Metabolic rates and thermoregulatory characteristics of Akodon azarae
(Rodentia: Sigmodontinae)

Tasas metabólicas y características termorregulatorias de Akodon azarae
(Rodentia: Sigmodontinae)

C. DANIEL ANTINUCHI¹ & CRISTINA BUSCH

ABSTRACT

In this work, we evaluated the energetic status, activity daily metabolic rate, thermoregulatory capacity and the existence of torpor in Akodon azarae (Fisher 1829). Basal metabolic rate (BMR) was similar to the expected value from Kleiber´s, and also according to the BMR of phylogenetically related species. No statistical difference in BMR between sexes was detected. For both males and females, activity daily metabolic rate was 235 % of BMR. Body temperature was not statistically different, neither at different ambient temperatures nor between sexes. In the thermoneutral zone, body temperature was 36.1 ± 1.5 °C. Thermal conductance was lower than the expected for mammals whereas thermoregulatory capacity was 136 % of the expected. Akodon azarae did not show evidence of torpor.

Key words: metabolic rates, thermoregulation, energetic, Akodon azarae.

RESUMEN

En este trabajo fueron evaluadas, para Akodon azarae (Fisher 1829), la tasa metabólica basal (BMR) y de actividad diaria así como la capacidad termorregulatoria y la posible existencia de torpor. La BMR no presentó diferencias estadísticas con la esperada por la ecuación de Kleiber así como con la esperada para especies filogenéticamente relacionadas. Tampoco fueron detectadas diferencias significativas en la BMR entre sexos. La tasa metabólica de actividad diaria para ambos sexos fue el 235 % de la BMR. La temperatura corporal no presentó variaciones entre individuos mantenidos a diferentes temperaturas ambiente ni tampoco entre sexos y en termoneutralidad fue de 36,1 ± 1,5 °C. Por otra parte, la conductancia térmica fue menor a la esperada mientras que la capacidad termorregulatoria fue el 136% de la esperada. Akodon azarae no presenta sopor.

Palabras clave: tasas metabólicas, termorregulación, energética, Akodon azarae.

INTRODUCTION

Endothermy is very expensive (McNab 1974) and endotherms expend most of the energy consumed to produce heat. Endotherms have sufficient fur insulation and thermogenic capacity to survive at low temperatures, but they need a regular food supply to do so. In consequence, endothermy allows mammals to maintain high body temperatures and independence of environmental conditions, but it becomes prohibitive in both unproductive and/or cold habitats (French 1992).

Body size measured as body weight (W), is the physical translator between the environment and the organism. Consequently, it constitutes the primary factor that determines the basal metabolic rate (BMR) and thermal conductance of individuals (C). Most vertebrates follow the Kleiber (1961) relation down to a "critical" W, below which the scaling of BMR must be changed to ensure the maintenance of endothermy. According to McNab (1983), the minimal W for endothermy in species that have a level of BMR similar to the Kleiber relation, is namely about 20 g. Contreras (1989), reported that, the minimal W to have a level of BMR similar to the Kleiber, is above 50 g. Therefore, a combination of a small body mass and a low BMR would cause animals enter in torpor. Kleiber´s equation has been discredited as a reference curve (Hayssen & Lacey 1985).

However, Kleiber´s equation has been used countless quantity of times in most of the research on energetics, for which it has a great comparative value. McNab (1988) discussed that Kleiber´s recommended curve have an acceptable level. Moreover, Bozinovic & Rosenmann (1988b) confirm the universality of the (-0.25) mass-specific allometric value reported by Kleiber and favor its use as a standard reference. On the other hand, independent of the influence of W, in mammals, food habits exert an important effect on BMR (McNab 1986).

Some endothermic species, when facing low temperatures, reduce substantially their heat production, entering torpor and thus balancing their total energy requirements. Thus, during daily torpor, animals drop their body temperatures between approximately 10°C and 25 °C and torpor bouts of less than 24 h (Hudson 1973). This mechanism, which allows species to inhabit both unproductive and/or cold areas, has been found in five species of South American rodents (Bozinovic & Contreras 1990, Caviedes-Vidal et al. 1990, Bozinovic & Marquet 1991). Bozinovic & Rosenmann (1988b), based on preliminary data set, hypothesized that other South American rodents such as Akodon azarae (Fisher 1829), Calomys ducilla, C. musculinus and Eligmodontia typus would show evidence of torpor at low ambient temperatures.

