50 YEARS AGO
in old home
Flying forward is hard enough, but flying
nowhere, just hovering, is so much harder.
Most bats and birds can manage the feat
for only a few frantic seconds.
Hovering means losing an aerodynamic
shortcut, says aerospace engineer and
biologist David Lentink. As a bat or bird
flies forward, its body movement sends
air flowing around the wings, which pro-
vides some cheap lift. For animals on the
scale of bats and birds, that’s a big help.
Without that boost, “you’re going to have
to move all the air over your wings by
moving it with your wings,” says Lentink,
of Stanford University. The power that’s
needed to stay in place by flapping wings
back and forth like a hummingbird “is
So how do nectar-sipping vertebrates,
for whom a lot of energy-sucking hovering
is part of life, manage? For the first direct
measurements of the wingbeat forces that
make hovering possible, Lentink’s Ph.D.
student Rivers Ingersoll spent three years
creating a flight chamber with exquisitely
responsive sensors in the floor and ceiling.
As a bird or bat hovers inside, the sensors
can accurately measure — every two-
hundredths of a second — tremors even
smaller than a nanometer caused by air
from fluttering wings.
UPDATE: The commission
said underground nuclear
testing would not harm
the otters, but the fears of
conservationists were well-founded: A test in 1971 killed
more than 900 otters on the
Aleutian island. Some otters
remained around Amchitka,
but 602 otters were relocated
in 1965–1972 to Oregon,
southeast Alaska, Washington
and British Columbia — areas
where hunting had wiped
them out. All but the Oregon
population thrived, and today
more than 25,000 otters live
near the coastal shores where
once they were extinct. “They
were sitting on the precipice,”
says James Bodkin, who is a
coastal ecologist at the U.S.
Geological Survey. “It’s been a
great conservation story.”
Excerpt from the
November 9, 1968
issue of Science News
A long-billed hermit
some help hovering by
twisting its wings
on the upstroke.
I T’S ALIVE
How nectar bats
When the [Atomic Energy
Commission] first cast its eye
on the island of Amchitka as
a possible site for the testing of underground nuclear
explosions, howls of anguish
went up; the island is part
of the Aleutians National
Wildlife Refuge, created to
preserve the colonies of nesting birds and some 2,500 sea
otters that live there….
Once the delicate techno-marvel of an
instrument was perfected, the researchers
packed it into 11 shipping cases and sent it
more than 6,000 kilometers to the wilds of
“Very difficult,” Ingersoll says. The Las
Cruces Biological Station is great for field
biology, but it’s nothing like a Stanford
engineering lab. Every car turning into the
station’s driveway set off the wingbeat sensors. Even the thick-walled room that held
the machine warmed up enough each day
to give the instrument a fever.
Babying the instrument as best he could,
Ingersoll made direct measurements for
17 hovering species of hummingbirds and
three species of bats, including Pallas’s
long-tongued bats (Glossophaga soricina).
“Their up-pointy noses made me think of
rhino faces,” he says.
Pallas’s bats specialize in nectar sipping
much as hummingbirds do. Comparing
wingbeats, bat vs. bird, revealed differences, though. Hummers coupled powerful
downstrokes and recovery upstrokes that
twist part of the wings almost backward.
The twist supplied about a quarter of the
energy it takes to keep a bird aloft, the
researchers reported in the September
Science Advances. The two kinds of nectar
bats got a little more lift from the upstroke
than did a bat that eats fruit instead of
strenuously hovering for nectar. Yet even
the specialist nectar bats relied mostly
on downstrokes: powerful, deeply angled
downstrokes of really big wings.
Those bat wings span proportionally
more area than hummer wings. So the bats
get about the same hovering power per
gram of body weight that hummingbirds
do. Supersizing can have its own kind of
design elegance. — Susan Milius
A Pallas’s long-tongued bat hovers
while sipping nectar.
Its skinny tongue is
tipped with nectar-catching microfingers.