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Natural History

ARTICLE REVIEW
Desert Ants on a Thermal Tightrope

by Martin J. Rosenberg

Herps, however, do not live isolated lives in nature. They often interact directly or indirectly not only with members of their own species, but also with other types of animals, both vertebrates and invertebrates, with plants, and with their physical environment. And this interaction, once it is revealed to a human's peering eyes (and inquiring mind), provides insight into the delicate balance between and among Nature's creatures.

A fascinating example of such an interaction appeared in a brief article published in Nature a number of years ago (Werner et al., 1992). It concerned the unique foraging strategy of the silver ant, Cataglyphis bombycina, which lives in the central Sahara desert in Africa. The authors of the article are researchers in the Department of Zoology at the University of Zurich in Switzerland (R. Wehner and S. Wehner) and the Department of Zoology at the University of Namibia in Namibia, Africa.

Most of us are aware that the daily cycle of an animal's activities are governed by a variety of factors, such as temperature, humidity, rainfall, searching for mates, availability of food, day/night cycle, and threat of predation. In the case of the silver ant, the two critical factors are predation by a reptile and exceedingly high temperatures.

According to this article, most desert ants escape from the desert sun when the surface temperature reaches the range of 95-113°F. Silver ants, however, only begin to forage for arthropods (insects, spiders, centipedes, and others which died from the heat of the sun) after other ants have headed for shelter, and even when the surface temperature reaches l40°F.

When the temperature "measured at ant height" reaches about 113°F, several hundred ants rush from the nest in a burst of activity, forage like mad, and then return to the nest. Their foraging activity lasts only about eight or ten minutes. That's it...for the whole day! And even during this period, the foraging ants must frequently seek areas (like the tops of stalks of dry vegetation) where they can dissipate some of the heat they have absorbed. The ants are so small that exposure to the desert sun for the full eight or ten minutes would cause them to exceed their upper lethal temperature of about 128.3°F. But the ants play it very close to the line -- they forage until their body temperature does reach the upper lethal temperature. A few more seconds in the sun, and they would probably not make it back to the nest -- and would be another ant's meal.

The question arises, of course, as to why the ants wait until the absolute hottest part of the day to emerge from their nest in order to seek food. Why not earlier in the morning or later in the afternoon, when it's cooler? The answer, as revealed by the authors of the paper, lies in the compromise the ants have "made" (actually that the processes of evolution and natural selection have made for them) between roasting to death or being eaten by a predator.

The predator which poses the most danger to these ants is the diurnal fringe-fingered (also called fringe-toed) lizard, Acanthodactylus dumerili (Grzimek, 1984; Obst et al., 1988). This 6-8" lacertid lizard, like some other desert lizards, is highly adapted for its unique habitat. The eyelids have interlocking scales to keep the sand out of the eyes and the fingers and toes have fringes to help them run across the surface of the shifting sands (Mattison, 1989). Their behavior is very much like racerunners in the U.S.

The favorite food of the lizard Acanthodactylus dumerili is...you guessed it...silver ants. However, the lizard is unable to forage during the absolute hottest time of the day when the ants are out and about. BUT, according to Wehner et al. (1992), there is a very small period of overlap in time when the temperature is just a smidgen below that which would fry the lizard, and just a smidgen above that at which the ants emerge. During these few seconds or minutes, both the lizards and the ants are foraging, each trying to find enough food to survive.

But within a few moments, the temperature reaches a point which would be fatal to the lizard -- its own critical thermal maximum. And so the lizards must retreat to their burrows, leaving the desert sands to the silver ants. The ants, however, can forage for only a few more minutes before the surface temperature reaches THEIR critical thermal maximum, which is the "highest critical thermal maximum value recorded so far for any terrestrial animal" (Wehner et al., 1992). So they, too, must retreat to their burrows.

In the words of the authors (Wehner et al., 1992), "The small overlap between the lizard's and the ant's natural foraging times indicates that the ant-eating lizard has ultimately shaped the ant's thermal, and thus temporal, foraging behavior... C. bombycina is forced to exploit an extremely narrow thermal window in which the upper and lower limits are set by heat stress and predatory pressure, respectively."

One more thing. As fascinating as this newly revealed predator-prey relationship is, it is equally fascinating to realize that it was probably discovered simply by being observant, by noticing perhaps incidentally that the ants were there...and then they were gone...until the same time the next day. I have no idea why these scientists were wandering around in the Sahara Desert during mid-day, when it was hot enough to fry an egg on the hood of their Jeep. For all I know they may have been studying the structure of sand dunes and their plants, or the thermal soaring of birds of the region. But whatever the reason, this article (I hope) will cause me to be just a bit more attentive to the plants and animals around me when I'm looking for herps in the woods.

LITERATURE CITED:
Grzimek, H. C. 1984. Grzimek's Animal Life Encyclopedia: Volume 6, Reptiles. Van Nostrand and Reinhold Co. Mattison, C. 1989. Lizards of the World. Facts on File, Inc. Obst, F. J., K. Richter and U. Jacob. 1988. The Completely Illustrated Atlas of Reptiles and Amphibians for the Terrarium. T. F. H. Publications, Inc. Wehner, R, A. C. Marsh, and S. Wehner. 1992. Desert ants on a thermal tightrope. Nature 357(18):$86-587.