A physiological perspective on nectar-feeding adaptation in phyllostomid bats

Nectar-feeding animals increase their food intake when nectar sugar concentration decreases. However, some species present physiological constraints that limit their energy intake when nectar is diluted. We hypothesized that gut capacities of bats affect the ability of these animals to acquire and store energy, modifying how they use food resources in the field. We measured the food intake and changes in body mass of the members of an assemblage of nectar-feeding bats (Choeronycteris mexicana, Leptonycteris yerbabuenae, and Glossophaga soricina) feeding on sucrose solutions of different concentrations (146, 292, 438, 584, 730, 876, and 1,022 mmol L(-1)). The three bat species presented differences in their food intake and their capacity to store energy. While C. mexicana was able to maintain a constant energy intake at all concentrations tested, G. soricina and L. yerbabuenae decreased their sugar/energy intake at the lowest sugar concentrations. Choeronycteris mexicana also increased body mass independent of sugar concentration, while G. soricina and L. yerbabuenae did not. On the basis of our results, we generated a model relating gut capacities and the use of food resources in the field. Our model's predictions and field data support the idea that digestive traits affect the way these animals use the food resources present in their environment.     

High activity enables life on a high-sugar diet: blood glucose regulation in nectar-feeding bats

High blood glucose levels caused by excessive sugar consumption are detrimental to mammalian health and life expectancy. Despite consuming vast quantities of sugar-rich floral nectar, nectar-feeding bats are long-lived, provoking the question of how they regulate blood glucose. We investigated blood glucose levels in nectar-feeding bats (Glossophaga soricina) in experiments in which we varied the amount of dietary sugar or flight time. Blood glucose levels increased with the quantity of glucose ingested and exceeded 25 mmol l(-1) blood in resting bats, which is among the highest values ever recorded in mammals fed sugar quantities similar to their natural diet. During normal feeding, blood glucose values decreased with increasing flight time, but only fell to expected values when bats spent 75 per cent of their time airborne. Either nectar-feeding bats have evolved mechanisms to avoid negative health effects of hyperglycaemia, or high activity is key to balancing blood glucose levels during foraging. We suggest that the coevolutionary specialization of bats towards a nectar diet was supported by the high activity and elevated metabolic rates of these bats. High activity may have conferred benefits to the bats in terms of behavioural interactions and foraging success, and is simultaneously likely to have increased their efficiency as plant pollinators.     

Intake Responses in Nectar Feeding Birds: Digestive and Metabolic Causes, Osmoregulatory Consequences, and Coevolutionary Effects

Nectar-feeding vertebrates respond to variation in nectar sugarcontent by modulating volumetric intake. In some nectar feedinganimals, the intake response to sugar concentration can be accuratelypredicted from simple mathematical models that rely on knowledgeof gut morphology, in vitro rates of sugar digestion, and dailyenergy expenditures. Because most of the floral nectars consumedby vertebrates are dilute, these animals ingest large amountsof water while feeding. The water turnover rates of hummingbirdsfeeding on dilute nectar are more similar to those of amphibiousand aquatic organisms than to those of terrestrial vertebrates.Dilute nectars can pose osmoregulatory challenges for nectarivores.Nectarivorous birds exhibit renal traits that are well suitedto dispose of large water loads and that appear inadequate toproduce concentrated urine. Nectar-feeding birds prefer concentratedover dilute sugar solutions. However, the concentration differencethat they can discriminate is smaller at low than at high concentration.We hypothesize that this pattern is a consequence of the functionalform of intake responses that often results in deceleratingsugar intakes with increasing sugar concentration. The diminishingreturns in floral attractivity that may result from increasednectar concentration may be one of the reasons why the nectarsof hummingbird pollinated flowers are dilute in spite of thepreference of birds for higher concentrations. The intake responsesof nectar-feeding birds capture the integration of a behavioralresponse with the physiological processes that shape it. Becausethe behavior of nectar-feeding birds can have consequences forthe plants that they visit, the intake response may also havecoevolutionary effects.     