Other energetic studies have been accomplished on South American rodents (Caviedes-Vidal et al. 1987, Bozinovic & Rosenmann 1988a). For A. azarae, Dalby & Heath (1976) obtained BMR values on a single observation, not allowing any type of statistical comparison on BMR and thermoregulatory capacity nor between the sexes. In the present study, BMR, activity daily metabolic rate, thermoregulatory characteristics, and the existence of torpor in Akodon azarae are evaluated. For small omnivores as A. azarae an intermediate BMR (90 to 110% of Kleiber) would be expected. Furthermore, rodents as A. azarae, which tolerate low temperatures during winter having both, low BMR and small body mass, should show torpor or low thermal conductance.

MATERIAL AND METHODS

Animals

Animals of both sexes (26 females and 27 males) were collected using Sherman live traps at Necochea, Buenos Aires Province, Argentina (38º29' S, 58º50' W). This region is known as the "Pampeana" biogeographic province (Cabrera & Willink 1973). Akodon azarae is a mouse of 24 g, strongly associated to natural grasslands, particularly to open formations (Bonaventura et al. 1992, Redford & Eisenberg 1992). This species, which is the most numerically representative rodent of the pampas grasslands, is found from southern Brazil to central Argentina (Redford & Eisenberg 1992).

Captured rodents were carried to the laboratory and housed individually in animal cages (0.3 x 0.22 x 0.15 m). Wood shaving and cotton for nesting material were placed on cage floors. All animals were maintained on a natural photoperiod (L:D = 10:14). Ambient temperature (T_a) ranged from 19 to 25 ºC, whereas relative ambient humidity ranged from 50 to 70 %. Animals were fed with pellets ad libitum (composition in %: minimum protein: 21, maximum fiber: 4.5, minimum fat: 8, average calcium: 1.8, phosphorous: 1.1, maximum ashes: 8). Tap water was provided ad libitum.

Energetics

The energetics variables were individually estimated by measurements of oxygen consumption (VO₂) in metabolic chambers at different T_a´s using a modified closed automatic system, based on the design of Kalabukhov-Skvortsof (Gorecki 1975). The metabolic chamber was a double wall aluminum cage with polyurethane between walls. Animals were put inside the chamber, which was closed with a 20 mm thick acrylic window-door to observe the animals regularly. A thermocouple was used to measure temperature inside the chamber. Heat was interchanged with the outside chamber automatically by two Pelttier intercoolers (Melcor, model CP-1.4-127-06l) connected to a PC and controlled by software. Ambient temperature inside the chamber was maintained with an accuracy of 0.1 °C. An internal and an external pressure sensor monitored air pressure. Oxygen influx was assessed through two electrovalves. Pressure sensors and electrovalves were connected to a PC and controlled by a software. CO₂ and H₂O were removed by ascarite and drierite. In order to achieve a post-absortive state, metabolic rates (MR) were measured in 24 h food deprived animals. Animals were weighted before each VO₂ measure. VO₂ was always measured in the morning. Data were discarded when activity was observed. To minimize the effect of stress, VO₂ records during the first 30 min were discarded. Basal metabolic rate was estimated from the minimal metabolic rate when it showed independence of T_a. For measuring activity daily metabolic rates (ADMR), animals were disposed into the respirometer during 24 h with food at thermoneutrality, provided with a treadmill.

VO₂ is reported as mass-specific metabolic rate (ml/gh). All results are presented as means ± standard deviation (SD). All measures of VO₂ were corrected to standard temperature and pressure of dry air. Nickerson et al. (1989) method, that identifies the best fit for a continuous n-phase regression, was used to represent the relationship between T_a and MR. This method estimates the point at which the relationship between independent and dependent variables change (i.e. threshold point). The best n-phase regression model was selected by comparing from, a three-phase regression models to a simple regression model.

