by Paul Racey
In late October each year, one of the great wildlife wonders of the world takes place in southern Africa. Straw-colored fruit bats (Eidolon helvum) begin to arrive in Kasanka National Park in Northern Zambia. The bats' numbers build over the following month until, by the last week of November, as many as 8 million of them are roosting in barely two and a half acres (one hectare) of evergreen swamp forest.
Fruit bats generally space themselves about a wingspan apart when roosting, but their roosting behavior at Kasanka is distinctive because they look from a distance like swarms of honey bees hanging from the boles and branches of the trees in dense clusters. So heavy are these clusters that tree branches frequently break under the weight and some of the bats fall to the ground, become injured and are eaten by crocodiles, monitor lizards, black mambas, pythons, civets and the occasional leopard.
The evening dispersal of these bats is one of the great wildlife wonders of the world - an amazing and beautiful spectacle set against the setting sun. The air is filled with bats dispersing in every direction around the roost for half an hour. The clouds of bats attract aerial predators: eagles, hawks, vultures and kites.
Bat numbers peak at the end of November, but by the end of December or early January, they have disappeared from the region.
So why do so many bats converge on such a small area? These are migratory bats that seasonally travel up to 600 miles (1,000 kilometers) to roosting sites.
I have been in many tropical forests, but never one with such a synchronized abundance of fruit. The main fruits are wild loquat, water berry and red milkwood. The trees are laden with these fruits and bats can clearly be seen feeding in the headlights of our vehicle after the evening dispersal. In the roost itself, bat droppings indicated the bats had been eating the purple-skinned water berry.
The importance of all that became clear when we examined the bats: The females were either pregnant or actually carrying newborn young. Pregnant females almost certainly are traveling to Kasanka, probably over great distances, because the abundant fruits support the increased energy demands of pregnancy and lactation.
Paul Racey, a BCI Scientific Advisor, is Regius Professor of Natural at the University of Aberdeen, Scotland.
BCI members can read the whole story of the remarkable Kasanka bat colony, and the BCI Ecotour to the site, in the Spring 2004 issue of BATS magazine.
