Extreme individual flexibility of heterothermy in free-ranging Malagasy mouse lemurs (Microcebus griseorufus)

Forest fragmentation and edge effects impact body condition, fur condition and ectoparasite prevalence in a nocturnal lemur community

Forest fragmentation and edge effects are two major threats to primate populations. Primates inhabiting fragmented landscapes must survive in a more degraded environment, often with lower food availability compared to continuous forests. Such conditions can have deleterious effects on animal physiological health, yet some primates thrive in these habitats. Here, we assessed how forest fragmentation and associated edge effects impact three different components of physiological health in a nocturnal primate community in the Sahamalaza-Iles Radama National Park, northwest Madagascar. Over two periods, 6 March 2019-30 October 2019 and 10 January 2022-17 May 2022, we collected data on body condition, fur condition scores and ectoparasite prevalence for 125 Mirza zaza, 51 Lepilemur sahamalaza, 27 Cheirogaleus medius and 22 Microcebus sambiranensis individuals, and we compared these metrics between core and edge areas of continuous forest and fragmented forest. Body condition scores for all species varied between areas, with a positive response to fragmentation and edge effects observed for M. zaza and L. sahamalaza and a negative response for C. medius and M. sambiranensis. Fur condition scores and ectoparasite prevalence were less variable, although M. zaza and L. sahamalaza had a significantly negative response to fragmentation and edge effects for these two variables. Interestingly, the impacts of fragmentation and edge effects on physiological health were variable-specific. Our results suggest that lemur physiological responses to fragmentation and edge effects are species-specific, and body condition, fur condition and ectoparasite prevalence are impacted in different ways between species. As other ecological factors, including food availability and inter/intraspecific competition, likely also influence physiological health, additional work is required to determine why certain aspects of lemur physiology are affected by environmental stressors while others remain unaffected. Although many nocturnal lemurs demonstrate resilience to fragmented and degraded habitats, urgent conservation action is needed to safeguard the survival of their forest habitats.

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.

Rare and Opportunistic Use of Torpor in Mammals-An Echo from the Past?

Torpor was traditionally seen as a winter survival mechanism employed by animals living in cold and highly seasonal habitats. Although we now know that torpor is also used by tropical and subtropical species, and in response to a variety of triggers, torpor is still largely viewed as a highly controlled, seasonal mechanism shown by Northern hemisphere species. To scrutinize this view, we report data from a macroanalysis in which we characterized the type and seasonality of torpor use from mammal species currently known to use torpor. Our findings suggest that predictable, seasonal torpor patterns reported for Northern temperate and polar species are highly derived forms of torpor expression, whereas the more opportunistic and variable forms of torpor that we see in tropical and subtropical species are likely closer to the patterns expressed by ancestral mammals. Our data emphasize that the torpor patterns observed in the tropics and subtropics should be considered the norm and not the exception.

Saving energy via short and shallow torpor bouts

Maintaining a high and stable body temperature as observed in most endothermic mammals and birds is energetically costly and many heterothermic species reduce their metabolic demands during energetic bottlenecks through the use of torpor. With the increasing number of heterotherms revealed in a diversity of habitats, it becomes apparent that triggers and patterns of torpor use are more variable than previously thought. Here, we report the previously overlooked use of, shallow rest-time torpor (body temperature >30 °C) in African lesser bushbabies, Galago moholi. Body core temperature of three adult male bushbabies recorded over five months showed a clear bimodal distribution with an average active modal temperature of 39.2 °C and a resting modal body temperature of 36.7 °C. Shallow torpor was observed in two out of three males (n = 29 torpor bouts) between June and August (austral winter), with body temperatures dropping to an overall minimum of 30.7 °C and calculated energy savings of up to 10%. We suggest that shallow torpor may be an ecologically important, yet mostly overlooked energy-saving strategy employed by heterothermic mammals. Our data emphasise that torpor threshold temperatures need to be used with care if we aim to fully understand the level of physiological plasticity displayed by heterothermic species.

Seasonal Differences in the Encounter Rate of the Fat-Tailed Dwarf Lemur (Cheirogaleus medius) in the Transitional Forests of Northwest Madagascar: Implications for Reliable Population Density Assessment

Primate encounter rates often vary throughout the year due to seasonal differences in activity, ecology, and behaviour. One notably extreme behaviour is continuous hibernation. Although a rare adaptation in primates, the dwarf lemurs of Madagascar (genus Cheirogaleus) enter obligate hibernation each year during the dry season, after spending the wet season consuming high-energy foods. Whilst seasonal changes in activity in some Cheirogaleus populations are well-known, many species remain little-studied, and there is no specific information on their encounter rates, nor when they enter and emerge from hibernation. This uncertainty critically affects reliable calculation of population density estimates for these highly threatened lemurs. In this study, we assessed how encounter rates of the fat-tailed dwarf lemur (C. medius) vary seasonally in the transitional forests of the Sahamalaza-Iles Radama National Park, northwest Madagascar, during a 4-year period. We established a system of line transects (N = 60) throughout our study area, on which we conducted distance sampling of C. medius. We then used our distance sampling data to calculate encounter rate and population density data. We found encounter rates of C. medius to be significantly higher during the wet season compared with the dry season. Furthermore, encounter rates of C. medius were particularly low from May–August. These results provide some evidence to suggest the time period that C. medius hibernate in Sahamalaza-Iles Radama National Park. These findings underpin the importance of careful study design when surveying threatened species with seasonal differences in activity, such as those that hibernate. This study also demonstrates the importance of species-specific behavioural data for accurate population density assessment, which is required to inform conservation action.