Fluid intake rates in ants correlate with their feeding habits

This study investigates the techniques of nectar feeding in 11 different ant species, and quantitatively compares fluid intake rates over a wide range of nectar concentrations in four species that largely differ in their feeding habits. Ants were observed to employ two different techniques for liquid food intake, in which the glossa works either as a passive duct-like structure (sucking), or as an up- and downwards moving shovel (licking). The technique employed for collecting fluids at ad libitum food sources was observed to be species-specific and to correlate with the presence or absence of a well-developed crop in the species under scrutiny. Workers of ponerine ants licked fluid food during foraging and transported it as a droplet between their mandibles, whereas workers of species belonging to phylogenetically more advanced subfamilies, with a crop capable of storing liquids, sucked the fluid food, such as formicine ants of the genus Camponotus. In order to evaluate the performance of fluid collection during foraging, intake rates for sucrose solutions of different concentrations were measured in four ant species that differ in their foraging ecology. Scaling functions between fluid intake rates and ant size were first established for the polymorphic species, so as to compare ants of different size across species. Results showed that fluid intake rate depended, as expected and previously reported in the literature, on sugar concentration and the associated fluid viscosity. It also depended on both the species-specific feeding technique and the extent of specialization on foraging on liquid food. For similarly-sized ants, workers of two nectar-feeding ant species, Camponotus rufipes (Formicinae) and Pachycondyla villosa (Ponerinae), collected fluids with the highest intake rates, while workers of the leaf-cutting ant Atta sexdens (Myrmicinae) and a predatory ant from the Rhytidoponera impressa-complex (Ponerinae) did so with the lowest rate. Calculating the energy intake rates in mg sucrose per unit time, licking was shown to be a more advantageous technique at higher sugar concentrations than sucking, whereas sucking provided a higher energy intake rate at lower sugar concentrations.     

Sugar preferences, absorption efficiency and water influx in a Neotropical nectarivorous passerine, the Bananaquit (Coereba flaveola)

Nectarivory has evolved repeatedly in a number of unrelated bird taxa throughout the world and nectar feeding birds, regardless of their taxonomic affiliation, display convergent foraging and food processing adaptations that allow them to subsist on weak sugar solutions. However, phylogeny influences sugar type preferences of nectarivores. We investigated sugar preferences, assimilation efficiency and water flux in a Neotropical honeycreeper, the Bananaquit (Coereba flaveola; Coerebidae), a member of a radiation of tanagers and finches. Bananaquits showed no preference for nearly equicaloric (25% w/v) sucrose, glucose, fructose or glucose-fructose mixtures in pair-wise choice tests. In agreement with this lack of preference, they were equally efficient at absorbing sucrose and both hexoses. Apparent assimilation efficiency of these sugars was around 97.5%. In pair-wise tests, Bananaquits displayed a strong preference for the most concentrated sucrose solution when the lowest concentration ranged from 276 to 522 mM. Between 522 and 1120 mM sucrose solution concentrations, Bananaquits were able to adjust their volumetric food intake in order to maintain a constant energy intake. At solution concentration of 276 mM, birds could not maintain their rate of energy intake by increasing food consumption enough. We consider that at low sugar concentrations, Bananaquits faced a physiological constraint; they were unable to process food at a fast enough rate to meet their energy needs. We also explored the possibility that dilute nectars might be essential to sustain high water needs of Bananaquits by allowing them to control osmolarity of the food. Between 276 and 1120 mM sucrose solution concentrations, average amount of free water drunk by Bananaquits was independent of food concentration. They drank very little supplementary water and did not effectively dilute concentrated nectars. The evidence suggests that water bulk of dilute nectars is a burden to Bananaquits.     

