Flexible energy-saving strategies in female temperate-zone bats

Intraspecific variation in metabolic responses to diverse environmental conditions in the Malagasy bat Triaenops menamena

Widespread species often display traits of generalists, yet local adaptations may limit their ability to cope with diverse environmental conditions. With climate change being a pressing issue, distinguishing between the general ecological and physiological capacities of a species and those of individual populations is vital for assessing the capability to adapt rapidly to changing habitats. Despite its importance, physiological variation across broad range distributions, particularly among free-ranging bats in natural environments, has rarely been assessed. Studies focusing on physiological variation among different populations across seasons are even more limited. We investigated physiological variation in the Malagasy Trident Bat Triaenops menamena across three different roost types in Madagascar during the wet and dry season, examining aspects such as energy regimes, body temperature, and roost microclimates. We focused on patterns of torpor in relation to roosting conditions. We hypothesized that torpor occurrence would be higher during the colder, more demanding dry season. We predicted that populations roosting in more variable microclimates would expend less energy than those in mores stable ones due to more frequent use of torpor and greater metabolic rate reductions. Our findings highlight complex thermoregulatory strategies, with varying torpor expression across seasons and roosts. We observed an overall higher energy expenditure during the wet season but also greater energy savings during torpor in that season, regardless of roost type. We found that reductions in metabolic rate were positively correlated with greater fluctuations in ambient conditions, demonstrating these bats' adaptability to dynamic environments. Notably, we observed diverse torpor patterns, indicating the species' ability to use prolonged torpor under extreme conditions. This individual-level variation is crucial for adaptation to changing environmental conditions. Moreover, the flexibility in body temperature during torpor suggests caution in relying solely on it as an indicator for torpor use. Our study emphasizes the necessity to investigate thermoregulatory responses across different populations in their respective habitats to fully understand a species' adaptive potential.

Positive consequences of group living among male bats during spermatogenesis

Energy can be limiting, especially for small animals with high metabolisms, particularly if they rely on ephemeral resources. Some energy-saving strategies, such as torpor, can impair physiological processes. Alternatively, group living can reduce energetic costs through social thermoregulation. This may allow individuals to maintain a high metabolism as well as processes such as gamete production. Although group living is common, its energetic benefits for heterothermic individuals during the season of sperm production have yet to be investigated. We remotely quantified the daily energy expenditure of individual parti-coloured bats (Vespertilio murinus) kept solitarily and in groups during the period of spermatogenesis, using high-resolution heart rate monitoring. The data showed that the energetic benefits of group living are complex. In groups, individual daily energy expenditure was more than 50% lower. Group roosting reduced the cost of thermoregulation during normothermia and allowed for a decrease in the depth but not the duration of torpor. Group living may enable bats to buffer unfavourable environmental conditions. Energy saved this way can then be invested in fitness-relevant processes, potentially making this a driver of the evolution of male sociality.

Field respirometry in a wild maternity colony of Bechstein's bats (Myotis bechsteinii) indicates high metabolic costs above but not below the thermoneutral zone

In a warming world, it is crucial to understand how rising temperature affects the physiology of organisms. To investigate the effect of a warming environment on the metabolism of heterothermic bats during the costly lactation period, we characterised metabolic rates in relation to roost temperature, the bats' thermoregulatory state (normothermia or torpor), time of day and age of juveniles. In a field experiment, we heated the communal roosts of a wild colony of Bechstein's bats (Myotis bechsteinii) every other day while measuring metabolic rates using flow-through respirometry. As expected, metabolic rates were lowest when the bats were in torpor. However, when bats were normothermic, colder temperatures had little effect on metabolic rates, which we attribute to the thermoregulatory benefits of digestion-induced thermogenesis and social thermoregulation. In contrast, metabolic rates increased significantly at temperatures above the thermoneutral zone. Contrary to our expectations, metabolic rates were not lower in heated roosts, where temperatures remained close to the bats' thermoneutral zone, than in unheated roosts, where temperatures were more variable. Our results show that torpor and digestion-induced thermogenesis are effective mechanisms that allow bats to energetically buffer cold conditions. The finding that metabolic rates increased significantly at temperatures above the thermoneutral zone suggests that the physiological and behavioural abilities of Bechstein's bats to keep energy costs low at high temperatures are limited. Our study highlights that temperate-zone bats are well adapted to tolerate cold temperatures, but may lack protective mechanisms against heat, which could be a threat in times of global warming.