Pronounced differences in heart rate and metabolism distinguish daily torpor and short-term hibernation in two bat species

Torpor, and its differential expression, is essential to the survival of many mammals and birds. Physiological characteristics of torpor appear to vary between those species that express strict daily heterothermy and those capable of multiday hibernation, but comparisons are complicated by the temperature-dependence of variables. Previous reviews have compared these different torpor strategies by measuring the depth and duration of torpor in multiple species. However, direct comparison of multiple physiological parameters under similar thermal conditions are lacking. Here, we quantified three physiological variables; body temperature, metabolic rate (MR) and heart rate (HR) of two small heterothermic bats (daily heterotherm Syconycteris australis, and hibernator Nyctophilus gouldi) under comparable thermal conditions and torpor bout durations. When normothermic and resting both MR and HR were similar for the two species. However, during torpor the minimum HR was more than fivefold higher, and minimum MR was 6.5-fold higher for the daily heterotherm than for the hibernator at the same subcutaneous T_b (16 ± 0.5 °C). The data show that the degree of heterothermy defined using T_b is not necessarily a precise proxy for physiological capacity during torpor in these bats and is unlikely to reveal accurate energy budgets. Our study provides evidence supporting a distinction between daily torpor in a daily heterotherm and short term torpor in a hibernator, at least within the Chiroptera with regard to these physiological variables. This exists even when individuals display the same degree of T_b reduction, which has clear implications for the modelling of their energy expenditure.

Disparate roost sites drive intraspecific physiological variation in a Malagasy bat

Many species are widely distributed and individual populations can experience vastly different environmental conditions over seasonal and geographic scales. With such a broad ecological reality, datasets with limited spatial and temporal resolution may not accurately represent a species and could lead to poorly informed management decisions. Because physiological flexibility can help species tolerate environmental variation, we studied the physiological responses of two separate populations of Macronycteris commersoni, a bat widespread across Madagascar, in contrasting seasons. The populations roost under the following dissimilar conditions: either a hot, well-buffered cave or within open foliage, unprotected from the local weather. We found that flexible torpor patterns, used in response to prevailing ambient temperature and relative humidity, were central to keeping energy budgets balanced in both populations. While bats’ metabolic rate during torpor and rest did not differ between roosts, adjusting torpor frequency, duration and timing helped bats maintain body condition. Interestingly, the exposed forest roost induced extensive use of torpor, which exceeded the torpor frequency of overwintering bats that stayed in the cave for months and consequently minimised daytime resting energy expenditure in the forest. Our current understanding of intraspecific physiological variation is limited and physiological traits are often considered to be fixed. The results of our study therefore highlight the need for examining species at broad environmental scales to avoid underestimating a species’ full capacity for withstanding environmental variation, especially in the face of ongoing, disruptive human interference in natural habitats.

Sportive lemurs elevate their metabolic rate during challenging seasons and do not enter regular heterothermy

Animals experience seasonal changes of environmental and ecological conditions in most habitats. Fluctuations in ambient temperature have a strong influence on thermoregulation, particularly on small endothermic mammals. However, different mammalian species cope differently with these changes. Understanding the physiological responses of organisms to different seasons and analysing the mechanisms that account for intra- and inter-specific differences and the ecological consequences of these variations is important to predict species responses to climatic changes. Consequences of climatic changes will be most pronounced in climatically already challenging habitats, such as the dry regions of western Madagascar. We aimed to identify the seasonal responses and adaptive possibilities in energy budgeting of Lepilemur edwardsi, a small primate of this habitat, by measuring metabolic rate (MR; open-flow respiratory) and skin temperature in the field during different seasons. Resting metabolism was generally low, but our study did not detect any signs of regular heterothermic episodes, despite the fact that these are known in other sympatrically living lemurs with a similar lifestyle. Surprisingly, L. edwardsi responded by elevating its resting MR in the poor-resourced dry season, compared to the better-resourced wet season, presumably to master detoxification of their increasingly toxic diet. As body mass decreased over this time, this strategy is obviously not energetically balanced on the long term. This is cause for concern, as it suggests that L. edwardsi has a very small leeway to adjust to changing conditions as experienced due to climate change, as dry season are expected to become longer and hotter, straining water budgets and food quality even more. Moreover, our findings highlight the importance of studying physiological parameters directly in the field and under differing climatic conditions.

Nightly torpor use in response to weather conditions and individual state in an insectivorous bat

Torpor is a well-known energy conservation strategy in many mammal and bird species. It is often employed when environmental conditions are unfavourable to maximize survival probabilities. However, torpor often carries with it the physiological costs of a low body temperature and of rewarming in addition to potential missed opportunities for foraging. Therefore, we hypothesised that decision making regarding when to use torpor should reflect the most important environmental conditions for species distributions, and thus how they may be impacted by ongoing climate change. We investigated how weather conditions affect nightly torpor patterns in the nocturnal insectivorous Australian eastern long-eared bat (Nyctophilus bifax). By measuring the skin temperature of 37 free-ranging individuals, we confirmed that torpor was used more frequently during the winter and at subtropical compared to tropical locations. Using mixed-effect models we show that lower ambient temperatures were the main driver of individual torpor use, probably due to lower roost temperatures and prey availability. However, increased rain, wind and humidity, and decreasing barometric pressure, as well as brighter moonlight, also led to more time spent torpid per night. We suggest that bats evaluate multiple environmental cues to make decisions regarding torpor use versus active foraging based upon their expectations of the energetic benefits, prey availability and relative predation risk. Interactions between some of these effects and body mass (whilst controlling for forearm length) indicate that individual variation in body size and/or state-dependent effects of energy reserves also partly determined the use of nightly torpor in these bats.

