CROSS RIVER GORILLA

Gorilla gorilla diehli

Geographic Distribution and Habitat

Cross River gorillas were originally discovered in 1904. In the decades that followed, scientists believed these giant creatures had been hunted to extinction—until their “rediscovery” in the 1980s. Scientists have since identified 11 family groups (or “troops”) living at eight sites with overlapping home ranges across the rugged, hilly terrain that straddles the Nigeria-Cameroon border on the African continent, known as the Cross-Sanaga-Bioko Coast Forest ecoregion.

Within this vast expanse of 4,633 square miles (12,000 square kilometers), the gorillas—considered a flagship species (or “icon”) of this ecoregion—are mostly concentrated within an area of 270 square miles (700 square kilometers). Lowland, submontane, and montane forests situated on the upper drainage of the Cross River on either side of the border provide habitat. Within these locales, the gorillas reside at elevations from 4,921 to 11,483 feet (1,500–3,500 meters). Bamboo forests provide an alternative habitat; here the gorillas reside at elevations of 8,202 to 9,843 feet (2,500–3,000 meters).

The prevalence of these great apes in secluded, hilly areas is thought to be a strategic, deliberate effort on their part to avoid contact with humans and activities associated with humans.

TAXONOMIC NOTES

Initially thought to be a new species, further research conducted in the 1980s led scientists to classify the Cross River gorilla as one of two subdivided subspecies of the western gorilla (Gorilla gorilla), with the western lowland gorilla (Gorilla gorilla gorilla) being the second subspecies. Scientists believe that these two distinct subspecies diverged genetically about 18,000 years ago; geographically, they are separated from one another by about 155 to 186 miles (250 to 300 kilometers) south of the Sanaga River.

The eastern gorilla (Gorilla beringei) is also subdivided into two subspecies: the mountain gorilla (Gorilla beringei beringei) and Grauer’s gorilla (Gorilla beringei graueri), formerly known as the eastern lowland gorilla.

Size, Weight, and Lifespan

These are formidable creatures. While their eastern gorilla cousins—Grauer’s gorillas—hold the distinction as the largest of all four gorilla subspecies, Cross River gorillas are far from diminutive. Males stand tall from 4.6 to 5.6 feet (1.4–1.7 meters) and weigh between 309 and 441 pounds (140–200 kilograms). Females are smaller, reaching a height of 4.6 feet (1.40 meters) with an average weight of 220 pounds (100 kilograms). Size difference in the subspecies is an example of sexual dimorphism.

Lifespan in the wild is from 35 to 50 years.

Appearance

Except for a subtle difference in skull size (their heads are smaller relative to body size), smaller tooth dimensions, and a slightly smaller mouth, Cross River gorillas closely resemble their western lowland gorilla kin. Like western lowland gorillas, Cross River gorillas are not quite as large as their eastern gorilla cousins. They have a more slender build, shorter and lighter-colored hair, longer arms, and a more prominent brow ridge (scientifically known as a “sagittal crest”). Compared to their human primate cousins, the sturdy skeleton of these great apes is defined by straighter vertebrae. Like all apes, they have no tail.

A flat and expressive hairless, dark face, characteristic of all gorillas, is almost disarming in its guilelessness. Brown eyes are deeply set beneath the furrowed brow (most prominent in adult males), evocative of deep contemplation. Wide nostrils flare downward to the philtrum (fancy name for the groove between the nose and top of the lip) above a narrow, unpretentious mouth.

The pelage (fur coat) is usually brownish gray to black, with auburn-colored fur covering the chest of Cross River gorillas. A reddish crest adorns a cone-shaped head (same as western lowland gorillas), and nondescript ears sit on either side beneath the temples. Adult males sport a silvery swath of hair down the center of their backs, a notable feature they share with all adult male gorillas, earning them the descriptive nickname “silverback.” Like the face, hands and feet are bare of fur.

Diet

These elusive giants of the forest adhere to a mostly vegetarian diet. They love fruits and travel great distances to pick from their favorite trees. Aframomum, a type of ginger plant that is widespread across tropical Africa, is a favorite delicacy for Cross River gorillas. And not just for the fruit. By peeling back the outer layer of the stem, the gorillas gain access to the soft and tasty center. Leaves, nuts, and berries from various plant species complement their meal plan.

Their highly seasonal habitat impacts the gorillas’ diet. When fruits are not in season, ground vegetation and tree bark provide sustenance. Compared to the dietary proclivities of western lowland gorillas, Cross River gorillas eat more liana (a woody vine) and tree bark throughout the year. Insects, along with the occasional lizard appetizer, might also show up on the gorillas’ seasonal menu. Their largely vegetarian diet provides them with their daily water requirement. Gorillas are also known to sip water that has pooled onto leaves.

