Jackson R. Phillips, Amanda E. Hewes, and Kurt Schwenk
We describe air-breathing mechanics in gray tree frog tadpoles (Hyla versicolor). We found that H. versicolor tadpoles breathe by ‘bubble-sucking’, a breathing mode typically employed by tadpoles too small to break the water’s surface tension in which a bubble is drawn into the buccal cavity and compressed into the lungs. In most tadpoles, bubble-sucking is replaced by breach-breathing (breaking the surface to access air) at larger body sizes. In contrast, H. versicolor tadpoles bubble-suck throughout the larval period, despite reaching body sizes at which breaching is possible. H. versicolor tadpoles exhibit two bubble-sucking behaviors: ‘single bubble-sucking’, previously described in other tadpole species, is characterized by a single suction event followed by a compression phase to fill the lungs. ‘Double bubble-sucking’ is a novel, apparently derived form of bubble-sucking that adds a second suction event. H. versicolor tadpoles transition from single bubble-sucking to double bubble-sucking at approximately 5.7 mm SVL, which corresponds to a period of rapid lung maturation when they transition from low to high vascularization (6.0 mm SVL). Functional, behavioral, and morphological evidence suggest that double bubble-sucking increases the efficiency of pulmonary gas exchange by separating expired, deoxygenated air from freshly inspired air to prevent mixing. H. versicolor and possibly other hylid tadpoles, may have specialized for bubble-sucking in order to take advantage of this increased efficiency. Single- and double bubble-sucking represent two- and four-stroke ventilation systems, which we discuss in the context of other anamniote air-breathing mechanisms.