ALLEN'S SWAMP MONKEY
Geographic Distribution and Habitat
The Allen’s swamp monkey is found in lowland forests throughout the central Congolian Basin, on both sides of the Congo River. Although the species has an apparently wide distribution, these monkeys are confined to areas near bodies of water, like swamp forests or riverine forests. Within the range depicted, only one-third of the area can be considered suitable habitat. The lowland forests along the central Congolian basin northwest of the Congo River comprise several watercourses and their tributaries, like the Oubangui, Likouala-aux-Herbes, and Sangha rivers. These watercourses are bordered by wide stretches of swamp and riparian forest, the preferred habitat of Allen’s swamp monkeys.
Populations of Allen’s swamp monkeys have been documented in the Democratic Republic of the Congo, in the Republic of the Congo and close to its northern border with Cameroon, and at one site in Central African Republic. This species possibly inhabits additional areas in Cameroon and the Central African Republic, but there have been no formal reports of these occurrences yet. Curiously, the Allen’s swamp monkey has been sighted on two islands outside of the Nouabalé National Park in the northern Republic of the Congo. They do not occupy areas within the park itself. These islands are entirely separate from the mainland and are without land bridges or tree access, meaning that access was gained by swimming!
The Allen’s swamp monkey is technically considered a guenon monkey and at one point was classified Cercopithecus nigrovirdis. Taxonomy changed over time; today this species is the only one that exists in its genus, Allenopithecus. Understanding the evolutionary background helps explain and contextualize some of this monkey’s physical features, relative to other closely related species. Guenons diverged from a group of other Old World monkeys, which included ancestral baboons, macaques, and mangabeys, around 10 million years ago. About 6 million years ago, additional radiations (divergences) occurred, giving rise to the distinctive, modern Cercopithecus monkeys. Most primatologists today consider Allen’s swamp monkeys to represent the most primitive extant guenon or cercopithecine. In other words, this species is thought to have speciated right around the 6-million-years-ago mark and thus serves as an intermediary between guenons and the group with baboons, macaques, and mangabeys. While the Allen’s swamp monkey possesses many physical characteristics typical of guenons in general, it also possesses other traits that are emblematic of the other group.
Size, Weight, and Lifespan
Compared with other guenons, the Allen’s swamp monkey has a stockier build and short-ish tail. Total length, including head and body, is around 18 in (45 cm), with tail-length just slightly longer at 20 in (50 cm). Consistent with other arboreal (tree-dwelling) quadrupeds, the swamp monkey has hindlimbs that are longer than its forelimbs. The sexes exhibit a marked degree of sexual dimorphism in overall body mass. Sexual dimorphism is distinct differences in size or appearance between the sexes of an animal in addition to the differences in their reproductive organs. Adult males are 13 lbs (6 kg), and their female conspecifics are typically around 8 lbs (3.7 kg). At birth, these monkeys are very small, roughly 0.5 lbs (220 g). Data regarding senescence (aging) and longevity of this species in the wild is currently non-existent, but captive monkeys are reported to live to at least 23 years old.
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The swamp monkey is relatively comparable in body proportions and size to guenons. Both males and females possess a dark-khaki haircoat with whitish undersides. Their faces are framed by long, dark gray cheek fur, which is outlined with dark lines. The skin on their throats is light gray. Both sexes have a patch of orange-red skin beneath the tail; females have distinctive estrus swellings (other female guenon species do not present with these), and males have light-blue scrotal skin. While the bright hues of their sexual skin may seem conspicuous, overall this coloration is relatively dull and drab among guenons. The mouths of Allen’s swamp monkeys include the dimorphic canines (males’ are longer and larger than females’) and the shortened third molar of the guenons, but this monkey has significantly broader molars than other cercopithecines, more similar to macaques. There are other features this species shares with macaques, like their leaf-shaped ears, broad hands and feet, and short limbs. Adult male swamp monkeys have well-developed sitting pads, which are located beneath the buttocks and feel hard to the touch. Within primatology, these built-in seat cushions are formally referred to as ischial callosities.
Newborns possess yellowish-brown fur and silky white facial hairs, both of which begin to be replaced at 10 weeks of age.
