Metabolic routing of dietary nutrients in birds: effects of diet quality and macronutrient composition revealed using stable isotopes

During fall migration many songbirds switch from consuming primarily insects to consuming mostly fruit. Fruits with more carbohydrates and less protein may be sufficient to rebuild expended fat stores, but such fruits may be inadequate to replace catabolized protein. We manipulated the concentrations and isotopic signatures of macronutrients in diets fed to birds to study the effects of diet quality on metabolic routing of dietary nutrients. We estimated that approximately 45% and 75%, respectively, of the carbon in proteinaceous tissue of birds switched to high- or low-protein diets came from nonprotein dietary sources. In contrast, we estimated that approximately 100% and 20%-80%, respectively, of the nitrogen in proteinaceous tissues of birds switched to high- or low-protein diets was attributable to dietary protein. Thus, the routing and assimilation of dietary carbon and nitrogen differed depending on diet composition. As a result, delta (15)N of tissues collected from wild animals that consume high-quality diets may reliably indicate the dietary protein source, whereas delta (13)C of these same tissues is likely the product of metabolic routing of carbon from several macronutrients. These results have implications for how isotopic discrimination is best estimated and how we can study macronutrient routing in wild animals.     

Ontogeny and nutritional status influence oxidative kinetics of nutrients and whole-animal bioenergetics in zebra finches, Taeniopygia guttata: new applications for (13)C breath testing

Rapidly growing animals or those that are recovering from nutritional stress may use exogenous nutrients differently from well fed adults. To test this possibility, we compared the rates of exogenous nutrient oxidation among fledgling, fasted adult, and refed adult zebra finches using a technique called breath testing, where animals are fed (13)C-labeled nutrients and (13)C in the exhaled breath is collected and quantified. In order to identify the possible mechanisms responsible for differences in oxidative kinetics of ingested nutrients, we also compared body mass (m(b)), organ mass, core body temperature (T(b)), and metabolic rate (MR). We found that fasted birds had lower T(b), relative liver and intestine masses, MR, and respiratory exchange ratios (RERs) than fed adults. Adult birds recovering from nutritional stress had much lower rates of exogenous nutrient oxidation than fed birds; this difference was particularly evident for fatty acids. Differences in oxidative kinetics were correlated with reduced RER, m(b), and liver mass, suggesting that previously fasted birds were using recently assimilated nutrients to replenish exhausted fuel stores. Rapidly growing fledglings oxidized exogenous nutrients as quickly as fed adults, despite their significantly lower m(b) and T(b). We suggest that fledglings had higher mass-specific rates of exogenous nutrient oxidation because they must compensate for the relatively low conversion efficiency of feather production and other lean tissue growth, which was not taking place in the adults. Although this study demonstrates that ontogeny and nutritional status influence the way that birds oxidize exogenous nutrients, it also underscores the likelihood that environmental and endogenous factors shape how other types of animals spend the nutrients they ingest.     

During stopover, migrating blackcaps adjust behavior and intake of food depending on the content of protein in their diets

During migration, birds undergo alternating periods of fasting and re-feeding that are associated with dynamic changes in body mass (m(b)) and in organ size, including that of the digestive tract. After arrival at a migratory stopover site, following a long flight, a bird must restore the tissues of its digestive tract before it can refuel. In the present study we examined how the availability of dietary protein influences refueling of migrating blackcaps (Sylvia atricapilla) during a migratory stopover. We tested the following predictions in blackcaps deprived of food and water for 1-2 days to induce stopover behavior: (1) birds provided with a low-protein diet will gain m(b), lean mass and fat mass, and increase in pectoral muscle size slower than do birds fed a high-protein diet; (2) since stopover time is shorter in spring, birds will gain m(b) and build up fat tissue and lean tissue faster than in autumn; and (3) if low dietary protein limits a bird's ability to gain m(b) and fat reserves, then birds that do not obtain enough protein will initiate migratory restlessness (Zugunruhe) earlier than will birds with adequate dietary protein. These predictions were tested by providing captured migrating blackcaps with semisynthetic isocaloric diets differing only in their protein content. Each day, we measured m(b), and food intake; also lean mass and fat mass were measured using dual energy X-ray absorptiometry. In addition, we monitored nocturnal activity with a video recording system. In both spring and autumn, birds fed diets containing either 3 or 20% protein increased in m(b), lean mass and fat mass at similar rates during the experiment. However, the group receiving 3% protein ate more than did the group receiving 20% protein. In support of our predictions, m(b), lean mass, fat mass, and intake of food all were higher in spring than in autumn. We also found that in spring all birds had higher levels of migratory restlessness, but birds fed 3% protein were less active at night than were birds fed 20% protein, possibly an adaptation conserving energy and protein. We conclude that protein requirements of migrating blackcaps during stopover are lower than expected, and that birds can compensate for low dietary protein by behavioral responses, i.e. hyperphagia and decreased migratory restlessness, that ensure rapid refueling.     

