Fatty acids and stable isotopes (δ13C and δ15N) reveal temporal changes in narwhal (Monodon monoceros) diet linked to migration patterns

Narwhals (Monodon monoceros) are sentinel species in the Arctic and to investigate marine food web changes from 1982–2011 we examined diet using fatty acids, δ¹⁵N, and δ¹³C, in narwhals from Baffin Bay (BB) and northern Hudson Bay (NHB). We predicted temporal changes would be greater in NHB due to a significant reduction in summer ice cover. In NHB, δ¹⁵N significantly increased, δ¹³C displayed a parabolic trend, and fatty acids gradually shifted, albeit not significantly, over time. δ¹⁵N was stable, δ¹³C decreased, and fatty acids significantly changed over time in BB. Stable isotope mixing models indicated a dietary reduction in capelin and increase in Greenland halibut from 1994–2000 to 2006–2011 in BB, while capelin was an important dietary component for narwhals in NHB in recent years (2006–2011). These dietary changes may be attributed to changes in sea ice and narwhal migration. Seasonal dietary changes, as evidenced by changes in blubber fatty acids and skin and muscle stable isotopes, were not as apparent in the NHB population, which may be indicative of a reduced migratory distance. Long‐term monitoring of narwhal diet and migratory patterns associated with reduced sea ice provides invaluable information about how the marine ecosystem will redistribute with global warming.     

Enhanced dietary fat clearance in postobese women

The objective of this study was to examine the postprandial response to an exogenous fat source in eight weight-stable postobese subjects (2;-3 years after gastric bypass) and eight matched control women, using a stable isotope, [13C]oleate. After a high fat breakfast meal (1,062 cal, 67% fat), [13C]oleate in triglyceride (TG)-rich lipoproteins (Sf >400 and Sf 20;-400) and nonesterified fatty acids (NEFA), and 13C in breath CO2, were monitored over 8 h. There were no differences in resting energy expenditure, thermic effect of food, carbohydrate/fat oxidation ratio, breath 13CO2 enrichment, or fecal fat content between postobese and control subjects. Postprandially, there was no difference in S(f) 20;-400 TG or NEFA, but postobese subjects had lower Sf >400 incremental area under the curve (AUC) (- 33%, P < 0.0025) and glucose [P < 0.01 by repeated measures analysis of variance (RM ANOVA)]. Postprandial 13C in Sf >400 TG returned to fasting levels 4 h earlier in postobese subjects and was lower than in control subjects at 4 and 6 h (P < 0.05 by RM ANOVA). The greatest difference was in the [13C]NEFA profiles. In control subjects [13C]NEFA increased markedly over 8 h; postobese subject [13C]NEFA remained close to fasting nonenriched values, and was strikingly lower than in control subjects (72% lower by AUC, P < 0.0001 by RM ANOVA). Finally, postobese subjects tended to have lower postprandial insulin (P < 0.01, 4 h), lower postprandial acylation-stimulating protein, and lower fasting leptin (-46%, P < 0.02). This study demonstrates clear metabolic differences in exogenous dietary fat partitioning in postobese women. These findings are compatible with an increased efficiency of dietary fat storage and suggest one possible mechanism for promotion of weight regain in postobese individuals.     

Nutrient routing in omnivorous animals tracked by stable carbon isotopes in tissue and exhaled breath

