The Cross River gorilla (Gorilla gorilla diehli) is a critically endangered type of western gorilla (Gorilla gorilla). It was named as a new species in 1904 by Paul Matschie, a scientist who classified animals at the Humboldt University Zoological Museum in Berlin. However, scientists did not study its populations in detail until 1987.

This gorilla is the westernmost and northernmost type of gorilla. It lives only in the forested hills and mountains near the border of Cameroon and Nigeria, where the Cross River begins. It is about 300 kilometers (190 miles) away from the nearest group of western lowland gorillas (Gorilla gorilla gorilla) and about 250 kilometers (160 miles) from the gorilla population in the Ebo Forest of Cameroon. In 2014, scientists estimated that fewer than 250 adult Cross River gorillas remain, making them the rarest great ape in the world.

These gorillas gather in 11 areas within a range of 12,000 square kilometers (4,600 square miles). Recent surveys found gorillas in places not previously known, suggesting they may live in more areas than thought. Genetic studies show that some groups stay connected because individuals occasionally move between them. In 2009, scientists recorded the first professional video of a Cross River gorilla on a mountain in Cameroon.

Description

The Cross River gorilla was first identified as a new type of western gorilla by Paul Matschie, a scientist who classifies animals, in 1904. Researchers confirmed its physical differences from other gorillas in 1987. Later studies of skull shape, teeth, bone size, and how bones are arranged showed that the Cross River gorilla is unique. It was officially recognized as a separate subspecies in 2000.

When comparing the Cross River gorilla to western lowland gorillas, it has smaller palates, smaller skull shapes, and shorter skulls. The Cross River gorilla is similar in body size and limb and bone length to western lowland gorillas. However, measurements from a male suggest it has shorter hands and feet and a larger opposability index than western lowland gorillas.

According to a study by Sarmiento and Oate published by the American Museum of Natural History, the Cross River gorilla has smaller teeth, smaller palates, smaller skull shapes, and shorter skulls than western lowland gorillas. The Royal Belgian Institute of Natural Sciences described the Cross River gorilla as having a large, barrel-shaped chest, even hair, a bare black face and chest, small ears, joined brow ridges, and raised nostril edges. 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: about 1.65 to 1.75 meters (5.4 to 5.7 feet).
• Average adult male weight: between 140 and 200 kilograms (310 to 440 pounds).
• Average adult female height: about 1.4 meters (4.6 feet).
• Average adult female weight: about 100 kilograms (220 pounds).

Evolution

In 2000, Esteban E. Sarmiento and John F. Oates suggested and supported the idea that the Cross River gorilla began to develop into a separate subspecies of Gorilla gorilla during a dry period in the African Pleistocene. This change happened because of less food and a greater focus on eating plants and living on the ground.

The researchers said that ancestors of the Cross River gorilla may have lived in the 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 became different from the Cross River gorilla by moving to areas south or east of the Sanaga River. Sarmiento and Oates stated 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 many other gorilla groups, lives in thick forests that are not used by humans. Because of their large size, these gorillas need big and varied forest areas to meet their needs. Like other endangered primates, their homes are in places where humans often live and use natural resources. The forests where Cross River gorillas live are at different heights, from about 100 to 2,037 metres (328 to 6,683 ft) 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 forest types, land shapes, weather, the availability of food plants over time and space, and the gorillas’ behaviors, diets, and group patterns. This information was gathered using indirect signs, such as paths where they fed, nests, and droppings.

The homes of the Cross River gorilla are harmed by large amounts of forest cutting and splitting into smaller pieces. These changes leave the gorillas with few places to live. Because of this, the number of animals the area can support has decreased, meaning the size of the areas they live in has become much smaller. The high number of people living nearby limits the resources available to the gorillas. Even though the loss of land seems like a big problem, studies show that enough rainforest still exists that is suitable for this group. However, if humans continue to cut down forests, these areas will keep shrinking until they no longer exist. Other human activities that harm Cross River gorillas and other animals include hunting, cutting trees, 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 size, they cannot easily adapt to new environments and reproduce slowly. Although research on Cross River gorillas is limited, enough studies show that they can still survive. However, the exact number of Cross River gorillas that exist is still unknown.

The Cross River gorilla is not only critically endangered, as listed by the IUCN, but also not well studied. Their limited living areas have caused them to live far away from other gorilla groups, about 200 kilometers (120 miles) apart. This area is near the Nigeria-Cameroon border, where highlands create natural barriers for the gorillas. In the 20th century, Cross River gorillas lived in lowland areas, but habitat loss and human activities, such as using natural resources, forced them to move to higher ground. This shift reduced the availability of food and living space. Most of the areas where Cross River gorillas live are legally protected because of their endangered status. However, some places, like 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 is fighting back against possible dangers from humans. Researchers found that gorillas sometimes throw sticks and clumps of grass when humans are nearby. This behavior is unusual because gorillas usually run away when they see humans and rarely charge at them.