The development of a synthetic diet for investigating the effects of macronutrients on the development of Plodia interpunctella

The use of chemically defined artificial diets has allowed researchers to examine questions within nutritional ecology about how macronutrients affect life‐history traits and resource‐based trade‐offs. Using a chemically defined diet, it is possible to manipulate both the total nutritional content and the ratio of macronutrients (i.e., proteins, carbohydrates, or lipids) within the diet. Studies using the geometric framework have made use of these diets to examine lifespan, fecundity, and immune responses. Here, we develop an artificial diet suitable for rearing lepidopteran larvae. We created diets with three proportions of non‐nutritive material (30, 50, and 70% indigestible cellulose) relative to protein and carbohydrate macronutrients, and compared these to standard wheat bran laboratory diet. We then examined the effects of variable nutrient content on lifespan and development time in Plodia interpunctella Hübner (Lepidoptera: Pyralidae). The artificial diets supported development (almost) as well as bran‐based laboratory diets. Total nutrient content affected development time: females that fed on the diet with the highest nutrient content took the longest time to reach eclosion. We also found evidence to support dietary restriction, with larvae receiving the fewest nutrients having the longest lifespan as adults. These findings are indicative of the usefulness of this diet as a tool to further investigate the effects of nutrient content and macronutrient imbalance on resource‐based trade‐offs and life‐history traits.     

GRACILARIA EDULIS (RHODOPHYTA) AS A BIOLOGICAL INDICATOR OF PULSED NUTRIENTS IN OLIGOTROPHIC WATERS

The response of the marine macroalga Gracilaria edulis (Gmelin) Silva to nutrient pulses of varying magnitude was investigated to test its applicability as a marine bioindicator at two oligotrophic locations. After exposure to nutrient pulses, algal amino acid, tissue nitrogen, and chlorophyll a content were assessed relative to algae incubated under control conditions (no nutrient enrichment). The smallest nutrient pulse involved a nutrient enrichment experiment conducted within a coral atoll, whereas two larger pulses resulted from sewage discharge to a tropical coastal bay. After exposure to the smallest nutrient pulse (10 × ambient), only changes in macroalgal amino acid concentration and composition were detected (mainly as increases in citrulline). At 100 × ambient concentrations, increases in tissue % nitrogen of the macroalgae were detected, in addition to responses in amino acids. Macroalgae exposed to the highest nutrient pulse (1000 × ambient) responded with increased chlorophyll a , tissue nitrogen, and amino acids within the three day incubation period. In contrast to these algal responses, analytical water sampling techniques failed to detect elevated nutrients when nutrient pulses were not occurring. The responses of this algal bioindicator to variable nutrient pulses may provide a useful tool for investigating the source and geographical extent of nutrients entering oligotrophic coastal waters.     

Tissue mass dynamics during egg-production in female Zebra Finches Taeniopygia guttata: dietary and hormonal manipulations

Changes in tissue masses associated with egg production were investigated in female Zebra Finches Taeniopygia gutatta using dietary and hormonal manipulations. We tested three hypotheses: that changes in organ masses, (a) reflect utilisation of endogenous nutrient stores due to inadequate daily dietary intake, (b) involve changes in organ structure or 'functional capacity', and (c) are initiated by onset of reproductive development (e.g. elevated plasma estrogen or yolk precursor levels, oviduct growth). Pectoral muscle lean dry mass was 18–22% lower in breeding females at the 1-egg stage compared to non-breeders, and this was independent of nutritional plane, i.e. similar changes occurred in birds provided with supplemental protein or egg food. Heart lean dry mass was also lower (16%) in breeding females, but only in birds on a low-quality seed diet, not in birds on supplemented diets. Decreases in total liver mass (14%) were due to changes in lipid content not lean dry mass, and were diet-dependent. These results demonstrate that changes in organ masses associated with egg production are complex, and do not simply reflect a general mobilisation of stored protein. We discuss why there is no hypertrophy of biosynthetic or metabolic 'machinery' associated with egg production in birds (cf. reproducing mammals). Exogenous 17β-estradiol induced plasma levels of yolk precursors typical of breeding birds, and initiated oviduct growth (to 31% of mature size). However, estradiol treatment caused no change in mass of pectoral muscle, heart or liver, demonstrating that there is no simple relationship between onset of reproductive development and associated tissue mass changes.     

