered, evolutionarily closely related to humans, and easily bred in captivity, vervets are a popular species for use in biomedical primate research. Specifically, vervets are important in studying high blood pressure and AIDS. They are one of the few species of nonhuman primates that naturally develops high blood pressure and simian immunodeficiency virus (SIV), the ancestor of human immunodeficiency virus (HIV), is widespread throughout wild populations.

There is compelling evidence that SIV was transferred from monkeys or apes in Africa to humans and led to the emergence of HIV/AIDS in humans. Studying naturally occurring SIV and the origins of HIV/AIDS in vervets and other African primates may help scientists discover a cure or vaccine for the disease.