Body temperature and thermal variables

Body (rectal) temperatures (T_b) and ambient temperature (T_a) were measured using a YSI probe (model 93k73545-402) connected to a Cole-Parmer instrument thermistor (model 8402-10). Body temperature, was recorded at the beginning (T_bb) and at the end (T_be) of each VO₂ measurement by inserting a thermocouple 2 cm into the rectum of the mouse for a period no longer than 30 s. Decrease in T_b after each VO₂ measurement was estimated as DT_b= T_be - T_bb. The slope of the first phase of the best fitted n-phase regression that related MR and T_a was used as an estimate of minimal thermal conductance (C_min). Thermoregulatory capacity was estimated by the equation DTm = BMR/C (McNab 1974). The equations of: Kleiber (1961): BMR = 3.4 W^-0.25; Morrison & Ryser (1951): C = 1.0 W^-0.5 ; McNab (1983) BMR (endothermic limit) = 15.56 W^-0.67; and the minimal thermal differential DTm = 15.56 W^-0.17; were taken as standard references for eutherian mammals. The equation of Hayssen & Lacey (1985): BMR = 5.73 W^-0.359 was taken as standard references for murid rodents.

Daily torpor

Since in the field sampling, during winter, mortality of 80 % was observed when animals were in the traps for more than 4 h at T_a lower than 10 °C, VO₂ was measured in individuals at 15 °C during 4 h.

Statistics

ANOVA was used both to test the null hypothesis of no differences among metabolic rates at different T_a between sexes and to test the null hypothesis of no differences among DT_b neither at different T_a nor between sexes. A posteriori Tukey´s test was used to identify differences among DT_b, both at different T_a and between sexes.

A t- test was used either to evaluate the null hypothesis of no differences between observed BMR and the expected one by Kleiber´s equation; to evaluate the null hypothesis of no differences between observed BMR and the expected one by Hayssen & Lacey´s equation (1985); to test the null hypothesis of no differences in ADMR between sexes; to test the null hypothesis of no differences between the BMR observed and the expected one by McNab´s equation on the euthermic limit; and finally for animals reared in animals room to test the null hypothesis of no differences in T_b between sexes. A t- test for comparisons between two slopes was used to test the null hypothesis of no differences between observed and expected C_min. An analysis of sensitivity was used to solve the possible meaningless of SD values of estimated BMR and C_min, since these parameters reflect the variance of W between the animals.

RESULTS

Metabolic measurement

No statistical differences among MR of males and females were detected at any T_a (F = 0.71, n = 70, df = 7, P > 0.5). To analyze the relationship between MR and T_a, data from both sexes were pooled. Mean MR was lowest at 35 °C (best fit: was a two-phase lineal regression, break point = 35 °C, Fig. 1, Table 1). Estimated BMR was not statistically different to the expected for a 22.5 g mammal neither by the Kleiber relation nor by the Hayssen & Lasey relation (Table 2). Sensitive analysis of the observed/expected BMR of A. azarae, shows that a bias of 347 % from Kleiber (t obs = t crit = 2.57, BMR expected = BMR observed ± 0.57, df = 5, P < 0.001) and a bias of 435 % from Hayssen & Lacey (t obs. = t crit. = 2.57, BMR expected = BMR observed ± 0.57, df = 5, P < 0.05) would be necessary to alter the latest conclusion.

The relation between VO₂ and T_a was (Fig. 1):

MR= 6.73 - 0.148 T_a if T_a ­ 35

MR= -57.98 + 1.68 T_a if T_a > 35

MR at 40 °C was estimated on only one individual since other mice exhibited thermal shock at this T_a.

Activity daily metabolic rate

No statistical differences between ADMR for males and females was detected (t = -0.77, n1=16, n2 = 7, df = 21, P > 0.45). For the pooled data, mean value was 4.04 ± 1.16 ml/gh (Table 2).

Body temperature and thermal variables

No statistical differences were detected between T_b of males and females (t = -0.70, n = 70, df = 68, P > 0.4). Body temperature of A. azarae, in the thermoneutral zone (TNZ), was 36.1 ± 1.5 °C (Table 2) whilst DT_b was statistically different between sexes only at 5 °C (DT_b females = -6.77 ± 2.18 and DT_b males = 0.42 ± 2.31; F = 9.29, n = 67, df = 7, P < 0.005). Thus, DT_b of males and females were pooled. No statistical differences between DT_b at different T_a were detected (F = 1.03, n = 68, df = 7, P > 0.4). On the other hand, C_min below TNZ was lower than expected from McNab´s equation (Table 2). Sensitive analysis of the observed/expected C_min shows that a bias of 156 % in the estimated C_min of A. azarae would be necessary to alter the latest conclusion (t obs = t crit = 2.01, BMR expected = BMR observed ± 0.25, df = 48, P < 0.001). In addition, DTm was 136 % of expected from McNab´s equation (Table 2).

