Does thermal history affect metabolic plasticity?: a study in three Phyllotis species along an altitudinal gradient.

(1) The aim of this study was to understand the effects of thermal history in metabolic features such as maximum (MMR) and basal (BMR) metabolic rates, as well as in metabolic plasticity, considered as the total variation of MMR and BMR during the acclimation period. (2) We studied three species of the genus Phyllotis, from different thermal environments, in an altitudinal gradient from sea level to 3800m.a.s.l. Animals were acclimated to contrasting temperatures of 5 and 30 degrees C. To determine the metabolic flexibility, MMR was measured at intervals of 6 days during the acclimation period, while BMR values were obtained at the end of acclimations. Aerobic scope and the rates of change of MMR were estimated in all populations. (3) High- and low-altitude rodents did not show differences in BMR. However, both upper and lower limits of MMR, as well as aerobic scope, were significantly different between high- and low-altitude species, indicating similar ranges of metabolic plasticity. On the other hand, the rates of change of MMR were similar in all populations. (4) Our results indicate that thermal history has a profound effect on the individuals' thermogenic capacity, probably in both phylogenetic and ontogenetic levels. Low-altitude species could not increase MMR to the same levels as high-altitude species, while the later were unable to decrease MMR to achieve the values of the low-altitude species.

Thermal biology of the fossorial rodent Ctenomys fulvus from the Atacama desert, northern Chile.

The Andean tuco-tuco, Ctenomys fulvus (Rodentia: Ctenomyidae) inhabits one of the most arid regions of the world, the Salar de Atacama, Northeast of Antofagasta, Chile (23 degrees 17'06"S, 68 degrees 05'43"W; 2.240 m.a.s.l). We found that a stable microclimate in burrows, a low evaporative water loss (EWL), and a diet of roots (59% water content) are the main factors that permit the survival of this fossorial species in harsh desert conditions. Large circadian variation in T(a) was observed above ground. Daily DeltaT(a) (T(a) max - T(a) min) = 37.9+/-0.2 degrees C in summer and in winter. In contrast, circadian variation of T(a) inside the burrows was only 5.8+/-0.5 degrees C in the same seasons. Relative humidity (RH) was 1.9-3.1% during the day, increasing to maximum values of 27% at night and early morning. Inside the burrows RH was higher and quite stable, ranging between 53.1 and 65%, independent of the time of day and season. EWL, measured between 10 and 25 degrees C, was low (1.26 mg/g h), and a moderate increase of 13-20% was observed at higher temperatures. The low EWL may prevent dehydration. However, because of the low heat loss capability, animals became hyperthermic (0.8-1.6 degrees C) in dry air at T(a)=30-35 degrees C. As T(a) during afternoon normally exceeded 35 degrees C, the microclimate of burrows provided the only way to avoid the lethal effects of hyperthermia.

Seasonal changes of blood values in the Andean mouse Abrothrix andinus.

1. Biomonthly measurements of hematological indices were conducted in freshly captured Abrothrix andinus (Rodentia:Cricetidae) over a period of one year. 2. All blood values were in the same range for adult males and females, but juveniles showed lower red blood cell counts (RBC) and erythrocytes of larger size. 3. High RBC numbers, a slight but significant increase of blood hemoglobin concentration (Hb), and a significant decrease in mean cell volume (MCV) were found during the winter months. 4. Hematocrit values (Hct) and the mean cell Hb concentration (MCHC) were relatively constant throughout the year. 5. Plasticity of MCV and an inverse correlation between size and the number of RBC may provide an enlarged diffusion area during winter. This mechanism appears to be a useful seasonal adjustment, since the increase of Hct and the concurrent increase in blood viscosity are avoided.

Energetics and geometry of huddling in small mammals.

Huddling in small mammals appears as an efficient response to low ambient temperatures with important consequences in thermoregulatory energy savings. These energy savings have been ascribed to a decrease in the exposed area in relation to the animal's volume. It has been proposed that during huddling reductions in the exposed area and in the metabolic rate are equal functions of the number of grouped individuals with a common exponent of -1/3. However, reported data shows a great variability of this exponent. In this paper we present a geometrical and energetic analysis on several huddling efficiencies in small mammals and in geometric bodies. Our theoretical analysis shows a variability in the efficiency of huddling, depending on the morphological characteristics of the geometric bodies. At the same time original and literature information show an analogous interspecific variability in small mammals. Finally, a general mathematical expression is proposed which represents and explains the energetic and geometric specific variations of huddling in small mammals.

