Head start
Question: When pigeons walk they move their heads rhythmically and
horizontally a considerable distance, I would estimate around 5 centimetres back
and forth. Is there a biological advantage associated with this movement and, if
not, why do they do it?
Answer: A pigeon鈥檚 head does not move backwards and forwards when it walks,
despite appearances to the contrary. What actually happens is that the head
remains stationary relative to the ground until the neck runs out of stretch
room. Then the head is pushed forwards to maximum stretch as fast as possible.
This activity ensures that for most of the pigeon鈥檚 walking time the head is
stationary.
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A similar effect, known as 鈥渟potting鈥�, is used by dancers, who quickly become
dizzy if they allow their heads to spin at the same steady rate as their bodies.
So they hold their head still while looking at a single object, only moving it
when their body has moved too far for them to keep it there, and then spin it
round as fast as possible to lock on to the same point again. This way they
avoid becoming dizzy. The characteristic nod of the pigeon indicates a similar
process is happening here.
Stephen Forbes
Leeds, West Yorkshire
Answer: Birds other than raptors generally have their eyes located on the
sides of their heads. This gives them near all-round vision, but because the
fields of view do not overlap they cannot enjoy stereoscopic vision and hence
cannot estimate distances accurately.
To compensate for this, they move their heads in a jerky fashion. They take a
鈥渟napshot鈥� before each jerk and memorise it. Comparing this with the image after
the jerk gives the birds a rough estimate of the distance of an object.
Raptors, on the other hand, need fast, real-time distance information, and so
have their eyes positioned at the front of the head. This gives them overlapping
fields of view and the true stereo vision that they need to capture their
prey.
George Sassoon
Lochbuie, Isle of Mull
Long-distance cone CALL
Question: As a child in New Zealand, I was told that the Krakatoa eruption in
Indonesia was heard in Auckland. (At the time, the sound was allegedly
interpreted as the Russian Navy having a spot of artillery practice). By what
mechanism could the sound of the eruption remain audible after being transmitted
for several thousand kilometres?
Answer: I understand that the explosion accompanying Krakatoa鈥檚 eruption was
caused by a large volume of water superheating and rapidly turning to steam. The
initial eruption created a red hot underwater cavern, which the sea water
penetrated. The cavern then resealed as the eruption progressed.
The power of such an explosion was brought home to me and many other keen
young scientists of the late 1960s by a lecturer from the University of
Nottingham, who toured the country giving a demonstration on explosives. Just
before the start of his talk, he placed a small sealed length of glass tube
filled with water over a Bunsen flame, surrounded this with thick safety glass,
then began his lecture. After several minutes, the audience was brought rapidly
to attention when the tube exploded with great noise and violence. The tube had
heated up until it reached a temperature at which the glass softened. At that
point, the water inside vaporised instantly. As they say on TV: don鈥檛 try this
one at home.
Andrew Dearing
The Hague, Netherlands
Answer: There are two parts to the stock answer. First, the eruption was very
loud to begin with. Second, the sound was channelled round the southern
hemisphere by the presence of an inversion layer of cold air above warmer air
next to the ocean. Such effects do occur on occasion, but usually not over such
large distances.
John Woodgate
Rayleigh, Essex
Stop talking
Question: What parameters, if any, limit the number of different words
available to us in English (or any other language)? Are we near to running out
of words?
Answer: The existence of polyglots suggests that the average person is far
from running out of storage space in the brain. And while there are major
structural constraints on word numbers, they too leave lots of room for
expansion.
First, there is some effect from the number of distinct sounds (phonemes) in
a language. Languages with fewer phonemes have some tendency to have longer
words.
English has about 40 phonemes and many short words, while Hawaiian, with only
13 phonemes, has many words of three or four syllables. Any language can
increase the potential size of its vocabulary by using longer words.
Much more limiting than the number of phonemes are a language鈥檚 phonotactic
patterns鈥攖he constraints on possible sequences of phonemes. In English,
words can begin with sequences like 鈥渟p鈥� or 鈥渟t鈥�, but in Spanish they cannot
hence the vowel at the beginning of Espa帽ol. In Greek, words can begin
with sequences like 鈥減n鈥� or 鈥減s鈥�, but in English they can鈥檛, so we pronounce
Greek loan words like pneumatic and psychology without the p.
Even so, we are a long way from using up all the permitted shapes: AIDS, fax
and ROM are all recent additions, but 鈥渟nizz鈥�, 鈥渨hask鈥� and literally thousands
of other possible English sequences are all unused.
D. Ladd
Department of Linguistics
Edinburgh University
Answer: Assuming that we strictly limit ourselves to only
consonant-vowel-consonant forms and do not include such extras as tones and
stress, the following provides a generous lower boundary to the number of words
available in English.
There are more than 50 possible initial consonants (including combinations
such as 鈥渢r鈥� and 鈥渟k鈥�). There are more than 10 distinct vowel sounds (consider:
rad, raid, red, rid, ride, rude, rod, reed, road and add in nonwords such as
roid (void) and rould (should).
There are more than 40 terminal consonants (including 鈥渞t鈥� and 鈥渓k鈥�). This
means that there are in excess of 20 000 (50 x 10 x 40) single syllables. If we
limit ourselves to using only two syllables per word we would still have more
than 400 million words to play with.
Francis Glassborow
Oxford
This week鈥檚 question
Rustle riddle: What generates the energy that makes the thin, white
supermarket bags so noisy?
Lucy Birkinshaw
Leicester