The nesting habits of Cross River gorillas depend on factors such as their habitat, climate, food availability, and risk of danger. Research on Cross River gorillas in the Kagwene Gorilla Sanctuary found that the location of their nests changes with the season. From April to November, they are more likely to build nests in trees, and 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 create more tree nests during the wet season. Daytime nests are more common, especially in the wet season. Reusing nesting sites is common, but this does not depend on the season. The average group size of nests is between four and seven individuals, though this number can vary by location.

Groups of Cross River gorillas usually include one male, six to seven females, and their offspring. Gorillas in lowland areas have fewer offspring than those in highland areas. This is likely because hunting is more common in lowland regions, leading to higher infant deaths. Highland groups are more densely populated than lowland groups.

The diet of Cross River gorillas mainly 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, like herbs and bark, during the dry season, which lasts about 4–5 months in northern regions. They eat more lianas and tree bark throughout the year and less fruit during dry periods when fruit is scarce.

Cross River gorillas usually live in small groups of four to seven 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 terrestrial herbs, tree bark, and leaves. Many of their food sources are seasonal, so their diet includes dense, nutritious plants found near their nesting areas. Studies of the Afi Mountain group showed that they eat Aframomum spp. herbs most often, but during the wet season, they prefer Amorphophallus difformis plants instead. This shows they choose seasonal foods and plants found only in their habitat.

The nesting habits of Cross River gorillas are influenced by environmental conditions, such as weather, predation risks, availability of plants, and nearby food sources. Researchers found that nesting locations, whether on the ground or in trees, depend on the season. During the dry season, most nests are built on the ground, while in the wet season, nests are mostly built in trees to stay dry. More nests are built during the day in the wet season, and about 35% of nesting sites are reused. The average group size is four to seven individuals, but nesting sites sometimes have up to 26 nests, showing that multiple gorilla groups may nest together.

In Kagwene Mountain, Cameroon, Cross River gorillas have been observed using tools, a behavior unique to this population. In three separate cases, gorillas threw grass at researchers, a detached branch, or responded to a man throwing rocks by throwing grass back. During these encounters, gorillas first observed researchers, made vocalizations, and then approached calmly before throwing objects. Researchers believe this behavior may result from human contact near the mountain and the gorillas’ cautious but non-aggressive attitude toward people, possibly due to local folklore that protects them from hunting.

Geographical distribution

The Cross River gorilla lives near the border between Nigeria and Cameroon in tropical and subtropical moist forests. These forests are also home to the Nigeria-Cameroon chimpanzee, another type of great ape. The Cross River gorilla is the westernmost and northernmost type of gorilla. It lives in forested hills and mountains along the border between Cameroon and Nigeria near the headwaters of the Cross River. It is about 300 km (190 mi) away from the nearest group of western lowland gorillas and about 250 km (160 mi) from the gorilla population in the Ebo Forest of Cameroon. These gorillas are found in 11 areas across a range of about 12,000 km (4,600 sq mi). Recent studies show they may live in more places than previously known. Genetic research supports this, showing that some groups of Cross River gorillas stay connected through occasional movement of individuals.

Cross River gorillas have been found in the Mbe Mountains and in the Afi River, Boshi Extension, and Okwanggo Forest Reserves of Nigeria’s Cross River State. They are also found in the Takamanda and Mone River Forest Reserves and the Mbulu Forest of Cameroon’s South West Province. These areas form a mostly continuous forest covering about 8,000 km (3,100 sq mi) from Afi Mountain to Kagwene Mountain, as noted in a 2007 conservation plan. Researchers think there may be another group near Bechati in the southeast. Today, their total range is estimated to cover about 12,000 km (4,600 sq mi). These gorillas stay in the Afi-to-Kagwene area because the rugged, high terrain keeps them away from humans.

A 2013 study found that Cross River gorillas also live in lower areas, such as the Mawambi Hills, which are about 552 m (1,811 ft) above sea level. This is much lower than their usual habitat, which averages about 776 m (2,546 ft) above sea level.

Cross River gorillas live in small groups separated from other groups of the same species. They occupy about 14 areas across a rugged landscape of about 12,000 km (4,600 sq mi) near the Nigeria-Cameroon border. In Nigeria, there are about 75–110 gorillas, and in Cameroon, about 125–185. Habitat loss is now a bigger threat than hunting. These gorillas live in dense, undisturbed forests, which are becoming rare due to human activity. Most of the remaining gorillas live in the Takamanda National Park and Kagwene Gorilla Sanctuary. Within the sanctuary, gorillas avoid areas disturbed by humans. Conservation workers and eco-guards are trained to enforce laws protecting wildlife. A planned highway near the Ekuri community forest was rerouted in 2017 to protect gorilla habitat.