A field lab method to determine urine concentration in small mammals.

1. The concentrations of 136 urine samples from four species of small mammals were compared using osmometry, refractometry and a colorimetric test for urea concentration. 2. To obtain a wide range of concentrations (430-3950 mOsm/kg), urine samples were collected under normal and dehydration conditions. 3. Regression analyses of paired values indicate that measurements of total solids concentration (refractometric method) permit evaluations of urine osmolality and estimations of the concentration of urea with a high degree of confidence.

Altitudinal distribution and blood values in the toad, Bufo spinulosus Wiegmann.

1. Red blood cell (RBC) count, RBC size, hematocrit, cell and blood hemoglobin concentrations and plasma total solid concentration were measured in 16 lowland (from near sea level up to 2700 m) and 18 highland (3200 up to close to 4500 m) adult toads (Bufo spinulosus). 2. Lowland toads showed higher hematocrit values than highland toads, but their blood hemoglobin concentration and plasma solid concentration were not significantly different. 3. Highland toads had smaller RBC size, higher corpuscular hemoglobin concentration, a trend toward larger RBC count and a considerably smaller body size. These features may contribute to their successful life at high altitude.

Comparative energetics of South American cricetid rodents.

1. The effects of body size, food habits and limits of endothermy on the energetics of seventeen species of South American cricetid rodents were examined. The combination of mixed diets, together with close to the expected basal rates of heat production, allows most cricetids studied to maintain effective endothermy; but those that have low basal metabolic rates and small sizes show a state of torpor. 2. Our observations do not support the hypothesis that the absence of torpor in South American cricetid rodents is due to phylogenetic characteristics. 3. The high ecological diversity, as well as the variety of life modes of these rodents, is explained on an energetic basis.

Comparative water economy of sympatric rodents in a Chilean semi-arid habitat.

1. Water requirements, water balance components and dependence on exogenous water were estimated in four species of sympatric rodents inhabiting a Chilean semi-arid region. 2. A significant increase in free water drinking was observed in all rodents when the diet composition was changed from 14 to 20% protein. 3. Under water balance conditions the cricetid species (Phyllotis darwini, Oryzomys longicaudatus and Akodon olivaceus) showed 1.66 to 1.88 times the weight-specific daily water gain of Octodon degus. 4. In the three cricetid rodents, evaporation was the largest water loss component (2/3 to 3/4 of total loss), while in O. degus evaporative water loss amounted to only 38% of the total. 5. Survival time during water deprivation ranged from 13.4 days in O. degus to only 4 days in O. longicaudatus. 6. All water variables combined indicate that water dependence of O. longicaudatus greater than P. darwini greater than A. olivaceus greater than O. degus.

[The critical pressure of oxygen]. / La presión crítica de oxígeno.

A review of the metabolic response (M) to hypoxia measured in different mammals shows that the critical ambient O2 pressure (Pc), is a linear function of the normoxic metabolic demand. Values for Pc at different metabolic loads may range from about 10 torr to close to 150 torr. However, the ratio M/Pc provides a useful index that seems to be independent of the metabolic demand. Critical ambient pO2 for reducing O2 uptake at comparable loads has been found to be lower in highland than in lowland species. Below the Pc the metabolism changes sharply or more gradually with further reduction in pO2. This reduction may be better expressed by a single hypoxic coefficient that is independent of the metabolic demand, representing the fractional reduction of M per torr. Regression analysis of Pc on body weight shows independence of the critical pO2 on body size within the range 8-480 g.

[Hematologic values and altitudinal distribution of Chilean amphibians]. / Valores hematológicos y distribución altitudinal de anfibios chilenos.

