Wary of humans, gorillas are notoriously hard to tally in the wild.
To assess their populations, WCS researchers instead used data on the numbers and ages of so-called sleep nests, temporary bedding made of leaves and branches.
Each group of lowland gorillas has a range of about 7.7 square miles (20 square kilometers), and the animals build the nests to sleep in each night before moving on in the morning.
The census work involved crossing hundreds of miles to count nests, then loading data into a mathematical model that estimated the number of gorillas living within a defined area.
In the 17,400-square-mile (28,000-square-kilometer) Ndoki-Likouala region, for example, the nest census found an estimated population density of 1.65 gorillas per square kilometer (equal to about 0.3 square mile).
This means that about 46,200 western lowland gorillas likely live in the area, which runs west of the Sangha River to the border of the Central African Republic.
An additional 6,000 gorillas reside in the region's 646-square-mile (1,040-square-kilometer) Batanga swamps. These wetlands, which are inaccessible to humans for more than half the year, house an estimated five to six apes per square kilometer.
"That's the highest density I've seen," Stokes said, adding that the data suggest Ndoki-Likouala is the subspecies' "largest remaining stronghold."
The discovery "shows that conservation in the Republic of Congo is working," said WCS president Steven Sanderson.
Almost half the surveyed area lies within officially protected zones or inside timber concessions where logging companies have banned transport of protected animals and weapons on their roads.
Researchers hope the latest census will encourage the government of Congo to establish a new national park in the Ntokou-Pikounda region.
The census was presented today at the International Primatological Society conference in Edinburgh, Scotland, and some of the data will appear in an upcoming issue of the conservation journal Oryx.
Perils of Counting Apes
Several experts greeted the survey findings with a mix of excitement and caution.
"If these new gorilla census figures are confirmed by further surveys, it would be the most exciting ape conservation news in years," said Craig Stanford of the Jane Goodall Research Center at the University of Southern California.
"Nest census data are notorious for varying from one method to the next, however, and I think we should be cautious before assuming the world's known gorilla population has just doubled."
Nesting data were among the factors used in a 2007 IUCN population assessment that placed the western lowland gorilla on the organization's Red List of Threatened Species.
IUCN estimated the gorillas had declined by more than 60 percent over the past 25 years, and its scientists projected the apes' population could fall to 50,000 as the deadly Ebola virus penetrated deeper into their habitat.
That report came with a caveat about the reliability of nest counts: "Technical problems with the conversion of ape nest density to estimates of gorilla density preclude a rigorous estimate of range-wide gorilla abundance."
Peter Walsh of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, led the 2007 IUCN assessment. He repeated those concerns when he learned of WCS's findings in northern Congo.
"It is not that I think that the numbers are necessarily too high," Walsh said. "It is just that I do not trust the assumptions made by the estimation models that are being used."
John Oates, professor emeritus of anthropology at Hunter College in New York, noted that "what does seem clear is that there are still plenty of western gorillas in northern Congo."
He remains cautious, however, about whether the new research should signal a change in status for the great apes.
In addition to habitat loss and hunting, in recent years Ebola has ravaged gorilla habitats bordering the Ntokou-Pikounda survey area, killing 60 percent of the apes in nearby Odzala National Park.
While WCS's Stokes said her survey found "no evidence of Ebola in Ntokou-Pikounda, our general philosophy is Ebola can hit anywhere, anytime."
And with a 90 percent mortality rate among infected gorillas, Stokes thinks the animals deserve all the protection they can get.
In general, the WCS findings demonstrate that our intensely observed planet still has its biological secrets, added Richard Bergl, curator of research at the North Carolina Zoo.
"It is extraordinary that in this day and age," he said, "there could be a population of a hundred thousand or more gorillas that were essentially unknown to science."
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