Summer World_ A Season of Bounty - Bernd Heinrich [78]
In my opinion the most extreme and the most beautifully elucidated thermal warrior strategy is one found during the extreme summer in the Sahara Desert. This is the story of the silver or “fast” ants, genus Cataglyphis, as unraveled by Rüdiger and Sibylle Wehner and colleagues from the University of Zurich, Switzerland. These ants are remarkable because they preferentially forage at midday, when ground surface temperatures reach 145°F. They tolerate a very high body temperature of 129°F, but because of their small size they would reach a lethal temperature within seconds after coming out of their subterranean nests and stepping onto the sand. They are much too small to cool off by the evaporation of water; instead, they survive by pausing frequently to cool off by climbing dry stalks that serve as thermal refuges. The question is: why don’t they go out at night like most other desert dwellers, when they would not desiccate so easily and would automatically escape the danger of being killed by heat?
The Wehners discovered the answer in the ant’s hunting strategy. These ants are fast, but not fast enough to run down live prey. They specialize in insects that have been incapacitated or killed by heat. The hitch is, however, that they cannot afford to leave their safe nests until the sand is hot enough to prevent their own major predators, lizards, from being out. As a result of this thermal tightrope, these ants must wait at their nest entrances, not risking a mass exodus, until it is hot enough for the lizards to have retired from the field but not so hot that the ants themselves would be incapacitated.
A focus of study for the Wehner group has been trying to understand the ant’s homing ability. The ants forage for insect prey incapacitated or killed by heat. This requires extensive searching, and taking many twists and turns in their paths. Moreover, at the end of a foraging trip—after finding prey or after temperatures rise to excessive levels—the ant has to find its way back. And it may have to get home in a hurry, since sand temperatures are often very high and the ants can stand only so much heat.
The ants’ safety margin, with regard to their physiological tolerances, depends on a combination of rapid running and unerring homing ability. The Wehner group determined that the ants’ homing ability is the result of an amazing mental feat. The ants compute where they are at any one time by integrating the turns and distances of their travel (“path integration”), and then use the sun’s location from the pattern of polarized light in the sky as a compass to determine the homeward direction and distance. Near the end of their run by the nest entrance, they also use landmarks, if any are available.
Fig. 33. The long-legged “fast ant,” Cataglyphis, of the Sahara Desert comes out of its relatively cool underground nest to go foraging on the sand surface in the daytime at near its thermal death point. It reduces direct solar radiation by being pseudo-erect, elevating its abdomen.
The ants’ lives outside their subterranean nests are hazardous, and they venture out only near the end of their lives. An analogous situation among humans would be drafting soldiers in old age because they have already lived and contributed to society, rather than sending youths into danger who have still much to do and give.
Various anthropologists and physiologists have remarked that we, too, are creatures that may have begun in an environment of extreme summer. We combine the extreme sweating response of the Apache cicada with the extraordinary running, hunting, and homing abilities of the Cataglyphis ants of North Africa and Asia (other genera of ants in the South African and Australian deserts have a similar lifestyle). In combination with imagination, they have given our distant ancestors (as well as some contemporary tribes) a