After feeding, vertebrates (Sievert et al., 1988; Chakraborty et al., 1992; Wang et al., 1995) and invertebrates (Bayne and Scullard, 1977) exhibit elevated metabolic rates. This phenomenon, known as specific dynamic action (SDA), is positively correlated with both the quantity of protein ingested and the size of the meal (Coulson and Hernandez, 1979; Chakraborty et al., 1992; Secor and Diamond, 1997). This increase in energy use is related to processing the nutrients ingested during the meal: producing digestive enzymes, transporting nutrients, producing nitrogenous wastes, synthesizing proteins, and in some cases up-regulating the gut (Wang et al., 1995; Secor and Diamond, 1997).

The increase in metabolic rate caused by feeding decreases the amount of energy an animal can gain from its meal. For ectotherms, the energy lost to SDA is normally a small fraction of the energy ingested in the meal, although for some infrequently feeding species, it can represent a substantial fraction of the energy ingested (Chakraborty et al., 1992; Secor and Diamond, 1997).

Very few data exist regarding SDA in amphibians, and much of what exists deals with frogs that eat large, infrequent meals (Secor and Phillips, 1997; Powell et al., 1999). Our purpose was to determine the duration and amplitude of SDA in Bufo woodhousii allowed to eat natural food equivalent to 5% of their body mass. These toads eat small, frequent meals in contrast to the dietary habits of most of the anuran species in which SDA has been studied.

Materials and Methods

Male, female, and juvenile B. woodhousii (n = 12, mean mass + SE = 82 ± 15 g; range = 25–150 g) were collected in Lyon County, Kansas. The toads were housed in groups of no more than three per clear plastic container (50.5 cm × 50.5 cm × 22.2 cm) with a ventilated lid. Retreats and water were available at all times, and a mixed diet of crickets (Acheta) and/or mealworm larvae (Tenebrio) was provided three times per week. The toads were acclimated to 20 ± 1 C with a photoperiod of LD 14:10 with photophase beginning at 0600 h CST. Basking lamps with 25 or 40 W bulbs above the plastic containers were on the same photoperiod. The toads were handled during cleaning and feeding to prepare them for handling during the experimental period. All toads were acclimated at least 10 days before testing.

After being induced to urinate and being weighed, each subject was placed alone in its home container along with a meal of Acheta and/or Tenebrio and allowed 20 min to eat a meal equivalent to 5% of its body mass. If the food was ignored or only partially eaten, the meal was removed and another subject was chosen. After eating a full meal, the toad was placed in an opaque plastic 750-ml holding container that was identical to the experimental chamber except that the bottom of the holding container was lined with a damp paper towel. The subjects were habituated in the chamber at 23 ± 1.5 C for up to 3 h before measurements were taken. An hour before each oxygen consumption measurement, the toad was placed in the experimental chamber. O₂ consumption was measured for 1 min every 11 min with an open-flow system Gas Sensor Model 180C (Columbus Instruments). During each minute of sampling, continuous measurements were taken and averaged to give a mean O₂ consumption value for the minute. Air flow into the experimental chamber was 47.5 ± 3.5 ml/min. Air traveling between the experimental chamber and the oxygen sensor passed through a column containing Drierite® and soda lime to remove water and CO₂, respectively. Oxygen consumption was measured for 12 toads at 3, 7, 28, and 120 h after feeding. The order in which the measurements were done varied among the toads. For seven of the toads, the first measurement was taken at 120 h postfeeding, and for the other five toads, the first measurement was taken 3 h after feeding. Because toads were moved between the holding container and the experimental chamber prior to each measurement period, toads were equally handled and habituated to their chambers all four times. It is likely that the toads moved around somewhat during the experiment. Because the toads were handled in the same fashion prior to all four O₂ consumption measurements, their response to handling could have slightly elevated their O₂ consumption but should have done so uniformly across all four measurements. We were able to crudely monitor activity within the experimental chambers by listening for movement. We found that the toads quickly stopped moving once placed in their chambers, and most sat quietly throughout the measurements. Five 1-min trials were averaged during each time period for each subject.

