Extreme Summer - Summer World: A Season of Bounty - Bernd Heinrich

Summer World: A Season of Bounty - Bernd Heinrich (2009)

Chapter 16. Extreme Summer

19 May 2006. APPLE BLOSSOMS WERE IN FULL GLORY today, and ruby-throated hummingbirds, bumblebees, and northern orioles were visiting the flowers for nectar. Blackflies were in full glory as well, when the sun was out. But the sunshine didn’t last long, and shortly after dark a thunderstorm blew in. It took only half an hour to pass, but it brought some fireworks. Lightning bolts crackled and ripped across the sky, lighting up the night to make a second of day. Booming crashes followed. One right after the other, they made the earth vibrate and shook the house. Then, after a slight pause, torrential rain gushed down through the trees and pounded the roof. I cannot imagine how birds live through this. How do the baby grackles in the bog keep warm?

SUMMER IS A TIME OF LIFE AND DEATH, IN AN ORCHESTRA of organisms interacting with each other. But summer is determined by two key external variables: temperature and moisture. One affects the other. Thunderstorms come from often distant areas where heat caused evaporation and built up clouds. Rain occurs as the clouds encounter temperatures below the dew point to cause condensation, and the change of the water from a gas to a liquid causes a reduction of volume of the air, which reduces air pressure. The air pressure gradients produce winds that help distribute moisture and cause temperatures to change over the globe.

Locally, the heat affects life directly. The higher the air temperature, the more water it can absorb and hold, and hence the greater is its drying power. In some very large parts of the Earth, those that we generally call deserts, there is almost no rain and what little does fall tends to be episodic. Desiccation created by high temperatures then poses a challenge for plants and animals, especially if they must maintain a body temperature below the temperature of the air and despite the added heat load of solar radiation. In moister regions summer warmth stimulates growth and sunshine provides the energy. But deserts have a surplus of both heat and solar energy and a scarcity of water, and that dearth of water retards or prevents the conversion of the plentiful energy from the sun into the chemical energy useful for life.

Life in deserts confronts hard-edged limits, though often in a context of intense beauty. Life exists there only because of intricate behavioral and physiological adaptations. Field trips into the Mojave and Anza Borrego deserts of southern California opened my eyes to this environment and its exotic animals, which I saw through the lens of work done with George Bartholomew in our lab at my graduate school alma mater, UCLA. “Bart” in turn led me to the research and writings of Knut and Bodil Schmidt-Nielsen, Ray Cowles, and eventually many others who came later. I would have little to say here without their revelations.

Few give a better account of deserts, especially those of southern California, than the pioneer of desert studies, the naturalist Raymond “Doc” Cowles. Cowles grew up in Zululand in South Africa, came to California in 1916 at age twenty, and eventually taught at UCLA. He became an expert in reptilian thermoregulation, and was an academic grandfather of many graduate students and professors who carried on the tradition.

Cowles proposed that the serenity and the severity of deserts makes people who are immersed in the loneliness of these regions into thinkers. In his own holistic views of nature and human ecology he speculated on the meaning of wilderness to society, and he lamented the experiences we were losing. He left for his friends a typed card signed on 1 November 1971. It read: “Raymond Bridgman Cowles, December 1, 1896, at Adams Mission Station, Natal, South Africa, has completed his tour of duty on [he here left a blank] and will now participate in the universal and unending recycling game. This gives notice that his name should now be removed from [reprint] mailing lists.” One of his daughters later inserted the date of his death: 7 December 1975.

Ray Cowles, in many ways my own academic grandfather, had experienced half a century of desert field trips on his feet and in his head before he wrote Desert Journal (published posthumously in 1977). In it he reminisced about “innumerable campfires and their evening sacrifice of incense from smoldering wood.” He was then “sadly reminded that such luxuries, such reverence for the gods of the open skies, are no longer ecologically excusable,” and said that “from now on the careful naturalist and his students must be content to enjoy fellowship and worship nature around a noisy hissing gasoline stove for as long as that store of onetime solar energy remains.” He anticipated the same “reverence for the gods” that is, as my friend from California recently reminded me, probably no longer possible even in the Maine woods.

