Scientists from the Wildlife Conservation Society and Max Planck Institute for Evolutionary Anthropology have released the first known photographs of gorillas performing face-to-face copulation in the wild. This is the first time that western gorillas have been observed and photographed mating in such a manner.

The photographs were part of a study conducted in a forest clearing in Nouabalé-Ndoki National Park in the Republic of Congo that appeared in a recent issue of The Gorilla Gazette.

“Understanding the behavior of our cousins the great apes sheds light on the evolution of behavioral traits in our own species and our ancestors,” said Thomas Breuer, a researcher at the Max Planck Institute for Evolutionary Anthropology and WCS and lead author of the study. “It is also interesting that this same adult female has been noted for innovative behaviors before.”

The western lowland gorilla is listed as Critically Endangered as a result of hunting by humans, habitat destruction, and health threats such as the Ebola virus.

The female gorilla in the photograph, nicknamed “Leah” by researchers, made history in 2005 when she was observed using tools – another never-before-seen behavior for her kind in the wild. Breuer and others witnessed Leah using a stick to test the depth of a pool of water before wading into it in Mbeli Bai, where researchers have been monitoring the gorilla population since 1995.

Researchers say that few primates mate in a face-to-face position, known technically as ventro-ventral copulation; most primate species copulate in what’s known as the dorso-ventral position, with both animals facing in the same direction. Besides humans, only bonobos have been known to frequently employ ventro-ventral mating positions. On a few occasions, mountain gorillas have been observed in ventro-ventral positions, but never photographed. Western gorillas in captivity have been known to mate face-to-face, but not in the wild, which makes this observation a noteworthy first.

“Our current knowledge of wild western gorillas is very limited, and this report provides information on various aspects of their sexual behavior,” added Breuer, whose study is funded by the Brevard Zoo, Columbus Zoo and Aquarium, Cincinnati Zoo and Botanical Garden, Max Planck Society, Sea World & Busch Gardens Conservation Fund, Toronto Zoo, Wildlife Conservation Society and Woodland Park Zoo. “We can’t say how common this manner of mating is, but it has never been observed with western gorillas in the forest. It is fascinating to see similarities between gorilla and human sexual behavior demonstrated by our observation.”

Scientists estimate that western gorillas have declined 60 percent in recent years due to habitat loss, illegal hunting, and Ebola hemorrhagic fever. The Wildlife Conservation Society, which is the only organization working to protect all four gorilla sub-species (also including the Cross River Gorilla, the mountain gorilla, and the Grauer’s gorilla), has been studying gorillas and other wildlife in the Republic of Congo since the 1980s. In 1993, the Congolese Government, working in tandem with technical assistance from WCS, established Nouabalé-Ndoki National Park.