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.
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.
This place attracts more forest animals - sitatunga, bongo, giant forest hogs, red river hogs and forest buffalo - than any other clearing in the region, and could hold the key to the future for forest elephants.
On any given day, up to 140 elephants lumber into the open.
The elephants come to these clearings, called "bais" by Central Africans, for nutrients they cannot get from the forest.
Vital salts lie in solution at the bottom of water-filled holes and the elephants are the most efficient 'miners', digging or pumping the mud to reach a water covered layer of dolerite.
Over the years, Andrea has identified over 4000 elephants, resulting in the first comprehensive data set detailing the life history and social behaviour of forest elephants.
But although bais are scattered throughout the Congo Basin, forest elephants spend less than 5% of their lives in these openings.
Most of the time they roam the rainforest in small groups.
Early warning system
So scientists from the Elephant Listening Project at Cornell University, Ithaca, US, have followed them into the forest, by deploying remote listening devices deep among the trees.
The success of this approach is now paying off: this month in the African Journal of Ecology, Drs Mya Thompson, Steven Schwager and Katharine Payne published details of the first acoustic survey of the elephants.
By using nine sensors listening to the sounds elephants make over 38 days, they estimate 294 to 350 forest elephants live in the Kakum Conservation Area of Ghana.
Crucially, that is a similar to the number of elephants estimated using dung surveys, genetic sampling and visual sightings.
But the benefit of counting elephants by the rumbles they make is that it can be done in impenetrable forest or across wetlands, where surveying by other means is near impossible.
"In addition, recordings from remote sensors capture the acoustic signals of many other species," write the scientists.
For example, the first acoustic survey of elephants also picked up the unique calls of at least two species of monkey, suggesting that the populations of other wildlife can be gauged at the same time.
It also recorded the noise of gunshots in the forest, as well as chainsaw and vehicle noise, potentially alerting wardens to the presence of poachers, loggers or traffic that might disturb the forest elephants.
'Forest Elephants; Rumbles in the Jungle' is broadcast at 2000GMT on Thursday 4 March on BBC Two, as part of the BBC Natural World documentary series.
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