Efficiency of facultative frugivory in the nectar-feeding bat Glossophaga commissarisi: the quality of fruits as an alternative food source

The efficiency of food exploitation correlates positively with the extent of dietary specialization. Neotropical nectar-feeding bats (Glossophaginae) have one of the most specialized diets among mammals, as floral nectar constitutes a sugar-rich and highly digestible but protein and fiber depleted food source. However, dietary constraints, such as a temporary scarcity of nectar, or protein demands may sometimes require the uptake of alternative food items. We investigated the influence of a diet switch from nectar to fruit on intestinal morphology, body mass, and energy budget in the nectar-feeding bat Glossophaga commissarisi and quantified feeding efficiency. We hypothesized that these nectar specialists depend on a constant supply of nectar, if they were lacking the ability for morphological and physiological plasticity in response to a fiber-rich diet. Although capable of harvesting infructescences of Piper hispidum, G. commissarisi was less efficient in extracting energy from fruits (48% digestive efficiency of total fruit energy content) than from nectar (c. 99% digestive efficiency). The intestinal morphology and organ masses did not change after bats were switched from nectar to fruits. Captive bats exhibited lower daily energy expenditures and flight activity when feeding on fruits than during nectarivory. Possibly, this may have been a deliberate regulation to balance reduced feeding efficiency, or simply the consequence of extended digestive pauses. The low digestibility of Piper, in combination with slow digestion and the bats’ inability for morphological and physiological plasticity may cause nectar-feeders to reduce their maximum energy expenditure when feeding on fruits. We argue that although fruits may substitute for nectar, they may cause restricted maximum energy assimilation compared with nectar.     

The effect of ambient humidity on the foraging behavior of the hawkmoth Manduca sexta

The foraging decisions of flower-visiting animals are contingent upon the need of an individual to meet both energetic and osmotic demands. Insects can alter their food preferences to prioritize one need over the other, depending on environmental conditions. In this study, preferences in nectar sugar concentrations (0, 12, 24 %) were tested in the hawkmoth Manduca sexta, in response to different levels of ambient humidity (20, 40, 60, and 80 % RH). Moths altered their foraging behavior when placed in low humidity environments by increasing the volume of nectar imbibed and by consuming more dilute nectar. When placed in high humidity environments the total volume imbibed decreased, because moths consumed less from dilute nectars (water and 12 % sucrose). Survivorship was higher with higher humidity. Daily foraging patterns changed with relative humidity (RH): moths maximized their nectar consumption earlier, at lower humidities. Although ambient humidity had an impact on foraging activity, activity levels and nectar preferences, total energy intake was not affected. These results show that foraging decisions made by M. sexta kept under different ambient RH levels allow individuals to meet their osmotic demands while maintaining a constant energy input.     

The intake responses of three species of leaf-nosed Neotropical bats

Flower-visiting bats encounter nectars that vary in both sugar composition and concentration. Because in the new world, the nectars of bat-pollinated flowers tend to be dominated by hexoses, we predicted that at equicaloric concentrations, bats would ingest higher volumes of hexoses than sucrose-containing nectars. We investigated the intake response of three species of Neotropical bats, Leptonycteris curasoae, Glossophaga soricina and Artibeus jamaicensis, to sugar solutions of varying concentrations (292, 438, 584, 730, 876, and 1,022 mmol L⁻¹) consisting of either sucrose or 1:1 mixtures of glucose and fructose solutions. Bats did not show differences in their intake response to sucrose and 1:1 glucose–fructose solutions, indicating that digestion and absorption in bat intestines are designed under the principle of symmorphosis, in which no step is more limiting than the other. Our results also suggest that, on the basis of energy intake, bats should not prefer hexoses over sucrose. We used a mathematical model that uses the rate of sucrose hydrolysis measured in vitro and the small intestinal volume of bats to predict the rate of nectar intake as a function of sugar concentration. The model was a good predictor of the intake responses of L. curasoae and G. soricina, but not of A. jamaicensis.     