Cowles’s love for the desert campfire of crackling and smoldering wood and his enjoyment of desert life are revealed in the following passage from a chapter in Desert Journal, titled “Around the Campfire”:

Summer or winter, there is something special about sundown and the coming night, and my desert camps were no exception. Not the least was the cessation of work, return to camp, and, in those days of fewer people, the gathering of scanty firewood. I often used cactus skeletons and the roots and stems of stunted shrubs. Soon my camp was rich with fragrance. Food cooked in the aromatic smoke from desert wood has a tang in this clear, unpolluted air unknown outside the arid world. Long before the summer sun has set, the first flight of bats commences, most often the little canyon, or pipistrelle, bat with pale silvery body, black wings and ears. Along the Colorado River they flicker across the sky, bent primarily on reaching water where they replenish the moisture lost during the day, even in their relatively cool rock-crevice retreats.

In the same locale nighthawks by the hundred appear soon after the heat begins to abate. They flutter and sail toward the river for the first drink of the day. During May and June, when many are still incubating or hovering over their eggs to protect them from the sun’s increasing heat, this first intake of water precedes feeding. The birds nest, or more accurately, lay their eggs, on the exposed ground. Throughout the day the relentless sun beats down. Air and ground temperatures may exceed 120°F for hours on end; the direct heat of the sun contributes to what for most creatures would be unendurable conditions. Insulated against heat by its feathers, each nighthawk sits in a self-made patch of shade and comfort. Plumage is as effective in shielding the skin and blood vessels from high temperatures as it is in containing body heat during cold weather.

From time to time they open their enormous mouths and flutter their gular pouch, evaporating some of their small supply of water to keep blood and body temperatures below damaging or lethal levels. But water is so scarce and the day so long that excessively prolonged cooling by this means would dehydrate the birds. I know of no other animals, however, not even the supposedly sun-tolerant lizards, that possess no feathery insulation, that can remain in direct sunlight for so long a time. Many of the smaller lizards will die in minutes under such conditions. Yet the nighthawks, warm-blooded, heat-generating birds, complete their incubation period and care for the young in the unrelenting heat of the desert until all can take flight to the surrounding environment.

In these three paragraphs alone, Ray Cowles eloquently and presciently summarizes volumes of research that came after him. I can add here only a few details to expand on the theme: that birds are preadapted for getting by on less water than mammals because they excrete their nitrogen wastes in a white uric acid paste and thus do not need large amounts of water to flush them out, and they also save water otherwise required for evaporative cooling because their body temperature is 2°F to 4°F higher than ours. Keeping the eggs cool has gone one step farther in the Egyptian plover, Pluvianus aegyptianus, which brings water back to the eggs and dampens them to cool them. Similarly, sand grouse in Africa have special feathers on the breast that soak up water so that it can easily be carried back to the nest. The young sip the water from the tips of the feathers, like baby mammals suckling on their mother’s teats.

As Cowles indicated, possibly the most effective way for animals to reduce heat input and save precious water is by behavioral adjustments. Desert birds are active mainly in the early morning and evening and take a long siesta in the middle of the day, although some of the larger birds—such as ravens, vultures, and hawks—may soar high in the air, where temperatures are lower than they are close to the ground. It is cooler at night, and most rodents, many reptiles, and many insects escape the heat by becoming nocturnal and staying in cool burrows during the heat of the day. Rodents that are generally diurnal, such as ground squirrels, are heated up temporarily when they venture to run across hot sand, but they then hurry back into their burrow to press their belly against the cool ground and unload their heat.

Avoiding the heat by becoming nocturnal also helps to alleviate the water shortage. The relative humidity is high within a burrow, and so the air cannot suck up moisture from the skin, or from the lungs through breathing. Death in the desert is seldom directly from heat. It comes from dehydration resulting from trying to keep cool. Australian Aborigines have adopted some of the same survival tricks used by other animals. On long walkabouts through hot country, they try to restrict travel to nighttime, and during the day they may bury themselves in the sand to keep from sweating and dying of thirst. Elizabeth Marshall Thomas relates similar strategies, which she learned about from her experiences with the Kalahari Bushmen during the hot dry season. The Bushmen go out early in the morning to hunt for perennial plants whose leaves die to reduce water loss and whose underground tubers are adapted to store water. After a tuber is located by the remnants of its dry vine on the ground, it is dug up and the pulp is scraped out of it and then squeezed to get water to drink. The people survive the heat and dryness of the day by burying themselves in pits dug in the shade. These pits are lined with the tuber shavings, which are then resoaked, but with urine, so that the evaporating water will not be from the precious body stores. At dusk, when temperatures drop, the Bushmen again venture forth to search for more tubers (Thomas 1958).

