Sunday, April 7, 2002		Nature

Yo-yos of the animal kingdom
Nutan Shukla

ANTELOPES and gazelles use alertness and speed to flee from predators, but they also exhibit another curious behaviour, known as pronking, which might intimidate or confuse a predator. They jump vertically into the air and appear to bounce along stiff legs, all four legs hitting the ground at the same time. The springbok of southern Africa is the champion pronker. As it leaps, the fan or crest of white hairs on the back are erected, and the head is bent forward and down almost to the feet. The hooves are kept together and the back is arched. At the moment it touches the ground, it springs back up again, sometimes jumping straight up, other times leaping forwards or jigging sharply to one side. Then, it lowers its fans, raises its head and races away at full speed.

As the predator nears and the chase begins, springbok warns the other antelopes by leaping straight-legged, up to 10ft high, into the air and releasing a scent from glands on its hindquarters.

This startling leap, the strong warning scent and the animal’s zigzag course together spell out an urgent message to the rest of the 250-strong herd — run!

They follow the airborne scent trail laid by the front-runners, keeping close together — if all the impalas ran in different directions the heard would fragment, and each individual would be more vulnerable to the hunter. Should a few animals become separated from the main herd, they can follow the scent trail to find their way back.

South African springboks also leap straight-legged into the air and emit a distinctive scent from a gland that stretches from midback to tail base. As a threatened animal leaps up, a pouch on its back opens, revealing long, erect, white hairs that draw the herd’s attention to its warning signals.

Why do springboks and impalas pronk or stot when they see a lion or cheetah? It is still a matter of controversy among biologists. It may be to tell the predator it has been seen, so the chase will be hard, or it may be to warn relatives within the herd. Another theory is that it may allow springboks to see over tall grass or bushes and locate other dangers, such as lions lying in ambush, before deciding which way to run. If forced to flee, these antelopes zigzag widely to make it harder for the predator to reach top speed.

Pronking is infectious. First one or two animals will start, and soon the rest of the herd will join in. Some biologists believe that the jumps are to indicate to the predator the animal’s fitness and its ability to escape. It is to encourage the predator to chase after other prey which it is more likely to get caught. The animal demonstrates that it is in full control of the situation and that the predator would be wasting time and energy in chasing it.

Another herbivore, the pronghorn of the open prairies of western North America and a member of the antelope-goats’ group, outruns wolves and coyotes and to do so it has developed not only speed that is rivalled by the cheetah, but also stamina. Pronghorns, which resemble antelopes, can run hard and long. Their top speed is about 55 mph (82km/h), achieved with 27ft (8.2m) strides of their slender legs, and 45mph (72km/h) can be maintained for mile after tortuous mile. This is made possible by modifications of the body.

Pronghorns have unusually large lungs and heart, which pumps more oxygenated blood to the muscles of the legs. They also have more of the cell organelles, known as mitochondria, in each muscle cell. These convert the additional oxygen into energy. Five time as much oxygen is available to a pronghorn than, say, a mountain goat.

This feature was published on March 31, 2002