Omnivorous animals feed on several food items that often differ in macronutrient and isotopic composition. Macronutrients can be used for either metabolism or body tissue synthesis and, therefore, stable C isotope ratios of exhaled breath (delta(13)C(breath)) and tissue may differ. To study nutrient routing in omnivorous animals, we measured delta(13)C(breath) in 20-g Carollia perspicillata that either ate an isotopically homogeneous carbohydrate diet or an isotopically heterogeneous protein-carbohydrate mixture. The delta(13)C(breath) converged to the delta(13)C of the ingested carbohydrates irrespective of whether proteins had been added or not. On average, delta(13)C(breath) was depleted in (13)C by only ca. -2 per thousand in relation to the delta(13)C of the dietary carbohydrates and was enriched by +8.2 per thousand in relation to the dietary proteins, suggesting that C. perspicillata may have routed most ingested proteins to body synthesis and not to metabolism. We next compared the delta(13)C(breath) with that of wing tissue (delta(13)C(tissue)) in 12 free-ranging, mostly omnivorous phyllostomid bat species. We predicted that species with a more insect biased diet--as indicated by the N isotope ratio in wing membrane tissue (delta(15)N(tissue))--should have higher delta(13)C(tissue) than delta(13)C(breath) values, since we expected body tissue to stem mostly from insect proteins and exhaled CO(2) to stem from the combustion of fruit carbohydrates. Accordingly, delta(13)C(tissue) and delta(13)C(breath) should be more similar in species that feed predominantly on plant products. The species-specific differences between delta(13)C(tissue) and delta(13)C(breath) increased with increasing delta(15)N(tissue), i.e. species with a plant-dominated diet had similar delta(13)C(tissue) and delta(13)C(breath) values, whereas species feeding at a higher trophic level had higher delta(13)C(tissue) than delta(13)C(breath) values. Our study shows that delta(13)C(breath) reflect the isotope ratio of ingested carbohydrates, whereas delta(13)C of body tissue reflect the isotope ratio of ingested proteins, namely insects, supporting the idea of isotopic routing in omnivorous animals.     

Isotope tracer measures of meal fatty acid metabolism: reproducibility and effects of the menstrual cycle

Oxidation and adipose tissue uptake of dietary fat can be measured by adding fatty acid tracers to meals. These studies were conducted to measure between-study variability of these types of experiments and assess whether dietary fatty acids are handled differently in the follicular vs. luteal phase of the menstrual cycle. Healthy normal-weight men (n = 12) and women (n = 12) participated in these studies, which were block randomized to control for study order, isotope ([3H]triolein vs. [14C]triolein), and menstrual cycle. Energy expenditure (indirect calorimetry), meal fatty acid oxidation, and meal fatty acid uptake into upper body and lower body subcutaneous fat (biopsies) 24 h after the experimental meal were measured. A greater portion of meal fatty acids was stored in upper body subcutaneous adipose tissue (24 +/- 2 vs. 16 +/- 2%, P < 0.005) and lower body fat (12 +/- 1 vs. 7 +/- 1%, P < 0.005) in women than in men. Meal fatty acid oxidation (3H2O generation) was greater in men than in women (52 +/- 3 vs. 45 +/- 2%, P = 0.04). Leg adipose tissue uptake of meal fatty acids was 15 +/- 2% in the follicular phase of the menstrual cycle and 10 +/- 1% in the luteal phase (P = NS). Variance in meal fatty acid uptake was somewhat (P = NS) greater in women than in men, although menstrual cycle factors did not contribute significantly. We conclude that leg uptake of dietary fat is slightly more variable in women than in men, but that there are no major effects of menstrual cycle on meal fatty acid disposal.     

Dietary fats and body lipid composition in relation to hibernation in free-ranging echidnas

Laboratory studies have shown that high levels of dietary unsaturated fatty acids prolong torpor and lower body temperatures in hibernating herbivorous rodents, which may in turn improve winter survival. The importance of nutritional ecology in relation to hibernation in insectivorous hibernators is unknown. We therefore studied fatty acid composition of dietary insects and the depot fat of echidnas Tachyglossus aculeatus (Monotremata) during the pre-hibernation season and compared depot fat fatty acid composition before and after hibernation. Echidna depot fat fatty acid composition during the pre-hibernation season was almost identical to that of the most abundant prey species, the ant Iridomyrmex sp. Oleic acid (C18:1) was by far the most common fatty acid in both Iridomyrmex sp. (60%) and echidna depot fat (62%). After about 5 months of hibernation and an 18% loss of body mass, echidna fatty acid composition had changed significantly. The percentage of the monounsaturated oleic acid (C18:1) and palmitoleic acid (C16:1) had declined, whereas that of the saturated fatty acids (C12:0, C16:0, C18:0) and the polyunsaturated linoleic acid (C18:2) had increased. Our study suggests that, unlike herbivorous rodent hibernators, echidnas rely to a large extent on monounsaturated fatty acids as fuel for hibernation, reflecting the most common fatty acid in their food. Moreover, it appears that the high concentration of monounsaturated fatty acids compensates for the moderate availability of polyunsaturates and enables them to hibernate at low body temperatures.     