Field Physiology: Studying Organismal Function in the Natural Environment

Continuous physiological measurements collected in field settings are essential to understand baseline, free-ranging physiology, physiological range and variability, and the physiological responses of organisms to disturbances. This article presents a current summary of the available technologies to continuously measure the direct physiological parameters in the field at high-resolution/instantaneous timescales from freely behaving animals. There is a particular focus on advantages versus disadvantages of available methods as well as emerging technologies "on the horizon" that may have been validated in captive or laboratory-based scenarios but have yet to be applied in the wild. Systems to record physiological variables from free-ranging animals are reviewed, including radio (VHF/UFH) telemetry, acoustic telemetry, and dataloggers. Physiological parameters that have been continuously measured in the field are addressed in seven sections including heart rate and electrocardiography (ECG); electromyography (EMG); electroencephalography (EEG); body temperature; respiratory, blood, and muscle oxygen; gastric pH and motility; and blood pressure and flow. The primary focal sections are heart rate and temperature as these can be, and have been, extensively studied in free-ranging organisms. Predicted aspects of future innovation in physiological monitoring are also discussed. The article concludes with an overview of best practices and points to consider regarding experimental designs, cautions, and effects on animals. © 2021 American Physiological Society. Compr Physiol 11:1979-2015, 2021.

On the modulation and maintenance of hibernation in captive dwarf lemurs

In nature, photoperiod signals environmental seasonality and is a strong selective "zeitgeber" that synchronizes biological rhythms. For animals facing seasonal environmental challenges and energetic bottlenecks, daily torpor and hibernation are two metabolic strategies that can save energy. In the wild, the dwarf lemurs of Madagascar are obligate hibernators, hibernating between 3 and 7 months a year. In captivity, however, dwarf lemurs generally express torpor for periods far shorter than the hibernation season in Madagascar. We investigated whether fat-tailed dwarf lemurs (Cheirogaleus medius) housed at the Duke Lemur Center (DLC) could hibernate, by subjecting 8 individuals to husbandry conditions more in accord with those in Madagascar, including alternating photoperiods, low ambient temperatures, and food restriction. All dwarf lemurs displayed daily and multiday torpor bouts, including bouts lasting ~ 11 days. Ambient temperature was the greatest predictor of torpor bout duration, and food ingestion and night length also played a role. Unlike their wild counterparts, who rarely leave their hibernacula and do not feed during hibernation, DLC dwarf lemurs sporadically moved and ate. While demonstrating that captive dwarf lemurs are physiologically capable of hibernation, we argue that facilitating their hibernation serves both husbandry and research goals: first, it enables lemurs to express the biphasic phenotypes (fattening and fat depletion) that are characteristic of their wild conspecifics; second, by "renaturalizing" dwarf lemurs in captivity, they will emerge a better model for understanding both metabolic extremes in primates generally and metabolic disorders in humans specifically.

Adenovirus infection is associated with altered gut microbial communities in a non-human primate

Adenovirus (AdV) infections are one of the main causes of diarrhea in young children. Enteric AdVs probably disrupt gut microbial defences, which can result in diarrhea. To understand the role of the gut microbiome in AdV-induced pathologies, we investigated the gut microbiome of a naturally AdV-infected non-human primate species, the Malagasy mouse lemur (Microcebus griseorufus), which represents an important model in understanding the evolution of diseases. We observed that AdV infection is associated with disruption of the gut microbial community composition. In AdV+ lemurs, several commensal taxa essential for a healthy gut microbiome decreased, whereas genera containing potential pathogens, such as Neisseria, increased in abundance. Microbial co-occurrence networks revealed a loss of important microbial community interactions in AdV+ lemurs and an overrepresentation of Prevotellaceae. The observation of enteric virus-associated loss of commensal bacteria and associated shifts towards pathobionts may represent the missing link for a better understanding of AdV-induced effects in humans, and also for their potential as drivers of co-infections, an area of research that has been largely neglected so far.

The Biological Clock in Gray Mouse Lemur: Adaptive, Evolutionary and Aging Considerations in an Emerging Non-human Primate Model

Circadian rhythms, which measure time on a scale of 24 h, are genetically generated by the circadian clock, which plays a crucial role in the regulation of almost every physiological and metabolic process in most organisms. This review gathers all the available information about the circadian clock in a small Malagasy primate, the gray mouse lemur (Microcebus murinus), and reports 30 years data from the historical colony at Brunoy (France). Although the mouse lemur has long been seen as a "primitive" species, its clock displays high phenotypic plasticity, allowing perfect adaptation of its biological rhythms to environmental challenges (seasonality, food availability). The alterations of the circadian timing system in M. murinus during aging show many similarities with those in human aging. Comparisons are drawn with other mammalian species (more specifically, with rodents, other non-human primates and humans) to demonstrate that the gray mouse lemur is a good complementary and alternative model for studying the circadian clock and, more broadly, brain aging and pathologies.

