Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
Energetic cost of a meal in a frequent feeding lizard
Introduction
Energy must be harvested by an organism from its environment to sustain life (Purves et al., 1992). Animals obtain energy by consuming food, but there are energetic costs associated with feeding; the energy spent in procuring a meal and specific dynamic action (SDA), which is the energetic cost associated with digestion and assimilation. SDA refers to the rise in metabolic rate that occurs in all animals as a result of the consumption of a meal (Kleiber, 1961). SDA incorporates the cost of digestion, absorption, and assimilation of food as well as prey handling and gastrointestinal growth (Cruz-Neto et al., 2001, Secor, 2001). SDA can be measured as the change in the rate of oxygen consumption () of an organism after feeding (Jobling, 1981, Wang et al., 2001). Typically reaches a peak soon after feeding, depending on size and composition of the meal, before gradually decreasing to prefeeding values (Jobling, 1981, Secor and Phillips, 1997). The duration of SDA and the peak achieved help to define the energetic cost of feeding and are, therefore, important ecological factors that can be quantified experimentally (Jobling, 1981).
The largest factorial changes in metabolism following feeding occur in large reptiles that feed infrequently (Wang et al., 2001). For example, SDA in the python (Python molurus) elicits an increase in that is greater than that resulting from muscular exercise (Secor and Diamond, 1997).
Comparative studies of SDA have used the response of fasted animals to a single meal to determine the energetic cost of digestion and assimilation (Jobling, 1981, Janes and Chappell, 1995, Sievert and Bailey, 2000). While this method may offer an accurate estimation of SDA for animals that feed infrequently, such as large sit-and-wait predators, it may be less accurate for animals that feed frequently. Most lizards, for example, typically never have completely empty guts when active (Halliday and Adler, 1987, Greer, 1989) and may therefore experience SDA for long periods. Hence, it was the aim of this study to compare the SDA in a lizard resulting from frequent feeding to that resulting from a single meal.
The concept of frequent and infrequent feeding can be problematic (Secor, 2001). While an animal that feeds weekly may be considered a frequent feeder when compared with an animal that feeds monthly, the same animal may be considered an infrequent feeder compared with an animal that feeds daily. In this study the term frequent feeding, as applied to the feeding habit of lizards, describes feeding while under the influence of an SDA elicited by a previous meal.
The omnivorous lizard Eulamprus quoyii (family: Scincidae) was used as an experimental model as it readily feeds in captivity (Weigel, 1988). We measured the in E. quoyii after they consumed mealworms (Tenebrio molitor), either as a single meal or as several small meals, to determine whether the rise in was sustained as a result of feeding behaviour, and to enable the calculation of the relative contribution of SDA to the resting metabolism of E. quoyii.
Section snippets
Animal maintenance and collection
Twenty-eight adult Eulamprus quoyii were captured by noosing (Simmons, 1987) in coastal bushland near Maroubra Beach, Sydney, (33°58′S, 151°16′E) in May, 2001. Lizards were housed individually in 450 mm×350 mm glass aquaria in a temperature regulated room at 20 °C. A 40 W light bulb, placed at one end of each aquarium, created a temperature gradient, with 41 °C at the warm end and 21 °C at the cool end. The heating lights were on from 09.00 h to 18.30 h. Room lights were set to a 12:12
Assimilation efficiency of energy (AE)
The average dry matter content of mealworms in this study was 41.0±1.1%. Gross energy content of mealworms was 27.7±0.4 kJ per gram of dry matter. The AE of E. quoyii fed six mealworms was 85.7±1.3%.
SDA in response to single feeding
The body mass of the experimental group (mean: 26.9 g) was not significantly different from that of the controls (mean: 27.0 g; F1,10=0.002, P=0.96) at the start of the experiment. changed significantly over time in both the experimental and control groups (experimental: F7,39=6.437, P<0.01;
Discussion
The lizard controls in both experimental treatments experienced significant variations in over time. These variations are the result of an endogenous circadian rhythm in the resting metabolic rate of E. quoyii which persists in the absence of an external light cue, but is dampened by constant light (Iglesias, 2001). All lizards showed similar variations in through time as a result of these circadian rhythms. However, was significantly greater in the digesting lizards compared
Acknowledgements
We thank K. Robert, J. Herbert, J. Sparrow, K. Rogers and C. Brown for all their help and advice. We also thank G. Packard for advice on statistics and S. Abubla without whom this study would never have taken place. This research had the approval of the NSW National Parks and Wildlife Service, and the University of Sydney Animal Care and Ethics Committee (LO4/5-2001/1/3380).
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