GALE,H.H.: Intermittent breathing in very large mammals.

Breath holding is common in reptiles. Intermittent breathing also occurs in very large mammals; whale, elephant, rhinoceros, hippopotamus, and giraffe. Both groups have low rates of metabolism. Ectothermy allows the low rates of reptiles. Endotherms have higher metabolic rates that are inversely related to body size. Hence the mass specific metabolic rate of a 4000 kg elephant is about the same as a 1kg monitor lizard. Low metabolic rates are coupled to low breathing frequencies, 6 breaths per minute (bpm) for the elephant and 3 bpm for the lizard. At low bpm, intermittent breathing reduces the cost of ventilation. Continuous slow breathing has lower airflow velocities that decrease the frictional resistance to both laminar and turbulent flow. But brief rapid discontinuous breaths use less energy overall because very slow respiratory muscle contraction is extremely inefficient. A female African elephant weighing about 4000 kg was videorecorded during rest. Thoracic movements were analyzed frame by frame with a potential accuracy of 1/30th sec. Breath frequency, bpm; and the durations of: inspiration Ti, expiration Te, and the pauses between breaths Tp; were measured. Average values were; bpm 5.8, Ti 3.1 sec, Te 1.9 sec, Tp 5.1 sec. Breath holding was in the inspiratory position. Initial observations of rhinoceros and giraffe were qualitatively the same, but with higher bpm and shorter durations that correlate with smaller body masses. This ventilatory pattern is reminiscent of reptilian inspiratory breath holding and active expiration of shorter duration than passive inspiration. Inspiratory pauses have the advantage of a larger air volume to draw upon during apnea. Whether the breathing pauses of very large mammals are also at respiratory system relaxation volume, thus requiring no muscular effort, remains to be tested.