The shadow of a small plane darkens a forest clearing in Congo's Nouabali-Ndoki National Park in 2003. The park's 1,620 square miles (4,200 square kilometers) of forest shelter elephants, chimpanzees, and buffalo, according to the Wildlife Conservation Society.
The park is also home to western lowland gorillas, two of which are seen mating face-to-face in unprecedented photos released on February 12, 2008.
Nouabali-Ndoki's marshy terrain may have played a role in the rare mating event, suggests Stony Brook University anthropologist Diane Doran-Sheehy, a National Geographic Society grantee.
"Perhaps," she said, "a female doesn't want to be face down in the swamp."
(The National Geographic Society owns National Geographic News.)
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Photograph by Michael K. Nichols/National Geographic/Getty Images