CROSS RIVER GORILLA
Gorilla gorilla diehli
Geographic Distribution and Habitat
Cross River gorillas are the rarest of all great apes and were unknown to science until the early 20th century. They were forgotten, then “rediscovered” after scientists had believed the gorillas had become extinct.
Classified as one of two subspecies of western gorilla (the western lowland gorilla being the other), Cross River gorillas inhabit the lowland montane forests and rainforests of Cameroon and Nigeria, an area of 3,000 square miles (7,770 sq km), on the African continent. Scientists have identified 11 family groups (or troops) living in eight sites across this rugged, hilly terrain in overlapping home ranges.
Cross River gorillas’ natural wariness of humans has forced scientists to resort to indirect methods, such as nest counts, to arrive at a total population count of only 100-250 of these gorillas remaining in the world.
Like all gorillas, Cross River gorillas are severely endangered in the wild. But of all the gorilla species and subspecies, Cross River gorillas face the greatest threat of extinction. Their low and falling numbers makes their survival extremely precarious.
Size, Weight, and Lifespan
Cross River gorillas stand 4.7 to 5.5 feet (1.4 to 1.7m) tall and weigh between 220 and 440 pounds (100 to 200kg). Lifespan in the wild is 35 to 50 years.
Except for subtle differences in skull size (their heads are smaller relative to body size) and tooth dimensions, Cross River gorillas closely resemble their western lowland gorilla kin. Like western lowland gorillas, Cross River gorillas are slightly smaller than the eastern gorilla subspecies, with a more slender build, shorter and lighter-colored hair, longer arms, and a more prominent ridge line.
They have flat, expressive faces that are characteristic of all gorillas, with what appears to be a permanently furrowed brow (evocative of deep contemplation), wide nostrils, and dark eyes that are deeply set. Their faces, hands, and feet are bare of fur. The coats of Cross River gorillas are typically brownish gray to black, and they have auburn-colored chests; a reddish crest adorns their cone-shaped heads (same as western lowland gorillas). Adult males sport a silvery swath of hair down their backs, a characteristic they share with all adult male gorillas, earning them the nickname silverback.
Like all gorillas, these elusive giants of the forest adhere to a mostly vegetarian diet. They love fruits and will 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. By peeling back the outer layer of stem, the gorillas gain access to the soft and tasty center. They also eat the fruit. Compared to the dietary proclivities of western lowland gorillas, Cross River gorillas eat more liana (a woody vine) and tree bark throughout the year.
Leaves, nuts, and berries from various plant species complement their meal plan. When fruits are scarce, insects might show up on their menu along with the occasional lizard appetizer. Their largely vegetarian diet provides Cross River gorillas, and all gorillas, with their daily water requirement. On occasion, gorillas will sip water that has pooled onto leaves.
Behavior and Lifestyle
Comparatively little study has been conducted on Cross River gorillas. 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 (in diet, ranging behavior, and grouping patterns) between Cross River gorillas and their lookalike subspecies kin, western lowland gorillas, might seem insignificant; yet these nuggets of information are creating an inroad into better understanding, and saving, this Critically Endangered primate. Differences in behavior between groups of Cross River gorillas are also being studied.
Scientists studying Cross River gorilla populations have employed an unlikely ally in their research: local hunters-turned-conservationists. These individuals—now dedicated to conserving these great apes and their habitats—are expert trackers. Not only are they able to nimbly navigate the gorillas’ rugged terrain, 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 has passed through the area. Nest sites are more obvious indicators of Cross River gorilla groups. By counting the number of nests (each gorilla individual creates a nest for his or her exclusive use, each night), conservationists can determine a troop’s population.
As with all gorillas, Cross River gorillas’ preferred mode of locomotion is walking on all fours, or “knuckle walking” (scientifically known as quadrupedalism); that is, 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, however, for brief periods.
In behavior that is more common to chimpanzees, Cross River gorillas have been observed throwing clumps of earth and tree branches at humans.
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’ 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, fashioning rudimentary tools that help them more effectively gather food, and flinging tree branches and handfuls of earth at humans.
An average troop is comprised of four to seven individuals; however, larger groups of 18 to 20 individuals exist. Group size is largely determined by habitat. More than other gorilla subspecies, Cross River gorillas’ habitat is highly seasonal, linked to the availability of certain foods. Additional determining factors for group size include limited opportunities for migration to other groups, and the threat of predators.
