Neurons in the dorsomedial hypothalamus promote, prolong, and deepen torpor

Torpor is a naturally occurring, hypometabolic, hypothermic state engaged by a wide range of animals in response to imbalance between the supply and demand for nutrients. Recent work has identified some of the key neuronal populations involved in daily torpor induction in mice, in particular projections from the preoptic area of the hypothalamus (POA) to the dorsomedial hypothalamus (DMH). The DMH plays a role in thermoregulation, control of energy expenditure, and circadian rhythms, making it well positioned to contribute to the expression of torpor. We used activity dependent genetic TRAPing techniques to target DMH neurons that were active during natural torpor bouts in female mice. Chemogenetic reactivation of torpor-TRAPed DMH neurons in calorie-restricted mice promoted torpor, resulting in longer and deeper torpor bouts. Chemogenetic inhibition of torpor-TRAPed DMH neurons did not block torpor entry, suggesting a modulatory but not a necessary role for the DMH in the control of torpor. This work adds to the evidence that a projection from the POA to the DMH forms part of a torpor-inducing circuit within the mouse hypothalamus.

Heterothermy as the Norm, Homeothermy as the Exception: Variable Torpor Patterns in the South American Marsupial Monito del Monte (Dromiciops gliroides)

Hibernation (i.e., multiday torpor) is considered an adaptive strategy of mammals to face seasonal environmental challenges such as food, cold, and/or water shortage. It has been considered functionally different from daily torpor, a physiological strategy to cope with unpredictable environments. However, recent studies have shown large variability in patterns of hibernation and daily torpor (“heterothermic responses”), especially in species from tropical and subtropical regions. The arboreal marsupial “monito del monte” (Dromiciops gliroides) is the last living representative of the order Microbiotheria and is known to express both short torpor episodes and also multiday torpor depending on environmental conditions. However, only limited laboratory experiments have documented these patterns in D. gliroides. Here, we combined laboratory and field experiments to characterize the heterothermic responses in this marsupial at extreme temperatures. We used intraperitoneal data loggers and simultaneous measurement of ambient and body temperatures (TA and TB, respectively) for analyzing variations in the thermal differential, in active and torpid animals. We also explored how this differential was affected by environmental variables (TA, natural photoperiod changes, food availability, and body mass changes), using mixed-effects generalized linear models. Our results suggest that: (1) individuals express short bouts of torpor, independently of TA and even during the reproductive period; (2) seasonal torpor also occurs in D. gliroides, with a maximum bout duration of 5 days and a mean defended TB of 3.6 ± 0.9°C (one individual controlled TB at 0.09°C, at sub-freezing TA); (3) the best model explaining torpor occurrence (Akaike information criteria weight = 0.59) discarded all predictor variables except for photoperiod and a photoperiod by food interaction. Altogether, these results confirm that this marsupial expresses a dynamic form of torpor that progresses from short torpor to hibernation as daylength shortens. These data add to a growing body of evidence characterizing tropical and sub-tropical heterothermy as a form of opportunistic torpor, expressed as daily or seasonal torpor depending on environmental conditions.

Temperature-dependent use of daily torpor attenuates the energetic consequences of habitat selection for a widespread bat

SummaryMany animals employ heterothermy to conserve energy during periods of inactivity, stress, or low resource availability. Unlike homeotherms, these heterotherms have some flexibility in body temperature. Unlike poikilotherms, heterotherms can maintain body temperatures independently from their environments. Heterotherms should thus exhibit fundamentally different responses to suboptimal environmental temperatures than either homeotherms or poikilotherms.In a species of heterothermic bat (Myotis thysanodes), we studied how daily torpor and roost selection could mitigate the energetic consequences of variation in ambient temperature. We then (1) quantified the relationship between ambient temperature and torpor use, (2) simulated daily energy expenditure over a range of roost temperatures, and (3) quantified the influence of roost temperature on roost selection.Bats did not select roosts with specific thermal characteristics, nor did ambient temperature alter patterns of roost selection. This was likely because bats could modulate use of torpor to maintain a consistent level of energy expenditure over the course of a day, irrespective of ambient temperature.Thermoregulatory processes in heterotherms differ from that of homeotherms and poikilotherms, including through behaviours as universal as habitat selection. Unlike homeotherms, bats face little pressure to select warm habitats to avoid heat loss during periods of inactivity—bats can use daily torpor to fully offset any increases in energy expenditure from maintaining homeothermy at colder temperatures.

Acclimation of intestinal morphology and function in djungarian hamsters (Phodopus sungorus) related to seasonal and acute energy balance

Small mammals exhibit seasonal changes in intestinal morphology and function via increased intestine size and resorptive surface and/or nutrient transport capacity to increase energy yield from food during winter. This study investigated whether seasonal or acute acclimation to anticipated or actual energetic challenges in Djungarian hamsters also resulted in higher nutrient resorption capacities due to changes in small intestine histology and physiology. The hamsters show numerous seasonal energy saving adjustments in response to short photoperiod. As spontaneous daily torpor represents one of these adjustments related to food quality and quantity, it was hypothesized that the hamsters’ variable torpor expression patterns are influenced by their individual nutrient uptake capacity. Hamsters under short photoperiod showed longer small intestines and higher mucosal electrogenic transport capacities for glucose relative to body mass. Similar observations were made in hamsters under long photoperiod and food restriction. However, this acute energetic challenge caused a stronger increase of glucose transport capacity. Apart from that, neither fasting-induced torpor in food-restricted hamsters nor spontaneous daily torpor in short photoperiod-exposed hamsters clearly correlated with mucosal glucose transport capacity.Both seasonally anticipated and acute energetic challenges caused adjustments in the hamsters’ small intestine. Short photoperiod appeared to induce an integration of these and other acclimation processes in relation to body mass to achieve a long-term adjustment of energy balance. Food restriction seemed to result in a more flexible, short-term strategy of maximizing energy uptake possibly via mucosal glucose transport and reducing energy consumption via torpor expression as emergency response.