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Summary 1.The role of nutrition in linking animals with their environment is increasingly seen as fundamental to explain ecological interactions. 2.The two currently predominant frameworks for exploring questions in nutritional ecology—Nutritional Geometry (NG) and Ecological Stoichiometry (ES)—share common features, but also differ in their goals and origins. NG originates from behavioural ecology using terrestrial insects as model organisms in tightly controlled feeding experiments, while ES originates from biogeochemistry focusing on the transfer of key elements across trophic levels, mainly in aquatic environments. 3.Here, we review the history of these two complementary frameworks, emphasizing the key concepts defining their respective aims, methodologies, and focal taxa to answer questions at different ecological scales. 4.We identify and explore homeostasis as a shared conceptual cornerstone of each framework that can be used to bridge knowledge gaps and for developing new hypotheses within nutritional ecology. 5.Expanding on the concept of homeostasis, we introduce dynamic energy budget (DEB) models as a general way to address homeostatic regulation at its fundamental level. 6.Specifically, we describe how a two-reserve DEB model can be used to track metabolic pathways of nutrients as well as elements and suggest that multi-reserve DEB models, when integrated and parameterized with NG and ES concepts, can form powerful components of agent-based models to predict how animal nutrition influences individual and trophic interactions in food webs. This article is protected by copyright. All rights reserved.

Summary 1. Body condition indices, measures of body ‘plumpness’ or mass relative to frame size, are often used as a proxy for lipid reserves or fitness-related traits of animals and assumed to be positively related to fitness. 2. The quantification and analysis of body condition indices has been the subject of debate for decades. Here, we summarize three additional concerns with the use of body condition indices. 3. First, body condition index is often poorly correlated with lipid content in animals. Second, even if body condition index and lipid content are correlated, lipid content of an animal may not be the most important aspect of body composition influencing fitness. Finally, neither body condition index nor lipid reserves are likely to be directly positively related to fitness in animals, as many animals homeostatically regulate intermediate levels of condition index or lipid reserves, with both higher and lower values incurring fitness costs. 4. A wide range of analytical methods, including some relatively inexpensive and simple measures, are available for more detailed measures of animal body composition or fitness-related traits. Replacing body condition indices with more direct measures of body composition – even relatively simple measures – can inform understanding of the physiological mechanisms underlying animal responses in a wide range of behavioural, ecological and evolutionary studies.

Summary Herbivores face various nutritional challenges in their life cycles, challenges that may become increasingly acute under ongoing environmental changes. Here, focusing on calcium, phosphorus and nitrogen, we used nutritional geometry to analyse individual-based data on foraging and extraction efficiencies, and combined these with data on reproduction and migratory behaviour to understand how a large herbivorous carnivore can complete its life cycle on a narrow and seemingly low quality bamboo diet. Behavioural results showed that pandas during the year switched between four main food categories involving the leaves and shoots of two bamboo species available. Nutritional analysis suggests that these diet shifts are related to the concentrations and balances of calcium, phosphorus and nitrogen. Notably, successive shifts in range use and food type corresponded with a transition to higher concentrations and/or a more balanced intake of these multiple key constituents. Our study suggests that pandas obligatorily synchronize their seasonal migration and reproduction with the disjunct nutritional phenologies of two bamboo species. This finding has potentially important implications for habitat conservation for this species and, more generally, draws attention to the need for understanding the nutritional basis of food selection in devising management plans for endangered species.

Models of the regulatory behaviour of organisms are fundamental to a strong physiologically-based understanding of species' responses to global environmental change. Biophysical models of heat and water exchange in organisms (biophysical ecology) and nutritionally-explicit models for understanding feeding behaviour and its fitness consequences (the Geometric Framework of nutrition, GF) are providing such an underpinning. However, temperature, water and nutrition interact in fundamental ways in influencing the responses of the organism to their environment, and a priority is to develop an integrated approach for conceptualising and measuring these interactions. Ideally, such an approach would be based on a thermodynamically-formalized energy and mass budgeting approach that is sparsely parameterised and sufficiently general to apply across a range of situations and organisms. Here we illustrate how mass-balance aspects of Dynamic Energy Budget theory can be applied to obtain first-principles estimates of fluxes of O2, CO2, H2O and nitrogenous waste. Then, using an herbivorous lizard (Egernia cunninghami) as a case study, we demonstrate how these estimates can be integrated with heat/water exchange models and environmental data to provide a holistic understanding of how foraging strategy, food availability, habitat and weather interact with heat, water and nutrient/energy budgets across the life-cycle. The analysis shows the potential importance of the water balance in affecting the energy budgets of ' dry skinned' ectotherms, especially early in ontogeny, and highlights a significant gap in our knowledge of the physiological and behavioural traits that affect water balance when compared with our knowledge of thermal traits. In general, the modelling approach we describe can provide the thermodynamically-constrained stage on which other evolutionary and ecological interactions play out; the 'thermodynamic niche'. This in turn provides a solid foundation from which to tackle key questions about organismal responses to environmental change. © 2012 British Ecological Society.