Large jungle cats and crocodiles are the gorillas’ only natural predators. The specter of humans, however, as hunters intent on killing them, remains in the psyche of these great apes. Therefore, no attempts to habituate Cross River gorillas (that is, get them used to/acclimated to human company) for detailed study and research have been made. Scientists in the field have only caught fleeting glimpses of these primates. Instead, the hunters-turned-conservationists, through their stealth approach and newfound reverence for this rarest of great apes, provide scientists with insight into the gorillas’ behavior. As example, these new conservationists reported on the gorillas’ seasonal patterns of movement, with the great apes seeking higher elevations during the rainy season and retreating to the valleys during the dry season.
Scientists studying a large group of Cross River gorillas over a three-month period have noted that the group breaks into smaller subgroups to forage over a wide area for one or more days. The gorillas reform as one group during rest periods.
Study of a second Cross River gorilla troop revealed similar behavioral patterns, although these gorillas did not forage quite as far. In both studied troops, however, scientists discovered that the gorillas, despite their enormous weight, more often constructed their nightly nest in trees, a practice that their western lowland gorilla kin mostly shun.
All gorillas communicate with one another through a variety of vocalizations, each with a specific meaning. Different sounds are used during play, mating, or when a predator is sighted. Scientist have determined 22 different gorilla vocalizations and have attested that gorilla behavior and emotions are similar to the human experience.
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.
As with all gorillas, Cross River gorilla troops are led and protected by an alpha male, who is a mature silverback. Males attain silverback status at about 12 years old; however, they continue to develop until age 15.
Belying their intimidating size, silverbacks often act as peacemakers. Besides protecting his troop from outside threats, a dominant male silverback will intervene when members of his harem become overly aggressive with one another as they compete for the silverback’s attention, or when an adult member of the troop behaves too roughly with one of the troop’s infants.
The alpha silverback male has special privileges with the females; he mates with each of them. Females reach breeding age at ten years old and after a gestation period of about 8.5 to 9 months, give birth to a single infant. Nursing mothers are unable to become pregnant and typically give birth every four or five years, after their infant is fully weaned.
Baby gorillas are tiny and vulnerable; they require much maternal care. 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. Grooming each another is a favorite gorilla pastime and establishes social bonds.
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. Males wishing to establish their own group and have their own female harem typically leave their birth group upon reaching puberty. Several may travel together for years in so-called “bachelor groups,” until they become silverbacks and realize their goal.
But for Cross River gorillas, who live in isolated habitats, their migration opportunities are lesser than other gorilla subspecies. Inbreeding is a consequence of this isolation and dilutes the genetic diversity of the Cross River gorilla subspecies.
Until her death in October 2016, after succumbing 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 named 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 their experience with Nyango, the husband and father of the family reverently wrote on his blog, “One can’t stay long in a gorilla’s presence and not be somehow changed.”
As with other primates who have a frugivorous diet, Cross River gorillas are important seed dispersers, playing a critical role in forest regeneration in their fragile ecosystem. Since they travel long distances, they disperse seeds far from the mother tree, adding to the diversity of the forest flora.
The Cross River gorilla is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2016), appearing on the IUCN Red List of Threatened Species. This highest threat level means that Cross River gorilla joins all gorilla subspecies in facing an “extremely high risk of extinction in the wild.”
Once hunted to near extinction by locals, habitat loss is now the looming threat to the survival of Cross River gorillas. Lush forests have been razed and transformed for agriculture, cattle grazing, and the creation of logging roads. Civil unrest has also impacted the small population of gorillas. And even though the hunting of gorillas is illegal in Cameroon and Nigeria, the gorillas are vulnerable to poaching. Inbreeding and the transmission of human diseases, specifically the Ebola virus, also threaten this great ape’s survival.
Today, the Cross River gorilla is considered one of the 25 most endangered animals on the planet. The subspecies’ habitat is considered a “biodiversity hot spot,” meaning that the region is home to a diverse number of species who live nowhere else in the world. Forest buffalo, forest elephants, other primates, birds, butterflies, and certain amphibians live in this threatened and diminished habitat.
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. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce.
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.
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 task that these “conservation canines” performed far more efficiently and successfully than human researchers. 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 Efforts
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 anti-poaching 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, alternative 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. The transformation of former hunters into conservationists is testament to the success of these educational efforts.
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.
International awareness remains key to the Cross River gorillas’ survival in our world. New England Primate Conservancy is dedicated to raising this awareness.
Written by Kathleen Downey, April 2017