Perhaps the most distinct physical characteristics of the Allen’s swamp monkey is the webbing present on their hands and feet between the second and fifth digits. This webbing is an adaptation that helps with swimming.
These monkeys have a highly varied diet. They are capable and frequent foragers in the trees, where they may find fruits, nuts, nectar, and blossoms, as well as on the ground, where they might encounter snails, shrimp, fish, and insect larvae to eat. They swiftly catch flying insects, spiders, and caterpillars. Animal or invertebrate prey accounts for around 20% of the Allen’s swamp monkey’s diet on a given day. Generally, these monkeys prefer yellow, orange, and red fruits over those that are green and brown. Possibly, visual cues from fruit color are used to differentiate between nutritional values of fruits, for brightly colored fruits are higher in proteins, while fruits with dull colors have high fiber content.
Their mouths are equipped with cheek pouches that store food. When fruits, insects, and leaves enter the mouth, they pass through a large and flexible slit-like hole into the cheek pouch, where mechanical processing starts. Cheek muscles churn the food within the pouches. High concentrations of the enzyme alpha-amylase within the pouches turn complex carbohydrates, like fiber in leaves, into simpler sugars. Further down in the digestive track, these simple sugars can be easily converted into energy. Cheek pouches are highly functional, as they also allow the monkeys to rapidly collect food in areas where predators pose a threat, or when competition over resources is high. The swamp monkeys can then retreat into a safer area to enjoy the fruits of their labor, and they may socialize with other group mates while doing so.
Relative to other guenons and to other similarly sized mammals, the Allen’s swamp monkey has long digestive times. That is, once food is consumed, it takes longer for the food to be processed as it moves through the gut before it is excreted as feces. While the mechanism that accounts for this difference has not been fully elucidated, researchers believe that these monkeys may have an expanded large intestine that allows for increased fermentation (breakdown by bacteria) of high-fiber foods while they slowly pass through the lower gut. Digestive retention, therefore, increases flexibility in diet. Food items that are typically avoided by most guenons, like leaves and stalks, can be eaten, digested, and converted into energy by Allen’s swamp monkeys.
Behavior and Lifestyle
In the swamp forests in the central Congo basin, Allen’s swamp monkeys move around on all fours through the thick vegetation along the water’s banks and are primarily active during the daytime. They walk along suspended plant roots and debris, wading or swimming through water when necessary. The waterways are often utilized as an escape route from predators; they are known to dodge threats by diving into the water and swimming away.
This monkey spends more time on the ground than in the trees. They sometimes uproot young palm saplings and dig into the soil below, sifting the litter to expose insects or animal prey. They will wade into water, using one hand to catch fish. When aquatic prey is not seen on the surface, they dive down for other aquatic organisms on which to feed.
Only when succulent ripe fruits are spotted above will they climb into low levels of trees to forage. Arboreal activities account for only 10% of total movement during the day.
In the swamp forest, Allen’s swamp monkeys may forage in smaller sub-groups during the daytime. At night, these groups converge into a larger troop that sleeps in the trees near the riverbanks. Sleeping site positioning is an effective anti-predation strategy and helps deter raptors, carnivores, and snakes from making swamp monkeys their nightly prey.
Primatologists use a metric called the intermembral index when thinking about how body size and proportions in primates correspond to lifestyle. It is used commonly in studies of both fossilized and extant primates to predict movement patterns. This index measures lengths of forelimbs relative to hindlimbs and equals (length of humerus + length of radius) x 100 / (length of femur + length of tibia). This proportion generates values under 100 or over 100, with certain ranges corresponding to specific movement types. Generally, leapers have long hindlimbs and a low intermembral index. Suspensory (tree-hanging and brachiating) primates have long forelimbs and a high intermembral index. The Allen’s swamp monkey is an intermediate species, possessing similar limb lengths and an intermembral index of 84, which indicates specialization for quadrupedal movement. Humans have a low intermembral index (around 67), which theoretically allows for upright walking, or bipedalism.
