Contribution of Gular Pumping to Lung Ventilation in Monitor Lizards Tomasz Owerkowicz, 1 * Colleen G. Farmer, 2 James W. Hicks, 2 Elizabeth L. Brainerd 3 A controversial hypothesis has proposed that lizards are subject to a speed- dependent axial constraint that prevents effective lung ventilation during mod- erate- and high-speed locomotion. This hypothesis has been challenged by results demonstrating that monitor lizards (genus Varanus) experience no axial constraint. Evidence presented here shows that, during locomotion, varanids use a positive pressure gular pump to assist lung ventilation. Disabling the gular pump reveals that the axial constraint is present in varanids but it is masked by gular pumping under normal conditions. These findings support the predic- tion that the axial constraint may be found in other tetrapods that breathe by costal aspiration and locomote with a lateral undulatory gait. When lizards walk and run, they generally use a lateral undulatory gait in which their bodies flex from side to side with each stride. It has been proposed that these lat- eral flexions prevent effective lung ventila- tion during moderate- and high-speed loco- motion (1). When lizards breathe at rest, intercostal muscles are active bilaterally to rotate the ribs, thereby expanding the tho- rax and aspirating air into the lungs (2). During locomotion, however, the intercos- tal muscles cannot produce adequate lung ventilation because they are active unilat- erally to flex the trunk (2, 3). These results support the hypothesis that lizards are sub- ject to a speed-dependent axial constraint on lung ventilation. According to this hy- pothesis, just when a lizard needs more oxygen, because it is running faster, it is actually able to breathe less effectively (1). The axial constraint hypothesis has re- mained controversial for over a decade. Met- abolic studies of green iguanas (Iguana igua- na) and monitor lizards (genus Varanus) have found that oxygen consumption increases and blood oxygen concentrations remain elevated as speed increases during locomotion (4, 5). These observations suggest that ventilation in lizards does not decrease as speed increases, although ventilation volume was not mea- sured directly in these studies. Predictions about the relation between ventilation volume (V ˙ E , milliliters per minute per kilogram) and speed of locomo- tion can be made from the axial constraint hypothesis (6 ). If the constraint is not present, V ˙ E should increase with speed until the maximum rate of oxygen consumption (V ˙ O 2 max ) is reached and then remain con- stant at this maximum value (V ˙ Emax ) as speed increases (Fig. 1, blue line). Above the speed at which V ˙ O 2 max is reached, liz- ards rely on their well-developed capacity for anaerobic metabolism to supply the ad- ditional energy required for locomotion (5). During the recovery period immediately after exercise, V ˙ E does not exceed the value for V ˙ Emax recorded during exercise, and, in most situations, V ˙ E is expected to decrease rapidly after cessation of exercise (5). If a speed-dependent axial constraint is present (Fig. 1, red line), V ˙ E increases above resting values at low speeds because the in- tercostal muscles are not constrained when lizards are walking slowly (1, 2). However, at moderate and high speeds, the mechanical constraint on the axial musculature begins to exert its influence, and V ˙ E decreases with increasing speed. Maximum ventilation is not reached during exercise, but rather V ˙ Emax is reached immediately after exercise when the mechanical constraint due to locomotion is lifted, but the animal’s oxygen requirements remain high to pay back the oxygen debt incurred during anaerobic activity (Fig. 1, red line). Recent experiments performed to test these predictions yielded conflicting results for two different species of lizards (6 ). Re- sults from green iguanas were consistent with the predictions of the axial constraint hypoth- esis: V ˙ E decreased with increasing speed, and V ˙ Emax was reached during the recovery period (Fig. 1, red line). Results from savannah monitor lizards (Varanus exanthematicus), however, agreed with the predictions of no axial constraint (Fig. 1, blue line). Ventilation increased with increasing speed up to 2 km hour -1 and leveled off between 2 and 3 km hour -1 , and V ˙ Emax was reached during loco- motion rather than during recovery. These findings cast doubt on the generality of the axial constraint hypothesis; green iguanas ap- pear to be subject to the constraint, but va- ranids do not, even though both lizards use lateral undulatory gaits (6 ). To determine whether differences in breathing mechanics might explain the con- flicting results obtained from varanids and iguanas, we used videoradiography to ob- serve lung ventilation in savannah monitors (V. exanthematicus) and green iguanas (I. iguana) during locomotion on a treadmill at speeds between 1 and 3 km hour -1 (7 ). In savannah monitors, a breathing cycle begins with exhalation (Fig. 2A), followed immedi- ately by a costal inspiratory phase that does not completely inflate the lungs (Fig. 2B). During costal inspiration, the hyobranchial apparatus is depressed to expand the gular cavity (Fig. 2B) and then elevated to com- 1 Museum of Comparative Zoology, Harvard Universi- ty, Cambridge, MA 02138, USA. 2 Department of Ecol- ogy and Evolutionary Biology, University of California, Irvine, CA 92697, USA. 3 Department of Biology and Program in Organismic and Evolutionary Biology, Uni- versity of Massachusetts, Amherst, MA 01003, USA. *To whom correspondence should be addressed. E- mail: towerkow@oeb.harvard.edu Table 1. Locomotor endurance times (20) of six monitor lizards (V. exanthematicus) with function- al (control) and impaired (experiment) gular pumps (values  SEM). Endurance times of gular- impaired lizards were statistically significantly shorter at 2 and 3 km hour -1 than the endurance times of the same animals under control condi- tions (paired t test; P  0.01). Treadmill speed (km hour -1 ) Endurance (seconds before exhaustion) Control Experiment 1* 900 900 2 346  55 225  25 3 140  14 89  5 *At 1 km hour -1 , exhaustion was not reached in either treatment during the 15-min exercise period. Fig. 1. Predictions of the speed-dependent axial constraint hypothesis. The axial constraint hy- pothesis predicts that, above a threshold speed, minute ventilation (V ˙ E ) will decrease with in- creasing speed and maximum minute ventila- tion (V ˙ Emax ) will be reached during the recovery period immediately after exercise. R EPORTS www.sciencemag.org SCIENCE VOL 284 4 JUNE 1999 1661