Koalas are shy creatures, which makes them difficult to count. And they're also quite hard to be spotted, as they silently sleep up in the top of the trees for 20 hours every day. But after a noisy camping trip was interrupted by the bellows of these bashful animals, Dr. Brad Law had a new idea. Now he records their distinctive mating calls, and listens to them by choice. In this way, the density of population can be tracked.