Balancing of lipid,protein, and carbohydrate intake in a predatory beetle following hibernation,and consequences for lipid restoration

Carnivorous animals are known to balance their consumption of lipid and protein, and recent studies indicate that some mammalian carnivores also regulate their intake of carbohydrate. We investigated macronutrient balancing and lipid restoration following hibernation in the ground beetle Anchomenus dorsalis, hypothesizing that carbohydrates might be important energy sources upon hibernation when predator lipid stores are exhausted and prey are equally lean. We recorded the consumption of lipid, protein, and carbohydrate over nine days following hibernation, as the beetles foraged to refill their lipid stores. Each beetle was given the opportunity to regulate consumption from two semi-artificial foods differing in the proportion of two of the three macronutrients, while the third macronutrient was kept constant. When analyzing consumption of the three macronutrients on an energetic basis, it became apparent that the beetles regulated lipid and carbohydrate energy interchangeably and balanced the combined energy intake from the two macronutrients against protein intake. Restoration of lipid stores was independent of the availability of any specific macronutrient. However, the energetic consumption required to refill lipid stores was higher when a low proportion of lipids was ingested, suggesting that lipids were readily converted into lipid stores while there were energetic costs associated with converting carbohydrate and protein into stored lipids. Our experiment demonstrates that carbohydrates are consumed and regulated as a non-protein energy source by A. dorsalis despite an expectedly low occurrence of carbohydrates in their natural diet. Perhaps carbohydrates are in fact an overlooked supplementary energy source in the diet of carnivorous arthropods.     

Responses to nutrients in farm animals: implications for production and quality

It is well known that any quantitative (energy and protein levels) and qualitative (nature of the diet, nutrient dynamic) changes in the feeding of animals affect metabolism. Energy expenditure and feed efficiency at the whole-body level, nutrient partitioning between and within tissues and organs and, ultimately, tissue and organ characteristics are the major regulated traits with consequences on the quality of the meat and milk produced. Recent progress in biology has brought to light important biological mechanisms which explain these observations: for instance, regulation by the nutrients of gene expression or of key metabolic enzyme activity, interaction and sometimes cross-regulation or competition between nutrients to provide free energy (ATP) to living cells, indirect action of nutrients through a complex hormonal action, and, particularly in herbivores, interactions between trans-fatty acids produced in the rumen and tissue metabolism. One of the main targets of this nutritional regulation is a modification of tissue insulin sensitivity and hence of insulin action. In addition, the nutritional control of mitochondrial activity (and hence of nutrient catabolism) is another major mechanism by which nutrients may affect body composition and tissue characteristics. These regulations are of great importance in the most metabolically active tissues (the digestive tract and the liver) and may have undesirable (i.e. diabetes and obesity in humans) or desirable consequences (such as the production of fatty liver by ducks and geese, and the production of fatty and hence tasty meat or milk with an adapted fatty acid profile).     

Towards a synthesis of frameworks in nutritional ecology: interacting effects of protein,carbohydrate and phosphorus on field cricket fitness

Phosphorus has been identified as an important determinant of nutrition-related biological variation. The macronutrients protein (P) and carbohydrates (C), both alone and interactively, are known to affect animal performance. No study, however, has investigated the importance of phosphorus relative to dietary protein or carbohydrates, or the interactive effects of phosphorus with these macronutrients, on fitness-related traits in animals. We used a nutritional geometry framework to address this question in adult field crickets (Gryllus veletis). Our results showed that lifespan, weight gain, acoustic mate signalling and egg production were maximized on diets with different P : C ratios, that phosphorus did not positively affect any of these fitness traits, and that males and females had different optimal macronutrient intake ratios for reproductive performance. When given a choice, crickets selected diets that maximized both lifespan and reproductive performance by preferentially eating diets with low P : C ratios, and females selected diets with a higher P : C ratio than males. Conversely, phosphorus intake was not regulated. Overall, our findings highlight the importance of disentangling the influences of different nutrients, and of quantifying both their individual and interactive effects, on animal fitness traits, so as to gain a more integrative understanding of their nutritional ecology.     

