The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Food limitation is an important issue in ecology because it can influence life history traits, population sizes, and community structure (through effects of competition). Work at the level of populations and communities has led to arguments that food limitation and competition are more important in winter than during the breeding season (e.g. 1, 2, 7, 8, 83, 85, 211, 234). In fact, it is commonly argued that food is superabundant during the breeding season (7, 8, 161, 213, 246, 250, 251, 308-311). However, such arguments are based on indirect rather than direct evidence of the effects of food on reproduction and survival (fitness) (149). Direct evidence for food limitation and competition in winter exists when survival is affected, and some experimental evidence suggests such effects (e.g. 75, 113, 122, 132). However, if current or future reproductive success is limited by food, then food limitation will also exist during the breeding season. Reproductive ecologists historically have argued that food limits reproductive success (e.g. 13, 297). Yet, even this school has included recent arguments against food limitation. For instance, Ettinger & King (77) think that perching time of birds commonly reflects loafing time because birds set their clutch and brood sizes based on years and periods of stringent (low food) conditions. However, perching time may not reflect loafing but rather an important time commitment to reproductive success (T. Martin, unpublished ms.) Thus, the status of food limitation in breeding birds is not clear. Here, I review evidence for food limitation in the context of life history theory because it provides a fundamental framework from which to interpret

Food as a limit on breeding birds: a life-history perspective

▪ Abstract The functional causes of life history trade-offs have been a topic of interest to evolutionary biologists for over six decades. Our review of life history trade-offs discusses conceptual issues associated with physiological aspects of trade-offs, and it describes recent advances on this topic. We focus on studies of four model systems: wing polymorphic insects, Drosophila, lizards, and birds. The most significant recent advances have been: (a) incorporation of genetics in physiological studies of trade-offs, (b) integration of investigations of nutrient input with nutrient allocation, (c) development of more sophisticated models of resource acquisition and allocation, (d) a shift to more integrated, multidisciplinary studies of intraspecific trade-offs, and (e) the first detailed investigations of the endocrine regulation of life history trade-offs.

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The Physiology of Life History Trade-Offs in Animals

© 2000 by Blackwell Science Ltd. All rights reserved. Issues of scale have become increasingly important to ecologists. This book addresses the structure of regional (large-scale) ecological assemblages or communities, and the influence this has at a local (small-scale) level. This macroecological perspective is essential for the broader study of ecology because the structure and function of local communities cannot be properly understood without reference to the region in which they are situated. The book reviews and synthesizes the issues of current importance in macroecology, providing a balanced summary of the field that will be useful for biologists at advanced undergraduate level and above. These general issues are illustrated by frequent reference to specific well-studied local and regional assemblages -- an approach that serves to relate the macroecological perspective (which is perhaps often difficult to comprehend) to the everyday experience of local sites. Macroecology is an expanding and dynamic discipline. The broad aim of the book is to promote an understanding of why it is such an important part of the wider program of research into ecology. Summarises the current macroecological literature. Provides numerous examples of key patterns. Explicitly links local and regional scale processes. Exploits detailed knowledge of one species assemblage to explore broad issues in the structuring of biodiversity.

http://www.gbv.de/dms/hebis-darmstadt/toc/93195257.pdf

Pattern and Process in Macroecology

Increasingly, levels of the ‘stress hormones’ cortisol and corticosterone are being used by ecologists as indicators of physiological stress in wild vertebrates. The amplitude of hormonal response is assumed to correlate with the overall health of an animal and, by extension, the health of the population. However, much of what is known about the physiology of stress has been elucidated by the biomedical research community. I summarize five physiological mechanisms that regulate hormone release during stress that should be useful to ecologists and conservationists. Incorporating these physiological mechanisms into the design and interpretation of ecological studies will make these increasingly popular studies of stress in ecological settings more rigorous. Vertebrates cope with unpredictable and noxious stimuli (STRESSORS; see Glossary) by mounting a STRESS RESPONSE

https://www.cell.com/trends/ecology-evolution/pdf/S0169-5347(04)00063-1.pdf

Physiological stress in ecology: lessons from biomedical research

Avian Energetics and Nutritional Ecology.

