Fruit bats: Africa’s greatest mammal migration

Forget the wildebeest of the Serengeti – Africa’s greatest mammal migration involves 8 million animals and takes place in the air. It is Zambia’s secret bat spectacle.

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African bat migration article spread

Forget the wildebeest of the Serengeti – Africa’s greatest mammal migration involves 8 million animals and takes place in the air. This is Zambia’s secret bat spectacle.

It’s just before dawn in Kasanka National Park, Zambia, and the forest is about to get a rude awakening – a vast host of straw-coloured fruit bats is heading its way. This gathering may comprise the greatest concentration of mammalian biomass in Africa, if not the world, yet only a lucky few have witnessed it.

Hoping to be one of them, I walk through the forest under cover of darkness. I can barely see my feet, let alone the snakes and crocodiles that patrol the forest floor. Finally, as the sun streams over the horizon, I climb a ladder into the canopy.

Soon, the spectacle begins. Whirling and tumbling, the bats arrive in bewildering numbers, shrieking and colliding as they return to roost in the trees. The animals migrate here every November and December to gorge on Kasanka’s seasonal abundance of fruit, but no one knows where they’ve come from or where they go afterwards.

Incredible journey

Straw-coloured fruit bats are a migratory species that live in colonies thousands or even millions strong on the edges of forests, towns and cities. Their range encompasses the tropical belt of Africa, and populations exist from Sudan south to Zambia. The species even reaches Nigeria and the Ivory Coast (well, their hotel menus at least…).

As I watch, the rising sun illuminates the bats’ wings, revealing networks of blood vessels. These supply the muscles that hold the skin membranes taut. With wingspans of up to 80cm – the broadest of any African bat – these creatures are an impressive sight. Squadron upon squadron flies in, spiralling around the trees before landing to roost.

But resting is dangerous. The bats are frequently attacked by birds of prey, such as fish eagles, while crocodiles snap up any unfortunate individuals that fall to the forest floor. The bats take no chances. If they spy a predator circling above, a squadron will mob it until it retreats. This activity continues until the sun has risen, when the bats finally settle to sleep.

Why migrate?

Biologist Heidi Richter from the University of Florida is determined to find out where the bats go after Kasanka, and why they come here in the first place. Through her research, she has discovered that several of the females are expecting. Some will have only recently conceived, but others are nearly full-term. “They must have migrated while heavily pregnant – a high-risk strategy because of the energy demands. There must essential nutrients at Kasanka to make such a trip worthwhile,” she explains.

Indeed, each November sees a ‘Big Bang’ of fruit production in the park, when hordes of hungry bats consume up to twice their bodyweight every night. Such massive consumption has a huge impact on the habitat, and bats are responsible for at least 60 per cent of the seed dispersal of Africa’s rainforest trees, including economically important timber and crops.

A mystery solved

But where do the bats go once the glut is over? In the 1980s, DW Thomas of Aberdeen University used sightings to estimate that these mammals travelled some 1,500km during the course of a year. But when Heidi monitored their movements, she discovered this to be a major underestimation.

She fitted four individuals with satellite-tracking collars, then plotted their movements on a map. She found that after their departure in December, her study bats had covered a whopping 1,000km in just one month. “They disappeared off the radar somewhere over the Democratic Republic of Congo (DRC),” says Heidi. “The collars could have been lost or damaged, or the bats eaten.”

However, previous data showed that one particular bat, named Hercules, had travelled 1,900km in six months. The DRC may not even have been his final destination – with a return trip to Kasanka later in the year, Hercules would have made a round trip of at least 3,800km, a staggering distance. The full route will not be known until more tracking studies are carried out.

For now, I’m content to enjoy the memory of the drama. As I retrace my steps out of the forest and leave the colony to settle, I think about the 2,500 tonnes of bat in the branches above me, and wonder which part of the continent they’ll head for next. 

 

Glimpse the spectacle of migrating fruit bats here

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