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Ants Find Ally in Sahara's Searing Heat : Adaptation: Researchers say species emerges to scavenge after sun has driven predators to their lairs. Insects work quickly and retreat before temperature hits 128.

June 18, 1992|THOMAS H. MAUGH II | TIMES SCIENCE WRITER

Noel Coward was wrong.

According to the English playwright, only "mad dogs and Englishmen go out in the noonday sun." But a team of Swiss researchers reports today that they have discovered a third species, an exotic strain of ants from the Sahara that also disport themselves during the midmorning heat, presumably to avoid being eaten.

At that hour, an army of worker ants gathers in an assembly area at the entrance to their burrow, while sentry ants monitor the temperature outside.

When the air becomes so hot that all other species have retreated to their lairs, the ants burst forth in a brief spasm of activity, scavenging the insects that have been injured by the heat, then retreating to their nest before the sun gets too hot for them.

This frenzy of activity, apparently unique in the animal world, illustrates dramatically a species' ability to adapt to a harsh environment, according to zoologists Rudiger and Sybil Wehner of the University of Zurich. The ants' unusual workday is driven by the forces of evolution: If the ants come out earlier, large numbers fall prey to desert lizards. If they come out later they die from the heat.

These "harvesters from hell," said Harvard University entomologist Edward O. Wilson, "give us a picture of how far evolution has been able to take life in challenging the physical environment."

The discovery, reported in the British journal Nature, is "wonderful work . . . that gives a real appreciation of evolutionary specialization," said entomologist Thomas Eisner of Cornell University in Ithaca, N.Y. "When you have an animal active under such harsh conditions, you have to ask yourself why it is active at this time." The Wehners and their colleague, zoologist A.C. Marsh of the University of Namibia, provide that answer, Eisner said.

On a summer day in the central Sahara, the silver ant is the only insect that forages in the "full midday sun," according to the Wehners. During its four-minute foray, it searches for other insects that were active during the night and early morning but that did not retreat to their lairs quickly enough when temperatures climbed.

To endure this heat, they found, the ants have evolved the ability to withstand body temperatures as high as 128 degrees. But what drove evolution in that direction? The answer seems to be desert lizards that make their nests close to silver ant colonies.

During the heat of the day, the lizards forage almost exclusively on silver ants. When the researchers placed silver ants outside while the lizards were active, the ants usually fell prey within five minutes.

But the lizard retreats to its own lair when the temperature rises to 116 degrees. And that is about the temperature at which the ants emerge from their burrow. They must collect all their food for the day before the surface temperature rises to 128 degrees, the point at which their bodies become badly stressed.

Finally, exhausted by the heat, they retire to their burrows to await the next day's burst of activity--a truly unique adaptation to an unusual environmental niche.

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