Dynamics of lipid accumulation by the fat body of Rhodnius prolixus: the involvement of lipophorin binding sites

In insects, lipids are stored in the fat body, mainly as triacylglycerol (TAG). In Rhodnius prolixus, a hematophagous hemipteran, lipids are accumulated after blood meal to be used later on. In adult females, at the second day after feeding, the amount of TAG was 57+/-17 microg/fat body, it increased almost five times and at fourth day it was 244+/-35 microg/fat body. TAG content remained constant until day 13, but it then decreased and, at day 20th it was very low (31+/-4.9 microg/fat body). Radiolabeled free fatty acid was used to follow lipid accumulation by the fat body, as it was previously shown that, in R. prolixus, injected free fatty acids associate with lipophorin, a major hemolymphatic lipoprotein. (3)H-palmitic acid was injected into the hemocoel of R. prolixus females. It disappeared from the hemolymph very rapidly, and radioactivity was incorporated by the fat body. Sixty minutes after injection, radioactivity in the fat body was found mainly in TAGs. The capacity of the fat body to incorporate fatty acids from the hemolymph varied according to the days after blood meal, and it was maximal around the fourth day. Lipophorin binding to specific sites in fat body membrane preparations also showed variation at different days. When membranes obtained from insects at the second, fifth and tenth days were compared, binding was highest at fifth day after feeding.     

The Role of Storage Lipids in the Relation between Fecundity,Locomotor Activity,and Lifespan of Drosophila melanogaster Longevity-Selected and Control Lines

The contribution of insect fat body to multiple processes, such as development, metamorphosis, activity, and reproduction results in trade-offs between life history traits. In the present study, age-induced modulation of storage lipid composition in Drosophila melanogaster longevity-selected (L) and non-selected control (C) lines was studied and the correlation between total body fat mass and lifespan assessed. The trade-offs between fecundity, locomotor activity, and lifespan were re-evaluated from a lipid-related metabolic perspective. Fewer storage lipids in the L lines compared to the C lines supports the impact of body fat mass on extended lifespan. The higher rate of fecundity and locomotor activity in the L lines may increase the lipid metabolism and enhance the lipolysis of storage lipids, reducing fat reserves. The correlation between neutral lipid fatty acids and fecundity, as well as locomotor activity, varied across age groups and between the L and C lines. The fatty acids that correlated with egg production were different from the fatty acids that correlated with locomotor activity. The present study suggests that fecundity and locomotor activity may positively affect the lifespan of D. melanogaster through the inhibition of fat accumulation.     

Diet preferences for specific fatty acids and their effect on composition of fat reserves in migratory Red-eyed Vireos (Vireo olivaceous)

Fatty acid composition of body fat in birds often differs between bird species and between seasons, and changes in diet may be responsible for this variation. We tested two related hypotheses using Red-eyed Vireos, a long-distance migratory songbird: (1) birds prefer diets with certain fatty acids, and (2) fatty acid composition of the diet primarily determines the composition of lipid reserves. During paired-choice experiments, vireos preferred semi-synthetic diets with triolein (81% digestive extraction efficiency) over diets with tristearin (54% digestive extraction efficiency) and, in general, ate more when offered diets with unsaturated fats compared to saturated fats. These results demonstrate that vireos can discriminate between diets differing only in fatty acid composition and prefer diets with long-chain unsaturated fatty acids. When vireos were fed one of two diets for 1 month, the primary fatty acids in each diet also predominated in the tissues of birds fed each diet. However, some fatty acids that were absent in the diet occurred in bird tissues (e.g., 22:4, 22:5) suggesting that selective metabolism of fatty acids along with diet composition determine the fatty acid composition of lipid reserves in migratory birds.     

