Hamlyn’s Monkey, Cercopithecus hamlyni
HAMLYN'S MONKEY
Cercopithecus hamlyni
Geographic Distribution and Habitat
Also known as owl-faced monkeys, Hamlyn’s monkeys are mainly found in the eastern Democratic Republic of Congo (DRC), with only a single isolated population in western Rwanda. Their range in the DRC begins at their western limit in the lowland forests along the Congo/Lualaba River and ends in the tropical and bamboo forests within the mountains of the Albertine Rift in the east. The Lindi-Nepoko River marks the northern limit of their range, which continues as far south as the remainder of the tropical moist forests. The Rwanda population is restricted to a 12.35 square miles (32 square km) region in the Nyungwe National Park. Other populations were known previously to exist in Rwanda, but they have now succumbed to habitat loss. It is possible that other unknown populations may exist within the country, however, with the extent of habitat loss and fragmentation in the region, their persistence is unlikely. Hamlyn’s monkeys inhabit lowland and montane forests, as well as bamboo or bamboo-forest mixes in Rwanda. They are most commonly found in primary forests which have not been disrupted by deforestation, and in older secondary forests that have had a chance to regrow after human disturbance. The persistence of populations isolated by historical habitat change (due to changes in climate) suggests that Hamlyn’s monkey may be able to adapt to fragmented environments caused by deforestation.
Size, Weight, and Lifespan
Similar to many other African primates, the Hamlyn’s monkey displays sexual dimorphism where one sex (in this case, the males) is much larger than the other. Males weigh 9.7–16.09 pounds (4.4- .3 kg), and females 5.7–9.9 pounds (2.6-4.5 kg). Males have a head-to-body length of 1.6–2.1 feet (500–630 mm), and females have a 1.4– .5 foot (430 – 480 mm) head-to-body length. Their tail length is 1.6–2.1 feet (490–630 mm).
Hamlyn’s monkeys likely have a similar lifespan as other guenons, surviving around 30 years.
Appearance
While guenons as a group are known for being some of the most colorful primates, Hamlyn’s monkeys display one of the simpler color palettes. The fur on most of their body is gray, fading into black on their arms and belly, and the tip of their tail. The most striking part of their appearance is the white stripe running up their long flat nose, connecting with their pale yellow brow ridge. Males are a little extra colorful in their appearance with the powder-blue skin on their genitals.
Diet
Similar to other guenons, Hamlyn’s monkey mostly eats fruits and seeds. Also similar to other guenons, they have special cheek-pouches that allow them to store extra food for later. Some populations have been recorded taking advantage of food sources unique to their range, such as specific tree saplings or bamboo shoots. They will also eat mushrooms and vegetation during dry seasons when fruit is scarce.
Behavior and Lifestyle
Because the Hamlyn’s monkey is a particularly shy species and difficult to observe in the wild, not much is known about their lifestyle. We do know that they are diurnal, meaning they are active during the day and sleep through the night. They also spend most of their time on the ground, making them primarily terrestrial, and will only sometimes climb among the trees to forage or evade potential predators. Unique among other more terrestrial primates, Hamlyn’s monkeys have particularly long fingers. This adaptation makes it easier to climb in trees, hence why it is unusual for a primarily terrestrial primate. Because Hamlyn’s monkeys are often found in bamboo forests, their exceptionally long digits help them climb among the smooth bamboo stalks.
Based on their location, Hamlyn’s monkeys are likely preyed on by leopards and crowned eagles.
Hamlyn’s monkeys have been observed in groups of 2-11 individuals. The average group size is estimated to be around 4 individuals. Because this species is so elusive in the wild, researchers think their group sizes are likely underestimated to what may actually be most common. Similar to most other guenons, Hamlyn’s monkeys have a single-male multi-female group structure. Once males reach maturity, they will leave their natal group (the group they were born into) and try to establish themselves as the resident male in another group. Females will remain in their birth group their entire lives.
Hamlyn’s monkeys also occur alongside Eastern chimpanzees, Angolan colobuses, L’Hoest’s monkeys, and blue monkeys, however due to their elusive nature, any interactions with these species are unclear.