Flight is the key to postprandial blood glucose balance in the fruit bats Eonycteris spelaea and Cynopterus sphinx

Excessive sugar consumption could lead to high blood glucose levels that are harmful to mammalian health and life. Despite consuming large amounts of sugar‐rich food, fruit bats have a longer lifespan, raising the question of how these bats overcome potential hyperglycemia. We investigated the change of blood glucose level in nectar‐feeding bats (Eonycteris spelaea) and fruit‐eating bats (Cynopterus sphinx) via adjusting their sugar intake and time of flight. We found that the maximum blood glucose level of C. sphinx was higher than 24 mmol/L that is considered to be pathological in other mammals. After C. sphinx bats spent approximately 75% of their time to fly, their blood glucose levels dropped markedly, and the blood glucose of E. spelaea fell to the fast levels after they spent 70% time of fly. Thus, the level of blood glucose elevated with the quantity of sugar intake but declined with the time of flight. Our results indicate that high‐intensive flight is a key regulator for blood glucose homeostasis during foraging. High‐intensive flight may confer benefits to the fruit bats in foraging success and behavioral interactions and increases the efficiency of pollen and seed disposal mediated by bats.     

Sucrose hydrolysis does not limit food intake by Pallas's long-tongued bats

Nectarivorous bats include very dilute nectar in their natural diet, and recent work with Pallas's long-tongued bat Glossophaga soricina showed that sugar (energy) intake rate decreased at dilute sucrose solutions. However, chiropterophillous nectar is composed mainly of the hexoses glucose and fructose. Because bats fed hexose nectar would save the delay of hydrolyzing sucrose, we hypothesized that sugar intake rate should be higher on this diet than on sucrose nectar. We compared intake response in Pallas's long-tongued bats offered 1 : 1 glucose-fructose (hexose) and sucrose diets at 5%, 10%, 20%, 30%, and 40% (mass/volume) sugar solutions. We also tested the hypothesis that sucrose hydrolysis limits food intake in bats. Intake response was the same in bats fed both types of diet: sugar intake rate was lower in dilute solutions and then increased with sugar concentration. Similar intake responses in both diets indicate that sucrose hydrolysis alone does not limit food intake and support the idea that the burden of processing excess water in dilute solutions plays a major role.     

Feeding activity in captive New Zealand lesser short‐tailed bats (Mystacina tuberculata)

Abstract

Lesser short‐tailed bats (Mystacina tuberculata) have been reported as commonly feeding on the ground, but few direct observations of the diet and foraging behaviour of these rare and secretive bats have been published. Here, we describe the feeding behaviour of six captive M. tuberculata at Wellington Zoological Gardens, in order to experimentally clarify and validate some of the feeding behaviours previously reported from anecdotal observations. In particular, we focused on food type choice and dependence on nectar. The bats emerged every night 80 ± 30 (mean ± SE) min after sunset, irrespective of the weather, and spent most of their time foraging in leaf litter on the ground (when available) and on the branches of trees. Larvae of meal worms (Tenebrio molitor) and cerambycid beetles (Prionoplus reticularis) were favoured in the diet, followed in diminishing order by adult meal worms, tree weta (Hemideina crassidens) and crickets (Teleogryllus commodus), some nectars, sugar solutions, and finally water. Nectar from Eucalyptus sp. was preferred, followed by nectar of pohutukawa (Metrosideros excelsa), wood rose (Dactylanthus taylorii), and kakabeak (Clianthus puniceus). Nectar of Acacia sp., Hebe sp., and synthetic Dactylanthus nectar were not taken. The bats visited 50 and 25% sugar solutions significantly more frequently (91% of visits) than 12.5 and 0% sugar solutions. Various meats (carrion) and fruit were not eaten. Faecal content fairly represented the invertebrates consumed the previous night, but the volumes of invertebrates consumed could not be reliably estimated from faecal analysis.     

