If an insect falls from a plant it is rarely damaged as it
has a low momentum and its body creates considerable friction drag as it falls.
The force of the impact is reduced as a result and the insect then usually
quickly scurries away.
Some larger animals are also adept at surviving falls and
the offspring of tree-nesting ducks drop to the ground, being cushioned by leaf
litter, by their down-covered bodies and by some flapping of their tiny wings.
[1] Adult birds have little problem as they can fly down and then use sweeping
movements of their wings to produce “reverse thrust” and thus decelerate gently,
using the downward displacement of air to effect an easy landing.
Cats leap from trees, fences and walls, and their landing is
cushioned by the shock-absorbing properties of the limbs, but surviving a fall
from considerable height is much less likely, as maximum acceleration then
results in high momentum. In arboreal mammals, swinging from the arms is a
common feature and, together with the grip of the hands (and feet - and even
tail in some monkeys) ensures that catastrophic falls are avoided. Yet some
animals are able to leap from high in one tree to the base of a nearby tree
while suffering no damage, thus moving more rapidly from one location to
another than could be achieved over the ground. Two well-known examples of
these “fliers” are tree frogs (in the genus Rhacophorus)
and flying squirrels (genus Glaucomys).
Tree frogs spend their adult life in forest canopies, but,
being amphibians, they must find water in which to breed. Some species use small
rain pools in the axils of leaves or in tree holes, while the females of some Rhacophorus build nests of foam attached to
tree branches overhanging ponds, the nest being created by rapid movements of the hind limbs (akin
to whisking) in secretions made by the frog. Eggs are laid within this mass and
these are fertilised by male frogs, hatching tadpoles then emerging into the
foam and dropping into the water below to complete larval life. Froglets leave
the pond and then climb adjacent trees, remaining in the canopy for the rest of
their lives. Although they have pads on their toes to provide excellent
adhesion to surfaces, adult frogs retain the webbed feet of the ancestral forms,
even though they are not used for swimming. The webs are especially
well-developed in some species and they act as parachutes to slow down the
descent when frogs move from one tree to another, or from one branch to a lower
one.
Flying squirrels also use parachutes and these are formed
from loose skin (termed a patagium), that runs between the fore and hind limbs, and between the fore limbs and the head.
I was fortunate to be able to watch Glaucomys
parachuting when visiting Dr Joe Merritt at the Powdermill Biological Station
of the Carnegie Museum of Natural History in Pennsylvania. Dr Merritt had been studying
these mammals for several years and laid out live traps so that we could then
see the squirrels close up. Traps containing animals were collected and each
trap was emptied into a cloth bag. In the photograph below you’ll see me (with
Dr Merritt on the right and the late Prof. Bjรถrn Malmqvist on the left) at the moment when a captured squirrel
bit me on the hand. I was told to take a firm grip of the skin over the neck of
the animal, but it was still able to turn its head easily to defend itself from
such unpleasantness. It was a lesson for me in just how much skin that flying
squirrels possess. I let go immediately and the squirrel ran up a nearby tree,
and then made a wonderful gliding flight before climbing rapidly, making
another flight and then disappearing. The parachute of loose skin was very
effective in slowing its descent, the squirrel covering tens of metres and with
good directional control provided by the tail and by changing the profile of the
patagium. It was most impressive and, in one way, I’m pleased that I was
bitten.
I was reminded of this incident when reading a quote from Jeb
Corliss in The Independent:
At the beginning, there were probably
only very few squirrels that even contemplated flying from tree to tree. The
other squirrels thought they were crazy. I imagine that hundreds of them died
in the attempt. But then, in the end, one of them managed it. Now that, to me,
is evolution. And now we are evolving, through technology and through skill. I
liken what we’re doing in proximity flying to the first animals that left the
water. We are evolving and growing. And becoming stronger. What else is the purpose
of life? [2]
Not quite the way I would express the likely evolution of
parachuting in flying squirrels, but Jeb Corliss is an expert wingsuit flyer,
not a Biologist. Of course, it is impossible for humans to control their
descent to the land without an external parachute and the earliest examples
have been transformed into steerable ‘chutes that enable precise landings. As
Jeb points out, wingsuit flying has close similarities to the flight of Glaucomys, with webbing between the
arms, legs and body, analogous to the patagium of flying squirrels, providing
steerable flight. Proximity flying capitalises on this level of control to
allow fliers to pass very close to objects, or the ground, while making their descent
(see the video clip at the end of this post).
Whereas Rhacophorus
and Glaucomys have both evolved
changed body forms to enable them to move from tree to tree for various reasons,
human use of wingsuits is solely for pleasure. The excitement comes from
exposure to danger, a sense of freedom, and the thrill of depending on a skill
where a small mistake can have disastrous results. Some of us are drawn to such
activities and proximity flying is addictive, even though the number of fatalities
is large relative to the number of those who fly. Wingsuit fliers, and others
involved in the most extreme sports, are only too well aware of the dangers and
most are not afraid of death, recognising that it is possible that flights
can go very wrong, even after meticulous preparation. They feel very alive as a
result, and their approach to death contrasts markedly with the fear of life ending that seems
to haunt others within the human population - and which is the basis of many
religions. While animals such as Rhacophorus and Glaucomys cannot be aware of danger in the same way as humans, I wonder
if they get a thrill from flying?
[1] http://www.bbc.co.uk/learningzone/clips/mandarin-ducklings-jump-from-their-tree-top-nest/10473.html
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