The influence of circadian rhythms on pre- and post-prandial metabolism in the snake Lamprophis fuliginosus

Measuring standard metabolic rate (SMR) and specific dynamic action (SDA) has yielded insight into patterns of energy expenditure in snakes, but less emphasis has been placed on identifying metabolic variation and associated energy cost of circadian rhythms. To estimate SMR, SDA, and identify metabolic variation associated with circadian cycles in nocturnally active African house snakes (Lamprophis fuliginosus), we measured oxygen consumption rates (VO2) at frequent intervals before and during digestion of meals equaling 10%, 20% and 30% of their body mass. Circadian rhythms in metabolism were perceptible in the VO2 data during fasting and after the initial stages of digestion. We estimated SMR of L. fuliginosus (mean mass=16.7+/-0.3 g) to be 0.68+/-0.02 (+/-SEM) mL O2/h at 25 degrees C. Twenty-four hours after eating, VO2 peaked at 3.2-5.3 times SMR. During digestion of meals equaling 10-30% of their body mass, the volume of oxygen consumed ranged from 109 to 119 mL O2 for SMR, whereas extra oxygen consumed for digestion and assimilation ranged from 68 to 256 mL O2 (equivalent to 14.5-17.0% of ingested energy). The oxygen consumed due to the rise in metabolism during the active phase of the daily cycle ranged from 55 to 66 mL O2 during digestion. Peak VO2, digestive scope, and SDA increased with increasing meal size. Comparisons of our estimates to estimates derived from methods used in previous investigations resulted in wide variance of metabolic variables (up to 39%), likely due to the influence of circadian rhythms and activity on the selection of baseline metabolism. We suggest frequent VO2 measurements over multiple days, coupled with mathematical methods that reduce the influence of undesired sources of VO2 variation (e.g., activity, circadian cycles) are needed to reliably assess SMR and SDA in animals exhibiting strong circadian cycles.