Arturo Cortés

Seasonal acclimatization in water flux rate, urine osmolality and kidney water channels in free-living degus : molecular mechanisms, physiological processes and ecological implications

SUMMARY The environmental modification of an organisms physiology in the field is often hypothesized to be responsible for allowing an organism to adjust to changing biotic and abiotic environmental conditions through increases in biological performance. Here, we examine the phenotypic flexibility of water flux rate, urine osmolality and the expression of kidney aquaporins (AQP; or water channels) in free-ranging Octodon degus, a South American desert-dwelling rodent, through an integrative study at cellular, systemic and organismal levels. Water flux rates varied seasonally and were significantly lower in austral summer than in winter, while urine osmolality was higher in summer than during winter. The observed water influx rate during summer was 10.3±2.3 ml day-1 and during winter was 40.4±9.1 ml day-1. Mean urine osmolality was 3137±472 mosmol kg-1 during summer and 1123±472 mosmol kg-1 during winter. AQP-2 medullary immunolabeling was more abundant in the kidneys of degus captured during summer than those captured during winter. This immunoreactivity was higher in apical cell membranes of medullary collecting ducts of degus in summer. AQP-1 immunostaining did not differ between seasons. Consistently, AQP-2 protein levels were increased in medulla from the summer individuals, as judged by the size of the 29 kDa band in the immunoblot. Here, we reveal how the integration of flexible mechanisms acting at cellular, systemic and organismal levels allows a small desert-dwelling mammal to cope with seasonal water scarcity in its semi-arid habitat.

Ambient temperature limits above-ground activity of the subterranean rodent Spalacopus cyanus

Abstract We studied daily activity patterns of the subterranean rodent Spalacopus cyanus , in the field during summer and winter, and under laboratory conditions at two different temperatures, 15°C and 30°C. This rodent exhibited nocturnal activity in the laboratory, but diurnal above-ground activity in the field. We suggest that this discrepancy between field and laboratory results is a consequence of differential space-use inside burrows during the day, and that low external ambient temperatures appear to constrain activity of S. cyanus outside their burrows during the night in summer and winter. In contrast, we hypothesize that high summer temperatures constrain above-ground activity at midday.

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.

Seasonal food habits of the endangered long-tailed chinchilla (Chinchilla lanigera): the effect of precipitation

Summary Based on the content of feces, we studied the food habits of the endangered rodent Chinchilla lanigera . On a seasonal basis, during two years of contrasting rain levels (1992 = 242 mm; 1993 = 123 mm), we collected chinchilla feces from El Cuyano ravine, adjacent to the Chinchilla National Reserve in north central Chile (31° 29′ 10.8″ S, 71° 03′ 43.9″ W). The main plant species eaten was the perennial graminoid Nassella chilensis . Chinchillas showed a broader trophic niche during the rainy year, than during the dry year, consuming 55.5 and 40.7% of the 38 and 27 plants available, respectively. Within the wet year the diet differed less between winter and summer (Horn similarity index R 0 = 0.58) than within the dry year (R 0 = 0.83). Between years, the diet differed more during winters (R 0 = 0.20) than during summers (R 0 = 0.52). Chinchillas are folivorous, using a feeding pattern of a generalist species. The opportunistic feeding behavior of chinchillas may be an adaptation to the harsh conditions and high variability in food availability triggered by fluctuations in rainfall among years in the arid north central Chile.

Seasonal variation in the range areas of the diurnal rodent Octodon degus

Abstract Both breeding activity and abundance and quality of available food are expected to influence daily movements of animals. Animals are predicted to range over large areas to meet high energy demands associated with reproduction (females) or to increase mating success (males). However, animals should expand their range areas whenever food conditions deteriorate. To examine the extent to which breeding activity versus food availability influence space use, we compared the size and location of range areas (home ranges) of the degu (Octodon degus), a diurnal rodent from semiarid environments of north-central Chile, during the austral winter and summer seasons. Degus produce young during the austral spring (September–October) when high-quality food is readily available. In contrast, degus do not breed during the austral summer (January–March) when food is scarce and of low quality. We predicted that degus would range over smaller areas in winter if the availability of food has a greater influence on space than breeding activity. Individuals were radiotracked in winter and the following summer over a 3-year period. Surveys of herbaceous cover were conducted during winter and summer to determine seasonal changes in the abundance and quality of primary food. In summer degus expanded and moved the location of their range areas to locations with available food. Given that preferred food was less abundant in summer than winter, we suggest that degu range areas are strongly influenced by food conditions.

