Metabolic Costs of Rodents Feeding on Plant Chemical Defenses: A Comparison Between an Herbivore and an Omnivore

Comp. Biochem. Physiol. Vol. 117A, No. 4, pp. 511–514, 1997 Copyright  1997 Elsevier Science Inc.

ISSN 0300-9629/97/$17.00 PII S0300-9629(96)00409-4

Metabolic Costs of Rodents Feeding on Plant Chemical Defenses: A Comparison Between an Herbivore and an Omnivore Francisco Bozinovic,1 and F. Fernando Novoa 2 1

Departamento de Ecologi´a, Facultad de Ciencias Biolo´gicas, P. Universidad Cato´lica de Chile, Casilla 114-D, Santiago, Chile, and 2 Departamento de Ciencias Ecolo´gicas, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile ABSTRACT. We analyzed the combined effect of subchronic dietary cellulose and tannic acid on the metabolic cost in the herbivorous burrowing caviomorph rodent Octodon degus, and the omnivorous Sigmodontine Phyllotis darwini. Both are sympatric rodent species living in open scrub subjected to summer droughts in the Mediterranean communities of northern and central Chile. We measured basal metabolic rate (BMR), maximum metabolic rate of thermoregulation (MMR) and the aerobic scope. As a consequence of the ingestion of comparative higher concentrations of dietary tannic acid and fiber, a higher MMR was observed in P. darwini than in the herbivorous O. degus. Metabolic costs were observed under maximal but not under basal conditions. comp biochem physiol 117A;4:511–514, 1997.  1997 Elsevier Science Inc. KEY WORDS. Tannins, fiber, metabolic costs, Octodon degus, Phyllotis darwini

INTRODUCTION The effects of variation of chemical defenses on food preferences and digestive efficiency have been explored in detail in many studies (18). The coupled, and likely synergistic, effects of subchronic levels of dietary fiber and tannins on feeding behavior and digestion, have received considerably less attention (9). Cork and Foley (6) proposed that the main physiological responses of mammals to deal with plant defenses are metabolic. McNab (13) postulated that at interspecific levels, the low basal metabolic rates found in folivorous and frugivorous mammals constitute an adaptation to low nutrient availability and high concentration of defensive compounds in plants. Species exploiting food with low energy contents and/or high cost of digestion generally have low, mass-independent metabolic rates. Nevertheless, experimental results on the effect of food quality on the level of metabolism in small mammals are contradictory (3,5,7,8,23,24). Low availability of energy in poor diets and lower metabolizable energy imposed by slower gut processing rates may limit energy expenditures and allocation to biomass production. In this paper, we investigate the energetics of two sympatric species of rodents inhabiting the Mediterranean region of Chile subjected to alternative experimental diets difAddress reprint requests to: Francisco Bozinovic, Departamento de Ecologı´a, Facultad de Ciencias Biolo´ gicas, P. Universidad Cato´lica de Chile, Casilla 114-D, Santiago, Chile. Tel. (56-2) 686-2618; Fax (56-2) 222-5515; E-mail: [email protected].

fering in cellulose or fiber (F) and secondary metabolites (the hydrolyzable tannin, tannic acid (TA)). These two species differ in diet, and likely have different metabolic characteristics to cope with plant defenses. We analyzed the combined effect of subchronic dietary cellulose and tannic acid on the metabolic rate of the herbivorous caviomorph rodent Octodon degus (degu), and the granivorous Sigmodontine Phyllotis darwini (leaf-eared mouse). Both species live in open scrub habitat subjected to summer droughts, but they have dietary differences in the relative proportions of shrub, grass foliage and seeds. Degus feed primarily on grasses, forb, shrub foliage, and conductive tissue. The leafeared mouse is mainly granivorous, but it also feeds on insects (14–16). MATERIAL AND METHODS Animals and Maintenance The experimental animals tested were non-reproductive; all were captured in Quebrada de la Plata, central Chile (70°50′W, 33°31′S). In the laboratory, they were first maintained on rabbit food pellet and later randomly assigned to dietary groups. The study was conducted during winter and spring. Powdered diets were prepared by adding and homogenizing cellulose and tannic acid (Sigma Chemical Company) to commercial rabbit food pellets. Diets were analyzed for neutral (NDF) and acid (ADF) detergent fiber, or simply fiber (2); nitrogen content of the diets was measured using a microKjeldhal method (1).