Behavior and Lifestyle

Cross River gorillas are shy animals who have a natural, and warranted, wariness of humans. Thus, comparatively little study has been conducted on this enigmatic subspecies. Most of what is known about general gorilla behavior and lifestyle comes from studies of mountain gorillas. But long-term studies currently underway at two Cross River gorilla habitat sites are already providing a peek into the ecology and social life of this rarest of all great apes. The differences that have been revealed (for example: in diet, ranging behavior, and grouping patterns) compared to other gorilla subspecies might seem insignificant; yet these nuggets of information are creating an inroad into a better understanding of these primates to hopefully save them from extinction. Differences in behavior between groups of Cross River gorillas are also being studied.

These great apes spend most of their time on the ground (making them terrestrial), and they are active during daylight hours (making them diurnal). Most of their day is spent eating so that they can maintain their massive size and keep up their strength. They also engage in mutual grooming sessions. Each night, these great apes create a new nest in which to sleep, individually. During the rainy season (April through November), they build these nests in trees, rather than on the ground, to keep themselves dry. This discerning, seasonal nesting behavior is distinctive to Cross River gorillas. They also exhibit seasonal movement patterns, seeking higher elevations during the rainy season and retreating to the valleys during the dry season.

Cross River gorillas’ preferred mode of locomotion is “knuckle-walking,” a form of quadrupedalism (walking on all four limbs); whereby they propel themselves forward by walking on the knuckles of their hands and on the soles of their feet. They are capable of walking upright for brief periods.

When foraging over a wide area for one or more days, larger groups often break into smaller teams—reforming as one group during rest periods.

Skeletal finds of Cross River gorillas have led scientists to speculate that these great apes may spend less time than western lowland gorillas chewing their food. This hypothesis relates to their skull size, which allows for a greater “biting force” and relates also to the shape of the Cross River gorillas’ fangs (incisors), which are typically more worn than those of western lowland gorillas, suggesting the utility of their incisors in snacking on harder and more abrasive foods.

Skeletal finds further suggest that Cross River gorillas possess more opposable thumb flexibility than western lowland gorillas, helpful in peeling the stems of their favorite ginger plant and in fashioning rudimentary tools to help them more effectively gather food. Their ability to create and successfully use tools is only one marker of their intelligence and problem-solving abilities.

Large jungle cats and crocodiles are the gorillas’ only natural predators. But the specter of humans as hunters, intent on killing them, remains in the psyche of these great apes.

Fun Facts

Gorillas are one of our closest living relatives, sharing with us at least 95 percent of their DNA.

World Gorilla Day is celebrated on September 24 to honor the iconic gorilla, including the enigmatic Cross River gorilla.

Daily Life and Group Dynamics

Gorillas live within complex social groups, occupying relatively small areas of land. Members display individual personalities and exhibit emotions we might associate more with human primates, such as grief, compassion, and empathy (the latter two of which seem to be in short supply these days in much of our human primate species).

True of all gorilla societies, Cross River gorilla troops are led and protected by an alpha male, who is a mature silverback. An average troop is composed of four to seven individuals who include the dominant (alpha) male, several adult females with whom the alpha breeds, and their offspring. Larger groups of 18 to 20 individuals might exist in areas of rich habitat. Additional determining factors for group size include predator threats and opportunities for male migration to other groups.

Home range, the area traversed in daily activities such as foraging, mating, and caring for young, can be as large as 11.5 square miles (30 square kilometers).

In gorilla culture, both male and female young gorillas might leave their birth groups. Females are inclined to leave their birth group if the dominant silverback dies and no other male can fill the role of protector and mating partner. Males wishing to establish their own group and harem typically leave their birth group upon reaching puberty. Several may travel together for years in so-called “bachelor groups,” until they become silverbacks at about 12 years of age. They continue to develop until the age of 15 when they have more luck at realizing their goal.

Sympatric species of Cross River gorillas include the Nigeria-Cameroon chimpanzee (Pan troglodytes ellioti), drill (Mandrillus leucophaeus), Preuss’s guenon (Allochrocebus preussi), crowned guenon (Cercopithecus pogonias), Preuss’s red colobus monkey (Procolobus preussi), leopard (Panthera pardus), African forest elephant (Loxodonta cyclotis), African forest buffalo (Syncerus caffer nanus), slender-snouted crocodile (Mecistops cataphractus), grey-necked rockfowl (Picathartes oreas), along with a myriad of bird species.

