Browsing by Author "Mejias, Carlos"

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    A Mesocosm Experiment in Ecological Physiology: The Modulation of Energy Budget in a Hibernating Marsupial under Chronic Caloric Restriction
    (2022) Nespolo, Roberto F.; Fonturbel, Francisco E.; Mejias, Carlos; Contreras, Rodrigo; Gutierrez, Paulina; Oda, Esteban; Sabat, Pablo; Hambly, Catherine; Speakman, John R.; Bozinovic, Francisco
    During the past 60 years, mammalian hibernation (i.e., seasonal torpor) has been interpreted as a physiological adaptation for energy economy. However, direct field comparisons of energy expenditure and torpor use in hibernating and active free-ranging animals are scarce. Here, we followed the complete hibernation cycle of a fat-storing hibernator, the marsupial Dromiciops gliroides, in its natural habitat. Using replicated mesocosms, we experimentally manipulated energy availability and measured torpor use, hibernacula use, and social clustering throughout the entire hibernation season. Also, we measured energy flow using daily food intake, daily energy expenditure (DEE), and basal metabolic rate (BMR) in winter. We hypothesized that when facing chronic caloric restriction (CCR), a hibernator should maximize torpor frequency to compensate for the energetic deficit, compared with individuals fed ad lib. (controls). However, being torpid at low temperatures could increase other burdens (e.g., cost of rewarming, freezing risks). Our results revealed that CCR animals, compared with control animals, did not promote heat conservation strategies (i.e., clustering and hibernacula use). Instead, they gradually increased torpor frequency and reduced DEE and, as a consequence, recovered weight at the end of the season. Also, CCR animals consumed food at a rate of 50.8 kJ d(-1), whereas control animals consumed food at a rate of 98.4 kJ d(-1). Similarly, the DEE of CCR animals in winter was 47.3 & PLUSMN;5.64 kJ d(-1), which was significantly lower than control animals (DEE=88.0 & PLUSMN;5.84 kJ d(-1)). However, BMR and lean mass of CCR and control animals did not vary significantly, suggesting that animals maintained full metabolic capacities. This study shows that the use of torpor can be modulated depending on energy supply, thus optimizing energy budgeting. This plasticity in the use of heterothermy as an energy-saving strategy would explain the occurrence of this marsupial in a broad latitudinal and altitudinal range. Overall, this study suggests that hibernation is a powerful strategy to modulate energy expenditure in mammals from temperate regions.
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    Body Composition and Energy Savings by Hibernation: Lessons from the South American Marsupial Dromiciops gliroides
    (2022) Mejias, Carlos; Navedo, Juan G.; Sabat, Pablo; Franco, Lida M.; Bozinovic, Francisco; Nespolo, Roberto F.
    Hibernation (i.e., seasonal or multiday torpor) has been described in mammals from five continents and represents an important adaptation for energy economy. However, direct quantifications of energy savings by hibernation are challenging because of the complexities of estimating energy expenditure in the field. Here, we applied quantitative magnetic resonance to determine body fat and body composition in hibernating Dromiciops gliroides (monito del monte). During an experimental period of 31 d in winter, fat was significantly reduced by 5.72 +/- 0.45 g, and lean mass was significantly reduced by 2.05 +/- 0.14 g. This fat and lean mass consumption is equivalent to a daily energy expenditure of hibernation (DEEH) of 8.89 +/- 0.6 kJ d(-1) , representing 13.4% of basal metabolic rate, with a proportional contribution of fat and lean mass consumption to DEEH of 81% and 18%, respectively. During the deep heterothermic bouts of monitos, body temperature remained 0.41 degrees C +/- 0.2 degrees C above ambient temperature, typical of hibernators. Animals shut down metabolism and passively cool down to a critical defended temperature of 5.0 degrees C +/- 0.1 degrees C, where they begin thermoregulation in torpor. Using temperature data loggers, we obtained an empirical estimation of minimum thermal conductance of 3.37 +/- 0.19 J g(-1) h(-1) degrees C-1, which is 107% of the expectation by allometric equations. With this, we parameterized body temperature/ambient temperature time series to calculate torpor parameters and metabolic rates in euthermia and torpor. Whereas the acute metabolic fall in each torpor episode is about 96%, the energy saved by hibernation is 88% (compared with the DEE of active animals), which coincides with values from the literature at similar body mass. Thus, estimating body composition provides a simple method to measure the energy saved by hibernation in mammals.
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    Communal nesting is the optimal strategy for heat conservation in a social marsupial: lessons from biophysical models
    (2022) Nespolo, Roberto F.; Pena, Isabella; Mejias, Carlos; Nunque, Abel; Altamirano, Tomaprimes; Bozinovic, Francisco F.