We used a one-way repeated measures analysis of variance (ANOVA; df = 11) followed by a Student-Newman-Keuls (S-N-K) pairwise comparison test to ascertain whether O₂ consumption varied with time. Mean values were considered significantly different at P ≤ 0.05.

Results

The ANOVA showed a significant effect of the time since feeding on O₂ consumption (P = 0.0005). The S-N-K test demonstrated that toads had a significantly higher O₂ consumption at 3 h after feeding than at 7 (P = 0.003), 28 (P = 0.003), or 120 h (P = 0.0006) after feeding (Fig. 1). Values at 7, 28, and 120 h postfeeding were not significantly different from each other.

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Oxygen consumption was 1.7 times higher in toads that had eaten 3 h previously than it was in fasted (120 h postfeeding) toads. In B. woodhousii, SDA was relatively short and was completed by 7 h postfeeding.

Discussion

Our toads displayed the expected rise in oxygen consumption following feeding. We chose 5% of body mass as a meal size because our toads readily consumed that amount. We did not know the size of a typical meal for B. woodhousii because the availability of food and conditions for hunting vary over the activity season. Five percent of body mass was near the upper limit of what we observed toads voluntarily eating in the lab. Because of the volume of Acheta or Tenebrio this represented, it is likely that toads do not normally eat larger meals in the field, although bufonids have the ability to eat very large meals (Bush and Manhinick, 1962).

Recent work with reptiles and amphibians shows that the increase in oxygen consumption after feeding is related to the feeding ecology of the species. Species that ingest large, infrequent meals such as the snake Python molurus (Secor and Diamond, 1996), the lizard Varanus albigularis (Secor and Phillips, 1997), or the anurans Ceratophrys ornata and Pyxicephalus adspersus (Secor and Diamond, 1996) exhibit a large SDA response compared to species that eat smaller, more frequent meals. Bufo woodhousii eats small, frequent meals and displayed only a 1.7-fold increase in oxygen consumption following feeding. This agrees with other data for frequently feeding, nonanuran species (Houlihan et al., 1990; Lyndon et al., 1992; Sievert and Andreadis, 1999) as well as with data from an anuran that eats frequently (Wang et al., 1995; Secor and Diamond, 1996).

Our results were similar to those of the congeneric Bufo marinus that displayed a twofold increase following feeding (Wang et al., 1995) and Rana catesbeiana that increased oxygen consumption by fourfold following feeding (Secor and Diamond, 1996).

Our results varied from those of other studies in that our peak in oxygen consumption occurred relatively quickly after the toads were fed. Ceratophrys cranwelli (Powell et al., 1999), Nerodia sipedon (Sievert and Andreadis, 1999), and Varanus albigularis (Secor and Phillips, 1997) exhibit peak oxygen consumption values approximately a day after feeding. Rana catesbeiana, another frequently feeding anuran, fed a meal of 5% its body mass showed peak oxygen consumption at one day postfeeding. The peak occurred later when the frogs were fed larger meals (S. Secor, pers. comm.). Bufo marinus fed a peptone diet showed peak oxygen consumption levels 5–6 h postfeeding (Wang et al., 1995), but those toads did not have to digest whole insects as our toads did. Because of the paucity of data available, it is impossible to determine whether our rapid peak in oxygen consumption is aberrant or typical of frequently feeding anurans.

The minor increase in oxygen consumption observed in B. woodhousii after feeding should not constrain simultaneous activities. Bufo boreas has a factorial aerobic scope of 10 (Hillman and Withers, 1979), which is well above the approximately twofold increase in oxygen consumption observed in B. marinus (Wang et al., 1995) and in our toads after feeding. Assuming the aerobic scope of B. boreas is similar to that of B. woodhousii, then even after a large meal a toad could increase oxygen consumption considerably, which would allow it to undertake aerobic activity to avoid predators or move to another area.

Recent work with reptiles shows that SDA is influenced by a number of factors related to the animal's natural history (Secor and Diamond, 1997; Secor and Phillips, 1997). With the paucity of data available on SDA in amphibians, it is too early to say with certainty how the feeding ecology of amphibians influences SDA. More work with a variety of species is needed if we are to understand the importance of SDA in amphibians.