The Cross River gorilla (Gorilla gorilla diehli) is a critically endangered subspecies of the western gorilla (Gorilla gorilla). It was named as a new species in 1904 by Paul Matschie, a scientist who studied mammals at the Humboldt University Zoological Museum in Berlin. However, its populations were not studied in detail until 1987.
This gorilla is the westernmost and northernmost type of gorilla. It lives only in the forested hills and mountains along the border of Cameroon and Nigeria, near the source of the Cross River. It is separated by about 300 kilometers (190 miles) from the nearest group of western lowland gorillas (Gorilla gorilla gorilla) and by around 250 kilometers (160 miles) from the gorilla population in the Ebo Forest of Cameroon. In 2014, experts estimated that fewer than 250 adult Cross River gorillas remain, making them the rarest great ape in the world. These gorillas gather in 11 specific areas across a range covering 12,000 square kilometers (4,600 square miles). However, recent field studies found gorillas in areas outside these known locations, suggesting they may live more widely within this range. Genetic research supports this idea, showing that some groups remain connected through the occasional movement of individuals. In 2009, professional video footage captured a Cross River gorilla on a forested mountain in Cameroon.
Description
The Cross River gorilla was first identified as a new type of western gorilla by Paul Matschie, a scientist who studies and classifies animals, in 1904. Researchers confirmed its physical differences from other gorillas in 1987. Later studies of skull shape, tooth size, bone length, and how these features are arranged showed that the Cross River gorilla is unique. It was officially recognized as a separate subspecies in 2000.
Compared to western lowland gorillas, Cross River gorillas have smaller mouth areas, smaller skull shapes, and shorter skulls. They are similar in body size and limb and bone length to western lowland gorillas. However, measurements from a male Cross River gorilla suggest they have shorter hands and feet and a larger opposability index, which measures how flexible their fingers and thumbs are.
A study by Sarmiento and Oate, published by the American Museum of Natural History, noted that Cross River gorillas have smaller teeth, smaller mouth areas, smaller skull shapes, and shorter skulls than western lowland gorillas. The Royal Belgian Institute of Natural Sciences described Cross River gorillas as having a rounded chest, evenly colored fur, a bare black face and chest, small ears, joined brow ridges, and raised nostrils. However, they are not the largest gorillas, and more research is needed to fully understand their physical differences. Other details include:
- Average adult male height: 1.65–1.75 m (5.4–5.7 ft).
- Average adult male weight: 140–200 kg (310–440 lb).
- Average adult female height: 1.4 m (4.6 ft).
- Average adult female weight: 100 kg (220 lb).
Evolution
In 2000, Esteban E. Sarmiento and John F. Oates proposed a theory that the Cross River gorilla began to develop into a separate subspecies of Gorilla gorilla during a dry time in Africa’s Pleistocene era. This change happened because food became less available, and the gorillas focused more on eating plants and living on the ground.
The researchers suggested that ancestors of the Cross River gorilla may have lived in forests near the headwaters of the Cross River or in the Cameroon highlands. They noted that these gorillas may not have moved far from these areas after becoming isolated. The ancestors of Gorilla gorilla gorilla separated from the Cross River gorilla by moving to areas south and/or east of the Sanaga River. Sarmiento and Oates explained that there is no evidence showing that G. g. gorilla and G. g. diehli live in the same area.
Habitat
The Cross River gorilla, like other gorilla subspecies, lives in thick forests where people do not live. Because of their large size, these gorillas need big and varied forest areas to meet their needs. Their natural homes are in places where humans often live and use the land for resources. The forests where Cross River gorillas live are at different heights, from about 100 to 2,037 meters (328 to 6,683 feet) above sea level. Between 1996 and 1999, researchers studied Cross River gorillas on Afi Mountain in Cross River State, Nigeria, for 32 months. They collected a lot of information, including details about the types of forests, the land's shape, the climate, the availability of tree and plant foods, and the gorillas’ behavior, diet, and how they group together. This information was gathered using indirect signs, such as paths where they feed, nests, and feces.