Communication

Gorillas communicate with one another through a vocabulary of 22 different vocalizations (according to wildlife biologists). Each call is assigned a specific meaning. As example, a specific call is used for play, another for mating, and still another to announce a predator threat. The plethora of other calls are used to convey a range of emotions and feelings that are similar to the human experience.

Grooming one another using their fingers is a favorite gorilla pastime and helps to establish social bonds with one another.

When it comes to body language, perhaps the most intimidating display of posturing is when a silverback stands upright and beats his chest while loudly roaring, in the face of a threat.

Besides acting as protector of his troop, a dominant male silverback uses his authority to keep his members in line. Should females become overly aggressive with one another as they vie for his attention, he intervenes. Likewise, should an adult behave roughly toward one of the troop’s infants, the alpha silverback puts a stop to the harassment.

While other gorilla subspecies are mainly peaceful toward humans, Cross River gorillas are known to be aggressive in these rare encounters. In behavior that is more common to chimpanzees, these great apes have been observed throwing clumps of earth, tree branches, and stones at humans.

Reproduction and Family

Because Cross River gorillas are so successful at eluding humans, wildlife biologists have had scant opportunity to observe the reproductive behavior of these great apes. Nevertheless, the general scientific consensus is that their behavior of is likely similar to that of other gorillas.

Gorillas live in a polygamous society; that is, breeding individuals have more than one mate. Females reach sexual maturity at about 7 or 8 years of age, but they don’t typically give birth until at least 10 years of age. Males lag behind a bit, reaching sexual maturity between 11 and 13 years of age, though they don’t typically sire young until they are between 15 and 20 years of age.

A troop’s dominant silverback enjoys special privileges with all the adult females in his harem; he mates with each. After a gestation period of about 8.5 to 9 months, a single infant is born. At birth, Cross River gorilla babies weigh a mere 4 pounds (1.8 kilograms). Mothers nurse their babies from four to five years of age, at which time the young are considered fully weaned. At this time, breeding females may again give birth (they are unable to become pregnant while nursing). Mothers teach their young important life skills, like foraging for food.

Baby gorillas are tiny and vulnerable. For the first five months of their lives, they are in constant physical contact with their mothers. Mothers keep close to the silverback for added protection. As the babies grow up, they begin playing with other members of the troop, including their silverback dad who is gentle with his offspring.

Ecological Role

Cross River gorillas are ambassadors of their ecosystem, playing a crucial role in keeping it healthy and thriving. Their poop is loaded with seeds from the many fruits they eat. Thus, the stupendous plops of scat these great apes drop throughout their environment as they go about their daily lives helps to regenerate their forested habitat. When their seasonal diet shifts and they start eating vegetation and peeling bark from trees, the gorillas help to prune overgrowth and encourage new seedlings to grow.

Conservation Status and Threats

The Cross River gorilla is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, January 2016), appearing on the IUCN Red List of Threatened Species; this highest threat level means that the Cross River gorilla faces an extremely high risk of extinction in the wild. Wildlife biologists estimate that only 200 to 300 Cross River gorillas remain in the world. Included in this dire number are 50 to 100 adolescents.

Its sibling subspecies, the western lowland gorilla, is also classified as Critically Endangered; its eastern gorilla cousins fare no better. While the mountain gorilla tenuously hangs onto an Endangered status (just beneath Critically Endangered), Grauer’s gorilla is Critically Endangered.

Of the four gorilla subspecies, the Cross River gorilla holds the ignoble distinction of being the most threatened and is regarded as one of the twenty-five most endangered animals on our planet. They are an “umbrella species” for their dwindling habitat—a “biodiversity hot spot” inextricably linked to their survival and to the survival of the diverse number of species who also live here and who are found nowhere else in the world.

Habitat loss is the gravest threat against these gorillas. Dense human settlements surrounding the areas where the gorillas live, conversion of pristine forest to farmland and into tracts for cattle grazing, commercial logging, and industrial agriculture (specifically, palm oil production) have made the gorillas’ future precarious. Related infrastructure, such as roadways, have left these great apes vulnerable to poachers who exploit access to these secluded beings and kill them for their flesh, known as “bushmeat,” or for the large animals’ body parts, to be sold on the black market. The subspecies may also be “incidentally” maimed or killed, caught in snares targeting other species. These great apes’ proximity to humans also leaves them susceptible to diseases such as Ebola, which is responsible for killing many of their western lowland cousins.