    Endothermy, understood as the maintenance of continuous and high body temperatures owing to the combination of metabolic heat production and an insulative cover, is severely challenged in small endotherms inhabiting cold environments. As a response, social clustering combined with nest use (=communal nesting) is a common strategy for heat conservation. To quantify the actual amount of energy that is saved by this strategy, we studied the social marsupial Dromiciops gliroides (monito del monte), an endemic species of the cold forests of southern South America. It is hypothesized that sociability in this marsupial was driven by cold conditions, but evidence supporting this hypothesis is unclear. Here, we used taxidermic models ('mannequins') to experimentally test the energetic benefits of clustering combined with nest use. To do this, we fitted and compared cooling curves of solitary and grouped mannequins, within and outside of a nest, at the typical winter ambient temperatures of their habitat (5 degrees C). We found that the strategy that minimized euthermic cost of maintenance was the combination of nest use and clustering, thus supporting communal nesting as a social adaptation to cope with the cold. Considering the basal metabolic rate of monitos, our estimates suggest that the savings represents almost half of energy consumption per day (in resting conditions). This study shows how simple biophysical models could help to evaluate bioenergetic hypotheses for social behavior in cold-adapted endotherms.
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    Natural history of the relict marsupial Monito del Monte at the most extreme altitudinal and latitudinal location
    (2021) Mejias, Carlos; Castro-Pastene, Carlos A.; Carrasco, Hector; Quintero-Galvis, Julian F.; Soto-Gamboa, Mauricio; Bozinovic, Francisco; Nespolo, Roberto F.
    Until a few years ago, the "monito del monte" (Dromiciops gliroides) was considered the sole surviving species of Microbiotheria, the sister group of Australidelphia (Australian marsupials). However, the recent identification of a new species relegated to the northern range (Dromiciops bozinovici) and newly discovered populations of Dromiciops beyond the known limits for the geographic range demands for new studies in extreme locations. In this report, we describe the northernmost population of D. bozinovici at a high Andean location (Reserva Nacional Altos de Lircay [RNAL], which represents a record of Dromiciops distribution both in latitude [35 degrees S] and in altitude [1696 m a.s.l.]). We live-trapped individuals, performed behavioral observations, and extracted mtDNA (Cytb) for analyzing phylogenetic affinities. Together with confirming the phylogenetic position of RNAL within the D. bozinovici clade, interesting aspects of this isolated population arose, such as the sympatry with a didelphid marsupial (Thylamys elegans), and phylogenetic closeness with populations 300 km southward coastal. Our trapping effort yielded approximately 9.3 individuals/ha, which is relatively high compared with other small mammal populations, but typical for Dromiciops. Using in situ thermographic imaging during a night transect, we detected free-ranging animals running vertically at similar to 3.6 km/h through the logs of mature trees (Nothofagus pumilio), which is fast compared with other small runners (e.g., cursorial rodents), and also compared with Dromiciops from other locations. We compared the critical time to escape, of RNAL individuals which resulted significantly higher than Dromiciops from Nahuelbuta and Valdivia, suggesting that animals from RNAL are more tolerant to human presence than other populations, which is probably due to the high level of isolation of this population. According to the IUCN the northernmost distribution of Dromiciops reaches Cauquenes, thus this report extends the distribution in 120 km to the North East. Interesting aspects, such as hibernation at freezing temperatures and the existence of a viable D. bozinovici population northward of RNAL, warrant further research on this interesting location.
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    Why bears hibernate? Redefining the scaling energetics of hibernation
    (2022) Nespolo, Roberto F.; Mejias, Carlos; Bozinovic, Francisco
    Hibernation is a natural state of suspended animation that many mammals experience and has been interpreted as an adaptive strategy for saving energy. However, the actual amount of savings that hibernation represents, and particularly its dependence on body mass (the 'scaling') has not been calculated properly. Here, we estimated the scaling of daily energy expenditure of hibernation (DEEH), covering a range of five orders of magnitude in mass. We found that DEEH scales isometrically with mass, which means that a gram of hibernating bat has a similar metabolism to that of a gram of bear, 20 000 times larger. Given that metabolic rate of active animals scales allometrically, the point where these scaling curves intersect with DEEH represents the mass where energy savings by hibernation are zero. For BMR, these zero savings are attained for a relatively small bear (approx. 75 kg). Calculated on a per cell basis, the cellular metabolic power of hibernation was estimated to be 1.3 x 10(-12) +/- 2.6 x 10(-13) W cell(-1), which is lower than the minimum metabolism of isolated mammalian cells. This supports the idea of the existence of a minimum metabolism that permits cells to survive under a combination of cold and hypoxia.