The homes of the Cross River gorilla are harmed by heavy cutting down of trees and breaking up of forests. These changes leave the gorillas with fewer places to live. Because of this, the areas they can live in have become much smaller. The large number of people living in the area limits the resources available to the gorillas. Even though the amount of land has decreased, studies show that enough rainforest still exists for these gorillas to live comfortably. However, if humans continue to cut down forests, these areas will shrink until they no longer exist. Other human activities that harm Cross River gorillas and other species include hunting, logging, farming, collecting firewood, clearing land for plantations, and using natural resources. Gorillas and other primates depend on many parts of their habitat to survive. Also, because of their large size, they cannot easily adapt to new environments and reproduce slowly. Although research on Cross River gorillas is limited, enough information exists to show that they can still survive. However, the exact number of Cross River gorillas that exist is still debated.
The Cross River gorilla is labeled as critically endangered by the IUCN, International Union for Conservation of Nature. It is also not well studied. The small areas where they live have kept them separated from other gorilla groups by about 200 kilometers (120 miles). This region is near the Nigeria-Cameroon border, where highlands create natural barriers. In the 20th century, Cross River gorillas lived in lowland areas, but due to habitat loss and human activities like resource use, they now live only in hilly areas. This has reduced the availability of food and land. Most of the areas where Cross River gorillas live are protected because of their endangered status. However, some places, such as between Kagwene Mountain and Upper Mbulu, and around Mone North, are not protected.
Behavior
A study published in 2007 in the American Journal of Primatology reported that a subspecies of gorilla was defending itself from humans. Researchers observed that the gorillas sometimes threw sticks or clumps of grass. This behavior is unusual because gorillas typically run away from humans and rarely attack them.
Cross River gorillas have nesting habits that depend on their habitat, climate, food availability, and risk of danger. Research on gorillas living in the Kagwene Gorilla Sanctuary found that the location of their nests changes with the season. From April to November, they often build nests in trees, while from November onward, they build nests on the ground. More nests built at night were on the ground than in trees. These gorillas also build more nests during the wet season than the dry season, and they create more tree nests during the wet season. Daytime nests were more common, especially in the wet season. Reusing nesting sites was common, but this did not depend on the season. The average number of gorillas in a nest group ranges from four to seven, though this number can vary based on where the gorillas live.
Groups of Cross River gorillas usually include one male and six to seven females and their offspring. Gorillas in lowland areas have fewer offspring than those in highland areas. This difference is likely due to higher hunting rates and infant deaths in lowland regions. Highland gorilla groups are more densely populated than lowland groups.
The diet of Cross River gorillas includes fruit, herb plants, lianas (vines), and tree bark. Their food choices depend on the season. Observations show that they prefer fruit but eat other foods during the dry season, which lasts about 4–5 months in northern regions. Throughout the year, they eat more lianas and tree bark than fruit, especially during dry periods when fruit is scarce.
Cross River gorillas live in small groups of 4–7 individuals, including a few males and females. Their diet is mostly fruit, but during months when fruit is scarce (August–September, November–January), they eat more ground plants, tree bark, and leaves. Many of their food sources are seasonal, so they eat dense, nutritious plants found near their nesting areas. Studies of the Afi Mountain group showed that they often eat Aframomum spp. herbs but prefer Amorphophallus difformis during the wet season when it is available.
The nesting habits of Cross River gorillas are influenced by environmental conditions such as climate, predation, plant availability, and nearby food sources. Researchers found that the location of nests—on the ground or in trees—depends on the season. During the dry season, most nests are on the ground, while in the wet season, nests are often built high in trees to stay dry. More daytime nests were built during the wet season, and nesting sites were reused about 35% of the time. The average group size was 4–7 individuals, but some nesting areas had up to 26 nests, suggesting that multiple groups sometimes shared nesting sites.