Blood values were measured on a series of Chilean species of frogs and toads from low and high altitudes, ranging from near sea level up to 4,500 m. Average values of red cell number, blood hemoglobin concentration and cell hemoglobin concentration were all higher in the high altitude group (6 species), while the average cell size was larger in the low altitude group (16 species). Hematocrits and cell hemoglobin content showed poor or no correlation with the altitudinal distribution of the examined species.

[Heat transfer, convection and altitudinal gradient]. / Transferencia de calor, convección y gradiente altitudinal.

Measurements of heat loss from fur covered aluminium cylinders were made under barometric pressures ranging from 760 to 368 torr (sea level up to 5.8 km, simulated altitudes). Heat transfer diminished at high altitudes and a relative greater diminution was observed when forced convection was applied. The virtual increase in thermal insulation at high altitudes may be useful to compensate the expected larger difference between body and ambient temperatures.

Thermal freedom of Graomys griseoflavus in a seasonal environment.

1. Thermal freedom, defined as the physiological potential to make full spatiotemporal use of the seasonal thermal macroclimate, was assessed in Graomys griseoflavus, a cricetid rodent inhabiting the Chaco Province in Argentina. 2. The extent of the physiological thermal range (58 degrees C) was larger than the seasonal thermal range (30-50 degrees C), favoring Graomys resistance to low winter temperatures. The upper physiological thermal limit was almost coincident with the highest extreme of summer temperatures. 3. Graomys thermal freedom is facilitated by a high evaporative water loss, high metabolic capability, wide thermal neutral zone and a 4 degrees C lability of body temperature. 4. Behavioral responses (thermal avoidance) are utilized by this species, but with no apparent loss in thermal freedom.

Norepinephrine thermogenesis in seasonally acclimatized and cold acclimated red-backed voles in Alaska.

The calorigenic response (millilitres O2 per gram pre hour) to injected norepinephrine (NE) was compared as an index of nonshivering thermogenesis (NST) in the following groups of the Alaska red-backed vole (Clethrionomys rutilus): (1) summer, (2) fall acclimatized, (3) winter acclimatized, (4) 20 degrees C acclimated and (5) 5 degrees C acclimated. The metabolic response was tested at thermoneutrality (25 degrees C) and during cold exposure (5 degrees C). Winter acclimatized voles showed a significantly greater metabolic response to NE than summer voles at both 25 degrees C and 5 degrees C. In summer or winter voles the total metabolic rate after NE (Mne) was similar at 25 degrees C and 5 degrees C but the fraction of the total caused by exogenous NE was lower at 5 degrees C. Thus, thermogenesis during cold exposure and resulting from exogenous NE appear to be based on the same mechanism, and NE has thermoregulatory significance in these voles. The magnitude of the NE response in winter voles was comparable to he highest values reported for bats and exceeded levels reported for other adult small mammal species. Summer acclimatized voles and those acclimatized to 20 degrees C in the laboratory were comparable in their response to NE but winter acclimatized voles were significantly more sensitive to NE than voles acclimated to 5 degrees C. The seasonal winter peak in MNE coincided with peaks previously found for maximum metabolic capacity (MMAX), maximum brown fat, and the period of coldest temperature in December-January. the ratio of MNE to Mmax was similar throughout the year. The results suggest that small arctic-subarctic rodents have a greater capacity for NE stimulated NST than rodents from temperate latitudes probably because they are acclimatized to colder seasonal condtions.

Impairment of metabolic capability in feral house mice by Klossiella muris invection.

Kidney infection by Klossiella muris in highland house mice from Peru was associated with lowered metabolic capability (57%) and endurance time (25%) at maximum metabolic rate. Administration of coccidiostatic preparations improved the metabolic capability from 7.8 to 10.5 cc 0-2 g hr and the endurance from 2.8 to longer than 10 min. Their colony-reared offspring, which were free from parasites, sustained maximum metabolic rates of 13.8 cc 0-2/g hr also for longer than 10 min. This rate was slightly higher than the values found in both healthy lowland controls: feral house mice from Arkansas and laboratory white mice [lab: HA(ICR)]. Although clinically inapparent under oridinary conditions, the effects of K muris infection were easily demonstrated at high metabolic loads stimulated by cold. Thus emphasizing the importance of such performance tests in assessing the status of experimental subjects.