3.5 Summary of Section 3
Behavioural mechanisms for reducing water loss are integrated with physiology. While Dipodomys rests in a cool burrow, the nasal counter-current heat exchanger cools exhaled air, conserving water vapour evaporated from respiratory surfaces. Long loops of Henle operate as counter-current multipliers, producing highly concentrated urine. Desert foxes use panting for evaporative cooling, but high rates of evaporative water loss cannot be sustained; hence the crucial importance of dens for cooling. Research suggests that desert birds have reduced BMR and FMR in comparison to mesic species. Such data need cautious interpretation, as studies on hoopoe larks demonstrated phenotypic flexibility in BMR, FMR and evaporative water loss.
Large endurers have difficulties losing body heat. Dipsosaurus dorsalis allows T b to rise: its enzymes function at 47°C. Overheated mammals cool by sweating, losing water and salts. Camels reduce the need for sweating by relaxed homeothermy, allowing T b to rise to 40°C, but they can function after a 30 per cent water loss from the body. Oryx leucoryx obtain water from eating plants. In summer, the water content of grasses is much lower than in winter, highlighting the importance for oryx of foraging at night when T a is relatively low, and resting in shade during the day.
The brains of oryx and camels are protected from overheating by the operation of a rete mirabile. In the ostrich, blood cooled during passage through complex nasal passages flows to the brain, keeping it 1.5°C lower than T b, a significant reduction when the bird allows T b to increase to 44° C.