Because of the gorillas’ inherent wariness toward humans, wildlife biologists have been forced to resort to indirect methods, such as nest counts, GPS tracking, DNA extracted from gorilla fecal samples, and camera traps to arrive at population numbers.

The subspecies’ low and plummeting population is compounded by their isolation. While the gorillas choose their remote habitats as self-preservation, their migration opportunities are lessened as compared to that of the other gorilla subspecies. The eradication of forest corridors due to anthropogenic activities has harmed the gorillas’ ability to mingle with and breed with other subpopulations. Thus, inbreeding becomes a consequence of this isolation and dilutes the genetic diversity of the Cross River gorilla subspecies.

Conservation Efforts

The Cross River gorilla is listed in Appendix I 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. The subspecies is also listed in Class A of the African Convention. Unfortunately, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce—even in so-called protected areas.

A modicum of safety can be found in the following national parks and wildlife sanctuaries, home to a segment of the Cross River gorilla population.

Cross River National Park, Nigeria. Defined by two separate sections, Okwangwo (established 1991) and Oban (established 1988), encompassing a total area of about 1,544 square miles (4,000 square kilometers), consisting mostly of primary moist tropical rainforests.
Tofala Hill Wildlife Sanctuary, Cameroon. Established in September 2014, encompassing about 31 square miles (80 square kilometers).
Kagwene Gorilla Sanctuary, Cameroon. Established in April 2008, encompassing just 7.5 square miles (19.44 square kilometers). Of this land, only about half is prime gorilla habitat, while the rest includes grassland or cultivation not suitable for the species.
Takamanda National Park, Cameroon. Established in 2008 with the specific intention of protecting Cross River gorillas, encompassing 269 square miles (697 square kilometers).

Conservationists have vociferously called for more stringent management within these protected areas to combat illegal activities against the resident wildlife species. The Wildlife Conservation Society (WCS) is one environmental group at the forefront of this ongoing effort.

Today, 16 uniformed eco-guards—local men trained by the WCS—conduct weeklong patrols of the Mbe Mountains Forest, which is jointly managed as a wildlife sanctuary by WCS and the Conservation Association of the Mbe Mountains. Covering an area of approximately 33 square miles (85 square kilometers), the Mbe Mountains form an important habitat corridor that links the Afi Mountain Wildlife Sanctuary with the Okwangwo Division of Cross River National Park, Nigeria.

Another 13 community rangers patrol the nearby Afi Mountain Wildlife Sanctuary, Nigeria, established in 2000, to provide enhanced protection to Cross River gorillas and other Critically Endangered species. Covering an area of approximately 38.6 square miles (100 square kilometers), the expanse includes lowland and submontane forests with rocky peaks rising to altitudes of 4,265 feet (1,300 meters). The sanctuary and the surrounding Afi River Forest Reserve form one of the largest tracts of forest remaining in Cross River State, outside of a national park.

Conservationists further call for the preservation of gorilla habitat outside of these sanctuaries and national parks and lobby for the eradication of the illicit trade in bushmeat and body parts.

One proposed solution in protecting Cross River gorillas, along with other endangered species, is to classify the ecoregion where they live as a UNESCO World Heritage Site. In 2020, UNESCO placed the Cross River-Korup-Takamanda National Parks on the organization’s “tentative” list, an action that signifies a promising foothold in wildlife species and habitat conservation.

Gorilla corridors . . . and more
Successful conservation requires relevant research that includes population surveys, health monitoring, and disease risk assessment; the creation of local educational programs that foster an awareness and appreciation of these rarest of great apes; and direct community engagement in helping to create safe habitat corridors between Cross River gorilla subpopulations—a crucial initiative since about 30 percent of the total Cross River gorilla population resides outside of protected areas. The most important conservation need for the long-term survival for this Critically Endangered gorilla subspecies, conservationists stress, is to allow the population to expand. Creating habitat corridors allows the gorillas to safely migrate and breed.

Unlikely conservationists
Scientists studying Cross River gorilla populations have employed an unlikely ally in their research: local hunters-turned-conservationists. These individuals, now with a newfound reverence for this rarest of great ape, are dedicated to conserving Cross River gorillas and their habitat. These new conservationists play a crucial role in providing scientists with insight into the gorillas’ behavior. They are expert trackers. Not only are they able to nimbly navigate the gorillas’ rugged terrain with a stealthy approach, their keen awareness of the local environment lets them pick up on subtle visual clues that scientists might miss. For example, broken branches, scratches in the dirt, and flattened vegetation are all possible indicators that a Cross River gorilla or gorilla has passed through the area.