Plasma fatty acids and [13C]linoleic acid metabolism in preterm infants fed a formula with medium-chain triglycerides

Most preterm infant formulas contain medium-chain triacylglycerols (MCT), but the effects of MCT on polyunsaturated fatty acid status and metabolism are controversial. Thus, we studied the effects of MCT on linoleic acid metabolism using stable isotopes. Enterally fed preterm infants were randomized to receive for 7 days 40% of fat as MCT (n = 10) or a formula without MCT (n = 9). At study day 5, infants received orally 2 mg/kg body weight of (13)C-labeled linoleic acid. Fatty acids in plasma lipid classes and (13)C enrichment of phospholipid fatty acids were measured and tracer oxidation was monitored. Compared with the control group, the MCT group showed lower breath (13)CO(2) and higher plasma triacylglycerol contents of octanoic acid, of decanoic acid, and of total long-chain polyunsaturated fatty acids (57.1 +/- 4.4 micro mol/l vs. 37.9 +/- 4.8 micro mol/l, P < 0.01). Concentrations of several polyunsaturated fatty acids in plasma phospholipids and non esterified fatty acids were higher in the MCT group. (13)C concentrations in phospholipid n-6 fatty acids indicated no difference in the relative conversion of linoleic to arachidonic acid. We conclude that oral MCT effectively reduce polyunsaturated fatty acid and long chain polyunsaturated fatty acid oxidation in preterm infants without compromising endogenous n-6 long chain polyunsaturated fatty acid synthesis.     

Selective partitioning of dietary fatty acids into the VLDL TG pool in the early postprandial period

Circulating triacylglycerol (TG) arises mainly from dietary fat. However, little is known about the entry of dietary fat into the major TG pool, very low-density lipoprotein (VLDL) TG. We used a novel method to study the specific incorporation of dietary fatty acids into postprandial VLDL TG in humans. Eight healthy volunteers (age 25.4 +/- 2.2 years, body mass index 22.1 +/- 2.3 kg/m2) were fed a mixed meal containing 30 g fish oil and 600 mg [1-13C]palmitic acid. Chylomicrons and VLDL were separated using immunoaffinity against apolipoprotein B-100. The fatty acid composition of lipoproteins was analyzed by gas chromatography/mass spectrometry. [1-13C]palmitic acid started to appear in VLDL TG 3 h after meal intake, and a similar delay was observed for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Approximately 20% of dietary fatty acids entered the VLDL TG pool 6 h after meal intake. DHA was clearly overincorporated into this pool compared with [1-13C]palmitic acid and EPA. This seemed to depend on a marked elevation of this fatty acid in the nonesterified fatty acid pool. In summary, the contribution of dietary fatty acids to early postprandial VLDL TG is substantial. The role of DHA in VLDL TG production will require further investigation.     

Tracing sources of carbon and hydrogen to stored lipids in the migratory moth,Mythimna unipuncta using stable isotopes (δ2H, δ13C)

Stable isotope measurements of insect tissues can be used to determine origin and migratory patterns. The isotopic links between diet and stored lipids in laboratory reared true armyworm moths (Mythimna unipuncta, Haw.) were investigated using δ¹³C and δ²H measurements. Newly emerged moths were fed synthetic nectars, consisting of different carbohydrate and water sources that were isotopically distinct from those in the larval diet. After 4 days of feeding, insects were sacrificed, and fat‐body lipids were extracted for isotopic analysis. When held on a constant nectar source, adult diet contributed ～87% of the C and 44% H of lipids. For H, 27% and ～ 17% of lipid was derived from carbohydrates and water, respectively, with the remainder presumably from the larval stage. When the isotopic composition of the nectar source was switched there was rapid and exponential temporal change in lipid isotopic profiles. The relevance of our findings to tracing origins of stored lipids in migrant Lepidoptera is discussed.     

