Respiration in Varanids
Lung morphology. Varanids (and helodermatids) are the only lizards that possess multichambered lungs (Perry, 1998). They are generally large and heterogene-ously subdivided into various chambers, with the apical chambers supplied with inspired air through a cartilage-reinforced secondary bronchus (Perry, 1998). The dorso-medial region of the varanid lung is where the majority of gas exchange occurs via dense parenchyma, while the caudal sacculated sections are poorly vascularized, highly compliant (Perry, 1998, 1983), and are believed to serve as ventilatory bellows (Klein and Owerkowicz, 2006). The dorsal and medial chambers of the lungs in varanids are essentially immobile due to the tight attachment of the dorsal lung surface to the body wall (Perry, 1992; pers. obs. 2009). In addition, a complete nonmuscular postpulmonary septum (PPS) that lies caudal to the lungs and cranial to the liver has evolved in varanids (Klein and Owerkowicz, 2006).
The lungs are primarily ventilated by costal aspiration; the craniolateral movement of the ribs is assisted by the PPS, which prevents the caudal regions of the lung from collapsing (Klein and Owerkowicz, 2006). The saccular caudal regions of the varanid lung are poorly vascularized (Perry, 1998) and generally do not participate in gas exchange; however, they are thought to aid in the ventilation of the immobile cranial portion of the lung that contains the dense respiratory parenchyma (Klein and Owerkowicz, 2006). Costal aspiration, however, is only completely effective when the animal is immobile (Owerkowicz et al., 1999). During locomotion, the intercostal muscles, M. external oblique and M. rec-tus abdominis actively produce lateral undulations of the trunk, diminishing their capacity to help ventilate the lungs (Ritter, 1996; Owerkowicz et al., 1999). This results in a reduced capacity to maintain the necessary blood oxygen concentrations required for intense activity; however, some derived lizards (i.e., Varanus exanthe-maticus) mitigate this axial constraint by using a gular pump in association with costal-driven respiration (Owerkowicz et al., 1999). Nevertheless, the lateral undulations associated with locomotion restrict squa-mate aerobic activity to short infrequent bursts (Carrier and Farmer, 2000a). Interestingly, varanids possess a cardiopulmonary system that permits oxygen uptake comparable to that of some mammals of similar size (Hicks and Farmer, 1999); however, their ability to ventilate the lung is greatly constrained by their axial morphology (Owerkowicz et al., 1999).
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