Physiologists have traditionally studied elevated metabolic states in isolation (e.g., the physiological response to physical activity). However, under natural conditions animals may perform more than one elevated metabolic state simultaneously (e.g., predator avoidance while processing a recently ingested meal) and the integrated response to such a condition is not well understood.
For my dissertation work, I used oviparous (Lamprophis fuliginosus) and viviparous (Thamnophis marcianus) snakes to test whether the oxygen delivery capacity of these species' cardiopulmonary systems would be sufficient to meet the oxygen demands from reproduction, physical activity, and digestion simultaneously (i.e., additivity). Flow-through respirometry was used to measure levels of gas exchange (i.e., oxygen consumption and carbon dioxide production) during standard conditions (i.e., standard metabolic rate, SMR), and then during reproduction, activity (while fasting), digestion of a large (i.e., 10 to 20% body mass) meal, and post-prandial activity.
Results indicate that reproduction causes significant increments in oxygen consumption for females of both species. Levels ranged from 1.4 to 3.2 fold above SMR. During physical activity (while fasting), males and females of both species exhibited peak factorial increments in oxygen consumption that ranged from ~6 to 9.6 fold above SMR; digestion resulted in peak factorial increments of ~3 to 6 fold above SMR. Contrary to predictions, males and non-reproductive females of both species exhibited a prioritization pattern of oxygen delivery during post-prandial activity (i.e., factorial increments in oxygen consumption were similar to or lower than levels attained during activity while fasting). When reproduction and digestion were combined, female Lamprophis fuliginosus exhibited a prioritization pattern of oxygen delivery, while Thamnophis marcianus exhibited a more-than-additive pattern of oxygen delivery (i.e., there was a 30-50% increase in oxygen consumption above non-reproductive levels). Contrary to predictions, during the combination of reproduction and post-prandial activity females of both species exhibited a prioritization pattern of oxygen delivery.
Overall, results from my dissertation work indicate that these species' cardiopulmonary systems' capacity for oxygen delivery is limited to increments of approximately 9 to 10 fold above SMR. In most cases, activity performance (i.e., time to exhaustion, TTE) was maintained, possibly at the expense of digestive and reproductive processes.