Deep in the forests of central Africa roams one of the world's largest, but most elusive land animals, the forest elephant.

Few are ever seen, and no-one knows how many exist: in fact, it was only a few years ago that scientists identified them as a unique species.

But researchers are now lifting the veil on the elephants' secretive lives, and they are doing so by listening to the rumbles in the jungle.

This month, scientists have published an acoustic survey of elephant numbers in the Kakum Conservation Area in Ghana. It found around 300 elephants live in the conservation area's forests.

More important, the survey is the first to gauge elephant numbers in the wild by listening to them, instead of seeing them.

A BBC natural history documentary, Forest Elephants: Rumbles in the Jungle, to be broadcast on Thursday 4 March also reveals how these same researchers are joining with elephant expert Andrea Turkalo to listen to forest elephants roaming dense forest in the Central African Republic.

Together, under the auspices of the Elephant Listening Project, their work is allowing these endangered elephants to speak out, granting us the opportunity to understand their mysterious lives and what they need to survive.

Forest elephants (Loxodonta cyclotis) were thought to be a subspecies of the African elephant, but recent genetic research suggests they are a separate species.

They are smaller in stature than the African savannah, or bush, elephant (Loxodonta africana), standing 50cm less tall on average at a height of 2m for females and 2.4m for males.

They also sport round ears and straight, downward pointing tusks made from ivory of a more pink, highly prized hue.

That puts them at increased risk of poaching, especially across central Africa where wars have increased the availability of firearms.

But our understanding of these endangered beasts is limited in part because of the dense terrain in which they live, and also because they live in smaller groups than their larger cousin.

Elephant oasis

So scientists working for the Elephant Listening Project have developed an ingenious array of techniques to study the forest elephant.

For 20 years, Andrea Turkalo has enlisted the help of few Ba'Aka Pygmies living in the Central Africa republic to track the elephants to see where they go.

Much of her research focused on studying forest elephants as they gathered at the Dzanga Bai, an elephant oasis in the middle of the rainforest in the Central African Republic and an extremely important site for wildlife conservation.