We can tolerate very high air (though not body) temperatures, as was demonstrated (Schmidt-Nielsen 1964) more than 225 years ago when Dr. Blodgen, then secretary of the Royal Society of London, and some friends, a dog, and some steak spent some time in a room heated to 260°F (48°F above the boiling point of water at sea level). They remained there for forty-five minutes, at which time the steak was cooked but the men and the dog were unharmed (their feet had been protected from touching the floor). Had the air been saturated with water, there would have been no evaporative cooling and it can be confidently said that they would have been cooked along with the steak.

We are not deterred by heat so much as by lack of water. In his book The Hunters or the Hunted, C. K. Brain notes that in southwestern Africa, all the Hottentot villages in the Namib Desert are scattered directly along the Kuiseb River. Here the people have dug wells from which they get their water when the river runs dry. Birds there get water from eating insects, and most insects get water from live plants. But one group of Namib beetles of the family Tenebrionidae are an exception. Some of them stay in water balance even when eating only dried plant detritus that blows around in the wind.

These beetles are ground-dwelling, usually large, and black (the melanin absorbs heat but is necessary to protect them from damage by ultraviolet light). They live on the sand surface. Those that live on the hottest sands have stiltlike legs to reduce heat input from below. Others reduce heat input from above, from the sun, by light-colored wax on their black backs. But even then there is still the problem of getting sufficient water, and there is no standing water and no rain when they are active. Although they are subjected to a desiccating environment during the daytime, at night temperatures in the Namib typically drop and wind from the Atlantic coast may sweep in with moisture-laden air. The beetles then orient themselves by standing on the sand dunes with the head straight down and the abdomen up into the air. Water condenses on the beetle’s back and flows down in droplets to its mouth.


Fig. 30. A Namib Desert tenebrionid beetle, which elevates itself above the most intense heat at the ground surface.


Fig. 31. A Namib Desert tenebrionid beetle that catches water from moist air blowing in from the Skeleton Coast by doing headstands. The water condenses on its back in tiny droplets, which then coalesce and run down to its mouth.

The beetle’s amazing behavior is cobbled together by evolution from structures and behaviors that previously had other functions. Their backs are modified wing covers (elytra) that no longer cover any wings but serve instead as physical protection for the body. But in these beetles the elytra have taken on an additional, very different, and novel function. All tenebrionid elytra are sculptured in various patterns. In these beetles they have a pattern of bumps that helps capture vapor molecules into tiny droplets. Waxy valleys between the bumps channel the water droplets so that they coalesce and roll down into the mouth. I recalled seeing similar tenebrionid beetles in our southwestern Mojave Desert, where they are sometimes derisively called “butt-head” beetles because here also they stand with their butt in the air. But in this case they do their headstands for a different purpose: defense. The headstanding exposes a gland in the tip of the abdomen from which the beetle can exude a foul liquid that may spread over the back and will repel most predators. The Namib beetle’s water-catching behavior was probably derived from similar defensive behavior that later became joined to an existing morphology.

Although I had often seen butt-head beetles in the Mojave, I was not fortunate enough to witness the dew-catching behavior of the African beetles when I was in Namibia with my graduate student James Marden to study desert insects. We stayed at the Namib Research Institute at Gobabeb next to the “shore” of the Kuiseb River. The riverbed was dry at the time, but it was the only place where we saw trees and shade. The trees’ roots tapped the groundwater, and that water fed insect fauna. We saw innumerable black tenebrionid beetles running in haste. They were mostly racing in pairs, with the female always in the lead. Jim made this curious phenomenon the focus of his study.

We saw no sign of free water. Yet during World War II two German geologists from nearby Windhoek—Henno Martin and Hermann Korn, with their dog, Otto—managed to hide out here undetected for two and a half years (to avoid being put in an internment camp). They lived like Robinson Crusoe during those years, and Martin later wrote a book about their experiences. In it, he describes the effect that water has on life in the desert. Martin and Korn had experienced a drought in the Namib for several years, and one night they heard thunder:

I had never before in my life heard [such thunder], or experienced such a cloudburst—and now the scorched and battered life began to raise its head again—within hours, bushes that had looked dead began to show shoots of green and in the shade of rocks ferns began to unroll delicate light green leaves. The desert was alive everywhere: seeds that had lain dormant for years came to life and pierced the crust of the earth; almost overnight the balsam bushes covered themselves with green leaves like young birches; the first yellow flowers opened their petals to the sun, and once again we found the speckled eggs of the quail amidst the grass and stones; and the lukewarm water on the pools swarmed with little crab-like insects whose eggs had survived the years of dryness and scorching sunshine.