Torpor energetics are related to the interaction between body mass and climate in bats of the family Vespertilionidae

Torpor is an adaptive strategy allowing heterothermic animals to cope with energy limitations. In birds and mammals, intrinsic and extrinsic factors, such as body mass and ambient temperature, are the main variables influencing torpor use. A theoretical model of the relationship between metabolic rate during torpor and ambient temperature has been proposed. Nevertheless, no empirical attempts have been made to assess the model predictions under different climates. Using open-flow respirometry, we evaluated the ambient temperature at which bats entered torpor and when torpid metabolic rate reached its minimum, the reduction in metabolic rate below basal values, and minimum torpid metabolic rate in 11 bat species of the family Vespertilionidae with different body mass from warm and cold climates. We included data on the minimum torpid metabolic rate of five species we retrieved from the literature. We tested the effects using mixed-effect phylogenetic models. All models showed a significant interaction between body mass and climate. Smaller bats went into torpor and reached minimum torpid metabolic rates at warmer temperatures, showed a higher reduction in the metabolic rate below basal values, and presented lower torpid metabolic rates than larger ones. The slopes of the models were different for bats from different climates. These results are likely explained by differences in body mass and the metabolic rate of bats, which may favor larger bats expressing torpor in colder sites and smaller bats in the warmer ones. Further studies to assess torpor use in bats from different climates are proposed.

Seasonal shifts in insect ephemerality drive bat foraging effort

Animal foraging is fundamentally shaped by food distribution and availability.1 However, the quantification of spatiotemporal food distribution is rare2 but crucial to explain variation in foraging behavior among species, populations, or individuals. Clumped but ephemeral food sources enable rapid energy intake but require increased effort to find,3 can generate variable foraging success,4 and force animals to forage more efficiently. We quantified seasonal shifts in the availability of such resources to test the proximate effects of food distribution on changes in movement patterns. The neotropical lesser bulldog bat (Noctilio albiventris) forages in a seasonal environment on emerging aquatic insects, whose numbers peak shortly after dusk.5,6 We GPS-tracked bats and quantified nocturnal insect distribution in their foraging area using floating camera traps across wet and dry seasons. Surprisingly, insects were 75% less abundant and swarms were 60% shorter lived (more ephemeral) in the wet season. As a result, wet season bats had to fly twice as far (total and maximum distance fromroost distances) and 45% longer (duration) per night. Within foraging bouts, wet season bats spent less time in each insect patch and searched longer for subsequent patches, reflecting increased temporal ephemerality and decreased spatial predictability of insects. Our results highlight the tight link between foraging effort and spatiotemporal distribution of food and the influence of constraints imposed by reproduction on behavioral flexibility and adaptations to the highly dynamic resource landscapes of mobile prey.7,8 Examining foraging behavior in light of spatiotemporal dynamics of resources can help predict how animals respond to shifts in food availability caused by escalating environmental changes.

Heart rate monitoring reveals differential seasonal energetic trade-offs in male noctule bats

Understanding how animals meet their daily energy requirements is critical in our rapidly changing world. Small organisms with high metabolic rates can conserve stored energy when food availability is low or increase energy intake when energetic requirements are high, but how they balance this in the wild remains largely unknown. Using miniaturized heart rate transmitters, we continuously quantified energy expenditure, torpor use and foraging behaviour of free-ranging male bats (Nyctalus noctula) in spring and summer. In spring, bats used torpor extensively, characterized by lowered heart rates and consequently low energy expenditures. In contrast, in summer, bats consistently avoided torpor, even though they could have used this low-energy mode. As a consequence, daytime heart rates in summer were three times as high compared with the heart rates in spring. Daily energy use increased by 42% during summer, despite lower thermogenesis costs at higher ambient temperatures. Likely, as a consequence, bats nearly doubled their foraging duration. Overall, our results indicate that summer torpor avoidance, beneficial for sperm production and self-maintenance, comes with a high energetic cost. The ability to identify and monitor such vulnerable energetic life-history stages is particularly important to predict how species will deal with increasing temperatures and changes in their resource landscapes.

