Sun-Tailed Monkey, Allochrocebus solatus
SUN-TAILED MONKEY
Allochrocebus solatus
Geographic Distribution and Habitat
Endemic to central Gabon, sun-tailed monkeys (Allochrocebus solatus) are believed to occupy a range of about 9,700 square miles (25,300 sq km), an area about the size of Vermont. They live in mature mixed forests and sometimes secondary lowland moist forests and prefer altitudes between 460 and 3,220 feet (140–980 m). Because sun-tailed monkeys forage in dense undergrowth, they tend to avoid very mature forests, which often have sparse undergrowth. They do well in logged forests, as the light gaps formed from logging often cause dense undergrowth.
Sun-tailed monkeys were first described by western scientists in 1988, and assigned to the genus Cercopithecus with other guenon monkeys. In 2013, they were moved to the genus Allochrocebus, the genus of the terrestrial guenons, although that designation remains controversial. They are believed to be the oldest species of their genus.
Sun-tailed monkeys, owing largely to their cryptic lifestyle and small range, are not well-studied. Most information about the species comes from initial observations when the species was first described, or is based on a single semi-free-ranging population at the Centre Internationale de Recherches Médicales de Franceville (CIRMF) in Gabon, observations about which may not accurately reflect fully wild populations.
Size, Weight, and Lifespan
Sun-tailed monkeys weigh between 8.8 and 20 lbs (4–9 kg), with females weighing about 60% as much as males. Length figures are not known, although other guenon species tend to have a combined head and body length of about 16–22 inches (42–56 cm). Based on other guenon species, it is likely that the sun-tailed monkey may live up to 20–30 years. One individual in captivity is estimated to be 18 years old.
Appearance
Sun-tailed monkeys sport a gray-brown pelage or coat over most of their bodies. Their backs are a rusty brown color and their tail fades to white, then to the bright orange for which they are named. The coloration of males is more vibrant than that of the females, and their pelage pattern has more contrast. Males also have a distinctive white throat ruff that is much less developed in females. Newborns are yellowish-brown all over, and eventually develop the coloring of adults, though it is less vibrant and contrasts less before they reach maturity.
Diet
Details about the sun-tailed monkey’s diet are very sparse, although they are known to be mainly omnivorous. One study found that the diet of the previously mentioned CIRMF semi-free-ranging group was composed of 27.5% invertebrates, 27.5% grasses, and 19.5% fruits; the rest was split almost evenly between mushrooms, flowers, seeds, pith, and leaves. They have also been known to crop-raid in nearby villages. Pith is the soft or spongy tissue of a plant or fruit, which is usually white or pale in color (i.e., the white part between the skin and fruit of an orange).
Behavior and Lifestyle
Sun-tailed monkeys are diurnal (active during daylight hours) and likely semi-terrestrial, living partially but not wholly on the ground. Their degree of terrestriality has been a point of debate among researchers, with early accounts labeling them a terrestrial species and others as more arboreal (tree-dwelling). One study of the CIRMF semi-free-ranging population found that sun-tailed monkeys spend about 40% of their time on the ground, making them semi-terrestrial and one of the most terrestrial of the guenon species. It is not certain that this behavior is reflective of fully wild populations.
Sun-tailed monkeys spend significantly more time on the ground during the dry season vs. the wet season. Foraging mainly occurs on the ground and begins at dawn. The daily travel distance of two studied groups was 0.6–1.2 miles (1–2 km). Sun-tailed monkeys mostly travel quadrupedally (walking on all fours). They also climb and leap in trees and occasionally walk bipedally, on two legs rather than all four limbs.
Sun-tailed monkeys have two daily peaks in activity: one right after dawn, and another from mid-afternoon until dusk, when the group goes to sleep in trees. During the middle of the day, the group rests. Every night, they locate new sleeping trees.
Although sun-tailed monkeys eat mainly fruit, grass, and invertebrates, young sun-tailed monkeys have been observed stalking larger prey, including duikers, a type of small antelope.
Groups are composed of one male and several females with 18 individuals on average. The sex ratio of adults is usually about five females for every male. Home range size is about 0.4–0.8 square miles (1–2 sq km) and may overlap slightly with other groups. Two studied groups had an overlap area of about 15%. Groups are not extremely territorial, but adult males do engage in aggressive interactions, such as chasing, to ward off rival groups.
Sun-tailed monkeys are relatively quiet, likely because, as semi-terrestrial animals, they are more at risk of predators than fully arboreal primates. Because they often move around in silence, they have an isolation call that helps them to relocate their group if they become lost. This call-type is used more frequently than in arboreal species. Adult males are known to “bark” and females and juveniles to “chirp” as a warning.
Females give birth approximately every 18 months, and births tend to occur during the two annual wet seasons, which occur in October-December and March-May. Birth weight is 13 oz (380 g) on average and one offspring is born at a time. Females reach sexual maturity at about four to five years of age and males at about five to seven years.
As fruit eaters, they are likely important seed dispersers.
Sun-tailed monkeys are listed as Near Threatened (IUCN, 2019) by the International Union for Conservation of Nature, appearing on the IUCN’s Red List of Threatened Species. Until the 1990s, the sun-tailed monkeys’ range was virtually uninhabited by people. In the last 20–30 years, however, logging has increased substantially in the area. Gabon’s forests cover most of the country, and, in addition to being vital habitat for wildlife, they also serve as extremely important carbon sinks that help to combat the effects of climate change.
While sun-tailed monkeys do well in logged forest, there is a close link between the logging industry and bushmeat hunting, which is a major threat to sun-tailed monkeys. When an area is opened up to logging, it creates a dense network of roads that open up previously inaccessible areas of forest. Because these roads are not carefully monitored, hunting camps are then able to operate in these remote regions. It is estimated that in one year, 1,200 logging employees consumed up to 80 tons of bushmeat at a logging camp near Lopé Reserve in central Gabon. The logging industry is not the primary market for bushmeat, but it is what fuels the trade. Because of the high demand for bushmeat in urban areas, hunting revenue may contribute up to 40% to a logging employee’s income, making it a lucrative venture in a country where 5% of the population lives on less than $1.90 per day. Unfortunately, sun-tailed monkeys are just one of the many species that suffer from this trade. Though sun-tailed monkeys are reportedly difficult to hunt, they are very susceptible to ground snares, which are common in the region for hunting other species, such as small antelope.
Unlike many arboreal species, sun-tailed monkeys are frequently made prey by larger species. Potential predators include leopards, African golden cats, African crowned eagles, and snakes.
Sun-tailed monkeys are listed under Appendix II of the Convention on International Trade in Endangered Species (CITES) and under Class B of the African Convention on the Conservation of Nature and Natural Resources, both of which prohibit collection and trade except under special circumstances. They have also been declared a totally protected species by the Gabonese government in 1994. About 16% of the sun-tailed monkey’s range is protected by national parks, mainly Lopé National Park, though this is the least population-dense part of their range. Most of their range covers the logging concessions of the Forêt des Abeilles, an area that is open to logging and bushmeat hunting (although hunting of sun-tailed monkeys is illegal, they are still poached in areas where legal hunting occurs). More research about sun-tailed monkeys is urgently needed, specifically about their distribution and biology to determine their conservation needs and potential threats. There is a semi-captive population of sun-tailed monkeys held at the Centre Internationale de Recherches Médicales de Franceville in Gabon. This organization has successfully carried out the reintroduction of a young sun-tailed monkey that was taken by hunters.
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Written by K. Clare Quinlan, April 2020