Adaptations of plants to deserts include dormancy and a variety of structural and behavioral adaptations. The majority of desert plants depend on a strategy that capitalizes on small size. They are annuals that spring up from dry, dormant, heat-resistant seeds. Some of these seeds may wait up to half a century before they are activated. The plants’ challenge is to be quick enough to respond to rain so that they can produce their seeds before the earth dries up again, while not jumping the gun to start growth until there is sufficient water for them to grow to maturity for seed production. Some achieve this balance on a tightrope by “measuring” rainfall. They have chemicals in their seeds that inhibit germination, and a minimum amount of rain is required before these are leached out. Others have seed coats that must be mechanically scarred to permit sufficient wetting for germination, and the scarring happens only when they are subjected to flash floods in the riverbeds where they grow. A plant in the Negev Desert releases its seed from a tough capsule only under the influence of water through a mechanism that resembles a Roman ballistic machine. Its two outer sepals generate sideways tension that can fling two seeds out of the fruit, but the two seeds are held inside by a lock mechanism at the top. However, when the sepals are sufficiently wetted, then the tension increases to such an extent that the lock mechanism snaps, and the capsule “explodes” and releases the seeds (Evenari et al. 1982).

In moist regions where it rains predictably (though not necessarily in abundance), we help agricultural plants to capture the precipitation by scarring the soil to facilitate the infiltration of the water into it, and hence into the roots. Least runoff and maximum water absorption are achieved by plowing the soil. However, such a strategy would not work in a true desert such as the Negev. A different program is required there because rain is infrequent and plowing would facilitate only the evaporation of scarce water from the soil. The solution applied by the peoples who inhabited the Negev in past centuries was a practice they called “runoff farming.” Farmers had mastered harnessing the flash floods that rush down into the gullies by catching the runoffs—not only by making terraces but also by building large cisterns into which the water was directed to be held for later use. Remnants of these constructions still exist.

Water-storage mechanisms have been invented by other organisms living in deserts, but mainly through modifications of body plan. Many plants, especially cacti and euphorbia, have the ability to swell their roots or stems with water stores. Possibly the most familiar is the saguaro cactus, Carnegiea gigantea, of the Sonoran desert in the American southwest. It has a shallow root system that extends in all directions to distances of about its height, fifty feet. In one rainstorm the root system can soak up 200 gallons of water, which are transferred into its tall trunk. This trunk is pleated like an accordion and can swell to store tons of water that can last the plant for a year. The cactus has no leaves, but the stem is green and can photosynthesize and produce nutrients as well as store water. The saguaro’s survival strategy requires it to grow extremely slowly. But it lives a century or more.

Some desert animals similarly store water. The frog Cyclorana platycephala, from the northern Australian desert, fills up and greatly expands its urinary bladder to use as a water bag before burying itself in the soil, where it spends most of the year waiting for the next rain. While in the ground it sloughs off skin and forms around itself a nearly waterproof cocoon that resembles a plastic bag and reduces evaporative water loss.

Desert ants of a variety of species (of at least seven different genera) in American as well as Australian deserts collectively called “honeypot ants” have evolved a solution that combines water storage with energy storage. Ants typically feed each other; and some of the larger worker ants may take up more liquid than the others, and others may bring more. Those that take the fluid may gorge themselves until they distend their abdomens up to the size of a grape, by which time they are unable to move from the spot. They then hang in groups of dozens to hundreds from the ceiling of a chamber in the ant nest, where they are then the specialized so-called repletes that later regurgitate fluid when the colony members are no longer bringing the fluid in but rather needing it. In western North America about twenty-eight species of one genus, Myrmecocystus, have adopted the storage strategy of water and sugary secretions that are secured from aphids, flower nectar, and other plant secretions when summer is not yet too severe.

Animals’ solutions to the extremes of desert summer have also been exploited by people. In the Australian deserts the Aborigines have learned to find and access the water-holding frogs and use this resource as a last resort in times of need. In central Australia also the repletes of one honeypot ant species, Camponotus inflatus, are large enough to be commonly used by Aboriginal peoples. Those of Myrmecocystus mexicanus in the southwest in North America, who store water or honey or both, were also used by native peoples (Conway 2008). The Bushmen of the Kalahari Desert, instead of exploiting the fluid stores of frogs and ants, use the shells of ostrich eggs as containers for underground water storage caches; but as mentioned previously, when they exhaust these stores they resort to water stored by plants in underground tubers.