The impact of dietary fats,photoperiod, temperature and season on morphological variables,torpor patterns,and brown adipose tissue fatty acid composition of hamsters,Phodopus sungorus

We investigated how dietary fats and oils of different fatty acid composition influence the seasonal change of body mass, fur colour, testes size and torpor in Djungarian hamsters, Phodopus sungorus, maintained from autumn to winter under different photoperiods and temperature regimes. Dietary fatty acids influenced the occurrence of spontaneous torpor (food and water ad libitum) in P. sungorus maintained at 18°C under natural and artificial short photoperiods. Torpor was most pronounced in individuals on a diet containing 10% safflower oil (rich in polyunsaturated fatty acids), intermediate in individuals on a diet containing 10% olive oil (rich in monounsaturated fatty acids) and least pronounced in individuals on a diet containing 10% coconut fat (rich in saturated fatty acids). Torpor in P. sungorus on chow containing no added fat or oil was intermediate between those on coconut fat and olive oil. Dietary fatty acids had little effect on torpor in animals maintained at 23°C. Body mass, fur colour and testes size were also little affected by dietary fatty acids. The fatty acid composition of brown fat from hamsters maintained at 18°C and under natural photoperiod strongly reflected that of the dietary fatty acids. Our study suggests that the seasonal change of body mass, fur colour and testes size are not significantly affected by dietary fatty acids. However, dietary fats influence the occurrence of torpor in individuals maintained at low temperatures and that have been photoperiodically primed for the display of torpor.Abbreviations BAT brown adipose tissue - bm body mass - FA fatty acid(s) - MR metabolic rate - MUFA monounsaturated fatty acid(s) - PUFA polyunsaturated fatty acid(s) - SFA saturated fatty acid(s) - T _a air temperature - T _b body temperature - T_s body surface temperature(s) - TNZ thermoneutral zone - UFA unsaturated fatty acid(s)     

Fractional oxidation of chylomicron-derived oleate is greater than that of palmitate in healthy adults fed frequent small meals

Differences in oxidation of individual dietary fatty acids could contribute to the effect of dietary fat composition on risk factors for non-insulin-dependent diabetes mellitus and cardiovascular disease. Using a novel stable isotope technique, we compared fractional oxidation of chylomicron-derived oleate and palmitate in 10 healthy adults in a crossover study. 1-(13)C-labeled oleate or palmitate was emulsified into a eucaloric formula diet administered each 20 min for 7 h to produce a plateau in excretion of (13)C label in breath CO(2). Unlabeled oleate and palmitate each provided 16% of dietary energy, and other fatty acids provided 8% of energy. Total dietary fat was 40% of energy, carbohydrate was 46%, and protein was 14%. Diet without tracer was fed for 2 h before beginning tracer administration to establish a baseline fed state. Relative oxidation of oleate versus palmitate was defined as fractional oxidation of oleate divided by fractional oxidation of palmitate. Relative oxidation averaged 1.21 (99.5% confidence interval = 1.03;-1.39), indicating that fractional oxidation of oleate was significantly greater than that of palmitate.     

A triple-isotope approach to predict the breeding origins of European bats

Despite a commitment by the European Union to protect its migratory bat populations, conservation efforts are hindered by a poor understanding of bat migratory strategies and connectivity between breeding and wintering grounds. Traditional methods like mark-recapture are ineffective to study broad-scale bat migratory patterns. Stable hydrogen isotopes (δD) have been proven useful in establishing spatial migratory connectivity of animal populations. Before applying this tool, the method was calibrated using bat samples of known origin. Here we established the potential of δD as a robust geographical tracer of breeding origins of European bats by measuring δD in hair of five sedentary bat species from 45 locations throughout Europe. The δD of bat hair strongly correlated with well-established spatial isotopic patterns in mean annual precipitation in Europe, and therefore was highly correlated with latitude. We calculated a linear mixed-effects model, with species as random effect, linking δD of bat hair to precipitation δD of the areas of hair growth. This model can be used to predict breeding origins of European migrating bats. We used δ(13)C and δ(15)N to discriminate among potential origins of bats, and found that these isotopes can be used as variables to further refine origin predictions. A triple-isotope approach could thereby pinpoint populations or subpopulations that have distinct origins. Our results further corroborated stable isotope analysis as a powerful method to delineate animal migrations in Europe.     