Seasonal intake responses in the nectar-feeding bat Glossophaga soricina

Food intake in nectar-feeding animals is affected by food quality, their energetic demands, and the environmental conditions they face. These animals increase their food intake in response to a decrease in food quality, a behavior named “intake response”. However, their capacity to achieve compensatory feeding, in which they maintain a constant flux of energy, could be constrained by physiological processes. Here we evaluated how both a seasonal change in environmental conditions and physiological constraints affected the food ingestion in the bat Glossophaga soricina. We measured food intake rate during both the wet/warm and dry/cool seasons at sucrose solutions ranging from 146 to 1,022 mmol L⁻¹. We expected that food intake and metabolic demands would be greater during the dry/cool season. Bats ingested ~20% more food in the dry/cool than in the wet/warm season. Regardless of season, bats were unable to achieve a constant flux of energy when facing the different sugar concentrations that we used in our experiments. This suggests that the rate of food intake is physiologically constrained in G. soricina. Using the digestive capacity of bats we modeled their food intake. The analytic model we used predicts that digestive limitations to ingest energy should have an important effect on the ecology of this species.     

Digestive capacities allow the Mexican long-nosed bat (Leptonycteris nivalis) to live in cold environments

Digestive capabilities of nectar-feeding vertebrates to assimilate sugars affect their ability to acquire and store energy and could determine the minimal temperatures at which these animals can survive. Here, we described the sugar digestive capability of Leptonycteris nivalis and related it with its capacity to live in cold environments. We measured the enzymatic activity, food intake rate and changes in body mass of bats feeding at four different sucrose concentrations (from 5 to 35% wt./vol.). Additionally, we used a mathematical model to predict food intake and compared it with the food intake of bats. L. nivalis was able to obtain ~ 111.3 kJ of energy regardless of the sugar concentration of their food. Also, bats gained ~ 2.57 g of mass during the experimental trials and this gain was independent of sugar concentration. The affinity (1 / K_m) of sucrase (EC 3.2.1.48) was one order of magnitude higher relative to that reported for its sister species Leptonycteris yerbabuenae (0.250 and 0.0189 mmol^? 1 L, respectively), allowing this species to have a higher energy intake rate. We propose that the high ability to acquire energy conferred L. nivalis the faculty to invade cold environments, avoiding in this way the ecological competition with its sympatric species L. yerbabuenae.     

On the mechanics and energetics of nectar feeding in butterflies.

A mechanistic model describing the mechanics and energetics of nectar-feeding in butterflies is developed. The butterflies Collas eurytheme and Danaus plexippus are used to illustrate the model. Simulation results indicate that there are mechanical limitations upon the range of nectar sugar concentrations and nectar extraction times available to butterflies. There is a unique optimum for net rate of energy gain at 20–25% nectar sugar concentration which is independent of the metabolic rate and of proboscis shape and size over the ranges found in butterflies. The optimal nectar extraction rate depends upon the size and shape of the proboscis. These results are discussed in relation to the design of nectar feeding structures, optimal foraging strategy, and the evolution of insect pollination.     

