Getting about
Question: Why do some birds walk and others hop? And are there any that do
both?
Answer: Birds use three distinct gaits. In walking, the left and right feet
move in turn, half a cycle out of phase with each other. Each foot is on the
ground for more than half the time, so there are times when both feet are on the
ground. In running, the feet also move half a cycle out of phase with each
other, but each foot is on the ground for less than half the time and there are
times when both feet are off the ground. In hopping, the feet move more or less
simultaneously. This inevitably means that for much of the time both feet are
off the ground. Footprints of birds hopping in snow show prints of the left and
right feet side by side, or only slightly staggered.
I have started with these definitions because the question is slightly
misleading: it is not hopping and walking that are alternatives, but hopping and
running. We humans walk to go slowly and run to go fast, making the change at a
speed of about 2 metres per second (or less in the case of small children).
Advertisement
Medium-sized and large birds walk at low speeds and either run or hop at
higher speeds. For example, a crow weighing half a kilogram walks at speeds of
up to about 0.7 metres per second and hops at higher speeds. A 23-kilogram rhea
walks at speeds of up to 1.7 metres per second and runs at higher
speeds. Big birds make the change from walking to the faster gait at higher
speeds than small ones. Theory suggests that the change should be made at a
speed proportional to the square root of leg length, but I do not have enough
data to test the theory. Small birds generally do not walk鈥攖hey seem never
to be travelling slowly enough for walking to be appropriate. For example, zebra
finches hop but never (in my experience) walk. Similarly, among the mammals,
mice seem always to trot or gallop, and never to walk.
Finches, tits, sparrows and crows are examples of birds that hop. Wagtails,
game birds and waders run. The general rule seems to be that the more time a
bird spends in trees, the more likely it is to hop. The traditional explanation
is that there may be wide gaps in a tree between one twig and the next, so
walking is not a particularly good option, while on each twig there is room to
set both feet down side by side, so hopping seems to make sense. However,
common-sense functional explanations like this should not be accepted too
hastily. I do not know of any formal study of the correlation between gait and
habit in birds that takes account of evolutionary history as well as
function.
Finally, you may have noticed I said that hopping birds move their feet more
or less simultaneously. Most birds set down their feet almost simultaneously
when hopping, but when crows and magpies are hopping they set one foot down
slightly before the other, like a child playing a skipping game. I have never
seen a plausible explanation for this.
R. McNeill Alexander
Research Professor
School of Biology, University of Leeds
Rise and fall
Question: Last week I had my weight measured. While standing on the scale I
saw that the measuring needle moved up and down. Is it because I鈥檓 breathing in
and out, or is there another explanation?
Answer: If, while standing on sensitive scales, you take your pulse at the
wrist, you will observe that the small upwards movement of the scale鈥檚 needle
coincides with the beat of your heart.
The answer lies in classical Newtonian mechanics. Every force causes an equal
and opposite force. The object exerting the force is the blood being ejected
down the descending aorta. The scales record the thrust of this rapid ejection
of a mass of blood, which makes the needle wobble.
Bron Lipkin
London
Answer: A medical technique called ballistocardiography involved recording
the body鈥檚 movement due to cardiac contraction, ejection of blood and
deceleration of blood flow in the large arteries. Five peaks and troughs were
traced on a graph with each beat of the heart. The amplitudes and shapes of
these peaks and troughs were thought to change in different types of heart
disease. As a young cardiologist in the early 1950s I wasted much time assessing
this technique, which ultimately proved to be of no diagnostic value.
Edwin Besterman
Department of Medicine, The University of the West Indies, Jamaica
This week鈥檚 questions
Random thought: How can there be such a concept as 鈥渞andom鈥�? Surely
everything has a structure if you look deeply enough. What does random actually
mean?
Tony Holkham
By e-mail, no address supplied
The morning before: What is the evolutionary advantage of morning sickness?
Why are so many pregnant women incapacitated and unable to eat sensibly for the
first three months of their pregnancy and how can this possibly help the
developing embryo? Do any other animals get morning sickness?
Diane Brown
National Childbirth Trust
Norfolk and Norwich Branch