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Every 10 years, America takes a census, but counting heads is also important to scientists who are interested in endangered species. Now, big as they are, few animals are more difficult to count than elephants. They move around a lot and they can be surprisingly hard to track. NPR's Gregory Warner talked to a former elephant polltaker to figure out how it's done.

GREGORY WARNER, BYLINE: Imagine you're flying in a two-seater plane over Africa, and your job is to count all the elephants you see. Over the savannah, or the grassland, that's easy. Just look down. But how do you peer into the forests where all you see is treetops? For years, zoologists who tried to do this just guessed, but in the late '80s, a man by the name of Richard Barnes devised a method to take an elephant census in the densest of forests. And to do this he recruited young scientists. Winnie Kiiru had just finished grad school in biology; she'd just returned to her native Kenya, and what she was told was to tie a piece of twine around her waist and to walk through the forest in a straight line.

WINNIE KIIRU: And you'd walk in a straight line. It didn't matter whether there was a river, a ravine, stinging nettles, buffalo, whatever. You just kept going.

WARNER: Just to her north, just to her south, are other young scientists also trailing their twine through the brush, literally slicing the forest into samples. And what they're supposed to count is not elephants, because elephants move around a lot and they tend to hide. They are on the lookout for evidence that is more settled.

KIIRU: Just by checking their poop. (Laughing) That was...

WARNER: Checking their poop?

KIIRU: Yes, just checking their poop out and counting it.

WARNER: The poop served as a kind of clock, a way of tracking the elephants in space and in time, allowing Barnes to build a model of elephants' numbers and their movement through the forest.

KIIRU: You'd then now figure out how old the dung pile was. So, if it is completely fresh, then you give it an A. (Laughing) If it was slightly more decayed, then you would give it a B. Then you'd give it a C if you could actually see the dung beetles already, because dung beetles, basically, what they do is they spread out the dung, so you'd see this one maybe has been there a little longer. So we graded the dung piles according to age.

WARNER: Not only age, but other factors like how much sunlight is there that could make the dung decay faster or how much vegetation might be hiding other dung on the forest floor that she could not see.

KIIRU: It was so exciting because here were these huge animals - a lot of times you walked through the forest and you didn't see them. You smelled them, but you didn't see them. And yet, you were so powerful you could actually count them or get a sense of how many they are.

WARNER: Think how exciting this was. Scientists since the time of van Leeuwenhoek in the 17th century have been looking into lenses to gaze at microscopic organisms, and yet up until this moment, near the end of the 20th century, something as proverbially unmissable as an elephant was for all intents and purposes invisible in its natural habitat.

KIIRU: In the forest, you don't see them.

WARNER: So did you feel like you, even though you haven't seen them, that you had seen them?

KIIRU: They were there. You knew they were there. Sometimes the dung was steaming. You knew that the elephant that dropped that poop was not very far.

WARNER: Winnie Kiiru is no longer walking through the forest herself, but other young scientists are. And those surveys have found that just in the past decade, 62 percent of Africa's forest elephants have died. Kiiru is now a doctor of biology and director of the group Conservation Kenya, trying to save the elephants that are left. But she still gets goosebumps remembering her days as a polltaker in the forest.

KIIRU: And then sometimes you're walking through this really thick forest and you know what? You step one step out and you're in this beautiful glade, like "Sound of Music" type of thing, like you have to stop and dance. (Laughing)

WARNER: Which song from the "Sound of Music" are you dancing to?

KIIRU: (Singing) Girls in white dresses with blue satin sashes.

(SOUNDBITE OF SONG, "MY FAVORITE THINGS")

JULIE ANDREWS: (Singing) Snowflakes that stay on my nose and eyelashes, silver white winters that melt into spring, these are few of my favorite things....

KIIRU: (Laughing) It's not very African, is it?

WARNER: Gregory Arthur Warner, NPR News, Nairobi.

(SOUNDBITE OF MUSIC)

SIMON: And tomorrow on WEEKEND EDITION, we trace a black market that stretches from Africa to Asia that trades an item as valuable as gold. It's the illegal trade of rhino horn and how it could lead to the extinction of one of Africa's iconic animals. That's tomorrow on WEEKEND EDITION.

(SOUNDBITE OF MUSIC)

SIMON: And you're listening to WEEKEND EDITION from NPR News.

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