Squareheads
Question: Why do hammerhead sharks have hammerheads?
Answer: The reason hammerhead sharks and their cousins the bonnetheads have
such strangely shaped heads is unknown, but several suggestions have been put
forward as to why they have evolved in this way.
First, it is possible that the 鈥渨ings鈥� that jut out on either side of the
shark鈥檚 head to create the hammer shape act as hydrofoils, aiding the shark鈥檚
movement and adding extra lift. This enables the shark to bank and make rapid
vertical movements more quickly. Secondly, the eyes of a hammerhead shark are at
either end of these wings and this could enhance the shark鈥檚 binocular vision,
allowing it to gauge distance more accurately.
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Another possibility takes into account the fact that, because the head is
widened, the ampullae of Lorenzini, the organs in a shark鈥檚 snout that it uses
to detect electrical signals generated by its prey, are spread out over a
greater area, possibly giving improved reception.
Finally, the nostrils of the shark are also further apart on hammerheads, so
the shark can sample a greater volume of water as it swims. This improves its
ability to smell over a larger area and range.
Matthew Oliver
Sunderland, Tyne and Wear
Answer: An interesting observation on two species of hammerhead shark,
Sphyrna tiburo and Sphyrna zygaena, was made in 1977 by Thomson
and Simanek in American Zoologist (vol 17, p 350). S. tiburo
has the larger head area and a smaller pectoral fin area, but the combined area
of head and pectoral fins is virtually the same in the two species. That fact,
in the opinion of the authors, confirms the importance of both the hammer and
pectoral fins as planing surfaces.
There are also reports that hammerhead sharks use their heads to 鈥渉ammer鈥� and
pin down stingrays, their favourite food.
Maria Bennett
University of Aberdeen
Crawling along
Question: How does the up-and-down kick of the legs that that is used in the
front crawl (freestyle) stroke propel the swimmer?
When swimmers practise this technique using a float to support their face and
arms, their progress along the pool is very slow. Why not use the more powerful
breaststroke kick instead?
Answer: The answer to this question is that the up-and-down leg kick used in
the front crawl stroke is not intended to propel the swimmer in the direction of
travel. Instead, the leg kick propels the swimmer鈥檚 body upwards. This is
essential if it is to remain parallel to the water鈥檚 surface, an important
condition for several reasons.
First, the propulsive force of the hands is naturally parallel to the
body鈥攊f the swimmer鈥檚 body is maintaining an angle to the water鈥檚 surface,
a substantial component of this propulsive force will no longer be in the
direction of travel and so will be lost.
Secondly, a swimmer parallel to the water鈥檚 surface only needs to push his
head and shoulders through the water (neglecting wave drag and similar factors).
By contrast, the 鈥渄iagonal 鈥� swimmer will be pushing his entire body through the
water鈥攖his is all but equal to walking through the pool. In practice,
these effects are so significant that the reader will find the execution of a
front crawl without leg kicks close to impossible.
Fabian Faltin
Vienna, Austria
Metal detector
Question: A member of my family is allergic to aluminium, and we have been
advised by his doctor to remove aluminium cooking utensils from our kitchen.
We have tried using a magnet to help us determine the difference between
stainless steel and aluminium, but it doesn鈥檛 stick to some items that we know
to be steel. Is there another way of finding out which items are aluminium and
which are stainless steel?
Answer: There are three main groups of stainless steel alloy: anstenitic,
ferritic and surgical (martensitic) steels.
Ferritic and surgical steels normally contain around 12 per cent chromium and
are magnetic: they can be distinguished from aluminium by a magnet.
Anstenitic stainless steel normally contains around 18 per cent chromium and
8 per cent nickel and is nonmagnetic. The sure way to decide between aluminium
and anstenitic stainless steel would be, in the manner of Archimedes, to measure
the density of the metal鈥攕teel is more than twice as dense as
aluminium.
In practice, the colour of the metal is quite a good guide: aluminium has a
greyish tinge whereas anstenitic stainless steel is bluish.
E. Billett
Brunel University, Middlesex
Answer: Aluminium alloys corrode vigorously on contact with caustic liquids,
while stainless steels remain unattacked at room temperature. Your correspondent
should be able to pick out the aluminium in his kitchen by putting a drop of
liquid lye or a drop of an oven cleaner containing a caustic substance on a
polished surface of the utensils to be tested. If they are aluminium, the spot
will be heavily etched in one or two minutes. If the spot stays bright your
utensil is made of stainless steel.
Dale McIntyre
Department of Materials Science
Cambridge University
This week鈥檚 questions
Irrational breakdown: Why is it that only micro-organisms can break down
things like cellulose, lignin and beeswax? Presumably it is just a question of
producing the right enzymes鈥攜et humans can鈥檛 do it, and even cows rely on
bacteria in their gut to digest their plant food for them. What a colossal
selective advantage it would offer, so why has evolution so far excluded it?
Martin Gregory
Katmandu, Nepal
Broth bother: Whenever I have a thick soup, or have made any kind of starchy
paste, I have noticed that if I give it a good swirl and then take out my spoon
something strange happens. For a while the liquid continues to revolve in the
direction of my stirring, but just before it slows and comes to a halt, its
direction reverses momentarily and it spins back the other way for a second. How
and why do such liquids do this?
Robbie Dickson
Newbury, Berkshire