An intra-population heterothermy continuum: notable repeatability of body temperature variation in food-deprived yellow-necked mice

Theoretical modelling predicts that the thermoregulatory strategies of endothermic animals range from those represented by thermal generalists to those characteristic for thermal specialists. While the generalists tolerate wide variations in body temperature (T _b), the specialists maintain T _b at a more constant level. The model has gained support from inter-specific comparisons relating to species and population levels. However, little is known about consistent among-individual variation within populations that could be shaped by natural selection. We studied the consistency of individual heterothermic responses to environmental challenges in a single population of yellow-necked mice (Apodemus flavicollis), by verifying the hypothesis that T _b variation is a repeatable trait. To induce the heterothermic response, the same individuals were repeatedly food deprived for 24 h. We measured T _b with implanted miniaturised data loggers. Before each fasting experiment, we measured basal metabolic rate (BMR). Thus, we also tested whether individual variation of heterothermy correlates with individual self-maintenance costs, and the potential benefits arising from heterothermic responses that should correlate with body size/mass. We found that some individuals clearly entered torpor while others kept T _b stable, and that there were also individuals that showed intermediate thermoregulatory patterns. Heterothermy was found to correlate negatively with body mass and slightly positively with the BMR achieved 1-2 days before fasting. Nonetheless, heterothermy was shown to be highly repeatable, irrespective of whether we controlled for self-maintenance costs and body size. Our results indicate that specialist and generalist thermoregulatory phenotypes can co-exist in a single population, creating a heterothermy continuum.

Individual variation of daily torpor and body mass change during winter in the large Japanese field mouse (Apodemus speciosus)

Daily torpor is a strategy used by some overwintering small endotherms to aid in energy conservation. However, the pattern of torpor varies among individuals within species and populations, even under the same environmental conditions, with significant implications for survival rate and reproductive success. Body mass is one factor that may influence this variation, especially in some small mammals that accumulate fat stores prior to overwintering. However, to our knowledge there has been no previous study examining the detailed relationships between torpor expression and body mass change in small mammals that hoard food as an energy resource during winter. The large Japanese field mouse, Apodemus speciosus, whose winter survival strategy depends on food caches instead of fat stores, displays daily torpor under artificial winter conditions (short-day photoperiod and cold). The present study clarifies the characteristics and patterns of daily torpor and body mass change in this species in the laboratory. Although expression of daily torpor was facilitated progressively as in other species, the observed patterns of torpor expression and body mass change showed considerable individual variation. Moreover, there was no obvious correlation between body mass and daily torpor expression. Therefore, it is suggested that in A. speciosus body mass may not contribute to individual variation of daily torpor during winter. Daily torpor during winter may be adjusted by not only mechanisms common to other small mammals, but also species-specific factors relating to the external or internal reserves of energy in small mammals.

Short and hyperthermic torpor responses in the Malagasy bat Macronycteris commersoni reveal a broader hypometabolic scope in heterotherms

The energy budgets of animal species are closely linked to their ecology, and balancing energy expenditure with energy acquisition is key for survival. Changes in animals' environments can be challenging, particularly for bats, which are small endotherms with large uninsulated flight membranes. Heterothermy is a powerful response used to cope with changing environmental conditions. Recent research has revealed that many tropical and subtropical species are heterothermic and display torpor with patterns unlike those of "classical" heterotherms from temperate and arctic regions. However, only a handful of studies investigating torpor in bats in their natural environment exist. Therefore, we investigated whether the Malagasy bat Macronycteris commersoni enters torpor in the driest and least predictable region in Madagascar. We examined the energy balance and thermal biology of M. commersoni in the field by relating metabolic rate (MR) and skin temperature (T_skin) measurements to local environmental characteristics in the dry and rainy seasons. Macronycteris commersoni entered torpor and showed extreme variability in torpor patterns, including surprisingly short torpor bouts, lasting on average 20 min, interrupted by MR peaks. Torpid MR was remarkably low (0.13 ml O₂ h^-1 g^-1), even when T_skin exceeded that of normothermia (41 °C). Macronycteris commersoni is thus physiologically capable of (1) entering torpor at high ambient temperature and T_skin and (2) rapidly alternating between torpid and normothermic MR resulting in very short bouts. This suggests that the scope of hypometabolism amongst heterothermic animals is broader than previously assumed and underlines the importance of further investigation into the torpor continuum.

