Joel Sartore, who snapped this photo of an African lion, is on a quest to document as many animal species as possible, before some disappear forever.
Hyenas —
"I would say I'm one of the few portrait photographers in the country in which my subjects routinely poop and pee right on my background, right in the open," he says, offering these hyenas as an example.
Martha —
"Martha the passenger pigeon is what really got me into this in the first place," says Sartore.
Gray gibbons —
"With the gray gibbon, there's only about 28 in captivity worldwide," says Sartore. "If there's not enough in captivity to keep the genetic lines going, they can't get any more from the wild, so the zoos are going to phase out these gibbons, and that means letting it go to extinction."
