Cardiovascular correlates of maximal oxygen consumption rates in anuran amphibians

Summary

• 1. Maximal oxygen consumption rates ($\dot V_{{\text{O}}_{\text{2}} }$ max; units, ml/g·h) were determined for four species of amphibians representing four families with habitat preferences varying from aquatic to terrestrial. Measured$\dot V_{{\text{O}}_{\text{2}} }$ max were:Xenopus laevis (aquatic), 1.33±0.16;Rana pipiens (semi-terrestrial), 0.54±0.10;Bufo cognatus (terrestrial), 1.91±0.26; andScaphiopus couchii (terrestrial), 1.91±0.26.

• 2. In order to assess possible cardiovascular bases for these interspecific differences, heart rate increments (differences between resting and active heart rates) and ventricle weights were measured to evaluate differential cardiac outputs. In order to assess possible differential blood oxygen capacities, hematocrits and hemoglobin concentrations were measured. Blood volumes were determined to assess total blood oxygen storage capacities.

• 3. Ventricle weights were statisticaly significantly different (p<0.01) between all species,B. cognatus>S. couchii>X. laevis>R. pipiens. These differences were closely positively correlated with the maximal metabolic rates of the species (Fig. 3a).

• 4. There were no differences in heart rate increments between the four species (Fig. 2).

• 5. Blood oxygen capacities were directly correlated with hemoglobin concentrations (Fig. 1). There were no interspecific differences in the amounts of oxygen bound per gram of hemoglobin (1.3 ml O₂/g Hb). Blood oxygen capacities were significantly different in the following sequence;X. laevis >S. couchii andB. cognatus>R. pipiens.

• 6. X. laevis had statistically significantly greater hematocrits than did the other three species.R. pipiens had significantly lower mean corpuscular hemoglobin concentrations.

• 7. Blood volumes were statistically significantly different between all species examined,S. couchii>B. cognatus>X. laevis>R. pipiens.

• 8. It is suggested that greater maximal oxygen consumption rates in anurans are correlated with 1) increased cardiac outputs based upon increased stroke volumes, 2) increased blood oxygen capacities due to either increased mean corpuscular hemoglobin concentration or increased hematocrit. Increased selective pressure for aerobic metabolism is also closely positively correlated with maximal blood oxygen storage capabilities.