1. Diet and health are intimately linked and recent studies have found that caloric restriction can affect immune function. However, when given a choice between diets that differ in their macronutrient composition, pathogen-infected individuals can select a diet that improves their survival, suggesting that the nutritional composition of the diet, as well as its calorie content, can play a role in defence against disease. Moreover, as individuals change their diet when infected, it suggests that a diet that is optimal for growth is not optimal for immunity, leading to trade-offs. 2. Currently, our knowledge of the effects of diet on immunity is limited because previous experiments have manipulated either single nutrients or the calorie content of the diet without considering their interactive effects. By simultaneously manipulating both the diet composition (quality) and its caloric density (quantity), in both naive and immune-challenged insects, we asked how do diet quality and quantity influence an individual's ability to mount an immune response? And to what extent are allocation trade-offs driven by quantity- versus quality-based constraints? 3. We restricted individuals to 20 diets varying in their protein and carbohydrate content and used 3D response surfaces to visualize dietary effects on larval growth and immune traits. Our results show that both constitutive and induced immune responses are not limited by the total quantity of nutrients consumed, but rather different traits respond differently to variation in the ratios of macronutrients (diet quality), and peak in different regions of macronutrient space. The preferred dietary composition therefore represents a compromise between the nutritional requirements of growth and immune responses. We also show that a non-pathogenic immune challenge does not affect diet choice, rather immune-challenged insects modify their allocation of nutrients to improve their immune response. 4. Our results indicate that immune traits are affected by the macronutrient content of the diet and that no diet can simultaneously optimize all components of the immune system. To date the emphasis has been on the effects of micronutrients in improving immunity, our findings indicate that this must be widened to include the neglected impact of macronutrients on defence against disease.

Summary 1. Marine herbivorous fishes are considered to be of critical importance in determining the biological structure of shallow reef environments, and by implication have key roles in carbon flux in reef ecosystems. Despite this, the nutritional processes that underpin these critical ecological roles have received comparatively little attention. 2. Here we give an overview of recent progress in the nutritional ecology of marine herbivorous fishes, and then examine two recent paradigms that we consider important in the development of the field: (i) the role of temperature in latitudinal gradients of diversity and abundance, and (ii) the impact of these fishes on coral reefs. Our aim is to illustrate how an integrative nutritional ecology approach can enrich insights gained from studies of fish herbivory, and to emphasize the distinctive differences between herbivory in marine and terrestrial systems. 3. We argue that much of the work on trophic biology in marine herbivorous fishes has focused on the ecological impacts of fishes on reefs, the deterrent properties of marine algae, and the morphological and mechanical aspects of ingestion. This has come at the expense of two of the elements necessary for an integrative understanding of feeding ecology, that is, food composition and the physiological processes involved in nutrient extraction and utilization. Together, these factors have hindered the development of the nutritional framework for analysing food resources, feeding patterns and evolutionary trends that has proved successful for terrestrial vertebrate herbivores. 4. The reef grazing and algal secondary metabolite paradigms, while both extremely productive, have failed to develop the clear predictive framework for diet choice required in broader contexts such as reef management and understanding the evolution of herbivory. The lack of a focus on nutritional factors has led to premature conclusions on the influence of temperature on algal digestion, both at the level of digestive processes and the biogeography of marine faunas. 5. Some marine herbivorous fishes appear to be bending the ‘rules’ of hindgut fermentation, especially with respect to temperature and fermentation substrates, and so the study of nutritional ecology in these animals has potential to generate novel insights for the field of vertebrate nutrition

Summary 1. The science of nutritional ecology spans a wide range of fields, including ecology, nutrition, behaviour, morphology, physiology, life history and evolutionary biology. But does nutritional ecology have a unique theoretical framework and research program and thus qualify as a field of research in its own right? 2. We suggest that the distinctive feature of nutritional ecology is its integrative nature, and that the field would benefit from more attention to formalizing a theoretical and quantitative framework for developing this. 3. Such a framework, we propose, should satisfy three minimal requirements: it should be nutritionally explicit, organismally explicit, and ecologically explicit. 4. We evaluate against these criteria four existing frameworks (Optimal Foraging Theory, Classical Insect Nutritional Ecology, the Geometric Framework for nutrition, and Ecological Stoichiometry), and conclude that each needs development with respect to at least one criterion. 5. We end with an initial attempt at assessing the expansion of our own contribution, the Geometric Framework, to better satisfy the criterion of ecological explicitness.

‘Nutritional ecology’ is a bit like ‘art’: easy to recognize but difficult to define. We believe that this is, in part, because the field has progressed on a broad (even diffuse) front, and has lacked cohesion or commonality of purpose. Our aim in this special feature is to bring together leading researchers drawn from across this spectrum and have them review what they consider to be the key issues in their respective research areas. The mandate for invited authors was broad, with invitations that were no more specific than: ‘contribute a paper that provides an overview of the achievements, opportunities and limitations of nutritional ecology studies of [respective taxon/ thematic research area]’. The special feature, we hope, will provide a lens that focuses attention on similarities and highlights disparities across the field. In this introduction we will attempt to define nutritional ecology and its relationship to functional ecology, justify why it warrants a special issue, and summarise the scope of the issue.

where C is the energy consumed, F is the energy content of the faeces and U is the energy content of the nitrogenous wastes (e.g. urine). Similar indices now appear in many entomological papers (Waldbauer 1968; Slansky & Scriber 1982, 1985). Ratios remain an ambivalent issue in biology, as is illustrated in the following quotation: '... there are some serious drawbacks to the use of ratios and percentages in statistical work... [but] ... we should emphasise that ratios may be the only meaningful way to interpret and understand certain types of biological problems' (Sokal & Rohlf 1981). Here I use the nutritional indices ADC and MEC to examine some of the issues involved in deciding on whether to analyse ratios or the parent variables. I conclude that there are considerable costs associated with the use of ratios and little justification for the claim that they provide information not available through analysis of the measured variables.

1. We recommend the use of bicoordinate plots, termed utilization plots, for the analysis of nutrient budgets. 2. Utilization plots explore the relationship between nutrient uptake (intake or absorption from the gut) and the various compartments to which ingested nutrients are allocated. 3. They provide more information, and are numerically less problematic, than ratio-based nutritional indices