In line with their discreet nature, scientists believe Hamlyn’s monkeys are more reliant on olfactory communication than vocalizations. Despite their flat noses, Hamlyn’s monkey’s long face creates more space for nasal receptors compared to other guenon species. They also have scent glands located on their chests, which they’ll rub against surrounding surfaces to mark their territory.
Compared to most other guenons, Hamlyn’s monkey tends to be fairly quiet. Males will emit a deep, booming call, which may be used to rally their group together and/or warn surrounding groups of their presence. Females have only been recorded to emit a low whimpering vocalization.
Due to the difficulties of studying Hamlyn’s monkeys in the wild, not much is known about their reproduction. Following the same trend as other guenons, individuals typically reach sexual maturity by 4 years of age. Their birth season is estimated to fall between June and November. Hamlyn’s monkeys likely give birth to a single offspring at a time, as do other guenons. Other than providing protection, males likely do not play a significant role in raising offspring, which is the norm among many primates, but there are always exceptions. Further research on the Hamlyn’s monkey is needed to understand what aspects of their reproduction may be unique to them.
Like other fruit-eating monkeys, Hamlyn’s monkeys play an important role in the forest ecosystem as seed dispersers. While the specifics of Hamlyn’s monkeys’ ecological role have not been researched, we do know that other seed-dispersing monkeys directly contribute to maintaining the biodiversity of their forest homes, making their role incredibly important.
The Hamlyn’s monkey is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2018), appearing on the IUCN Red List of Threatened Species. While the existing population in Rwanda has been limited, Hamlyn’s monkeys are abundant overall where they do occur. However, with continued habitat loss, the population is predicted to decline. Researchers say that, if current trends continue, there will likely be a 30% population decline seen from 2003 to 2033. The main drivers of habitat loss that the Hamlyn’s monkey faces are due to human population growth, which requires expansion of urban developments and agriculture, and the expansion of artisanal mining. They are also hunted for bushmeat and are particularly vulnerable to trapping due to their terrestrial nature.
The eastern DRC is abundant with a variety of minerals, including gold and cobalt, which are needed for the production of rechargeable lithium-ion batteries (such as the ones found in electric vehicles, smartphones, and laptops). Not only does mining directly contribute to deforestation, but the areas spanning outside of mining sites are also indirectly impacted as forests are destroyed for agricultural production and settlements for the influx of people coming to make their living on artisanal mining.
Hamlyn’s monkeys are also impacted by illegal bamboo harvesting. Many of us recognize bamboo as a sustainable resource due to its quick regrowth. However, when over-exploited through illegal harvesting, bamboo forests will struggle to recover. Bamboo is harvested for use in construction, making spits for roasting meat, medicinal purposes, and even making bicycles. The shoots are also harvested for food. Being able to sell bamboo and bamboo products can greatly increase the household income of those in poverty, making the temptation to illegally harvest it a necessity for many.
With the ongoing civil conflict in the DRC, many local people have become displaced and forced to expand their settlements into Hamlyn’s monkeys’ range. This conflict also drives many of the illegal resource harvesting activities impacting Hamlyn’s monkey and other local wildlife.
Hamlyn’s monkeys are listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. They are also included on the DRC’s list of nationally protected species. However, species protection laws are only enforced within protected areas. Fortunately, there are many protected areas within Hamlyn’s monkey’s range.
Researchers are calling for further study of these elusive animals in the wild to better understand them and what conservation strategies will be effective. There are also efforts aiming to educate locals on sustainable bamboo harvesting practices. Because many of the activities that drive deforestation are the result of people being forced into desperate situations, efforts to support the local people should also allow them to live more harmoniously with the local wildlife including Hamlyn’s monkeys. Efforts to educate local people on the importance of native species and how they can support themselves without harming wildlife are an important action for conservation efforts.
- https://www.iucnredlist.org/species/4219/166615690
- https://cites.org/eng/app/appendices.php
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Written by Lina Rademacher, October 2024