The Economist June 4th 2022 Science & technology 73ing moving surfaces attached to the wing
with hinges, mechanical actuators will
change the shape of a semi-flexible surface
on the trailing edge. These “morphing”
surfaces will be multifunctional, meaning
that by moving them up or down they can
be used either as ailerons (which allow the
pilot to bank and turn a plane) or as flaps
(which provide extra lift). Pop-up spoilers
that emerge from the top surface of the
wing will conversely reduce lift, and help
slow the aircraft during landing.
The wing itself will have a high aspect
ratio—in other words, a long, thin shape
that helps (at the expense of manoeuvra-
bility) to reduce aerodynamic drag. The
problem with long wingspans on passen-
ger aircraft, however, is that a plane might
not fit into the gates at airports. Hence the
idea is to fold the wing tips up once the
plane is on the ground.
Hinge and bracket
Folding wings are not a new idea. Naval air-
craft, which already have a low aspect ratio,
have long folded their wings in order to fit
into the cramped confines of aircraft-carri-
ers. And Boeing, an American firm that is
Airbus’s chief rival, is developing a new
version of its 777 aircraft, the 777X, which
will also fold its wing tips. (When unfold-
ed, these will add 3.4 metres to the length
of each wing.)
Airbus, though, is giving this idea a new
twist, by borrowing a trick from the alba-
tross. During long flights, an albatross
locks the elbow joints of its extended
wings to make them rigid. Thus fixed, they
work much like those of a glider (a type of
aircraft that has among the highest aspect
ratios of all). The bird unlocks its wings
and flaps them when it needs to manoeu-
vre or cope with gusty conditions.
The Airbus wing tips will do something
similar. When unlocked in flight, they will
be capable of flapping freely up and down
during gusts of wind or periods of turbu-
lence. In this way, says Oliver Family, who
leads the project, aerodynamic loads on
the wing will be reduced, allowing the
wing to be made lighter—which, in turn,
improves fuel economy. The flapping tips
will also help provide a smoother flight. In
addition, the project will explore the use of
sensors that could spot gusty conditions
ahead of the plane and prepare the wing
tips for flapping.
The converted Citation, which is due to
fly by 2024, will be operated remotely by a
pilot on the ground. This is a safety mea-
sure, because Airbus intends to use its test
flights to push the aircraft to its limits. The
company says it is not committed to using
these new systems in future aircraft. But if
one or other of them proves its worth, pas-
sengers gazing idly out the window of fu-
ture airliners are likely to see the wings on
their aircraft moving in unusual ways.
Conserving amphibiansFroggie went
a sportin’
C
aptivity is acushy number compared
with the rigours of the wild. No preda-
tors. Little risk of disease. And a guaran-
teed food supply that you don’t have to
work for. But that makes you soft. And if
the purpose of your captivity is eventual
reintroduction into a natural habitat, be-
cause you are a member of a rare species
that human beings would rather did not
become extinct, then having to make your
own way in the world when that moment
arrives can come as a rude awakening.
This is a problem faced by the mountain
yellow-legged frogs which are part of a cap-
tive-breeding programme run by San Diego
Zoo that is intended to boost that species’
numbers in the mountain streams of Cali-
fornia. But, as she reports in the Journal of
Applied Ecology, Talisin Hammond of the
San Diego Zoo Wildlife Alliance, the arm of
the zoo in charge of the programme, has a
plan to do something about it. She is lim-
bering up her charges prior to their release
by putting them on the aqueous equivalent
of a treadmill.
Ecologists have long understood that
animals raised in captivity can be confused
and disoriented when liberated. Breeding
programmes therefore go out of their way
to provide artificial habitats that are as nat-
uralistic as possible. Amphibian vivaria in-
tended for this purpose are thus supplied
with plants, branches, soil and water simi-
lar to those found in the wild. Yet Dr Ham-
mond and her colleagues suspected theymight need something more: a Jacuzzi.
In the wild, yellow-legged frogs live in
streams that, in the summer at least, have
powerful currents. So presumably they
have to develop the strength to swim
against these. She therefore set about in-
troducing such currents in captivity. She
took 146 one-year-old (ie, juvenile) captive-
bred frogs and 110 two-year-olds (ie, sub-
adults), tagged them all and divided each
group in two. She then put half into enclo-
sures equipped with a Jacuzzi jet that acted
as a sort of aquatic treadmill; the others,
which served as controls, had a conven-
tional filtration pump.
Five weeks later, she and her colleagues
measured the amphibians and gave them a
30-second swimming test, which they
filmed. The resulting videos recorded how
far each frog travelled per stroke of its
limbs, and also the total distance it moved
in the half minute available. Shortly there-
after, they released their charges into the
wild and monitored them for four subse-
quent months.
The results were striking. Though the
two-year-olds gained no noticeable benefit
from having an aquatic treadmill in their
enclosure, the one-year-olds definitely
did. After just five weeks in the Jacuzzi-jet
enclosures, their legs were proportionally
longer, compared with the sizes of their
bodies, than the limbs of one-year-olds
raised in the control enclosures. The one-
year-olds from the Jacuzzi-jet enclosures
also became better swimmers, travelling
more per stroke in the test, relative to their
overall body size, than the control frogs.
Crucially, one-year-olds that had had
access to the aquatic treadmill also sur-
vived better in the wild. Monthly attrition
rates were 65% for the control juveniles but
only 49% for the Jacuzzi-jet juveniles.
Though the two-year-olds had a better
overall prognosis after release than the
one-year-olds, which was expected, be-
cause they were bigger, and so better able
to look after themselves, there was no sig-
nificant difference between the attrition
rates of the Jacuzzi-jet animals (38% per
month) and the controls (40%).
These findings lead Dr Hammond and
her colleagues to suggest that there is a
critical developmental window, closed by
the time an animal is two years old, when
frogs’ anatomies can change in response to
their circumstances, and that this knowl-
edge can be used to toughen them up for
release at an appropriate moment.
Strictly speaking, the researchers
would, to prove the point, need to repeat
the experiment, having first allowed both
sets of one-year-olds to mature into sub-
adults, to see if the survival difference per-
sisted. But, on the face of things, it looks as
if a regular workout in a Jacuzzi does in-
deed improve a yellow-legged frog’s pros-
pects of making it in the real world. How to toughen up zoo-bred animals
for release into the Big Bad WorldSurvival of the fittest