Cooling for Posterity

Dinosaurs may have died out because they could not control the temperature of their reproductive organs. So says Dr. Raymond B. Cowles, of the University of California. Dr. Cowles is a leading authority on animal airconditioning. This week, writing in Science magazine, he turned his probing mind to bats and their special heat-regulating problems.

Dr. Cowles believes (with most zoologists) that warm blood is a key factor in evolution. Like diesels and steam engines, animal mechanisms are more efficient at high temperatures. This is one of the chief reasons why mammals and birds, both warm-blooded, dominate the earth.

But there is a gimmick in high-temperature operation. The "spermatogenic" cells, which produce the male sperms, will not tolerate as much heat as the somatic (body) cells. When sperm cells get too hot, they stop working, causing sterility. Even the normal temperature of the blood is often too high for them to function.

Warm-blooded animals get around this difficulty in various ways. Some of the more primitive creatures become cooler at certain seasons, so that their testes can manufacture sperm. In higher animals the testes, contained in the scrotum, outside the body, are cooled by the air to a temperature lower than the body's.

Way of a Bird. The scrotum system does not work with birds, because it might cause supersonic disturbances during flight. Birds' testes must be "faired into" the neatly streamlined body. But many birds breed in warm weather, even though their blood may get hotter than 42DEG C (107.6DEG F). How do they do it? Have their spermatogenic cells learned to work at high temperatures?

Dr. Cowles thinks not. Last year, while dissecting Brewer's blackbirds (Euphagus cyanocephalus) he discovered that their testes moved downward and backward during the warm spring breeding season. In the new position they were "enveloped between the two dorsal folds of the abdominal air sacs." Cooled by circulating air, they could function properly, though the general body temperature might be much too high for them.

Bat Radiators. Like birds, bats are streamlined, with their testes inside their bodies. When inactive, their temperature may fall very low, but when they fly, as they do every night to gather food, they are as hot as birds. How do their spermatogenic cells withstand this nightly cooking? Dr. Cowles examined bats, both active and resting, in the breeding and the non-breeding seasons. He found that, while they are at rest, their hairless wings are pallid, almost bloodless. But when they raise their temperature to the flying level, which they must do by an effort of will before they can take off, their wings fill up with blood, and become as efficient in heat dissipation as automobile radiators.

Their testes, too, behave as he expected they would, moving backward during the breeding season toward the tail membrane. Dr. Cowles postulates that the venous blood, returning from the air-cooled membranes, keeps their temperature down. Next step will be to prove it with accurate observations. Fellow zoologists cheer him on, but predict that he will have trouble when he tries to take the temperature of a bat's testes while it is flying.

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