Knut Schmidt-Nielsen, one of the all-time greats of animal physiology, died on 25 January 2007 aged 92..
He studied life in deserts, where heat and drought makes survival difficult. The most effective means of keeping cool depend on evaporation of precious water, either in the breath or as sweat, but kangaroo rats survive in the Arizona Desert with nothing at all to drink. They keep reasonably cool by spending the day in burrows and emerging only at night. But even at night they would lose much too much water in their breath if it were not for their remarkable noses. The air they breathe in is relatively cool and dry, but it is inevitably warmed in the body and becomes saturated with water vapour. To minimize water loss, this air must be cooled before it leaves the body to condense out most of the vapour. Schmidt-Nielsen showed that the incoming air cools the surfaces of the nasal cavity, which, in turn, cool the air when it is breathed out again. He showed that the same principle operates in other mammals and birds, but is particularly effective in kangaroo rats because their nasal surfaces are enlarged by elaborate nasal bones the turbinals. They also save water by producing exceptionally con concentrated urine.
In contrast dogs overheated by exercise, need to let water evaporate to cool themselves. With colleagues, Schmidt-Nielsen showed that panting dogs avoid the effect by breathing in through the nose but out through the mouth.
Camels are too big to shelter in burrows in the heat of the day. . Schmidt-Nielsen showed that camels avoid the water loss by allowing their bodies to heat up by day and cool by night. A camel may start the morning with a body temperature of only 34 °C, but warms to 41 °C during the afternoon. This strategy would be ineffective for small animals such as kangaroo rats, because they would quickly heat up to lethal temperatures. Schmidt-Nielsen then showed that camels’ noses also effectively conserve water .
Sea birds face a different problem: they drink sea water, which has a much higher osmotic concentration than their blood. Instead of producing urine even more concentrated than sea water, they have glands opening into their nostrils which secrete droplets of concentrated salt solution that are shed by a shake of the head.
In the later part of his career, Schmidt-Nielsen studies the energy budgets of animals with other. They measured the rates of oxygen consumption of various mammals, ranging from mice to horses, running at different speeds. They calculated the quantity of oxygen used per kilogram of animal moving a distance of one metre , this was less for large animals than small. Implying that the muscles of small animals are less efficient.
Similar relationship of oxygen consumption and body mass was shown for flying birds and swimming fish. Flight is cheaper than running (for the same body mass), and swimming is cheaper still.

Obituary in Nature Alexander Nature 2007, vol 446, p 744

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