Bird pollinators differ in their tolerance of a nectar alkaloid

Although the function of nectar is to attract and reward pollinators, secondary metabolites produced by plants as anti‐herbivore defences are frequently present in floral nectars. Greater understanding is needed of the effects of secondary metabolites in nectar on the foraging behaviour and performance of pollinators, and on plant–pollinator interactions. We investigated how nectar‐feeding birds, both specialist (white‐bellied sunbirds Cinnyris talatala) and generalist (dark‐capped bulbuls Pycnonotus tricolor and Cape white‐eyes Zosterops virens), respond to artificial nectar containing the alkaloid nicotine, present in nectar of Nicotiana species. Preference tests were carried out with a range of nicotine concentrations (0.1–300 μM) in two sucrose concentrations (0.25 and 1 M), and for bulbuls also in two sugars (sucrose and hexose). In addition, we measured short‐term feeding patterns in white‐bellied sunbirds that were offered nicotine (0–50 μM) in 0.63 M sucrose. Both nicotine and sugar concentrations influenced the response of bird pollinators to nicotine. The birds showed dose‐dependent responses to nicotine; and their tolerance of high nicotine concentrations was reduced on the dilute 0.25 M sucrose diet, on which they increased consumption to maintain energy intake. White‐bellied sunbirds decreased both feeding frequency and feeding duration as the nicotine concentration in artificial nectar increased. Of the three species, bulbuls showed the highest tolerance for nicotine, and sugar type (sucrose or hexose) had no effect. The indifference of bulbuls to nicotine may be related to their primarily frugivorous diet. However, the response of white‐eyes to nicotine in the dilute sucrose solution was very similar to that of sunbirds, even though white‐eyes are generalist nectar‐feeders. Additional testing of other avian nectarivores and different secondary metabolites is required to further elucidate whether generalist bird pollinators, which utilise dilute nectars in which secondary metabolites have stronger deterrent effects, are more tolerant of ‘toxic’ nectar.     

Changes in nectar concentration: how quickly do whitebellied sunbirds (Cinnyris talatala) adjust feeding patterns and food intake?

Nectarivorous birds encounter varying nectar concentrations while foraging on different food plants and must adjust their consumption to maintain constant energy intake. We determined how rapidly captive whitebellied sunbirds (Cinnyris talatala) adjust their volumetric intake and feeding patterns after changes in diet concentration. On four consecutive days, birds were fed sucrose diets alternating between a standard diet of 16% w/w and test diets of 2.5, 8.5, 16 or 30% w/w, respectively, for 1.5 h periods. Feeding events were recorded with an infrared photo-detection system and food intake and body mass were monitored continuously by electronic balances interfaced to a computer. Generally, birds demonstrated a measurable increase in feeding frequency and food intake within 10 min after a decrease in sucrose concentration. However, individuals responded differently to the most dilute diet (2.5%): while most increased their food intake, others stopped feeding for a short while, appearing to dislike this diet. Furthermore, the number and duration of feeding events increased in the first 5 min after the switch from 2.5% back to 16%, as the birds attempted to compensate for previous reduced sugar intake. Daily sugar intake was lower when birds alternated between 2.5 and 16% diets than on other test days, but birds were able to maintain body mass, presumably through behavioural adjustments.     

Hummingbirds pay a high cost for a warm drink

Endotherms must warm ingested food to body temperature. Food warming costs may be especially high for nectar-feeding birds, which can ingest prodigious volumes. We formulated a mathematical model to predict the cost of warming nectar as a function of nectar temperature and sugar concentration. This model predicts that the cost of warming nectar should: (1) decrease as a power function of nectar concentration, and (2) increase linearly with the difference between body temperature and nectar temperature. We tested our model on rufous hummingbirds (Selasphorus rufus). A typical experiment consisted of feeding birds nectar of a given concentration at 39°C (equivalent to body temperature) and then at 4°C, and vice versa. We used the percentage change in metabolic rate between the two food temperatures to estimate the cost of warming nectar. The model's predictions were accurately met. When birds had to hover rather than perch during feeding bouts, estimated food-warming costs were only slightly lower. The cost of warming nectar to body temperature appears to be an important yet overlooked aspect of the energy budgets of nectar-feeding birds. Hummingbirds feeding on 5% sucrose solutions at 4^oC have to increase their metabolic rate by an amount equivalent to that elicited by a 15°C drop in ambient temperature.Abbreviations AE assimilation efficiency - C nectar concentration - H' cost of warming food to body temperature - SDA specific dynamic action - T_a ambient temperature - T_b body temperature - T_n nectar temperatureCommunicated by: G. Heldmaier     

Does gut function limit hummingbird food intake?