Fig. 32. The Apache cicada is active during the hottest part of the day in the summer, when most animals try to escape the heat.

For those who solve the water problem, the desert can be a haven. For peoples living in the American southwest, the Namib, and the Negev, the desert has often been a refuge from persecution. Under what circumstances except necessity would people be so ingenious and hardworking as to try to make the desert bloom and grow crops? Why would animals live where they are physiologically tested to the limits of their endurance? Where else except where they were severely tested would they evolve to extend their tolerances? The Apache cicada, Diceroprocta apache, of the Sonoran Desert of Arizona, is one such animal. It not only tolerates the severe summers there; it courts the heat.

As with the cicadas in New England, and those at numerous other places all over the globe, the larvae of the Apache cicada live underground, where they are relatively safe, and they could potentially emerge as adults at a time of their choosing. Here in New England, cicadas wait until late summer when temperatures are, by our standards, benignly pleasant. Not so for the Apache cicadas of Tucson, Arizona. They emerge in the hottest part of summer, and become active at the hottest part of the day, at nearly 110 to 116°F.

Insects are generally renowned for their ability to retain water. Where other animals die of thirst, they can stay hydrated, mostly by avoiding heat, by their ability to conserve water because their watertight exoskelton is covered with layers of waterproofing lipids and waxes, and by their excretion of nitrogen wastes as uric acid that requires negligible water to excrete. The Apache cicada has, literally, prominent holes in it, and it has glands that excrete water from these holes. Not knowing more, one might suppose that this insect is active at the wrong time and also physiologically unsuited to life in extreme summer. Its design seems inefficient and backward.

It took two biologists, James E. Heath from the University of Illinois and Eric C. Toolson from Arizona State University, to unravel the cicada story, which is one of exquisitely elegant desert adaptation. Heath deduced from his studies that the cicada’s seemingly anomalous active time is the time when potential predators—both birds and wasps—have fled the field because they can’t stand the heat. Toolson found out that cicadas can stand high temperatures because they have glands that function like sweat glands to provide evaporative cooling in emergencies, as when the males are exerting themselves by calling deafeningly to attract females for sex.

The cicadas’ ability to defy the summer extremes, and thereby to escape enemies, would not be possible without a constant, reliable source of water. And as members of Homoptera—the aphids and their relatives—cicadas are preadapted to get that water. Near Tucson in the summer, the Apache cicadas perch all day in the shade of a paloverde branch in an arroyo and tap water from deep down in the soil. The means of access to that water are the deep roots of the bush, which grow as much as sixty feet down to the water level. The water is piped to the twigs that the cicadas tap into with their sucking mouthparts.

Thermal wars are also waged directly, in combat between one insect and another. The Asian honeybee Apis cerana japonica faces a serious predator, the giant hornet Vespa mandarinia japonica. Hornet scouts invade beehives and if successful recruit their nest mates to come in force and devastate a beehive. The hornet is far too big and heavily armored to be killed forcibly by the much smaller bees. However, these honeybees have evolved a strategy that compensates for their size deficit. They pin a hornet down by clustering in hundreds around it to form a ball, and then they shiver and produce enough heat to raise the temperature at the center of the ball, where the hornet is, to 118°F. That temperature kills the hornet but is still a degree or two below the upper tolerance of the bees (Ono et al. 1995).

A slightly different story is played out by thermal warriors during the summer close to my home in Vermont and the woods in Maine. In this case the white-faced hornets, Dolichovespula maculate, whose summer colony strategies I discussed previously, are the beneficiaries of the thermal strategy. It is often hot in the daytime during our summers, but in early and late summer nighttime temperatures commonly dip to 38°F or lower, and they may remain there early in the morning. Such temperatures are so low that many small insects hunted by these hornets cannot fly. The hornets hunt by cruising over the foliage and pouncing on any contrasting object that may be a fly or some other unsuspecting insect. They have their best chance of success when their prey has slower reaction times or cannot fly off, or both; and that is in the early morning, when it is still cool. This is when the hornets, with a muscle temperature near that of our own, leave their warm insulated paper nests in force to hunt. They are larger than their prey, and have the additional advantage of flying at low temperatures because their exercise, both flying and shivering to prepare to fly, results in more heat retention than is possible for the much smaller prey.

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 thermal advantage to avoid being eaten by predators and to run down antelope.