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Thursday, Oct. 4, 2001
Marveling at mammalian masters of flight
By MARK BRAZIL
I have dreamed of flying since childhood, and perhaps that is why I am obsessed with flying creatures. As ground-hugging humans, we readily identify with our fellow terrestrial mammals, assuming, easily enough, that being earthbound is a natural state for life on earth. But, think again. Even among the plants, the most obviously rooted species on Earth, an astonishing number not only reach for the skies and life-giving light, but also have airborne stages.
They reach for wind currents that carry their minute pollen grains aloft. They reach for gusts and billows of wind that scatter and blow their winged seeds for tens of metres in the case of maple trees, or for hundreds of kilometers in the case of the feathery seeds of the willow.
Many other species are more indirectly aerial, relying on insects to carry their pollen aloft for them.
Insects, of course, make up the bulk of species on Earth, and among them flight is the norm, with the flying varieties numbering in the millions. Among birds, those 10,000 or so species of diminutive dinosaurian derivatives, flight is the expected form of locomotion, though there are some spectacularly flightless ones such as the ostrich and kiwi.
It is only when we consider the far scarcer mammals, amphibians and reptiles that we find flight is rare. But then even the occasional snake, frog and lizard has adopted gliding flight as an effective means of movement among tropical forest habitats.
As for the mammals, including our closest relatives, the other primates, all are terrestrial or, at best, arboreal. Even the most spectacularly brachiating gibbons hurtling through the canopy of the Southeast Asian rain forest are only very momentarily airborne, though what a thrill that seems to bring them -- and in dreams of flight that is what I imagine most strongly.
There are only about 4,000 species of mammals on Earth right now, and though we lean toward considering only the larger of these, in fact, the majority of them are small. One large group consists of terrestrial, even subterranean rodents, while another collection of small creatures is that surprising group that has adopted flight every bit as effectively as birds -- the bats. In fact, of all the known mammals, one in five is a bat; so why do we know so little about them?
Prejudice and misinformation are rife about this group, dating back to early assumptions that all small mammals were mice of one sort or another, and bats were therefore "flittermice."
Their nocturnal habits, enormously overblown associations with blood-drinking, and the astonishing and irrational fear of them becoming entangled in our hair, have contributed to emotions in humans ranging from hatred to panic -- hardly conducive to encouraging a general interest in them or a pleasure in observing them.
However, bats are astonishing and fascinating animals. Here in Japan, there is a wide range of species, albeit poorly known, from tropical bats in the southernmost islands to northern cool-temperate species in Hokkaido, their habitats spanning the whole of Northern Asia to Europe. Physiologically, bats are not merely equipped for flight. Many of them are also capable of, or dependent on, hibernation for their survival. All feed on high-energy foods and most eat insects.
To see them leaving their tropical roosting sites as dusk falls is an astounding experience. I recall one amazing evening at a forest reserve close to Kuala Lumpur, where the day shift of swifts and swiftlets, those birds that glean diurnal insects over the tropical canopy, came hurtling back in clouds, just as a thrumming murmuration began underground.
The deep whirring of the wings of thousands of bats was surprising. At first, in a wisp of darkness like a rising coil of smoke, they began to issue from their underground roost. As the moments passed, the wispy coil billowed into a roiling dark cloud of leathery-winged creatures. A few minutes later, they were gone -- in search of the night shift of insects.
With their evolutionary origins in the tropics, bats have used the benefit of flight to accomplish feats of dispersal usually only possible for birds. For example, two ancestral species of bats were the only nonmarine mammals to colonize New Zealand naturally, while North American species have colonized the remote Hawaiian Islands.
Nevertheless, their anatomical adaptations for flight are derived from the familiar mammalian skeletal structure. Were you to dissect a bat, you would find that the bones of its wings are essentially similar to those of our own hands and forearms. The membrane that spans their wings and connects to their hind limbs is living skin, both flexible and elastic like ours.
In Botswana, I watched, entranced, a small colony of fruit bats clustered beneath the thatched roof of a game lodge. They seemed to be furled in black, leathery capes as they hung, with only their noses visible, from their hooked toes among the reeds. Then, in a moment, their "flying fox" faces were revealed along with their brown body-fur as their skin capes stretched into the finely tuned flight membrane capable of supporting them in their nocturnal hunt for flowering and fruit-bearing trees. But you don't need to travel to Malaysia or Botswana to encounter bats. Wander along a riverbank or a lakeside path in Japan in the evening, and you are quite likely to sense the ultrasonic aerial navigation calls of bats as they patrol over water, hunting for insects.
Their highly honed echolocation involves bouncing beams of high-pitched sounds off their mobile prey, rather like certain dolphins. However, if you visit the warm islands of Okinawa or Ishigaki, you may encounter that other group of bats known as the fruit bats. These are confined to the warmest parts of the world and include the largest of all the bats.
On first arriving in Katmandu, I was astonished by the enormous, dark fruit clusters in a number of large trees visible from my hotel, when my binoculars revealed that they were, in fact, enormous fruit bats at their daytime roost. I was convinced that they had to be the largest bats in the world.
Only years later did I encounter the true giant fruit bats of Indonesia. From a tiny mangrove forest-covered islet off Komodo, I witnessed the afternoon emergence of the fruit bats; their wingspans were in the region of 1.5 to 1.7 meters -- easily as large as that of an eagle.
But the bat family includes not only insectivores and frugivores. One species in northern South America fishes at night with its long gafflike claws trawling the surface of ponds and streams, while another species from that region has a name that tells it all -- the frog-eating bat.