Innovative technology: GPS tracking
This once-forgotten gorilla is now a priority with conservation groups. The Wildlife Conservation Society (WCS), in partnership with the North Carolina Zoo, is using GPS tracking technology to help conserve Cross River gorillas. (GPS is the abbreviation for “global positioning system” and is the same technology used in cars to help drivers know where they’re going.) In Cross River gorilla habitat, researchers are using rugged, hand-held computers that allow them to enter important information such as location of gorilla nest sites, evidence of poaching and logging, and other crucial details. GPS technology receives this information in a central database and automatically creates maps of the detailed areas to share with other conservationists. GPS technology has already proven helpful in detecting illegal activities at several Cross River gorilla sites.

Smile! You’re on camera!
Camera traps have revolutionized the field of wildlife conservation. Although camera trap technology had been in use for decades in the ecoregion home of Cross River gorillas, previous images captured only an occasional adult male gorilla. This situation changed dramatically in July 2020. A family of Cross River gorillas, including several adolescents and infants, walking through the Mbe Mountains Forest triggered the motion-sensitive camera, capturing the gorilla family’s images to the wild delight of scientists. WCS has called the images “a promising sign of the species’ population recovery.”

A coordinated camera-trapping campaign is underway in Mbe Mountains, neighboring Afi Mountain Wildlife Sanctuary, and in Cross River National Park with a goal of establishing an updated baseline for population size and health status for this Critically Endangered gorilla subspecies.

Beyond gaining insight into the lives and behavior of these rarest of great apes, images released to the public have sparked interest in this subspecies’ conservation—cultivating international awareness along with much-needed fundraising dollars.

Conservation canines
The North Carolina Zoo is at the forefront of another innovative conservation effort, this time in a partnership with the group Working Dogs for Conservation. A team of specifically trained working dogs was brought to Cameroon to locate dung samples of Cross River gorillas, a feat that these “conservation canines” performed efficiently and successfully. Dung specimens allow scientists to glean important DNA analysis and also help them to determine the threat level of human diseases upon these critically endangered great apes.

Additional conservation organizations/efforts
The Environment and Rural Development Foundation (ERuDeF), founded in 1999 and based in Cameroon, has significantly contributed towards restoring the fragile ecoregion that is home to Cross River gorillas. ERuDeF also actively promotes environmental education and empowers rural communities through innovative economic and livelihood development programs.

The World Wildlife Fund (WWF) is working with the governments of Cameroon and Nigeria to create a protected area for the Cross River Gorilla that spans the border of these two nations. WWF has established ranger posts within this area, fitting these posts with communication equipment for antipoaching staff. A future conservation project aims to establish protected corridors of forest habitat that will allow safe migration of gorillas between different gorilla groups. Additionally, WWF is working with local logging companies to foster sustainable management of the gorillas’ habitat and to promote safe economic development initiatives.

Fauna & Flora International is an organization helping to educate local communities so that citizens feel a sense of pride for their native gorilla subspecies. Today, most locals consider it “taboo” to hunt Cross River gorillas. In fact, part of the Cross River gorilla conservation plan is to have local communities act as guardians for these great apes.

Other conservation groups working on behalf of the Cross River gorilla include the African Conservation Foundation (ACF), the Environmental and Rural Development Foundation, and the Cross River Gorilla Campaign, a collaborative effort to raise awareness and funds for the conservation of the Cross River gorilla, Africa’s most endangered great ape.

Nyango’s Story
Until her death in October 2016 at age 24.5, after she succumbed to a protracted illness, Nyango had been the only known Cross River gorilla in captivity. She lived at the Lumbe Wildlife Center in Cameroon.

The primate had been rescued in 1994 by a kind-hearted couple who discovered, by chance, the then 2-1/2-year-old gorilla being held captive in a local village. They brought Nyango, as they called her, home to live with them.

Nyango became a member of their family, bonding with the couple’s three children. But having a wild animal inside a human household presents challenges. When that wild animal is a gorilla, the challenges are enormous, as the well-intentioned family found out—particularly as Nyango grew. Releasing her to the wild was not an option, so they ended up relinquishing Nyango to the Lumbe Wildlife Center. Only then did the family learn that Nyango was a Cross River gorilla, the rarest of all great apes.

Reflecting on his family’s experience with Nyango, the husband and father reverently wrote on his blog, “One can’t stay long in a gorilla’s presence and not be somehow changed.”

To read Nyango’s full story, visit: https://crossrivergorilla.org/nyango-gorilla-in-our-midst.

References:

http://www.iucnredlist.org/details/39998/0
http://www.crossrivergorilla.org
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Written by Kathleen Downey, June 2023