Cross River gorillas in Kagwene Mountain, Cameroon, have been observed using tools. In three separate cases, researchers saw the gorillas throw grass, a branch, or fistfuls of grass at people. These interactions began with the gorillas vocalizing and observing the researchers, then calming down before approaching and throwing objects. Scientists believe this behavior may have developed due to human activity near the mountain, such as farming. Local beliefs that gorillas are protected may also influence their cautious but non-aggressive behavior toward humans.
Geographical distribution
The Cross River gorilla lives near the border of Nigeria and Cameroon in tropical and subtropical forests that also support the Nigeria-Cameroon chimpanzee, another type of great ape. This gorilla is the westernmost and northernmost group of gorillas and is found in the forested hills and mountains along the Nigeria-Cameroon border near the headwaters of the Cross River. They are about 300 km (190 miles) away from the nearest group of western lowland gorillas and about 250 km (160 miles) from another gorilla population in the Ebo Forest of Cameroon. These gorillas are active in 11 specific areas across a range of about 12,000 km² (4,600 square miles). Recent studies found gorillas outside these known areas, suggesting their range may be larger. Genetic research supports this, showing that some groups remain connected through occasional movement of individuals.
Cross River gorillas are confirmed to live in the Mbe Mountains and the Afi River, Boshi Extension, and Okwanggo Forest Reserves in Nigeria’s Cross River State, as well as in the Takamanda and Mone River Forest Reserves and the Mbulu Forest in Cameroon’s South West Province. These areas form a mostly continuous forest covering about 8,000 km² (3,100 square miles) from Afi Mountain to Kagwene Mountain, as noted in a 2007 conservation plan. Researchers also suggest there may be a smaller group near Bechati in the southeast. Today, their total range is estimated to cover about 12,000 km² (4,600 square miles). These gorillas prefer the rugged, high-altitude terrain of the Afi-to-Kagwene landscape, which keeps them away from human activity.
A 2013 study found that Cross River gorillas also live in lower-altitude areas, such as the Mawambi Hills, which are about 552 meters (1,811 feet) above sea level, much lower than their usual habitat of around 776 meters (2,546 feet).
Cross River gorillas live in small, separated groups across about 14 areas in a rugged landscape of roughly 12,000 km² (4,600 square miles) near the Nigeria-Cameroon border. Their population is estimated at 75–110 in Nigeria and 125–185 in Cameroon. Habitat loss is now a greater threat to their survival than hunting. They live in dense, undisturbed forests that are becoming rare due to human use of land. Most surviving gorillas live in the Takamanda National Park and Kagwene Gorilla Sanctuary. Nesting is avoided in disturbed areas of the park, even though laws exist to protect wildlife. Conservation workers are trained to enforce these laws. A planned highway near the Ekuri community forest was rerouted in 2017 to avoid harming gorilla habitat.
Researchers estimate that 250–280 Cross River gorillas remain in 10 separate areas. Human population growth and the expansion of grasslands have split the species into many small groups. Factors like farming, human settlement, and limited suitable habitat have caused this fragmentation. This isolation has reduced genetic diversity, which could harm the species long-term. A study found that gene sharing between western lowland and Cross River gorillas stopped about 400 years ago, likely due to increased human activity. The recent decline in Cross River gorilla numbers is likely due to growing human pressure over the past few centuries.
A recent increase in hunting for bushmeat has also harmed the population. Hunting is more common in lowland areas, possibly pushing gorillas into highlands where their numbers are smaller. Despite laws against hunting, it continues due to local demand and trade. These laws are rarely enforced, and any hunting has a major impact on the species. All hunting of Cross River gorillas is unsustainable.
Decline
The number of Cross River gorillas dropped by 59% between 1995 and 2010, a faster decrease than any other type of great ape. These apes show problems in their environment and help other animals survive. Their numbers began to fall 30 years ago and have continued to drop quickly. Because of hunters, these gorillas now fear humans, making them hard to see.
