Heterothermic Species Research Articles

Hibernation and daily torpor minimize mammalian extinctions

Small mammals appear to be less vulnerable to extinction than large species, but the underlying reasons are poorly understood. Here, we provide evidence that almost all (93.5%) of 61 recently extinct mammal species were homeothermic, maintaining a constant high body temperature and thus energy expenditure, which demands a high intake of food, long foraging times, and thus exposure to predators. In contrast, only 6.5% of extinct mammals were likely heterothermic and employed multi-day torpor (hibernation) or daily torpor, even though torpor is widespread within more than half of all mammalian orders. Torpor is characterized by substantial reductions of body temperature and energy expenditure and enhances survival during adverse conditions by minimizing food and water requirements, and consequently reduces foraging requirements and exposure to predators. Moreover, because life span is generally longer in heterothermic mammals than in related homeotherms, heterotherms can employ a 'sit-and-wait' strategy to withstand adverse periods and then repopulate when circumstances improve. Thus, torpor is a crucial but hitherto unappreciated attribute of small mammals for avoiding extinction. Many opportunistic heterothermic species, because of their plastic energetic requirements, may also stand a better chance of future survival than homeothermic species in the face of greater climatic extremes and changes in environmental conditions caused by global warming.

Adaptive response of brain tissue oxygenation to environmental hypoxia in non-sedated, non-anesthetized arctic ground squirrels

The present study examined the physiological mechanisms of the responses of brain tissue oxygen partial pressure (P tO 2), brain temperature ( T brain), global oxygen consumption (V̇ O2), and respiratory frequency ( f R) to hypoxia in non-sedated and non-anesthetized arctic ground squirrels ( Spermophilus parryii, AGS) and rats. We found that (1) in contrast to oxygen partial pressure in blood (P aO 2), the baseline value of P tO 2 in summer euthermic AGS is significantly higher than in rats; (2) both P tO 2 and P aO 2 are dramatically reduced by inspired 8% O 2 in AGS and rats, but AGS have a greater capacity in P tO 2 to cope with environmental hypoxia; (3) metabolic rate before, during, and after hypoxic exposure is consistently lower in AGS than in rats; (4) the respiratory responding patterns to hypoxia in the two species differ in that f R decreases in AGS but increases in rats. These results suggest that (1) AGS have special mechanisms to maintain higher P tO 2 and lower P aO 2, and these levels in AGS represent a typical pattern of adaptation of heterothermic species to and a brain protection from hypoxia; (2) AGS brain responds to hypoxia through greater decreases in P tO 2 and decreased f R and ventilation. In contrast, rat brain responds to hypoxia by less reduction in P tO 2 and increased f R and ventilation.

The “minimal boundary curve for endothermy” as a predictor of heterothermy in mammals and birds: a review

According to the concept of the "minimal boundary curve for endothermy", mammals and birds with a basal metabolic rate (BMR) that falls below the curve are obligate heterotherms and must enter torpor. We examined the reliability of the boundary curve (on a double log plot transformed to a line) for predicting torpor as a function of body mass and BMR for birds and several groups of mammals. The boundary line correctly predicted heterothermy in 87.5% of marsupials (n = 64), 94% of bats (n = 85) and 82.3% of rodents (n = 157). Our analysis shows that the boundary line is not a reliable predictor for use of torpor. A discriminate analysis using body mass and BMR had a similar predictive power as the boundary line. However, there are sufficient exceptions to both methods of analysis to suggest that the relationship between body mass, BMR and heterothermy is not a causal one. Some homeothermic birds (e.g. silvereyes) and rodents (e.g. hopping mice) fall below the boundary line, and there are many examples of heterothermic species that fall above the boundary line. For marsupials and bats, but not for rodents, there was a highly significant phylogenetic pattern for heterothermy, suggesting that taxonomic affiliation is the biggest determinant of heterothermy for these mammalian groups. For rodents, heterothermic species had lower BMRs than homeothermic species. Low BMR and use of torpor both contribute to reducing energy expenditure and both physiological traits appear to be a response to the same selective pressure of fluctuating food supply, increasing fitness in endothermic species that are constrained by limited energy availability. Both the minimal boundary line and discriminate analysis were of little value for predicting the use of daily torpor or hibernation in heterotherms, presumably as both daily torpor and hibernation are precisely controlled processes, not an inability to thermoregulate.

