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running makes them completely so. Studying the common green iguana (Iguana iguana), Carrierconfirmed that the rib muscles are active during locomotion, and that the lizard holds its breathwhile sprinting.Now, any athlete can tell you that holding your breath while running will seriously cut down on yourendurance. So Carrier posited that lizards (not unlike me) are restricted to short bursts of anaerobicexercise (less than thirty seconds), followed by prolonged panting to pay back the oxygen debt. (Anoxygen debt accrues when muscles work without oxygen; the result is that lactic acid accumulates,and it must be oxidized after the work is done.)Carrier's hypothesis was controversial, particularly among respiratory physiologists. Otherinvestigators had discovered that monitor lizards--a distant relative of Carrier's iguana--have highmetabolic rates. That is, unlike most so-called cold-blooded animals, monitors burn a lot of energyrapidly. A good example is the savannah monitor (Varanus exanthematicus), an African monitorlizard weighing about ten pounds, which spends most of its day patrolling its territory for tastyinsects. Its oxygen consumption is as high as that of such mammals as the armadillo, and so themonitor can't afford to hold its breath while moving. On the contrary, the animal should ventilate asoften and as vigorously as a metabolically equivalent mammal. But if the lizard can't rely on its ribmuscles to breathe while it walks, how does the monitor spend all day walking?The resolution to this apparent paradox required the joint efforts of physiologists and biomechanists. Tomasz Owerkowicz of Harvard University and Beth Brainerd of the University ofMassachusetts at Amherst trained savannah monitors to trot on a treadmill in front of an X-raymachine coupled to a video camera. The X-ray movies demonstrated that, as Carrier had predicted,when the animal ran relatively fast, respiration relying on the sub atmospheric pressures generatedby expansion of the rib cage was supplanted by a different method of breathing. Long, thin bonesbelow the tongue and in the neck seemed to be causing the lizard's throat and the floor of its mouthto expand and contract: the animal was "gulping" air on the run.This kind of lung ventilation, well known in frogs and salamanders, is called gular pumping [seeillustration below]. In fact, the use of head muscles rather than trunk muscles to power respirationpredates the evolution of lungs. Fish, for example, pump water across their gills with their headmuscles. But until the work of Owerkowicz and Brainerd, gular pumping had not been consideredan important factor for lung ventilation in reptiles.To show that gular pumping is the key to the monitor's endurance, Brainerd and Owerkowicz took agroup of treadmill-trained lizards on a road trip to the University of California, Irvine. There, togetherwith the physiologists James W. Hicks and Colleen Farmer, they custom-fitted the animals withsmall face masks, which enabled the biologists to measure the lizards' oxygen consumption whilethe animals ran a treadmill. First each lizard ran normally; then a plastic tube was inserted into themouth to keep the animal's mouth open and prevent gular pumping. And sure enough, when thegular pumping was eliminated, the monitor lizards acted more like Carrier's green iguanas.

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carrier, Monitor lizard, Savannah monitor

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