Arctic waders and the capital‐income continuum: Further tests using isotopic contrasts of egg components

Birds migrating annually to high‐latitude breeding grounds may benefit from the transport of endogenous nutrient reserves that ultimately contribute to reproduction. Shorebirds represent a diverse group of Arctic breeders that typically arrive on the breeding grounds with body reserves enriched in ¹³C and ¹⁵N due to wintering and staging in marine or estuarine habitats. Such isotopic differences between endogenous macronutrient reserves and local foodwebs allow the use of stable isotopes to test for the source of nutrient allocations to eggs. We examined δ¹³C and δ¹⁵N values in lipid‐free yolk and albumen and δ¹³C values in yolk lipid of first clutches of ten species of sandpiper and plover breeding near Churchill, Manitoba, Canada in 2003. Most birds had egg isotope values indicating a C3 terrestrial biome, which fits primarily an income (exogenous) breeding strategy. Two exceptions were single sandpiper and plover with strong marine isotope values. Among species, strong positive relationships for each isotope between egg tissue components suggest that egg proteins and lipids tended to be derived from the same isotopic source. Correlations of egg δ¹³C values for lipids and proteins approached theoretical relationships expected for exogenous breeding strategies, based on captive studies. Significant positive correlations between clutch initiation date and δ¹³C values of egg lipids and albumen suggest some endogenous nutrient contributions to later laid eggs but the circumstances under which this may occur are unstudied. Where possible, we recommend that researchers use blood and fat biopsies from laying females as a means of anchoring endogenous and exogenous endpoints for modeling of each reproductive event. We encourage the isotopic analysis of egg albumen, yolk and yolk lipids among individuals and species and tests of correlations among these components as a means of inferring origins of nutrients to eggs.     

Mortality of bats at wind turbines links to nocturnal insect migration?

This note is based on a literature search and a recent review of bat mortality data from wind farms in Europe (published elsewhere). We suggest that mortality of bats at wind turbines may be linked to high-altitude feeding on migrating insects that accumulate at the turbine towers. Modern wind turbines seem to reach high enough into the airspace to interfere with the migratory movements of insects. The hypothesis is consistent with recent observations of bats at wind turbines. It is supported by the observation that mortality of bats at wind turbines is highly seasonal (August–September) and typically peaks during nights with weather conditions known to trigger large-scale migratory movements of insects (and songbirds). We also discuss other current hypotheses concerning the mortality of bats at wind turbines.     

Meal fatty acid uptake in adipose tissue: gender effects in nonobese humans

We tested for gender differences in dietary fatty acid metabolism in 12 nonobese men and 12 nonobese women using the meal fatty acid tracer/adipose tissue biopsy study design. In addition to determining body composition, measurements of regional adipose tissue lipoprotein lipase activity, blood flow, and fat cell size were performed to place the meal fatty acid kinetic studies in perspective. Twenty-four hours after ingesting the test meal, the concentration of meal fatty acids was greater (P < 0.05) in abdominal subcutaneous than in thigh adipose tissue in both men (0. 61 +/- 0.12 vs. 0.45 +/- 0.09 mg/g) and women (0.59 +/- 0.10 vs. 0. 43 +/- 0.05) but was not different between men and women. A greater percentage of dietary fat was stored in subcutaneous adipose tissue in women than in men (38 +/- 3 vs. 24 +/- 3%, respectively, P < 0. 05), and a greater portion of meal fatty acid disposal was unaccounted for in men. Significant gender differences in regional adipose tissue blood flow after meal ingestion were noted; the differences were in the direction that could support greater nutrient storage in lower body fat in women.     

Monoacylglycerol and diacylglycerol acyltransferases and the synthesis of neutral glycerides in Manduca sexta

The insect fat body and the adipose tissue of vertebrates store fatty acids (FA) as triacylglycerols (TG). However, the fat body of most insects has the unique ability to rapidly produce and secrete large amounts of diacylglycerol (DG). Monoacylglycerol acyltransferase (MGAT), which catalyzes the synthesis of DG from MG, and a diacylglycerol acyltransferase (DGAT), which catalyzes the synthesis of TG from DG, are key enzymes in the metabolism of neutral glycerides. However, very little is known about these acyltransferases in insects. In the present study we have cloned two predicted MGATs and a DGAT from Manduca sexta and compared their sequences with predicted MGAT and DGAT homologs from a number of insect species. The comparison suggested that insects may only have a single DGAT gene, DGAT1. The apparent absence of a DGAT2 gene in insects would represent a major difference with vertebrates, which contain DGAT1 and DGAT2 genes. Insects seem to have a single MGAT gene which is similar to the MGAT2 of vertebrates. A number of conserved phosphorylation sites of potential physiological significance were identified among insect proteins and among insect and vertebrate proteins. DGAT1 and MGAT are expressed in fat body, midgut and ovaries. The relative rates of utilization of FAs for the synthesis of DG and TG correlated with the relative expression levels of MGAT and DGAT suggesting that regulation of the expression levels of these acyltransferases could be determining whether the fat body secretes DG or stores fatty acids as TG. The expression patterns of the acyltransferases suggest a role of the monoacylglycerol pathway in the production and mobilization of DG in M. sexta fat body.     

