杏吧原创

Walking tall

During a recent visit to Kenya I noticed that giraffes tend to walk with a 鈥減acing鈥� gait. The two legs on each side of the animal move at the same time, unlike horses and other four-legged animals. As far as I am aware no other ruminant (aside from the camel and okapi) walks in such a fashion. Does anybody know the biomechanical reasoning behind this and indeed whether it is unique to the giraffe and camel? Is this form of locomotion more efficient than a conventional gait?

鈥� Giraffes and camels have long legs, relatively short bodies and large feet. A common explanation for their unusual gaits is that they prevent fore and hind feet getting in each other鈥檚 way.

If you refer to feet by their initials, LF for left fore, RH for right hind, and so on, you can write down the walking patterns of particular animals. When most mammals walk, they move their feet in turn, always in the same order, at more or less equal intervals of time:

LF^**RH^**RF^**LH^**LF^**RH^**RF^**LH and so on. The asterisks indicate the time intervals, ^**** for long and ^* for short.

In trotting, which is a faster gait, the legs move two at a time, in diagonally opposite pairs:

(LF+RH)^****(RF+LH)^****(LF+RH)^****(RF+LH) and so on.

Camels, however, do something different. Instead of trotting, they pace, moving the two feet on the same side of the body together:

(LH+LF)^****(RH+RF)^****(LH+LF)^****(RH+RF) and so on.

The questioner says that walking giraffes move the two feet of the same side of the body at once, like the camel, but that isn鈥檛 quite true. Films analysed by the American zoologist Milton Hildebrand showed that walking giraffes move their feet like this:

LF^***RH^*RF^***LH^*LF^***RH^*RF^***LH and so on.

Long and short time intervals alternate and the fore legs move slightly after the hind legs of the same side.

In trotting, a foreleg swings back while the hind leg on the same side swings forward, so there is a danger of the feet colliding, if the legs are long. In a pacing walk, the legs on one side of the body both swing forward, then both swing back, so the fore and hind feet are kept well out of each other鈥檚 way.

The fact that some long-legged breeds of dog also pace instead of trotting supports this explanation of the giraffe鈥檚 pacing walk.

In horses, there is less danger of fore-hind collisions in the standard walk than in the trot. The giraffe鈥檚 pace-like walk reduces the danger further. This may explain the giraffe鈥檚 unusual gait. But I feel bound to point out that both camels and giraffes gallop successfully. In galloping, both fore legs swing back while both hind legs are swinging forward, offering plenty of scope for collisions.

Hildebrand recorded giraffe-like walking gaits for the cheetah, a hyena and the gerenuk (a long-legged antelope). No experiments seem to have been done to find out whether there is any difference in energy costs between trotting and pacing, or between the horse鈥檚 and the giraffe鈥檚 styles of walking, but I would expect any differences to be small.

R. McNeill Alexander

Emeritus Professor of Zoology

University of Leeds, UK

Cold surface

I have heard that a common way to catch a cold is if somebody with the virus touches your hand before you touch your own nose or eyes. Apparently it can even be passed on via a third surface such as a door handle. How long can a cold virus or any other pathogen live on a surface? Does it depend on the surface and does moisture make a difference?

It depends on the surface. Cool, moist glass in the shade, for instance, might retain many kinds of rhinovirus or coronavirus for days.

Brass which is dry, sun-baked and covered with verdigris and zinc compounds, on the other hand, might be germ-free within half an hour of being touched. Such compounds are bad for most microbes, so filthy lucre, especially coins made of copper alloys, is not nearly as horribly germy as one might expect.

By and large, rhinoviruses are the most common causes of colds. They are picornaviruses, which are generally only moderately stable. Desiccation and ultraviolet light in open sunshine should render most surfaces safe quite quickly. A cosy damp pocket handkerchief, though, might harbour the germs for days, unless it is infested with decay bacteria that digest viruses along with the nutritious secretions donated by the owner.

To avoid infection in a viral epidemic, it makes sense to avoid touching your face as far as possible and to wash your hands before doing so.

Jon Richfield

Somerset West, South Africa

This week鈥檚 question

Against the grain?

Why do sand particles on a beach or dunes seem to reach a certain grain size and then reduce no further? After millions of years shouldn鈥檛 most sand have become dust?

Keith Minto

Holt, Australian Capital Territory