How Many Ugandan Mountain Gorillas?

Mountain gorillas are rare and endangered, and they have the misfortune to live in a part of the world wracked by human violence. In the magazine in 2007, we focused on the gorillas of Congo and Rwanda, giving little attention to the 350 living in neighboring Uganda. But the Ugandan gorillas may not be doing as well as we thought.

In Uganda, researchers have usually estimated mountain gorilla numbers through counts of nests (the gorillas build the nests each night and sleep in them). But a group of scientists used a new method to count the gorillas within Bwindi Impenetrable National Park in Uganda and discovered that the traditional counting method may be inaccurate (the results were published last month in the journal Biological Conservation). The scientists, led by Katerina Guschanski of the Max Planck Institute for Evolutionary Anthropology, obtained DNA from 384 fecal samples collected at the nest sites so they could assign individuals to each nest (as least by their dung sample). They report: “We found that both groups and lone silverbacks were double-counted in the field and that individuals constructed multiple nests with an overall rate of 7.8%, resulting in the overestimation of the population size in the absence of genetic data.” By their calculations, there are only 302 mountain gorillas in the park.

From New Scientist:

"We assumed that each individual constructs a single nest, but genetic analysis shows that several individuals construct more than one nest," says Guschanski. This has been observed in studies of lowland gorillas, who construct more than one nest if the original nest starts leaking during a rainstorm, or if a youngster finds the one that it has just built uncomfortable, she adds…. It might also mean that the gorilla population in the park is not growing after all--a census in 1997 found 300 gorillas, while one in 2003 found 320 individuals, but these figures may also be inaccurate. "Now we don't really know what is happening with this population," says Guschanski. "Probably the safest thing is to assume that the population is stable, but we will need to wait for another four to five years to assess how it is changing."

As for the mountain gorillas in Congo, we may have new population numbers soon. The rangers of Virunga National Park are now conducting a census of the mountain gorillas under their care (and they are blogging the results of their searches). Among the good news—the Rugendo family that we reported had been massacred in 2007 has grown from five individuals to nine.

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Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.