Torpor

No statistical differences were detected between BMR and the energetic limit of euthermia expected from McNab (1983) (2.02 ± 0.46 ml/gh; t = -1.02, t crit = 2.23, n = 12, df = 10, P > 0.005; Fig. 2). The lowest MR measured at 15 °C of T_a was 2.66 ± 0.49 ml/gh. Then, A. azarae at experimental conditions did not enter in torpor.

DISCUSSION

The inadequacy of W alone to account for the variation in standard energetic led some biologists to examine the effect of both, environment and food habits on mammalian energy expenditure (McNab 1992). The type of diet is crucial since it places limitations on the net amount of energy that can be assimilated and therefore affects metabolic rate. Akodon azarae is an omnivore rodent eating different parts of plants and including insects in its diet (Bilenca 1993). Then, the value of BMR found for A. azarae was according either, to the expectation of the Kleiber relation and, to the expectation of the Hayssen & Lacy (1985) relation for murid rodents.

Males and females of A. azarae, had the same BMR. According to Heusner (1982) similarity in oxygen consumption between males and females would imply similarity in body composition. This prediction is in agreement with the fact that both, males and females of A. azarae have the same body composition (del Valle personal communication).

Furthermore, in the same sense, similarities in ADMR for males and females of Akodon azarae in non-reproductive condition could imply similar selection pressures on the amount of energy allocated to behavioral activities. In accordance, Bilenca (1993) has reported that males and females of A. azarae do not have differences in habitat use. Activity daily metabolic rate in A. azarae was 235 % higher than BMR, suggesting that the cost of locomotion is as expensive as the reproductive cost of lactation which, as stated by Sikes (1995), could be 66 to more than 200 % over non reproductive requirements.

Akodon azarae is a good thermoregulator. As microtine rodents (Hart 1971), its thermoregulatory capacity was higher than the expected. Moreover, it has a lower than expected minimal thermal conductance and in consequence, its differential in body temperature did not show differences with changes in ambient temperature. As it was found a high mortality in traps at low field temperatures, torpor was evaluated in periods shorter than 24 h. We found that this species did not evidence torpor states, as happen with other Southamerican species such as Calomys musculinus, Eligmodontia typus, C. venustus, and Phyllotis darwini rupestris (Bozinovic & Rosenmann 1988a, b, Caviedes-Vidal et al. 1990, Bozinovic & Marquet 1991). In spite of the short time that individuals were exposed to cold temperatures, ours results are consistent with both, the high thermal insulation observed and with a BMR not lower than the expected for an euthermic mammal. The high field mortality found in traps, could be due to the impossibility to support in winter, a high cost of termorregulation for long times at low temperatures with limited food resource. Thus, Bozinovic & Rosenman (1988b) predictions, about the possible existence of torpor in Akodon azarae are not supported by our data.

Burrow construction (Hodara et al. 1997) and temporal selectivity in habitat use (Antinuchi 1996) in A. azarae has been discussed as a mechanism that allow the avoidance of extreme low ambient temperatures. Although this behaviour would contribute to diminish the quantity of energy assigned to thermorregulation, it may also be important in minimizing the risk of predation. On the other hand observed decreasing field activity of A. azarae during warm hours during summer (Bilenca 1993) could be attributed mainly to thermal characteristics of this species. The avoidance of high T_a in the field was in accordance with the fact that, MR responses to ambient temperatures above TNZ was abrupt and reached its highest value at 40 °C. A. azarae is the most numerically important species from pampas grassland and the above mentioned adaptations would contribute to the succes of this species.

Further studies on energetic of the reproduction, thermoregulatory acquisition, and metabolism in the development of newborn of A. azarae will allow a better understanding of the relationship among energetics, life history traits, and behavior in this species.

(Received May 17, 1999; accepted October 13, 1999; managed by F. Bozinovic)

ACKNOWLEDGMENTS

The authors wish to express their gratitude to Lopez Mañanes, Malizia, Zenuto, Kittlein, Vassallo and Iribarne for improving the manuscript with their comments and suggestions. This work was granted by Universidad Nacional de Mar del Plata - 1998 subsidio N° 2, Agencia Nacional de Promociones Científicas y Tecnológicas - subsidio N° 01-00000-01348 and CONICET - PEI N° 6429.

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