Summer World_ A Season of Bounty - Bernd Heinrich [73]
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