The nutritional significance of a winter‐flowering succulent for opportunistic avian nectarivores

The winter‐flowering succulent Aloe marlothii provides nectar for many opportunistic avian nectarivores in southern African savannas. We assessed the importance of A. marlothii nectar sugar for opportunistic nectarivores by analysing temporal changes in stable carbon isotope ratios (δ¹³C) in the tissues of birds in Suikerbosrand Nature Reserve, South Africa. The blood of the 11 most common non‐granivorous opportunistic nectarivores at our site was enriched in ¹³C by 3.4 ± 1.5‰ during the flowering period of A. marlothii, reflecting the enriched crassulacean acid metabolism (CAM) isotopic signature of nectar (?12.6 ± 0.5‰). This relatively small contribution of A. marlothii nectar to assimilated carbon in whole blood contrasted with that of exhaled CO₂ in African Red‐eyed Bulbuls Pycnonotus nigricans and Cape White‐eyes Zosterops capensis. In both these species, the δ¹³C of breath samples was significantly enriched compared with blood and feathers, and closely resembled that of the nectar, revealing combustion of ingested nectar rather than assimilation. Although our analysis was complicated by the presence of C₄ grasses, whose δ¹³C values are similar to those of CAM photosynthesizers, when considered with previously published feeding observations our data reveal that opportunistic nectarivores feeding on A. marlothii nectar obtain a relatively small fraction of their assimilated carbon, but most of their metabolized carbon, from this seasonally available carbohydrate food resource. Because the δ¹³C values of insects associated with C₃ plants also became enriched during the flowering season, some insect‐eating opportunistic nectarivores may have assimilated A. marlothii carbon indirectly from insects. This study highlights the importance of understanding isotopic routing when assessing the nutritional significance of specific dietary items to consumer communities.     

Move that fatty acid: fuel selection and transport in migratory birds and bats

The metaphor of marathon running is inadequate to fully capture the magnitude of long-distance migratory flight of birds. In some respects a journey to the moon seems more appropriate. Birds have no access to supplementary water or nutrition during a multi-day flight, and they must carefully budget their body fat and protein stores to provide both fuel and life support. Fatty acid transport is crucial to successful non-stop migratory flight in birds. Although fat is the most energy-dense metabolic fuel, the insolubility of its component fatty acids makes them difficult to transport to working muscles fast enough to support the highly aerobic exercise required to fly. Recent evidence indicates that migratory birds compensate for this by expressing large amounts of fatty acid transport proteins on the membranes of the muscles (FAT/CD36 and FABPpm) and in the cytosol (H-FABP). Through endogenous mechanisms and/or diet, migratory birds may alter the fatty acid composition of the fat stores and muscle membranes to improve endurance during flight. Fatty acid chain length, degree of unsaturation, and placement of double bonds can affect the rate of mobilization of fatty acids from adipose tissue, utilization of fatty acids by muscles, and whole-animal performance. However, there is great uncertainty about how important fatty acid composition is to the success of migration or whether particular types of fatty acids (e.g., omega-3 or omega-6) are most beneficial. Migratory bats provide an interesting example of evolutionary convergence with birds, which may provide evidence for the generality of the bird model to the evolution of migration by flight in vertebrates. Yet only recently have attempts been made to study bat migration physiology. Many aspects of their fuel metabolism are predicted to be more similar to those of migrant birds than to those of non-flying mammals. Bats may be distinct from most birds in their potential to conserve energy by using torpor between flights, and in the behavioral and physiological trade-offs they may make between migration and reproduction, which often overlap.