The Mechanism Enabling Hibernation in Mammals

Some rodents including squirrels and hamsters undergo hibernation. During hibernation, body temperature drops to only a few degrees above ambient temperature. The suppression of whole-body energy expenditure is associated with regulated, but not passive, reduction of cellular metabolism. The heart retains the ability to beat constantly, although body temperature drops to less than 10 °C during hibernation. Cardiac myocytes of hibernating mammals are characterized by reduced Ca²⁺ entry into the cell membrane and a concomitant enhancement of Ca²⁺ release from and reuptake by the sarcoplasmic reticulum. These adaptive changes would help in preventing excessive Ca²⁺ entry and its overload and in maintaining the resting levels of intracellular Ca²⁺. Adaptive changes in gene expression in the heart prior to hibernation may be indispensable for acquiring cold resistance. In addition, protective effects of cold-shock proteins are thought to have an important role. We recently reported the unique expression pattern of cold-inducible RNA-binding protein (CIRP) in the hearts of hibernating hamsters. The CIRP mRNA is constitutively expressed in the heart of a nonhibernating euthermic hamster with several different forms probably due to alternative splicing. The short product contained the complete open reading frame for full-length CIRP, while the long product had inserted sequences containing a stop codon, suggesting production of a C-terminal deletion isoform of CIRP. In contrast to nonhibernating hamsters, only the short product was found in hibernating animals. Thus, these results indicate that CIRP expression in the hamster heart is regulated at the level of alternative splicing, which would permit a rapid increment of functional CIRP when entering hibernation. We will summarize the current understanding of the cold-resistant property of the heart in hibernating animals.

Effects of food store quality on hibernation performance in common hamsters

Hibernating animals can adjust torpor expression according to available energy reserves. Besides the quantity, the quality of energy reserves could play an important role for overwintering strategies. Common hamsters are food-storing hibernators and show high individual variation in hibernation performance, which might be related to the quality of food hoards in the hibernacula. In this study, we tested the effects of food stores high in fat content, particularly polyunsaturated fatty acids (PUFAs), on hibernation patterns under laboratory conditions. Control animals received standard rodent pellets only, while in the other group pellets were supplemented with sunflower seeds. We recorded body temperature during winter using subcutaneously implanted data loggers, documented total food consumption during winter, and analysed PUFA proportions in white adipose tissue (WAT) before and after the winter period. About half of the individuals in both groups hibernated and torpor expression did not differ between these animals. Among the high-fat group, however, individuals with high sunflower seeds intake strongly reduced the time spent in deep torpor. PUFA proportions in WAT decreased during winter in both groups and this decline was positively related to the time an individual spent in deep torpor. Sunflower seeds intake dampened the PUFA decline resulting in higher PUFA levels in animals of the high-fat group after winter. In conclusion, our results showed that common hamsters adjusted torpor expression and food intake in relation to the total energy of food reserves, underlining the importance of food hoard quality on hibernation performance.

Two New Species of Sucking Lice (Phthiraptera: Anoplura: Polyplacidae) From Endangered, Hibernating Lemurs (Primates: Cheirogaleidae)

Lemurpediculus robbinsi sp. nov. is described from Crossley's dwarf lemur, Cheirogaleus crossleyi A. Grandidier, and Lemurpediculus claytoni sp. nov. is described from Sibree's dwarf lemur, Cheirogaleus sibreei Forsyth Major, from Madagascar. Both sexes of each new louse species are illustrated and distinguished from the two previously known species of Lemurpediculus: L. verruculosus (Ward) and L. petterorum Paulian. With the addition of two new species to the genus, an amended description of Lemurpediculus is provided. The two hosts of the new louse species are morphologically similar, endangered, obligately hibernating lemurs. These two species of lemurs are sometimes sympatric in rainforests in eastern Madagascar. Despite the morphological similarity of the two host species, their lice are morphologically distinct and are easiest to identify based on the shape of the subgenital plate of the female and the shape of the genitalia in the male. Both new species of lice should be considered to be endangered because their hosts are endangered. It is not known if either of the new species of lice are vectors of pathogens or parasites to their hosts.

More functions of torpor and their roles in a changing world

Increased winter survival by reducing energy expenditure in adult animals is often viewed as the primary function of torpor. However, torpor has many other functions that ultimately increase the survival of heterothermic mammals and birds. In this review, we summarize new findings revealing that animals use torpor to cope with the conditions during and after natural disasters, including fires, storms, and heat waves. Furthermore, we suggest that torpor, which also prolongs longevity and was likely crucial for survival of mammals during the time of the dinosaur extinctions, will be advantageous in a changing world. Climate change is assumed to lead to an increase in the occurrence and intensity of climatic disasters, such as those listed above and also abnormal floods, droughts, and extreme temperatures. The opportunistic use of torpor, found in many heterothermic species, will likely enhance survival of these challenges, because these species can reduce energy and foraging requirements. However, many strictly seasonal hibernators will likely face the negative consequences of the predicted increase in temperature, such as range contraction. Overall, available data suggest that opportunistic heterotherms with their flexible energy requirements have an adaptive advantage over homeotherms in response to unpredictable conditions.

Torpor patterns in common hamsters with and without access to food stores

Hibernating species significantly reduce energy expenditure during winter by entering torpor. Nevertheless, the various benefits of hibernation might be counteracted by negative effects of torpor such as immune depression, oxidative stress, or neuronal impairment. Considering these trade-offs, adequate energy reserves could allow animals to reduce the time spent in torpor or the extent of metabolic depression. Common hamsters use food stores during hibernation and previously documented high individual variations in body temperature patterns during winter could, therefore, be related to differences in external energy reserves. In this study, we manipulated the availability of food stores under laboratory conditions to investigate potential effects on hibernation patterns. Female hamsters were kept in artificial burrows in climate chambers and subcutaneous temperature was recorded using implanted data loggers. One group had access to large food stores, whereas another group received daily food portions which were removed on the next day if not consumed. Almost all hamsters without access to food stores hibernated, while less than half of the individuals with food stores entered deep torpor. Individuals without food hoards additionally expressed more short torpor bouts and exhibited lower minimum subcutaneous temperatures during torpor than those with food stores. Thus, individuals confronted with lacking food reserves were more likely to hibernate and additionally saved energy by entering short torpor bouts more frequently and remaining at lower subcutaneous temperature both during torpor and euthermic periods. In conclusion, our results demonstrate that food store availability affects torpor expression and also highlight variation in torpor patterns and energy-saving strategies in common hamsters.