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Four groups of five O. degus and four P. darwini each were maintained on diets with high TA-high F, low TA-low F, high TA-low F, and low TA-high F, respectively. Following Karasov et al. (11) we choose as high TA a 4% addition of TA into the diet because this concentration is sufficient to cause a reduction in apparent digestive efficiency, but not as high as to cause mass loss and thus nonspecific damage to intestinal function. Low dietary TA was 1%. The level of F incorporated into the diet was 52%-NDF and 32%ADF for high fiber, and 43%-NDF and 20%-ADF for the low fiber diet, following Bozinovic (3). This author documented that at Quebrada de la Plata, the grass consumed by O. degus contained 61.1% NDF during the dry season (summer), and 37.3% NDF during the wet season (fall-winter). After chemical and caloric analysis, the energy content, and the nitrogen and fiber (ADF and NDF) composition of the experimental diets were: 1) high TA-high F: 18.00 kJ/g, 2.36% N, 32.04% ADF and 51.92% NDF; 2) low TA-low F: 17.92 kJ/g, 2.31% N, 30.63% ADF and 51.50 NDF; 3) high TA-low F: 17.95 kJ/g, 2.59% N, 20.49% ADF and 43.60% NDF; and 4) low TA-high F: 18.05 kJ/g, 2.63% N, 20.25% ADF and 42.68% NDF. Energy Metabolism To test whether the alteration of metabolic rate would operate as a physiological mechanism to decrease the activity of plant defenses and to find out whether the level of metabolic rate and aerobic scope is correlated with the amount and interaction of plant defensive compounds, the basal rate of energy metabolism of each individual was determined after dietary acclimation by measurement of oxygen consumption within the thermoneutral zone of these two species [30°C, see (4,19)] using a computerized closed automatic system (17). CO 2 and H2 O in the metabolic chamber were absorbed with Ba(OH)2 and CaCl 2, respectively. Ambient temperature was controlled in a thermoregulated bath. Each measurement was conducted for 2 h in postabsortive individuals during the resting phase of their activity cycle. According to standard procedures, the average of three minimum oxygen consumption values were considered for each individual. Maximum resting metabolic rate for thermoregulation (MMR) was measured at Ta 5 5°C with mixtures of 20% O2 and 80% He, a 4-fold more conductive medium than air which greatly facilitates heat transfer and elicits maximum thermogenic resting metabolism (20). After approximately one hour in which O2-consumption was measured, metabolic chambers were flushed with six times their volume of the He-O 2 mixture while O2-consumption was continuously monitored. Rodents were removed when their O2-consumption declined, and body temperature (T b ) was immediately measured. All animals removed exhibited hypothermia, which indicated that MMR had been attained.

Body mass (6 0.1 g) was recorded before and after each O2-consumption measurements. Metabolic rates were measured after 1 week of treatment with the experimental diets. Observed values of metabolic rate were compared to the expected values of basal metabolic rate (BMR) and maximum metabolic rate (MMR) predicted for rodents—i.e., BMR 5 4.296 m20.29 and MMR 5 27.14 m20.32 (3) where b, b mb is body mass in grams and metabolic rate is expressed in mLO 2/g hr. Statistics The significance of the effect of dietary TA and F on variables was assessed by one-way and multifactorial analyses of variance (ANOVA), using the a posteriori Tukey test for multiple comparisons between groups; mb was used as a covariate when appropriate. In the ANOVA we evaluated F and TA and the interaction (*) between factors (21). Results are given as means 6 1 SD. RESULTS AND DISCUSSION Body mass was different between species, but did not differ significantly between treatments and within individuals of the same species, during or at the end of O2-consumption measurements (see Table 1). In both species the minimal or basal rate of metabolism showed no significant change due to treatment effect. In all cases, the observed values were similar to those expected from body mass (Table 1). Both the maximum metabolic rate for thermoregulation and consequently the aerobic scope increased significantly (18.8 and 20.6%, respectively) due to the effect of dietary TA, but not by effects of F and the interaction F*Ta (Table 1). The results of our study may be used to answer the following questions. 1) What are the combined, and perhaps synergistic, effects of tannic acid and fiber (the most common type of exposure of animals to plant food) on the metabolic rate in herbivorous and omnivorous rodents? and 2) What is the metabolic variability in omnivorous and herbivorous rodents when consuming different concentrations of plants defenses? The answers to these questions are important for understanding the physiological and ecological significance of animals when feeding on chemically-defended plants. The value of foods can be measured in terms of metabolizable energy. Theoretically, the low availability of energy in low-quality or defended diets and the lower metabolizable energy imposed by lower gut processing rates may limit metabolic energy expenditure. However, we previously documented no effect of dietary fiber on the BMR of the herbivorous O. degus (4), whereas Veloso and Bozinovic (23) documented an increment in BMR in this species acclimated to high lipid, high protein and low fiber diets. Thomas et al. (22) measured a 14–23% increase in BMR

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TABLE 1. Effect of four experimental diets on the metabolic rates and aerobic scope in the herbivorous Octodon degus and

the omnivorous Phyllotis darwini (X 6 1 SD)

