Deep in a remote corner of Zambia, a forest stirs with activity. A huge number of animals are coming to the end of an epic migration. Few have witnessed this annual event, yet it may well comprise the greatest concentration of mammalian biomass on the continent, if not the world.
I approach under the cover of darkness. Changwe, my Zambian guide asks to keep the headlamp switched off to avoid disturbing the animals. With no lamp and no moonlight I can barely see my legs, nevermind the mambas or crocodiles that roam the forest floor. My feet crunch into a deep leaf litter with every blind step, and I frequently stumble over fallen logs or bash my head on branches.
As the sun streams over the horizon, I take position high in the canopy after clambering up a rickety ladder of branches and timber.
Whirling and tumbling on every side, the animals arrive in bewildering numbers, shrieking and colliding as they return from a night feeding. They are straw-coloured fruit bats Eidolon helvum and about eight million of them roost in the skeletal trees found in this corner of Kasanka National Park, on the border with the Democratic Republic of Congo. They arrive every November and December to gorge themselves on the abundance of fruit in the Park – but the mystery surrounding exactly where they’ve come from and where they’re going is gaining them a reputation as Africa’s best-kept wildlife secret.
Straw-coloured fruit bats are a migratory species living in large, conspicuous colonies on the edge of forests, towns and cities with a their range covering the tropical belt of Africa. Colonies exist in Sudan south into Zambia through Uganda, Tanzania, western Kenya and the Democratic Republic of Congo, with hundreds of thousands to millions of bats. The species even reaches the Nigeria and Ivory Coast – well, it turns up on the menus of hotels there at least.
What makes the Kasanka colony so different is the record breaking scale and the mystery that surrounds their movements. Every night from the compact roost bats pour out across the sky in every direction across a radius of 40 miles or more.
The dawning African sun illuminates the bats' intricate wings showing up a network of blood vessels supplying the muscles that hold the skin membranes taught between elongated fingers. With a wingspan of up to 80cm – the broadest wingspan of any African bat, they make an impressive sight as squadron after squadron of bats returns from foraging and spirals around the roost. Landing is less graceful; each bat flies up to a branch and grabs it with its feet before falling headlong onto a bundle of leathery wings. It finally ends up hanging upside down.
Bat knees are turned around 180o from most mammals, the hinge bending the opposite direction compared to our own, to help them navigate in flight using their tail membrane (a flap of skin stretched between their feet). This adaptation means they must hang upside down, a special tendon means they can do so without effort.
As I watch, a passing fish eagle, spotted by hundreds of attentive eyes in the cluster, causes a flurry of activity, which settles down after the culprit is chased away by a mob of bats. Many birds of prey attack the bats here and even crocodiles predate on individuals that fall to the forest floor. This pattern is repeated until the sun is high above the horizon and the bats settle. The intense activity eventually strips the trees bare and kills them.
Bat biologist Heidi Richter, from the University of Florida, first saw the nightly migrations when she was a Peace Corps volunteer in the nearby town of Serenje returning to complete a masters degree and now for her PhD tracking where the bats spend the other 10 months of the year when they leave the park. Her first question was why do they come here in the first place?
“Bats caught from the roost showed females at different stages of pregnancy which is a high risk strategy because of the dangers related to migration- the greater energy and nutritional demands of carrying offspring, unless there is a large reward,” she explains, “While mating and gene exchange are benefits of the gathering, the risk of migrating during pregnancy suggested the bats find specific nutrients at Kasanka.” Bat reproduction is triggered by environmental cues such as the start of the rains or the abundance of food. The variation in natal development suggests that colonies from different geographical locations had come together at Kasanka.
Heidi describes a "Big Bang" of fruit production in the park during November acting like a bat magnet. Kasanka means ‘place where people come to harvest’ and man has historically traveled here to enjoy its abundant meat, fish and fruit.
The synchronous fruiting of wild loquat, red milkwood and water berry enables the millions of bats to feed, consuming up to twice their body weight- an estimated 5000 tonnes of fruit every night! And that is just the straw coloured fruit bat; there are seven other species of fruit bat in the park, of the 24 bat species recorded here.
Two major groups of bat exist in the order of Chiroptera- meaning hand wing. Microchiroptera are insectivorous and echolocating bats while megachiroptera use visual and oral cues eating mainly fruit. Megachiroptera, as the name suggests are generally larger than their micro relatives.
Bats hold an extraordinary importance for the future of African forests responsible for at least 60% of seed dispersal of tropical rain forest trees including economically important timber and cash crops.
In West Africa, one study showed some 300 million seeds of the valuable hardwood the Iroko were spread over hundreds of square miles in just one night by straw coloured fruit bats. They also disperse cashew, mango and figs among the 300 other bat dependant species. The economic and environmental impact of bats is only now becoming clear.
The Straw Coloured Fruit bat is master of migration. Previous studies showed the bats would travel some 900 miles during the year but new research from Kasanka using satellite tracked bats show this is an underestimates its abilities.
Using a specially designed collar attached to their neck, complete with solar panel and rechargeable battery, the device had to be lightweight so as not to hinder flight or expose the bat to predation. Four male bats were chosen as their larger size (around 300g) can cope with the added weight of the 12g transmitter.
Moving up to 60 miles in a night to feed around the reserve, one tagged bat was tracked for around 600 miles in just one month after leaving Kasanka in December.
“They disappeared somewhere over the Democratic Republic of Congo- either there was not enough sun for the charge, they chewed the collar off or they were chewed themselves”.
Not before data showed a final distance of 1200 miles over 6 months for the aptly named bat ‘Hercules’. The dispersal spread the males to different locations across a radius of 360 miles. If this was the final destination, and there is no reason to suggest that they will not go on further, would amount to a round trip of 2400 miles.
The full route of this migration, or migrations, will not be fully known until more bats are tagged and tracked in the future.
“I like the mystery,” says Heidi, “there is still so much to be discovered and I am sure further research will only confirm the importance of this natural across Africa”.
South Luangwa National Park, one hour's flight away, may have the “Big Five” and an international reputation but Kasanka, a smaller gem at a mere 350km2, has a nightly spectacle to rival any in the natural world. By day the sight is no less dramatic.
As we retrace our steps out of the forest leaving the colony to settle in the warm sun, I imagine 500 elephants, the equivalent of 2500 tonnes of bat, spread out in the branches above me. The thought is sobering. Walking under a tree holding one elephant would seem an unwise thing to do but Changwe reassures me he has never been hit by a falling bat. Or a falling elephant for that matter.
Published in The Herald Magazine and BBC Wildlife magazine.
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