Energetics, thermoregulation and torpor in the Chilean mouse-opossum Thylamys elegans (Didelphidae)

In this paper we studied the energetics and thermoregulation of the Chilean mouse-opossum Thylamys elegans (Dielphidae) a nocturnal small marsupial, endemic of southern South America. We studied their standard energetic and determined whether they exhibit shallow daily torpor or deep prolonged torpor as a function of ambient temperature and food availability. Thylamys elegans partially supports the hypothesis that Neotropical marsupials have somewhat a higher basal metabolic rate (BMR) and thermal conductance (Cm) than Australian ones. In fact, BMR was higher but Cm was lower than expected for their body mass. The higher mass-independent BMR of the Chilean mouse-opossum may be explained by its insectivorous food habits and its low Cm by its temperate habitats. Euthermic Chilean mouse-opossum showed daily fluctuations in body temperature being significantly higher during night time. In addition T. elegans entered in daily torpor and aroused spontaneously only was food was absent. That is, this species display a facultative type of daily torpor because propensity to enter in torpor was dependent of the combination of food absence and low ambient temperature. No torpor was observed when food was available. During torpor ambient temperature was slightly above ambient temperature between 0.3 to 0.5 °C. Torpor in this species as well as in marsupials in general, appears to be a flexible and an opportunistic response to unpredictable environmental conditions.

Phenotypic Flexibility at the Molecular and Organismal Level Allows Desert‐Dwelling Rodents to Cope with Seasonal Water Availability

We examined the phenotypic flexibility of field urine osmolality (Uosm) in response to seasonal rainfall and the experimental expression of renal aquaporins (AQPs) in the leaf‐eared mouse Phyllotis darwini, a South American desert‐dwelling rodent, through an integrative study at both the cellular and the organismal level. Field Uosm was higher in summer than in winter. Fall and winter Uosm were not significantly different. During a rainy year, winter Uosm was 2,140 ± 82.3 mOsm kg−1; the corresponding value in a dry year was 2,569 ± 61.3 mOsm kg−1. During the summer, the mean Uosm in a rainy year was 3,321 ± 71.5 mOsm kg−1, and in a dry year it was 3,604 ± 107.2 mOsm kg−1. The distribution of AQP‐2, AQP‐3, and AQP‐4 was similar to that described for mouse and rat kidneys and confined to principal cells in cortex and inner medullary collecting‐duct cells. AQP‐4 immunoreactivity was unaltered by the state of water balance. Relative to water loading, dehydration induced an increase in AQP‐2 immunoreactivity and protein abundance. Although more discrete, AQP‐3 immunolabeling was also increased by dehydration. We now reveal how the integration of flexible renal mechanisms acting at the cellular and organismal level allow a small desert‐dwelling mammal to cope with seasonal and yearly (El Niño) water availability in its semiarid habitat.