Seasonal adaptations in energy budgeting in the primate Lepilemur leucopus

The spiny forest of South Madagascar is one of the driest and most unpredictable habitats in Africa. The small-bodied, nocturnal primate Lepilemur leucopus lives in this harsh habitat with high diurnal and seasonal changes in ambient temperature. In this study, we investigated seasonal adaptions in energy budgeting of L. leucopus, which allow it to live under these conditions by measuring resting metabolic rate using open-flow respirometry. No signs of heterothermy were detected, and resting metabolic rate was significantly lower in the warmer wet season than in the colder dry season. In fact, L. leucopus possesses one of the lowest mass-specific metabolic rates measured so far for an endotherm, probably the result of adaptations to its habitat and folivorous and potentially toxic diet. Surprisingly, we identified a shift of the thermoneutral zone from between 25 and 30 °C in the wet season to between 29 and 32 °C in the cool dry season. L. leucopus seems to be more affected by the hot daytime temperatures during the dry season and thermoregulation seems to be more costly during this time, which makes this shift of the thermoneutral zone advantageous. Our findings suggest that L. leucopus has a very small scope to unfavorable conditions, making it highly vulnerable, e.g., to changing conditions due to climate change.

Primate Torpor Expression: Ghost of the Climatic Past

Torpor, the controlled depression of virtually all bodily function during scarce periods, was verified in primates under free-ranging conditions less than two decades ago. The large variety of different torpor patterns found both within and among closely related species is particularly remarkable. To help unravel the cause of these variable patterns, our review investigates primate torpor use within an evolutionary framework. First, we provide an overview of heterothermic primate species, focusing on the Malagasy lemurs, and discuss their use of daily torpor or hibernation in relation to habitat type and climatic conditions. Second, we investigate environmental characteristics that may have been involved in shaping the high variability of torpor expression found in lemurs today. Third, we examine potential triggers for torpor use in lemurs. We propose the “torpor refugia hypothesis” to illustrate how disparate primate torpor patterns possibly evolved in response to environmental cues during glacial periods, when animals were restricted to different refuge habitats along riverine corridors. For example, individuals enduring harsher conditions at higher altitudes likely developed seasonal hibernation, whereas those inhabiting lower elevation river catchments might have coped with unfavorable conditions by employing daily torpor. The ultimate stimuli triggering torpor use today likely differ between the different habitats of Madagascar. The broad diversity of torpor patterns in lemurs among closely related species, both within the same and in distinctly different habitat types, provides an ideal base for research into the stimuli for torpor use in endotherms in general. Our hypothesis highlights the importance of considering the environmental conditions under which ecosystems and species evolved when trying to explain physiological adaptations seen today.

How to spend the summer? Free-living dormice (Glis glis) can hibernate for 11 months in non-reproductive years

Edible dormice are arboreal rodents adapted to yearly fluctuations in seed production of European beech, a major food source for this species. In years of low beech seed abundance, dormice skip reproduction and non-reproductive dormice fed ad libitum in captivity can display summer dormancy in addition to winter hibernation. To test whether summer dormancy, that is, a very early onset of hibernation, actually occurs in free-living dormice, we monitored core body temperature (Tb) over ~12 months in 17 animals during a year of beech seeding failure in the Vienna Woods. We found that 8 out of 17 dormice indeed re-entered hibernation as early as in June/July, with five of them having extreme hibernation durations of 11 months or more (total range: 7.8-11.4 months). Thus, we show for the first time that a free-living mammal relying on natural food resources can continuously hibernate for >11 months. Early onset of hibernation was associated with high body mass in the spring, but the distribution of hibernation onset was bimodal with prolonged hibernation starting either early (prior to July 28) or late (after August 30). This could not be explained by differences in body mass alone. Animals with a late hibernation onset continued to maintain high nocturnal Tb's throughout summer but used short, shallow torpor bouts (mean duration 7.44 ± 0.9 h), as well as occasional multiday torpor for up to 161 h.