Number of individuals Octodon degus Phyllotis darwini Body mass (g) Octodon degus Pyllotis darwini P-values Metabolic rate (mLO 2 /g hr) BMR Octodon degus BMR/4.29 m b20.29 (%) Phyllotis darwini BMR/4.29 m b20.29 (%) P-values MMR Octodon degus MMR/27.14 m b20.32 (%) Phyllotis darwini MMR/27.14 m b20.32 (%) P-values Aerobic Scope (MMR–BMR) Octodon degus Phyllotis darwini P-values

High fiber—high TA

Low fiber—low TA

Low fiber—high TA

High fiber—low TA

5 4

5 4

5 4

5 4

185.8 6 6.1 50.4 6 4.9 Main 5 0.989

183.2 6 8.5 61.8 6 6.0 F 5 0.988

174.0 6 6.4 49.8 6 3.2 TA 5 0.888

176.7 6 6.9 52.9 6 4.2 F*TA 5 0.757

0.87 6 0.04 91.87 6 4.34 1.29 6 0.05 94.15 6 3.93 Main 5 0.303

0.84 6 0.02 87.15 6 1.13 1.14 6 0.05 89.03 6 5.05 F 5 0.535

0.81 6 0.07 84.06 6 6.35 1.22 6 0.12 87.88 6 7.50 TA 5 0.159

0.79 6 0.04 94.15 6 3.93 1.16 6 0.07 85.09 6 2.92 F*TA 5 0.009

5.78 6 0.73 120.39 6 21.82 7.54 6 0.70 94.84 6 5.98 Main 5 0.019

5.14 6 0.30 105.74 6 6.34 5.98 6 0.68 75.88 6 8.39 F 5 0.777

4.89 6 1.95 93.91 6 6.16 7.53 6 0.43 96.44 6 3.68 TA 5 0.006

3.88 6 0.12 77.17 6 4.26 5.98 6 0.69 77.84 6 7.79 F*TA 5 0.06

4.89 6 0.73 6.25 6 0.69 Main 5 0.023

4.30 6 0.29 4.84 6 0.67 F 5 0.719

4.08 6 0.26 6.31 6 0.39 TA 5 0.008

3.09 6 0.13 4.82 6 0.66 F*TA 5 0.09

BMR 5 basal metabolic rate, MMR 5 maximum metabolic rate, mb 5 body mass. P values are from multifactorial analysis of variance (ANOVA) with m b as a covariate. The effects of fiber (F), tannic acid (TA), and interactions are shown.

of the vole Microtus pennsylvanicus associated with the ingestion of diets with 6% dry-mass phenol gallic acid. According to those authors this increased metabolic cost may be associated with increased protein synthesis for detoxification and tissue repair. We did not observe any effect of dietary treatment on BMR in either of the two species studied. Nevertheless, although nonapparent under basal conditions, the metabolic cost by effect of 4% dietary TA was elicited at high metabolic loads stimulated by cold (Table 1). Comparing MMR between high fiber-high TA and high fiber-high TA treatments, we observed a 32.8% increase in O. degus and a 20.7% increase in P. darwini under high fiberhigh TA treatment. Comparing MMR values between low fiber-low TA and low fiber-low TA treatments, a 4.9% increase was observed in O. degus and a 20.6% increase in P. darwini under the low fiber–high TA treatment. If this difference is indicative of a general difference between herbivores and omnivores, then the following consequences may apply, the effect of TA was more significant in the omnivorous P. darwini rather than in the herbivorous O. degus, giving support to our hypothesis that herbivores, not omnivores, are able to deal with, and neutralize, plant defensive compounds more efficiently. According to Foley and McArthur (9), these results suggest a carry-over effect of TA, and perhaps of allelochemical compounds in general, affecting the MMR and therefore

the aerobic scope, as a consequence of the cost of detoxification. Apparently, the effect of TA reflects a species-specific physiological dependence. Based on our results, omnivores may have higher metabolic cost than herbivores when dealing with TA, probably as a consequence of tissue repair of digestive tract mass loss produced by tannins. Nevertheless, the usual difficulty with these studies is that the consequences are generally indirect and therefore the conclusions are inferential because we are dealing with a system that is potentially influenced by many factors. Traditional theoretical frameworks dealing with the effects of defensive compounds on herbivores have analyzed separately their physiological consequences under in vitro conditions. Nevertheless, and according to McArthur et al. (12), it is necessary to consider that in nature tannins and fibers may act together, reducing animal nutritional balance and increasing their metabolic costs through their combined effects. Observations on small mammals feeding on nutritionally poor and highly defended plants in the field during a specific period of the year, indicate that feeding on low quality food is rather an environmental constraint than a choice (3,10). Under such conditions, herbivorous mammals appear to be better equipped to neutralize almost every kind of chemical barrier, even if detoxification may be a metabolically costly process.

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This work was funded by a Fondo de Desarrollo Cientı´fico y Tecnolo´gico, FONDECYT grant 1950394 to F.B. Patricio A. Camus, F. M. Jaksic, and M. Rosenmann made useful comments on an earlier draft of this manuscript.

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