Differential responses of plants,reptiles and birds to grazing management,fertilizer and tree clearing

The relative effects of tree clearing, increased livestock densities and nutrient enrichment have rarely been compared across markedly different organism types, but negative effects are generally predicted. In contrast, adoption of rotational grazing is thought to benefit biodiversity in pastures but there are few supporting data. We examined the response of native plants, birds and reptiles to livestock management in south‐eastern Australia. We selected 12 pairs of rotationally and continuously grazed farms. Two 1‐ha plots were established in native pastures on each farm, one cleared and the second still retaining woodland tree cover. Stocking rates, fertilizer histories and landscape tree cover varied among farms. The abundance and richness of all taxa was lower in cleared pastures. The less mobile organisms (reptiles and plants) were positively correlated with tree cover at landscape scales, but only when trees were present at the plot scale. This pattern was driven by a few observations in landscapes with approximately 50% tree canopy cover. Neither bird abundance nor richness was correlated with stocking rates or nutrient enrichment, but plant richness responded negatively to both. The response of reptiles varied, declining with nutrient enrichment but positively correlated with livestock densities. These responses may be partly interpreted within the context of prior filtering of species pools through long‐term grazing pressure. No taxa responded positively to rotational grazing management. We predict that reductions in livestock density and soil nutrients will directly benefit plants and less so reptiles, but not birds. Indirect benefits are predicted for birds and reptiles if management increases persistence of trees within paddocks. Although some forms of rotational grazing can increase woodland tree recruitment, rotational grazing in itself is unlikely to enhance diversity.     

Fire and soil-plant nutrient relations in a pine-wiregrass savanna on the coastal plain of North Carolina

Summary Changes in soil and plant nutrient conditions were evaluated following various burn and clip treatments in a longleaf pine-wiregrass savanna in Bladen Co., N.C., USA. Ground fires were found to add substantial quantities of N, P, K, Ca, and Mg to the soil, though not necessarily in forms immediately available to plants. Less than 1% of the total nitrogen in the charred residue (ash) is present as nitrate or ammonium. Considerable quantities of all nutrients examined were lost to the atmosphere during burning. Green leaf tissue in recently burned areas was consistently higher in N, P, K, Ca, and Mg compared to unburned areas. Howerver, when compared to similar tissues from clipped plots, burned area tissues were significantly higher in N, Ca, and Mg only. Data presented here suggest that tissue age significantly affects nutrient content and must be considered in any analysis of tissue nutrient content following burning. Within 4–6 months following fire, burned-area tissue nutrient content decreases to concentrations found in the unburned area. Burning resulted in initial enrichment of available soil nutrients including PO₄, K⁺, Ca⁺⁺, and Mg⁺⁺, however, NO₃ ^-, and NH₄ ⁺ concentrations in burned soil were not significantly different from unbruned soil. Soil and plant nutrient changes in an area burned two years in succession indicate that repeated burning may diminish nutrient availability. Plant response to various nutrient enrichment treatments of the soil indicated that nitrogen is limiting growth in both burned and unburned soils and that burning may alter some factors other than nutrients which may retard plant growth in unburned areas.     

Nutrients in fruits as determinants of resource tracking by birds

Fruit pulp is an important source of nutrients for many bird species. Fruit‐eating birds use a variety of strategies to cope with changes in the availability of fruits, exhibiting a remarkable ability to track resources. We assessed the role of nutrient availability in the fruiting environment as a factor driving resource tracking by fruit‐eating birds. Fruit consumption by the four most common frugivorous species in a 6‐ha plot in the Southern Yungas montane forest of Argentina was assessed. We determined the content of selected nutrients (soluble carbohydrates, proteins, phenols, ascorbic acid and essential minerals) in 22 fruiting plant species eaten by birds, and measured fruit–frugivore interactions and the availability of nutrients and dry fruit pulp mass over 2 years. There was strong temporal covariation in the availability of the selected nutrients in fruits across the study period. Similarly, the availability of nutrients in the fruiting environment covaried with pulp mass. Fruit consumption by the four commonest bird species and the abundance of most species were positively associated with nutrient availability and dry pulp mass. Nutrient availability was a good predictor of temporal fruit tracking by three of the four commonest frugivores. Despite large differences in particular nutrient concentrations in fruits, overall nutrient (and pulp) quantity in the fruiting environment played a greater role in fruit tracking than did the nutritional quality of individual fruits. While overall nutrient availability (i.e. across fruit) and total pulp mass were important determinants of fruit tracking, we suggest that plant species‐specific differences in fruit nutrient concentration may be important in short‐term foraging decisions involved in fruit choice and nutritional balance of birds.     