Daily torpor and hibernation in birds and mammals

Many birds and mammals drastically reduce their energy expenditure during times of cold exposure, food shortage, or drought, by temporarily abandoning euthermia, i.e. the maintenance of high body temperatures. Traditionally, two different types of heterothermy, i.e. hypometabolic states associated with low body temperature (torpor), have been distinguished: daily torpor, which lasts less than 24 h and is accompanied by continued foraging, versus hibernation, with torpor bouts lasting consecutive days to several weeks in animals that usually do not forage but rely on energy stores, either food caches or body energy reserves. This classification of torpor types has been challenged, suggesting that these phenotypes may merely represent extremes in a continuum of traits. Here, we investigate whether variables of torpor in 214 species (43 birds and 171 mammals) form a continuum or a bimodal distribution. We use Gaussian-mixture cluster analysis as well as phylogenetically informed regressions to quantitatively assess the distinction between hibernation and daily torpor and to evaluate the impact of body mass and geographical distribution of species on torpor traits. Cluster analysis clearly confirmed the classical distinction between daily torpor and hibernation. Overall, heterothermic endotherms tend to be small; hibernators are significantly heavier than daily heterotherms and also are distributed at higher average latitudes (∼35°) than daily heterotherms (∼25°). Variables of torpor for an average 30 g heterotherm differed significantly between daily heterotherms and hibernators. Average maximum torpor bout duration was >30-fold longer, and mean torpor bout duration >25-fold longer in hibernators. Mean minimum body temperature differed by ∼13°C, and the mean minimum torpor metabolic rate was ∼35% of the basal metabolic rate (BMR) in daily heterotherms but only 6% of BMR in hibernators. Consequently, our analysis strongly supports the view that hibernators and daily heterotherms are functionally distinct groups that probably have been subject to disruptive selection. Arguably, the primary physiological difference between daily torpor and hibernation, which leads to a variety of derived further distinct characteristics, is the temporal control of entry into and arousal from torpor, which is governed by the circadian clock in daily heterotherms, but apparently not in hibernators.

The Hibernation Continuum: Physiological and Molecular Aspects of Metabolic Plasticity in Mammals

Mammals are often considered to be masters of homeostasis, with the ability to maintain a constant internal milieu, despite marked changes in the environment; however, many species exhibit striking physiological and biochemical plasticity in the face of environmental fluctuations. Here, we review metabolic depression and body temperature fluctuation in mammals, with a focus on the extreme example of hibernation in small-bodied eutherian species. Careful exploitation of the phenotypic plasticity of mammals with metabolic flexibility may provide the key to unlocking the molecular secrets of orchestrating and surviving reversible metabolic depression in less plastic species, including humans.

Unpredictable environments, opportunistic responses: Reproduction and population turnover in two wild mouse lemur species (Microcebus rufus and M. griseorufus) from eastern and western Madagascar

Small-bodied, nocturnal mouse lemurs (Microcebus) are widespread across diverse forest habitats in Madagascar. They are strict seasonal breeders and can, depending on the habitat and species, undergo daily or prolonged torpor to minimize energy expenditure during periods of food and water scarcity. Duration of reproduction, number of litters per season and timing of births vary across individuals and species. The "polyestry-seasonality" hypothesis proposes that the duration of reproduction and number of litters per year are positively correlated with rainfall but negatively correlated with longevity, whereas the "hypervariability" hypothesis suggests that the duration of reproduction is negatively correlated with the degree of predictability of food resources. We test these hypotheses in two mouse lemur species inhabiting contrasting habitats, the brown mouse lemurs, Microcebus rufus, from Ranomafana (a less seasonal and more climatically predictable habitat) and the gray-brown mouse lemurs, M. griseorufus, from Beza Mahafaly (a more seasonal and less climatically predictable environment). We use capture/mark/recapture techniques and records of female reproductive status. We found evidence of polyestry at both study sites but faster population turnover and longer duration of the reproductive season at Beza Mahafaly. The "polyestry-seasonality" hypothesis is not supported but the "hypervariability" hypothesis could not be rejected. We conclude that reproductive output cannot be tied to climatic factors in a simple manner. Paradoxically, polyestry can be expressed in contrasting habitats: less seasonal forests where females can sustain multiple reproductive events, but also highly seasonal environments where females may not fatten sufficiently to sustain prolonged torpor but instead remain active throughout the year by relying on fallback resources. Am. J. Primatol. 77:936-947, 2015. © 2015 Wiley Periodicals, Inc.

When to initiate torpor use? Food availability times the transition to winter phenotype in a tropical heterotherm

Timing of winter phenotype expression determines individual chances of survival until the next reproductive season. Environmental cues triggering this seasonal phenotypic transition have rarely been investigated, although they play a central role in the compensation of climatic fluctuations via plastic phenotypic adjustments. Initiation of winter daily torpor use-a widespread energy-saving phenotype-could be primarily timed according to anticipatory seasonal cues (anticipatory cues hypothesis), or flexibly fine-tuned according to actual energy availability (food shortage hypothesis). We conducted a food supplementation experiment on wild heterothermic primates (grey mouse lemurs, Microcebus murinus) at the transition to the food-limited dry season, i.e. the austral winter. As expected under the food shortage hypothesis, food-supplemented individuals postponed the seasonal transition to normal torpor use by 1-2 month(s), spent four times less torpid, and exhibited minimal skin temperature 6 °C higher than control animals. This study provides the first in situ experimental evidence that food availability, rather than abiotic cues, times the launching of torpor use. Fine-tuning of the timing of seasonal phenotypic transitions according to actual food shortage should provide heterotherms with a flexible adaptive mechanism to survive unexpected environmental fluctuations.