Water economy in rodents: evaporative water loss and metabolic water production

Para evaluar la eficiencia en la regulacion hidrica de roedores granivoros deserticolas se usa la perdida de agua por evaporacion (EWL) y la produccion de agua metabolica (MWP), estando esta expresada por el modelo Ta @ MWP = EWL, que representa el valor de una temperatura ambiente (Ta) al cual (@) MWP/EWL = 1. En este trabajo se evalua este modelo (1- r2) y se estima la eficiencia en la regulacion de agua basados en el costo energetico (MR) de mantencion del balance hidrico (WB), es decir MR-WB. Se midio EWL y se estimo MWP en las siguientes nueve especies de roedores de diferente localidades del norte y centro de Chile (Regiones II y IV): Octodon degus (Od) y O. lunatus (Olu) (Octodontidae), Abrothrix olivaceus (Ao), A. longipilis (Al), A. andinus (Ad), Phyllotis darwini (Pd), P. rupestris (Pr), P. magister (Pm), Oligoryzomys longicaudatus (Ol) (Muridae) and Chinchilla lanigera (Cl) (Chinchillidae). Se analizo ademas informacion de la literatura de roedores de habitats xericos y mesicos. Los resultados indican que: 1) Cl posee la menor EWL (0,58 mg H2O/g h), seguido por Od < Olu < Ao < Pd < Pm < Pr < Ad < Al < Ol. 2) EWL y masa corporal (M) se relacionan a traves de ecuaciones independientes que consideran dos habitats distintos: EWL (xerico)= 5,968 M-0,416 (r = -0,89; P < 0,001) y EWL (mesico) = 17,272 M -0,532 (r = -0,85; P < 0,001). 3) MWP and M se relacionan por la ecuacion: MWP = 14,256 M-0,539 (r = - 0,98; P < 0,001). 4) A un nivel intraespecifico, MWP/EWL y Ta se relacionan a traves de una ecuacion exponencial negativa: MWP/EWL=a 10-bTa (r = -0,95; P < 0,001). 5) Ta @ MWP = EWL y M se relacionan a traves de la ecuacion: Ta @ (MWP = EWL) = 26,799 M-0,142, (r = - 0,49, P < 0,02). 6) MR-WB y M, se relacionan por ecuaciones habitats-dependientes y son: MR - WB (xerico) = 34,627 M-0,339 (r = - 0,93; P < 0,001) y MR-WB (mesico) = 68,132 M-0,381 (r = -0,86; P < 0,001). Estas dos ultimas ecuaciones poseen ventajas comparativas con respecto a las anteriores pues incluyen roedores con diferentes habitos dietarios, son capaces de discriminar patrones de regulacion hidrica en funcion del habitat (xerico y mesico), y son capaces de evaluar el costo energetico del balance hidrico

Energy metabolism and thermoregulation in Chinchilla brevicaudata

Chinchilla brevicaudata lives at 3500-5000 m, with high ambient temperatures during the day but cold at night. In this Andean habitat there is also low availability of food and water resources. Physiological attributes that may minimize their energetic cost as well as the water requirements are: (1) Low values of basal metabolic rate (67.2%) and thermal conductance (51.0%) compared to predicted values. (2) The aerobic metabolic expansivity was 5.1, while the calculated theoretical critical lethal temperature was extremely low (� 67.8 � C). (3) The energetic cost for maintenance of water balance was 85.3% of the predicted value for xeric rodents of similar size.

Feeding habits of guanacos Lama guanicoe in the high Andes of North-Central Chile

The guanacoLama guanicoe Muller, 1776 has a wide distribution along the Andes and Patagonia. We studied the feeding behaviour of a guanaco population that lives over 4100 m altitude in the Andes of north-central Chile. By contrasting the diet of guanacos during a dry year with that of a wet year and comparing it with the plant availabilities in the field, we tested the hypothesis that the guanaco is a generalist herbivore. We predicted that under such extreme habitat conditions guanacos should consume whatever plant species are available in the environment, especially in a dry year, when vegetation is scarcer. In addition, we compared its diet at three different age classes. We estimated the diet through the microhistological analysis of plant remains found in guanaco pellets collected during January of 1997 (ie after a dry year) and 1998 (ie after a wet year; 41 vs 495 mm, respectively). Then, we computed dietary preferences, food niche-breadth, and food-niche overlap between years and among age classes. Vegetation cover and plant species richness were higher during the wet than during the dry year. The most common plants in the environment were perennial graminoids and legumes. Contrary to our prediction, the guanaco preferred a few plant species, showing a relatively narrow diet breadth that changed little between years differing in plant abundances. The diet proportions differed among the three age classes, however. Our data indicate that at least in this high-elevation population, guanacos are selective and non-opportunistic herbivores. This specialized feeding behaviour is puzzling given the energetic demands of living in a harsh environment with low availabilities of resources. The hypothesis that this is due to the lower palatability of the plants not eaten, remains to be tested.