Pulmonary compliance and lung volume varies with ecomorphology in anuran amphibians: implications for ventilatory-assisted lymph flux

Vertical movement of lymph from ventral regions to the dorsally located lymph hearts in anurans is accomplished by specialized skeletal muscles working in concert with lung ventilation. We hypothesize that more terrestrial species with greater lymph mobilization capacities and higher lymph flux rates will have larger lung volumes and higher pulmonary compliance than more semi-aquatic or aquatic species. We measured in situ mean and maximal compliance (Δvolume/Δpressure), distensibility (%Δvolume/Δpressure) and lung volume over a range of physiological pressures (1.0 to 4.0 cmH(2)O) for nine species of anurans representing three families (Bufonide, Ranidae and Pipidae) that span a range of body masses and habitats from terrestrial to aquatic. We further examined the relationship between these pulmonary variables and lymph flux for a semi-terrestrial bufonid (Rhinella marina), a semi-aquatic ranid (Lithobates catesbeianus) and an aquatic pipid (Xenopus laevis). Allometric scaling of pulmonary compliance and lung volume with body mass showed significant differences at the family level, with scaling exponents ranging from ∼0.75 in Bufonidae to ∼1.3 in Pipidae. Consistent with our hypothesis, the terrestrial Bufonidae species had significantly greater pulmonary compliance and greater lung volumes compared with semi-aquatic Ranidae and aquatic Pipidae species. Pulmonary distensibility ranged from ∼20 to 35% cmH(2)O(-1) for the three families but did not correlate with ecomorphology. For the three species for which lymph flux data are available, R. marina had a significantly higher (P<0.001) maximal compliance (84.9±2.7 ml cmH(2)O(-1) kg(-1)) and lung volume (242.1±5.5 ml kg(-1)) compared with L. catesbeianus (54.5±0.12 ml cmH(2)O(-1) kg(-1) and 139.3±0.5 ml kg(-1)) and X. laevis (30.8±0.7 ml cmH(2)O(-1) kg(-1) and 61.3±2.5 ml kg(-1)). Lymph flux rates were also highest for R. marina, lowest for X. laevis and intermediate in L. catesbeianus. Thus, there is a strong correlation between pulmonary compliance, lung volume and lymph flux rates, which suggests that lymph mobilization capacity may explain some of the variation in pulmonary compliance and lung volume in anurans.