Geographic variation in the use of torpor and roosting behaviour of female western long‐eared bats

AbstractThe costs and benefits of torpor may vary across the range of widespread heterothermic species, resulting in different thermoregulatory patterns and other behaviours for individuals inhabiting different environments. We compared torpor use and roosting behaviour for rock‐roosting populations of western long‐eared bats Myotis evotis living in the mountains and prairies of Alberta, Canada. We monitored body temperatures and located roosts for pregnant and lactating females using radio‐telemetry. We also took roost measurements, and assessed roost microclimate using temperature dataloggers. Females in the mountains entered torpor less frequently and spent less time in torpor than bats in the prairies, supporting our hypothesis that reproductive time constraints in the mountains outweigh the benefits of torpor. Alternatively, increased torpor use in the prairies may be attributed to a greater need for water conservation. Females in the mountains minimized thermoregulatory costs and maintained homeothermy by selecting exposed roosts with warm microclimates during pregnancy, and then switching to clustering with other individuals during lactation when conditions were cooler. In the prairies, females roosted alone in cooler, less exposed roost types, which facilitate the use of torpor. Our results illustrate the importance of considering geographic variation in behaviour across the range of a species.

ECOLOGICAL, TAXONOMIC, AND PHYSIOLOGICAL CORRELATES OF CAVE USE BY MEXICAN BATS

Roost use by bats is likely affected by their water balance and thermoregulatory abilities. To test this hypothesis, we explored the relationship between 4 traits of different species of bats (body size, general food habits, taxonomic group, and thermoregulatory pattern) and microclimates at roosts (temperature and humidity). We recorded roost variables and presence of bats in 18 caves from 5 contrasting biomes in central Mexico. There was little evidence of microclimatic specificity among the 23 species studied, but maternity colonies used warmer roosts and hibernating bats used cooler roosts. Heterothermic species (Vespertilionidae) used colder caves with the widest temperature range (1.6–29.8°C), whereas homeothermic species (Emballonuridae, Mormoopidae, Phyllostomidae, and Natalidae) occupied warmer roosts (14.5–37.5°C). Within these caves, precise (narrow body temperature range) homeotherms occupied slightly cooler roosts than more labile homeotherms. Body size alone was not associated with cave use patterns. However, when homeotherms and heterotherms were examined separately, body size and temperature were negatively correlated. The smallest homeothermic insectivorous species (<10 g) consistently occupied roosts with temperatures >20°C (more often >25°C), whereas only the largest homeothermic insectivores were found as low as 16°C. Frugivorous, nectarivorous, and sanguivorous bats were found in a wide range of temperatures (14.5–37°C), but often at <20°C. Humidity in roosts was highly variable for most species and we did not detect any trend regarding this factor. Our data suggest that the thermoregulatory ability resulting from the complex interaction of body size, type of food, and taxonomic affiliation constrains species with respect to types of roosts that they can successfully exploit. Our results support the hypothesis that temperature is the most important physical factor influencing roost selection in bats.

Differences in the thermotropic behaviour of mitochondrial membrane respiratory enzymes from homeothermic and heterothermic endotherms

1. The thermal response of liver and heart mitochondrial succinate oxidase and succinate cytochromec reductase of eleven homeothermic and six heterothermic species was compared. 2. Arrhenius plots of succinate oxidase and succinate cytochromec reductase of homeotherms exhibited a meanT* (Arrhenius critical temperature) of 22.6 °C. In normothermic heterothermsT* occurred at a mean temperature of 20.1 °C. During the season in which they entered torpor, two of the heterothermic mammals exhibited a meanT* at 10.9 °C, whilst for three species noT* was observed in the temperature range 5 °C to 40 °C. 3. The loweredT* in the heterothermic species was accompanied by an increase in theEa and Q10 both above and below the discontinuity in the Arrhenius plot. The increase Q10 of the mitochondrial enzymes isolated from animals during torpor was identical to the Q10 for O2 consumption for animals entering torpor. This may represent an important factor for energy conservation during torpor.

Differences in temperature sensitivity of the orcadian systems of homoiothermic and heterothermic neotropical bats

1. Comparative studies of two neotropical chiropteran species—one homoiothermic ( Phyllostomus discolor, Phyllostomidae) and one heterothermic ( Molossus ater, Molossidae)—show that their activity rhythms are differently affected by the ambient temperature (AT). 2. 1. Whereas the period of the free-running circadian rhythm of Molossus at 20°C.is much different from that at 30°C, and is positively correlated with AT (Fig. 2), no temperature-dependent changes in the period of Phyltostomus were observed (Fig. 4). 3. 2. Correspondingly, trapezoidal temperature cycles 10°C in amplitude do not entrain the free-running rhythm of the homoiothermic bat, whereas they act as a Zeitgeber for Molossus ater. Even for M. ater, however, the Zeitgeber strength of such temperature cycles is relatively slight. 4. 3. These results are inconsistent with a hypothesis of Bünning (1977) concerning the phylogenetic adaptation of temperature sensitivity of circadian rhythms of warm-blooded animals. They are consistent both with the hypothesis that heterothermic species in general have circadian systems with appreciable temperature sensitivity, and with the proposal that the more plastic circadian systems are in general more strongly influenced by AT than rigid systems.