Regional Intraspecific Differences of Thermal Biology in a Marsupial Hibernator

AbstractDuring periods of torpor, hibernators can reduce metabolic rate (MR) and body temperature (Tb) substantially. However, to avoid physiological dysfunction at low temperatures, they defend Tb at a critical minimum, often between ~0°C and 10°C via an increase in MR. Because thermoregulation during torpor requires extra energy, individuals with lower Tb's and thus minimal MR during torpor should be selected in colder climates. Such inter- and intraspecific variations occur in some placental mammals, but for the evolutionary separate marsupials, available information is scarce. Marsupial eastern pygmy possums (Cercartetus nanus; ~22 g body mass), widely distributed along the Australian southeastern coast including subtropical to alpine areas, were used to test the hypothesis that the defended Tb of torpid individuals is related to the climate of their habitat. Possums were captured from five regions, 1,515 km apart, with midwinter (July) minimum environmental temperatures (min Tenv's) ranging from -3.9°C to 6.6°C. Captive possums in deep torpor were slowly cooled with ambient temperature (Ta), while their MR was measured to determine the minimum torpor metabolic rate (TMR), the Ta at which their MR increased for thermoregulation (min Ta), and the corresponding minimum Tb (min Tb). Partial least squares regression analysis revealed that Ta and Tenv were the strongest explanatory variables for the min Tb. The min Tb and Ta were also correlated with latitude but not elevation of the capture sites. However, the best correlations were observed between the min Tenv and the min Tb and Ta for individuals experiencing min T env > 0 ° C ; these individuals thermoconformed to min Ta's between -0.8°C and 3.7°C, and their min Tb ranged from 0.5°C to 6.0°C and was 0.5°C-2.6°C below the min Tenv at the capture site. In contrast, individuals experiencing a min Tenv of -3.9°C regulated Tb at 0.6 ° C ± 0.2 ° C or 4.5°C above the Tenv. The minimum TMR of all possums did not differ with Ta and thus did not differ among populations and was 2.6% of the basal MR. These data provide new evidence that thermal variables of marsupials are subject to regional intraspecific variation. It suggests that min Tb is a function of the min Tenv but only above 0°C, perhaps because the T b - T a differential for torpid possums in the wild, at a min Tenv of -3.9°C, remains small enough to be compensated by a small increase in MR and does not require the physiological capability for a reduction of Tb below 0°C.

Heterothermic Migration Strategies in Flying Vertebrates

Migration is a widespread and highly variable trait among animals. Population-level patterns arise from individual-level decisions, including physiological and energetic constraints. Many aspects of migration are influenced by behaviors and strategies employed during periods of stopover, where migrants may encounter variable or unpredictable conditions. Thermoregulation can be a major cost for homeotherms which largely encounter ambient temperatures below the lower critical temperature during migration, especially during the rest phase of the daily cycle. In this review we describe the empirical evidence, theoretical models, and potential implications of bats and birds that use heterothermy to reduce thermoregulatory costs during migration. Torpor-assisted migration is a strategy described for migrating temperate insectivorous bats, whereby torpor can be used during periods of inactivity to drastically reduce thermoregulatory costs and increase net refueling rate, leading to shorter stopover duration, reduced fuel load requirement, and potential consequences for broad-scale movement patterns and survival. Hummingbirds can adopt a similar strategy, but most birds are not capable of torpor. However, there is an increasing recognition of the use of more shallow heterothermic strategies by diverse bird species during migration, with similarly important implications for migration energetics. A growing body of published literature and preliminary data from ongoing research indicate that heterothermic migration strategies in birds may be more common than traditionally appreciated. We further take a broad evolutionary perspective to consider heterothermy as an alternative to migration in some species, or as a conceptual link to consider alternatives to seasonal resource limitations. There is a growing body of evidence related to heterothermic migration strategies in bats and birds, but many important questions related to the broader implications of this strategy remain.

Contrasting Torpor Use by Reproductive Male Common Noctule Bats in the Laboratory and in the Field

Metabolic processes of animals are often studied in controlled laboratory settings. However, these laboratory settings often do not reflect the animals' natural environment. Thus, results of metabolic measurements from laboratory studies must be cautiously applied to free-ranging animals. Recent technological advances in animal tracking allow detailed eco-physiological studies that reveal when, where, and how physiological measurements from the field differ from those from the laboratory. We investigated the torpor behavior of male common noctule bats (Nyctalus noctula) across different life history stages using two approaches: in controlled laboratory experiments and in the field using calibrated heart rate telemetry. We predicted that non-reproductive males would extensively use torpor to conserve energy, whereas reproductive males would reduce torpor use to promote spermatogenesis. We did not expect differences in torpor use between captive and wild animals as we simulated natural temperature conditions in the laboratory. We found that during the non-reproductive phase, both captive and free-ranging bats used torpor extensively. During reproduction, bats in captivity unexpectedly also used torpor throughout the day, while only free-ranging bats showed the expected reduction in torpor use. Thus, depending on life history stage, torpor behavior in the laboratory was markedly different from the wild. By implementing both approaches and at different life history stages, we were able to better explore the limitations of eco-physiological laboratory studies and make recommendations for when they are an appropriate proxy for natural behavior.