Maintaining microendemic primate species along an environmental gradient - parasites as drivers for species differentiation

Understanding the drivers of species adaptations to changing environments on the one hand and the limits for hybridization on the other hand is among the hottest questions in evolutionary biology. Parasites represent one of the major selective forces driving host evolution and at least those with free-living stages are at the same time dependent on the ecological conditions of their host's habitat. Local immunological adaptations of host species to varying parasite pressure are therefore expected and might represent the genetic basis for ecological speciation and the maintenance of recently diverged species. Madagascar provides one of the rare examples where two partially sympatric primate species (Microcebus griseorufus, M. murinus) and their hybrids, as well as an allopatric species (M. cf rufus) live in close proximity along a very steep environmental gradient ranging from southern dry spiny bush to gallery forest to evergreen eastern humid rain forest, thus mimicking the situation encountered during extensions and retreats of vegetation formations under changing climatic conditions. This system was used to study parasite infection and immune gene (MHC) adaptations to varying parasite pressure that might provide selective advantages to pure species over hybrids. Parasite burdens increased with increasing humidity. M. griseorufus, M. murinus, and their hybrids but not M. rufus shared the same MHC alleles, indicating either retention of ancestral polymorphism or recent gene flow. The hybrids had much higher prevalence of intestinal parasites than either of the parent species living under identical environmental conditions. The different representation of parasites can indicate a handicap for hybrids that maintains species identities.

Torpor during reproduction in mammals and birds: dealing with an energetic conundrum

Torpor and reproduction in mammals and birds are widely viewed as mutually exclusive processes because of opposing energetic and hormonal demands. However, the reported number of heterothermic species that express torpor during reproduction is ever increasing, to some extent because of recent work on free-ranging animals. We summarize current knowledge about those heterothermic mammals that do not express torpor during reproduction and, in contrast, examine those heterothermic birds and mammals that do use torpor during reproduction. Incompatibility between torpor and reproduction occurs mainly in high-latitude sciurid and cricetid rodents, which live in strongly seasonal, but predictably productive habitats in summer. In contrast, torpor during incubation, brooding, pregnancy, or lactation occurs in nightjars, hummingbirds, echidnas, several marsupials, tenrecs, hedgehogs, bats, carnivores, mouse lemurs, and dormice. Animals that enter torpor during reproduction often are found in unpredictable habitats, in which seasonal availability of food can be cut short by changes in weather, or are species that reproduce fully or partially during winter. Moreover, animals that use torpor during the reproductive period have relatively low reproductive costs, are largely insectivorous, carnivorous, or nectarivorous, and thus rely on food that can be unpredictable or strongly seasonal. These species with relatively unpredictable food supplies must gain an advantage by using torpor during reproduction because the main cost is an extension of the reproductive period; the benefit is increased survival of parent and offspring, and thus fitness.

Torpor is not the only option: seasonal variations of the thermoneutral zone in a small primate

The reddish-gray mouse lemur (Microcebus griseorufus) is one of only a few small mammals inhabiting the spiny forest of southwestern Madagascar. In this study we investigated the physiological adjustments which allow these small primates to persist under the challenging climatic conditions of their habitat. To this end we measured energy expenditure (metabolic rate) and body temperature of 24 naturally acclimatized mouse lemurs, kept in outdoor enclosures, during different seasons (summer, winter, and the transition period between the two seasons). Mouse lemurs displayed two main physiological strategies to compensate seasonal and diurnal fluctuations of ambient temperature. On the one hand, individuals entered hypometabolism with decreasing ambient temperature (T a) during the transition period and winter, enabling them to save up to 21 % energy per day (92 % per hour) compared with the normal resting metabolic rate at comparable T a. On the other hand, euthermic mouse lemurs also showed physiological adjustments to seasonality when resting: the lower critical temperature of the thermoneutral zone decreased from summer to winter by 7.5 °C, which allowed mouse lemurs to keep energy demands constant despite colder T as during winter. In addition, the basal metabolic rate was substantially lowered prior to the winter period, which facilitated accumulation of fat reserves. The combination of physiological modifications during euthermia in addition to hypometabolism, which can be individually adjusted according to external parameters and respective body condition, is important as it allows M. griseorufus to cope with the environmental variability of an energetically challenging habitat.

Body temperature and thermal environment in a generalized arboreal anthropoid, wild mantled howling monkeys (Alouatta palliata)

Free-ranging primates are confronted with the challenge of maintaining an optimal range of body temperatures within a thermally dynamic environment that changes daily, seasonally, and annually. While many laboratory studies have been conducted on primate thermoregulation, we know comparatively little about the thermal pressures primates face in their natural, evolutionarily relevant environment. Such knowledge is critical to understanding the evolution of thermal adaptations in primates and for comparative evaluation of humans' unique thermal adaptations. We examined temperature and thermal environment in free-ranging, mantled howling monkeys (Alouatta palliata) in a tropical dry forest in Guanacaste, Costa Rica. We recorded subcutaneous (Tsc ) and near-animal ambient temperatures (Ta ) from 11 animals over 1586.5 sample hours during wet and dry seasons. Howlers displayed considerable variation in Tsc , which was largely attributable to circadian effects. Despite significant seasonal changes in the ambient thermal environment, howlers showed relatively little evidence for seasonal changes in Tsc . Howlers experienced warm thermal conditions which led to body cooling relative to the environment, and plateaus in Tsc at increasingly warm Ta . They also frequently faced cool thermal conditions (Ta  < Tsc ) in which Tsc was markedly elevated compared with Ta . These data add to a growing body of evidence that non-human primates have more labile body temperatures than humans. Our data additionally support a hypothesis that, despite inhabiting a dry tropical environment, howling monkeys experience both warm and cool thermal pressures. This suggests that thermal challenges may be more prevalent for primates than previously thought, even for species living in nonextreme thermal environments.