For a mammal or a reptile, the desert is a continuous arithmetic problem. Every breath carries moisture out of the body; every movement in the sun adds heat that will eventually have to be released; every meal of dry plant tissue supplies less water than it costs to digest. Desert animals solve these problems through a combination of physiology, behaviour and anatomical design that has been refined over many millions of years.

The kangaroo rat, a small nocturnal rodent of the American southwest, is often cited as the extreme case. It never drinks. Instead, it derives all the water it needs from the metabolic breakdown of the dry seeds that form its diet. Its kidneys, unusually, can produce a urine several times more concentrated than that of a laboratory rat, reducing the volume of water lost in waste. Its nasal passages are cooler than the core of its body, so that air exhaled through them is partially stripped of moisture before it leaves the animal. And it spends the day in a humid burrow whose atmosphere, saturated with the animal's own breath, slows further evaporation. Physiologist Dr. Camilla Ortega has shown that a kangaroo rat active at night at 25 degrees loses around a tenth of the water a similar-sized laboratory rat would lose under the same conditions.

The camel takes a different approach on a larger scale. Unlike the kangaroo rat, the camel drinks, and it can consume enormous quantities at a single sitting - more than a hundred litres in a few minutes after a long dry period. What is unusual is the camel's ability to withstand a drop in body water of up to a quarter of its mass without suffering circulatory collapse. Most mammals die if they lose more than around twelve percent. The camel's oval red blood cells remain functional at higher blood concentrations than those of other mammals, and the animal can also tolerate a body temperature that rises by several degrees during the hottest part of the day, reducing the need for evaporative cooling through sweating.

Reptiles have simpler strategies, because their skin is less permeable to water than that of mammals. The fringe-toed lizard of the Mojave Desert keeps its body temperature within a narrow working range by moving between sun and shade during the day and by burrowing at night. When temperatures become impossible, it enters a form of summer dormancy called estivation, slowing its metabolism and waiting for cooler weather. The tortoises of the Sonoran Desert use deep burrows in a similar way, spending roughly ninety-five percent of the year underground.

Birds have to manage heat without the option of burrowing, and many desert species have adapted instead by being active at cool times of day and by choosing nesting sites that are sheltered from direct sun. The sandgrouse of central Asia has a more specialised trick: the male's breast feathers are modified to absorb water, which the bird soaks up at a waterhole and then carries back to the nest, where chicks drink from the damp plumage. Dr. Ortega describes this as 'feathered water transport', and notes that it has evolved independently in several sandgrouse species.

All of these strategies require trade-offs. The kangaroo rat's concentrated urine is metabolically expensive to produce; the camel's tolerance of high body temperatures depends on an ability to dump heat quickly during the cooler night; the lizard's estivation involves weeks of lost feeding opportunity. Desert animals are not simply efficient; they are adapted to a particular balance between gain and loss that works only under specific conditions.

When those conditions change, the balance can be broken. Several studies in the past decade have shown that rising average temperatures, as well as more frequent extreme heat events, are pushing some desert species towards the edge of their physiological range. Dr. Ortega's team at Arizona State University has documented a measurable fall in the body condition of kangaroo rats during particularly hot summers, despite abundant seed crops. If nights fail to cool sufficiently, the animals cannot shed the heat they have absorbed, and even their remarkable water economies are not enough. The deserts that look, to a casual visitor, unchangeable are in fact systems whose tolerances are narrow and whose margins are shrinking. Their inhabitants, having already invested so much in staying alive in difficult conditions, have limited further room to manoeuvre.