Fun_People Archive
9 Jun
The Mystery of Sticky Geckos

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Subject: The Mystery of Sticky Geckos

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Clues to mystery of sticky geckos

By Keay Davidson
EXAMINER SCIENCE WRITER
June  8, 2000

"Super Glue with legs," you might call them.

They're Tokay geckos -- big, sometimes nasty-tempered lizards that resemble
miniature dinosaurs. Their feet are almost unbelievably sticky, which allows
them to scamper across walls and ceilings with ease.

With so much ease, in fact, that scientists at Berkeley, Stanford and
elsewhere are investigating an astonishing possible explanation for their
stickiness based on invisible micro-forces of the atomic world.

If verified, the hypothesis might lead to products, such as super-sticky
adhesives for use in outer space and the ocean bottom, says UC-Berkeley
biologist Robert J. Full.

The Tokay gecko is "a large, nasty animal whose bite can be quite severe,"
says one of Full's colleagues, biologist Kellar Autumn of Lewis and Clark
College in Portland.

Its feet are marvels of evolution -- pads covered with millions of
microscopic hairs that stick to ceilings with astounding force.

In Thursday's issue of Nature, Full, Autumn and their colleagues report
the first measurements of the strength of gecko hairs, technically known
as setae (pronounced "see tee"). The Nature article's authors are Full,
Autumn and two engineers, Ron Fearing at UC- Berkeley and Thomas Kenny of
Stanford University.

"The hairs themselves are so sticky that if we were able to fit a million
or two of them onto the surface of a dime, it would be enough to lift a
small child," Autumn said in a phone interview.

That's so unbelievably strong that it might require a bizarre explanation,
the scientists say. While most other creatures might use well-understood
means -- such as Velcro-like hooks -- to clamber up trees, walls and other
surfaces, the gecko's unbelievably strong stickiness may require a more
exotic explanation, the scientists say.

Since at least the 1960s, some scientists have suggested that setae tips
are so tiny that they can exploit molecular-size forces that are normally
unavailable to living creatures.

One possibility is van der Waals forces, which are negligible on the scale
of everyday life, but are fairly strong in the short distances between
molecules. (Van der Waals forces are named for Johannes Diderik van der
Waals, a resident of the Netherlands who won the 1910 Nobel Prize in
physics.)

Of course, the van der Waals forces between a single gecko hair tip and
molecules in, say, a wall, would be extremely weak -- too weak to support
a foot-long gecko as it scampers around a ceiling like Donald O'Connor in
"Singin' in the Rain."

But add up the combined van der Waals forces of millions of hairs on a
gecko foot, and you've got, well, one really sticky gecko.

The scientists caution that the van der Waals explanation remains unproven.
But it once might have seemed crazy, and now it has gained credibility,
thanks to their measurements.

The hypothesis is feasible because the measured strength "falls right in
the middle of the range of van der Waals" forces that one would expect for
millions of gecko hairs, says Full, head of the Poly-PEDAL (Performance,
Energetics, Dynamics, Animal Locomotion) Laboratory at UC-Berkeley. While
that doesn't prove the van der Waals hypothesis beyond a doubt, it certainly
makes it a prominent contender in the gecko-hypothesizing department.

Another possibility is that a different type of inter-molecular force is
involved: electrical attraction between oppositely charged particles in
the hair tips.

Whatever its cause, the gecko's stickiness is a dramatic example of Mother
Nature's creativity and cunning.

Evolutionary biologists have long marveled at how nature continually finds
ecological niches for animals to exploit, then gives them amazing tools
with which to exploit them. Consider bacteria, which orient themselves in
liquids by detecting Earth's magnetic field.

So when humans invented buildings with ceilings, geckos quickly recognized
the ceilings as an unexploited ecological niche -- blissfully free of
predators and deliciously crowded with edibles, such as bugs. As
energetically as pioneers entering the Old West, geckos entered the world
of walls and ceilings thanks to sticky powers developed over millions of
years for clambering up less arduous structures, such as trees.

The great evolutionary biologist Charles Darwin argued that life forms
evolved when nature eliminated those that were less fit. The stickiest
geckos were presumably more likely to survive than those that kept slipping
off trees (perhaps into some predator's jaws).

That's especially true in tropical lands where hurricanes are common, Autumn
says.

"A lot of geckos live in areas prone to hurricanes," Autumn says. "Perhaps
their (stickiness) prevents them from being blown away."

Geckos -- there are about 850 types, ranging in size from a few inches to
a few feet -- aren't the only creatures blessed with super-stickiness; so
are insects called kissing bugs.

And that, say the scientists, illustrates another one of Mother Nature's
traits: When she likes a particular biological function, she invents it
again and again in unrelated life forms.

The most celebrated example of such convergent evolution is the eye, which
was independently evolved in numerous unrelated life forms.


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© 2000 Peter Langston