Cross River gorillas avoid living in grasslands and farms, which causes the forest where they live to break into smaller, separate pieces. Their habitat has become damaged and broken into parts. Some models can’t explain why they live in scattered groups even though their area seems large and suitable. When habitats break into pieces, gorilla groups can no longer move between them, leading to more inbreeding. This causes a loss of genetic variety, which harms the long-term survival of the group and the whole population. Scientists study their genes to learn more about the population. Specific parts of their DNA helped researchers understand how the gorillas are divided and how genes spread between groups. Surveys suggest there are about 300 gorillas living in about 10 separate areas with little contact between them. In addition to habitat problems, they are hunted for meat and for their bones used in fake medical practices. Some African communities have used primates in rituals or as symbols, and tested them for medicine.
Another danger is the illegal pet trade. Only one Cross River gorilla is known to be in captivity at the Limbe Wildlife Center. Even though this number seems small, the pet trade has harmed other gorilla species before and could harm Cross River gorillas too. Hunters often kill adult gorillas to take baby gorillas as pets.
Cross River gorillas are critically endangered because of hunting and disease, such as Ebola. Even if hunting and Ebola deaths stopped, the population would likely not recover quickly. These gorillas have a low reproduction rate, and it may take 75 years for their numbers to fully recover. Their habitat is also lost due to mining, farming, and cutting down trees.
Despite these challenges, conservationists are hopeful after filming several adult and baby gorillas in spring 2020.
Conservation status
The western gorilla is classified as Critically Endangered, with the western lowland gorilla facing additional risks from the Ebola virus. Among all African apes, the Cross River gorilla is the most endangered. A 2014 survey estimated that fewer than 250 adult Cross River gorillas remained in the wild. However, a 2012 survey by Conservation International found that the Cross River gorilla was not listed on "The World's 25 Most Endangered Primates List." Conservation efforts have focused on uniting scattered gorilla populations to reduce inbreeding. A major challenge is that these gorillas live near human communities, which increases threats such as hunting for bushmeat and habitat destruction. Their protected habitats along the Nigeria-Cameroon border are near hunting areas, raising the risk of extinction. Cross River gorillas are vital to their ecosystem because they help spread seeds for tropical plants that might otherwise go extinct.
In 2007, a survey in five villages aimed to study local taboos against hunting and eating these gorillas. In Cameroon’s Lebialem division, 86% of people supported conservation, viewing the gorillas as important relatives to humans. The decline of these taboos among younger people, particularly those aged 18 to 25, was linked to the gorilla population’s decrease. Despite this, the taboo remains strong and continues to discourage hunting. These traditions are seen as essential for the gorillas’ survival, especially in areas with weak law enforcement. Reviving these beliefs helps conservation efforts and preserves local culture, though some practices must be carefully chosen to avoid encouraging harm to the species. Over the past 15 years, no hunting incidents involving Cross River gorillas have been reported, showing the effectiveness of these taboos.
In 2001, a conservation workshop organized by the Wildlife Conservation Society and the Nigerian Conservation Foundation in Nigeria aimed to improve the Cross River gorilla’s survival chances. The workshop highlighted the species’ unique traits and recommended steps to protect them, including regular meetings between governments and conservation groups in Cameroon and Nigeria.
In 2008, Cameroon established Takamanda National Park on the border with Nigeria to protect Cross River gorillas. The park is part of a shared protected area with Nigeria’s Cross River National Park, safeguarding about 115 gorillas—roughly a third of the total population—and other rare species. Conservationists hope this will allow gorillas to move between the two parks.
The Kagwene Gorilla Sanctuary, created by Cameroon’s government on April 3, 2008, is part of the IUCN’s plan to save Cross River gorillas. It covers 19.44 km² (7.51 sq mi) in western Cameroon, located between the Mbulu and Nijikwa forests. The area includes rugged mountains and reaches an elevation of 2,037 meters (6,683 feet). Only about half of the sanctuary is suitable gorilla habitat, with the rest being grasslands or farmland. As a protected area, it is expected to have staff and guards to enforce wildlife laws.