In the wild, the social system and group structure of Allen’s swamp monkeys are unclear. Population densities and group sizes are believed to vary wildly. At Salonga National Park, groups of 23 to 57 individuals have been documented, while at Lomako, only groups of four individuals were seen. However, researchers are fairly confident that these groups are regularly multi-male, multi-female, with a low ratio of adult males to adult females. For example, at Salonga, there were four males within a group of 32 total members. Studies of their social behavior are particularly difficult because most field observations occur on forested land routes where these monkeys are not regularly found. More complete studies are needed within the swamp forests to discern the group and social activities of Allen’s swamp monkeys.
Adult male swamp monkeys possess physical traits that allow for specialized communication. They have cutaneous glands on their chests that are used for scent marking tree trunks and branches, and their vocal sacs are enlarged, allowing for loud-calls or “gobbles” that are distinct to their sex. The significance of this male-performed scent-marking behavior is unknown, but in captivity, adult Allen’s swamp monkey males scent mark when new structures are introduced into their enclosure. Captive females do not scent mark but often sniff the scent-marked areas.
In the wild, when group members come together, all calls are used in the monkeys’ vocal repertoire. Cohesion contact “grunts” are audible, as well as warning “chirps,” adult male “loud-calls,” young adult isolation calls, and “screams” from both sexes. During copulation, adult females will elicit a “quaver” vocalization. When predators are sighted, alarm calls are loud and specific to signal the impending threat.
Reproduction and Family
There are currently no data on courtship, mating, or parent- and group-young interactions in the wild-living populations. What is known is that mating typically coincides with the main dry season, in July through August. Gestation is 5.5 months long. The birth season subsequently falls in the months of February and April, which coincides with the short dry season. Adult females give birth to one infant per season, and this infant is near fully weaned at three months of age.
A study of captive Allen’s swamp monkeys at the Cleveland Park Zoo documented that though the neonates were vulnerable and entirely dependent on the care of the mother at first, by the third month of life, infants spent only one fifth of the previous amount of time suckling, markedly less time resting, and more time exhibiting solitary active behaviors. These developing active behaviors included somersaults, erratic movements, parental harassment (met with cuffs or mild threat behaviors from annoyed parents), exploring, and examining objects in the environment. Curiously, the infants were the predominant regulators of physical distance from their mothers. Despite parental response to their young’s harassment, no instances of aggression were observed in the colony in captivity.
Data pertaining to this monkey’s distribution and behaviors is sparse; consequently, little is known about their ecological role in the swamp forest habitat. Given the that their diet includes fruit and nuts, one might assume that they play some role in seed dispersal and, therefore, reforestation.
The International Union for Conservation of Nature classifies the Allen’s swamp monkey as Least Concern (IUCN, 2018). However, wild populations are declining. The major threat to their survival is hunting and trapping by humans for the bushmeat trade. While these monkeys may evade other carnivorous predators by sleeping on the tree branches that overhang the Congo River and other nearby watercourses, their resting sites are easy to locate and shoot by hunters traveling in canoes. Additionally, when occupying forests close to villages, the monkeys feed on oil palm nuts. In doing so, they are often captured in baited traps. While the monkeys’ preference for swamp forest makes them somewhat less susceptible to habitat destruction through logging, their inflexibility in habitat renders them an inherently at-risk species.
The Allen’s swamp monkey is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) and in Class B of the African Convention on the Conservation of Nature and Natural Resources. Studies into the impact of hunting are needed. In addition, they occur in the following protected areas: Salonga National Park and Lac Tumba-Ledima Forest Reserve (DRC), Dzanga-Sangha Special Reserve (CAR), and Lac Télé Community Reserve (Congo).
Their conservation status is generally stable in areas where Allen’s swamp monkeys are not sought out by hunters or where the bushmeat trade has been restricted. For example, in the upper Sangha River, these monkeys are not an important target. Furthermore, the Noubalé-Ndoki Conservation Project formed agreements with local people to allow for hunting for the purpose of local consumption but not for export outside of the village. However, in the Likouala-aux-Herbes and Congo Rivers, these monkeys are intensively hunted for the bushmeat trade. Since 1981, extensive allowances, rather than restrictions, for hunting have been granted in the Congo Basin. Species survival is likely threatened as populations decline in these regions. Ecological surveys and hunting studies are currently lacking, so it is difficult to know the true impact on demographics throughout the zones that they inhabit.
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Written by Cookie Koch, January 2021