Researchers estimate that about 250–280 Cross River gorillas remain in 10 separate areas. Human activity has caused the gorilla population to split into smaller groups. Factors like farming, human settlement, and lack of suitable habitat have made this happen. Because of this, gorilla groups are isolated, and genetic diversity is decreasing. A study found that gene movement between western lowland gorillas and Cross River gorillas stopped about 400 years ago, likely because of human activity. The recent decline in Cross River gorilla numbers is probably due to increased human pressure over the past few hundred years.

A recent change is the commercial hunting of bushmeat, which has greatly affected the gorilla population. Hunting is more common in lowland areas and may explain why gorillas are more concentrated in highland areas with smaller groups. Although hunting is illegal, it continues because of local demand and trade to other countries. Laws are not always enforced, and any hunting has a major impact on the gorillas’ survival. All hunting of Cross River gorillas is not sustainable.

Decline

The number of Cross River gorillas decreased by 59% between 1995 and 2010. This is the largest drop in population among all great ape types. Apes like the Cross River gorilla show when there are problems in their environment and help other animals survive. The population began to decrease 30 years ago and has kept falling quickly. Hunters have caused these animals to fear humans, so they are rarely seen.

Cross River gorillas avoid living in grasslands and farms, which makes the forests where they live broken into smaller pieces. Their habitat has become damaged and split into parts. Simple models that scientists use to study animal movement do not explain why the gorillas live in separate groups even though their habitat seems large and suitable. When forests are split, gorillas in different areas cannot move between them, which leads to more breeding within the same group. This causes a loss of genetic diversity, which harms the long-term survival of the species. Scientists study the DNA of these gorillas to learn more about their population. Specific parts of their DNA helped scientists understand how their genes are spread across different groups. Surveys show there are about 300 Cross River gorillas in total, living in about 10 separate areas with little contact between them.

In addition to habitat loss, these gorillas are hunted for bushmeat and for their bones, which are used in fake medical practices. Some African communities have traditions that include using primates as symbols or for testing medicines, which has led to laws to protect them.

Another threat is the illegal pet trade. Only one Cross River gorilla is in captivity at the Limbe Wildlife Center. Even though this number is small, the pet trade has harmed other gorilla species before and could harm Cross River gorillas. Hunters often kill adult gorillas to take baby gorillas as pets.

The Cross River gorilla is critically endangered because of hunting and the Ebola virus. Even if hunting and Ebola deaths stopped, the population would not recover quickly. These gorillas reproduce slowly, and it could take 75 years for their numbers to return to normal. Their habitat is also lost due to mining, farming, and cutting down trees for wood.

Despite these challenges, conservationists are hopeful. In spring 2020, they filmed adult and baby gorillas, showing that some individuals still survive.

Conservation status

The western lowland gorilla is very close to extinction, partly because of the Ebola virus. However, the Cross River gorilla is the most endangered of all African apes. A 2014 study found that fewer than 250 adult Cross River gorillas remain in the wild. In 2012, a survey by Conservation International showed that the Cross River gorilla was not listed as one of the world’s 25 most endangered primates. Conservation efforts have focused on bringing scattered gorilla groups together to avoid inbreeding. A major challenge is that these gorillas live near human communities, which increases risks like hunting for bushmeat and habitat loss. Their protected areas near the Nigeria-Cameroon border are close to hunting zones, making extinction more likely. Cross River gorillas are important for the ecosystem because they help spread seeds for tropical plants that might otherwise die out.

In 2007, a survey in five villages in Cameroon studied local beliefs about protecting endangered species. In the Lebialem division, 86% of people supported conservation, seeing the gorillas as similar to humans. Some believed that if gorillas disappeared, it might harm cultural traditions linked to them. A decline in young people following these traditions was seen as a reason for the gorilla population drop. However, these traditions still discourage hunting. Conservationists believe these beliefs are vital for the gorillas’ survival, especially where laws are not strictly enforced. These traditions have helped prevent hunting for 15 years, showing their value as a conservation tool.

In 2001, a workshop organized by the Wildlife Conservation Society and the Nigerian Conservation Foundation aimed to help Cross River gorillas survive because of their unique traits. Key outcomes included recommendations for conservation actions and the need for regular meetings between governments and conservation groups in Cameroon and Nigeria.

In 2008, Cameroon created 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, which helps safeguard about 115 gorillas—about a third of the total population—and other rare species. The goal is to allow gorillas to move between the two parks.

The Kagwene Gorilla Sanctuary, established by Cameroon in 2008, is part of a plan to protect Cross River gorillas. It covers 19.44 km² of land in western Cameroon, located between the Mbulu and Nijikwa forests. The area has rugged, mountainous terrain and reaches an elevation of 2,037 meters above sea level. Only about half of the sanctuary is suitable for gorillas, as the rest includes grasslands or farmland. The sanctuary is expected to have staff and guards to enforce wildlife protection laws.