滞育昆虫营养物质的积累、转化与调控

滞育是昆虫度过不良环境的一种生存适应策略,营养物质的利用、积累和转化影响昆虫的滞育。滞育期间,昆虫体内发生一系列生理生化变化,包括脂类、碳水化合物和氨基酸等内源营养物质呈特异性地积累或转化,保障了滞育个体在逆境中存活及滞育解除后发育的能量需求。外源营养物质对昆虫滞育的影响较复杂,其种类、丰度、质量能通过"食料-寄主-天敌"营养关系进行传递,影响昆虫体内营养物质的积累和转化,改变昆虫的耐寒性、滞育率等,制约昆虫的滞育深度、滞育存活率。滞育昆虫营养物质积累、利用机制复杂多样,胰岛素信号通路-叉头转录因子以及激脂激素的调控效应在滞育诱导及营养物质转化中起到了关键的调控作用,但滞育的系统性调控仍需进一步深入研究。     

Tracking Nutrient Routing in Avian Consumers in a Subtropical Desert

The nutrients animals ingest are allocated to serve different functions. We used contrasting C stable isotope signatures of dominant vegetation types in a North American subtropical desert to decipher how avian consumers allocate nutrients to fuel oxidative metabolism and to construct tissues. We conducted C stable isotope analysis of breath and feathers collected from nectarivores (hummingbirds) and of breath, plasma, and red blood cell samples collected from frugivores, granivores, and insectivores. Based on varying nutrient characteristics of food sources, we expected that for frugivores and granivores, CAM‐derived food (RC_CAM) would have similar importance for oxidative metabolism and for tissue building, that RC_CAM in nectarivores and insectivores would be more important for fueling metabolism than for generating tissues, and that (although low) RC_CAM in insectivores would be higher for sustaining metabolism than for building tissues. Our predictions held true for nectarivores and granivores, but RC_CAM use in tissue building was lower than expected in frugivores and higher than expected in insectivores. Our examination at the trophic guild, population, and individual levels showed that in general, nutrients used to sustain oxidative metabolism and tissue construction had a uniform isotopic origin. This finding suggests that the avian community under investigation does not route different food groups to fulfill different needs. However, we found some exceptions, indicating that birds can use different food sources for different functions, irrespective of trophic guild.     

Effect of high-fat diet on the expression of proteins in muscle, adipose tissues, and liver of C57BL/6 mice

In the present study, the effect of a high fat diet on the expression of proteins in insulin target tissues was analyzed using a proteomic approach. Gastrocnemius muscle, white and brown adipose tissue, and liver were taken from C57BL/6 mice either fed on a high-fat or a chow diet. Expression levels of approximately 10 000 polypeptides for all the four tissues were assessed by two-dimensional gel electrophoresis (2-DE). Computer-assisted image analysis allowed the detection of 50 significantly (p < 0.05) differentially expressed proteins between obese and lean mice. Interestingly, more than half of these proteins were detected in the brown adipose tissue. The differentially expressed proteins were identified by tandem mass spectrometry. Several stress and redox proteins were modulated in response to the high-fat diet. A key glycolytic enzyme was found to be downregulated in adipose tissues and muscle, suggesting that at elevated plasma fatty acid concentrations, fatty acids compete with glucose as an oxidative fuel source. Furthermore, in brown adipose tissue there were significant changes in mitochondrial enzymes involved in the Krebs tricarboxylic acid (TCA) cycle and in the respiratory chain in response to the high-fat diet. The brown adipose tissue is an energy-dissipating tissue. Our data suggest that the high-fat diet treated mice were increasing energy expenditure to defend against weight gain.     

Ideal nutrient productivities and nutrient proportions in plant growth

Abstract I propose that one single formulation can be applied to relate growth and content of several nutrients, including the most important macronutrients, of most plant species. The plant growth rate is proportional to the nutrient content minus a given minimal concentration of the nutrient in minimum. The proportionality factor, the nutrient productivity, and the minimum concentration are species specific properties. The nutrient productivity formulation is shown to apply for very different plant species and for different nutrients.     