Many nectar-feeding bird species decrease food intake when sugar concentration in food is increased. This feeding response can be explained by two alternative hypotheses: compensatory feeding and physiological constraint. The compensatory feeding hypothesis predicts that if birds vary intake to maintain a constant energy intake to match energy expenditures, then they should increase intake when expenditures are increased. Broad-tailed hummingbirds were presented with sucrose solutions at four concentrations (292, 584, 876, and 1,168 mmol L(-1)) and exposed to two environmental temperatures (10 degrees and 22 degrees C). Birds decreased volumetric food intake in response to sugar concentration. However, when they were exposed to a relatively sudden drop in environmental temperature and, hence, to an acute increase in thermoregulatory energy expenditures, they did not increase their rate of energy consumption and lost mass. These results support the existence of a physiological constraint on feeding intake. A simple chemical reactor model based on intestinal morphology and in vitro measurements of sucrose hydrolysis predicted observed intake rates closely. This model suggests that intestinal sucrose hydrolysis rates were near maximal and, thus, may have imposed limits to sugar assimilation. Although sugar assimilation was high (95%), the proportions of excreted sucrose, glucose, and fructose found in excreta differed significantly. The monosaccharides glucose and fructose were about eight and three times more abundant than sucrose, respectively. Broad-tailed hummingbirds are small high-altitude endotherms that face unpredictable weather and the energetic expense of premigratory fattening. Digestive processes have the potential to impose severe challenges to their energy budgets.     

Selective choice of sucrose solution concentration by the hovering hawk mothMacroglossum stellatarum

To determine the preference of the hovering hawk mothMacroglossum stellatarum for different sugar concentrations, the foraging behavior of adults were analyzed under laboratory conditions. Six sucrose concentrations (range, 10–60%, w/w) were simultaneously offered in six artificial ab libitum feeders. The number of feeding bouts and the duration of each visit were automatically recorded and stored in a computer. Results showed that the frequency of visits to the feeders did not vary among the different solutions offered, but the gathered volume by the group attained a maximum at between 20 and 50% (w/w). Moths invested more time in front of the feeder with the more concentrated sugar solutions. It was assumed that factors different from maximizing energy intake, such as water balance and viscosity of concentrated nectars, have to be considered in order to understand the observed patterns of nectar choice.     

How to budget metabolic energy: torpor in a small Neotropical mammal

Neotropical nectar-feeding bats (Glossophaginae) are highly specialized in the exploitation of floral nectar and have one of the highest mass-specific metabolic rates among mammals. Nevertheless, they are distributed throughout the tropics and subtropics over a wide elevational range, and thus encounter many extreme and energetically challenging environmental conditions. Depressing their otherwise high metabolic rate, e.g., in situations of food restriction, might be an important adaptive physiological strategy in these dietary specialists. We investigated the thermoregulatory behavior of captive 10-g nectar feeding bats (Glossophaga soricina; Chiroptera, Phyllostomidae) under variable ambient temperatures (T _a) and feeding regimes and predicted that bats would use torpor as an energy-conserving behavior under energetic constraints. All tested animals entered torpor in response to energetic restrictions and the depth of torpor was dependent on the body condition of the animals and hence on their degree of physiological constraints. Periods of torpor with body temperatures (T _b) below 34°C were precisely adjusted to the photoperiod. The median length of diurnal torpor was 11.43 h. The lowest T _b measured was 21°C at a T _a of 19°C. Estimated energy savings due to torpor were considerable, with reductions in metabolic rate to as low as 5% of the metabolic rate of normothermic bats at the same T _a. However, contrary to temperate zone bats that also employ diurnal torpor, G. soricina regulated their T _b to the highest possible levels given the present energetic supplies. To summarize, G. soricina is a precise thermoregulator, which strategically employs thermoregulatory behavior in order to decrease its energy expenditure when under energetic restrictions. This adaptation may play a crucial role in the distribution and the assembly of communities of nectar-feeding bats and may point to a general capacity for torpor in tropical bats.