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Physiological Diversity and Its Ecological Implications

The physical environmental factors (air temperature, solar radiation, wind speed) that define specific microclimates and their effects on water and energy budgets of small birds are of major importance to our understanding of avian thermal biology. We examined the effects of solar radiation, wind speed, and their interaction on metabolic rates in the Verdin, Auriparus flaviceps. Daytime resting metabolic rates and evaporative water loss rates as a function of air temperature, as well as basal metabolic rate, were also measured to allow estimation of water and energy flux rates in diverse microclimates. In the absence of solar radiation, as wind speed was increased from 0.4 to 3.0 m/s, metabolic rate increased 14%. Exposure to simulated solar radiation significantly reduced metabolic heat production at all wind speeds measured except 3.0 m/s. Solar heat gain (SHG) was estimated for an irradiance of 1000 W/m2, similar to that commonly observed in nature. At 0.4 m/s wind speed and 1000 W/m2 irradiance, SHG may reduce metabolic rate by 46%. SHG declines precipitously as wind speed is increased, and at 3.0 m/s, metabolic rate is only reduced by 3%. Analyses of changes in thermostatic costs associated with microclimate selection in winter suggest that Verdins may reduce metabolic rate by as much as 50% by shifting from a shaded, windy site to one protected from the wind and exposed to 1000 W/m2 solar radiation. Similar analyses for Verdins during the summer suggest that microsite selection can result in significant water savings. By remaining out of the sun and wind, Verdins can reduce their rate of evaporative water loss by at least a factor of four. This analysis clearly dem- onstrates the potential importance of daytime microclimate selection to balancing water and energy budgets in small birds.

Thermal Effects of Radiation and Wind on a Small Bird and Implications for Microsite Selection

Fitness tradeoffs are difficult to examine because many fitness variables are correlated and vary in the same direction. Phenotypic manipulation circumvents many of these difficulties, and here we used this technique to examine mechanisms for tradeoffs between increased aggression (territorial defense) and survivorship. The behavioral phenotype of male mountain spiny lizards (Sceloporus jarrovi) was manipulated with testosterone to increase territorial defense, a sexually selected trait. We previously demonstrated that increased territorial defense results in a decrease in survival caused by a lower ratio of energy intake to energy expenditure. Here we measured energy consumption of increased territorial aggression using the doubly labeled water technique in the field and compared males with and without testosterone implants (Fig. 1). In a supplementary study we measured standard metabolic rate using captive lizards given similar testosterone implants to examine if an increase in energy expenditure was a result of only an increase in standard metabolic rate (Fig. 3). Our results indicated that a primary contribution to tradeoffs between increased territorial defense and survivorship could be made by a 31% increase in energy expenditure in the field that is not due to an increase in standard metabolic rate.

Increased energy expenditure due to increased territorial defense in male lizards after phenotypic manipulation

Indirect calorimetry is a commonly used method for estimating an animal’s metabolic energy expenditure based on rates of oxygen consumption or carbon dioxide production. An important complication in such analyses is that the relationship between respiratory gas exchange and power consumption is variable; the thermal equivalent of gas consumed or produced varies with the metabolic pathways used and the substrate catabolized. Workers must therefore deduce or assume what mixture of materials constitutes the metabolic substrate. The index that is customarily used in this deduction is the animal’s respiratory quotient (RQ) of CO2 produced to O2 consumed. (Although RQ is formally the exchange ratio of these gases at the cellular level, we here follow the convention for intact vertebrates and assume that it equals the gas exchange ratio at the lungs.) For example, an RQ of 1.00 characterizes carbohydrate catabolism. An RQ of 0.71 is the minimum ratio expected and is taken as indicating reliance on lipid substrates. For animals catabolizing only lipids and carbohydrates, the ratio of these materials in the substrate used can be calculated from values of RQ between 0.71 and 1.00 (Kleiber, 1961). Possible catabolism of proteins presents difficulties because of the variable effects of nitrogen excretion. Therefore, in animals such as birds and mammals, whose dominant nitrogenous

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Glenn E. Walsberg

Papers

Avian Energetics and Nutritional Ecology.

Physiological Diversity and Its Ecological Implications

Thermal Effects of Radiation and Wind on a Small Bird and Implications for Microsite Selection

Increased energy expenditure due to increased territorial defense in male lizards after phenotypic manipulation

Variation in the respiratory quotient of birds and implications for indirect calorimetry using measurements of carbon dioxide production