Chemical differences between seeds and elaiosomes indicate an adaptation to nutritional needs of ants

Ant-dispersed plants usually produce seeds with appendages (elaiosomes) as reward for ants. Plants that produce high-quality elaiosomes benefit because ants preferentially disperse their diaspores. We therefore hypothesized that seeds and elaiosomes differ in chemical composition in ways that make elaiosomes of high nutritional quality for ants, capable of providing essential dietary components that explain the increased fitness and higher gyne production documented for colonies with elaiosome consumption. To test the hypothesis we analysed the content and composition of lipids, amino acids, soluble carbohydrates, proteins and starch in seeds and elaiosomes of 15 central European ant-dispersed plants. After separating the different fractions, total lipids were determined gravimetrically, fatty acids and soluble carbohydrates were detected by gas chromatography (GC) and GC–mass spectrometry, free amino acids by an amino acid analyser while starch and protein were analysed photometrically. Seeds accumulated high molecular weight compounds such as proteins and starch, whereas elaiosomes accumulated more easily digestible low molecular weight compounds such as amino acids and monosaccharides. Analysis of similarities and similarity percentages analysis demonstrated that the composition of fatty acids, free amino acids and carbohydrates differed markedly between elaiosomes and seeds. The most important difference was in total amino acid content, which was on average 7.5 times higher in elaiosomes than in seeds. The difference was especially marked for the nitrogen-rich amino acid histidine. The availability of essential nutrients and, in some species, the higher nitrogen content in elaiosomes suggest that their nutritional value for larvae plays a key role in this interaction. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Abnormal antioxidant defence in some tissues of congenitally obese mice.

The concentration of lipoperoxides (estimated as thiobarbituric acid-reactive material) and some components of the antioxidant defence system have been compared in various tissues of lean and congenitally obese mice. NADPH-stimulated lipoperoxide generation in vitro was significantly higher in microsomes (microsomal fractions) prepared from obese hepatic tissue than lean. Plasma, liver and brain lipoperoxide concentration was significantly higher in obese mice. In blood derived from obese mice the concentration of non-enzymic antioxidants including caeruloplasmin and vitamin A was higher, but hepatic retinol concentration was lower in these animals. In all the tissues assayed the glutathione peroxidase activity against H2O2 was less than its activity against cumene hydroperoxide. Assayed with either substrate, glutathione peroxidase activity was significantly higher in the brain and blood of obese mice than their lean counterparts. Conversely, liver glutathione peroxidase was decreased in obese animals, representing 43% of the activity of the lean-mouse liver enzyme against H2O2 and 81% of the cumene hydroperoxide-reducing activity. The liver of obese mice had significantly less, and the kidneys more, oxidized glutathione than the corresponding tissues of lean mice. Further investigations on hepatic tissue indicated that glutathione reductase activity was lower in the obese animals, but there was no significant difference between glucose-6-phosphate dehydrogenase activity in obese and lean mice.     

Predators buffer the effects of variation in prey nutrient content for nutrient deposition

Predator feeding behavior and digestion regulate the flow of nutrients through ecosystems by determining the fate of prey nutrients. Most predators feed on a diversity of prey items, which differ widely in traits including their nutrient content. Yet, relatively little is known of the mechanisms through which variation in prey nutrient content affects the form by which nutrients are deposited into the environment. The overall goal of this study was to test how variation in the nutrient content of prey affected the fate of nutrients following predation by an arthropod carnivore, the Carolina wolf spider Hogna carolinensis. We manipulated the macronutrient content of prey by varying the diet on which crickets were fed to produce prey treatments that differed in lipid and protein content. Nutrients were measured as both macronutrients and elements in prey and elements in excreta. We found that there was no effect of diet treatment on the amounts of elements or macronutrients in prey carcasses and excreta despite significant variation in the nutrient content of those prey. This is in contrast to studies of some aquatic systems where mass balance by consumers results in variation in excreta content depending on the nutrient content of food. Wolf spiders assimilated the majority of prey nutrients and deposited relatively small and similar amounts of nutrients following feeding. Hence, while prey can vary widely in nutrient content, our findings suggest that this variation has little effect on the amounts of nutrients deposited by predators.